## Vol. 64, No. 21 (2015)

##### 2015-11-05

###### SPECIAL ISSUE—Progress in Research of Superconductivity and Correlated Systems

2015, 64 (21): 214701.
doi: 10.7498/aps.64.214701

Abstract +

One-dimensional carbon chains are expected to show outstanding optical and mechanical properties. But synthesis of the compounds containing one-dimensional carbon chains is a challenging work, because of the difficulty in saturating the dangling bonds of carbon atoms. Recently, the transition from the Immm phase to the Cmcm one at a transition pressure 5 GPa has been predicted for Li2C2 by density-functional theory calculations. In Cmcm-Li2C2, there are one-dimensional zigzag carbon chains caged by lithium atoms. Under ambient pressure, the electronic structure of Cmcm-Li2C2 is as follows: The hybridization among 2s, 2py, and 2pz orbitals of carbon atoms results in three sp2-hybridized orbitals that are coplanar with the zigzag chains of these carbon atoms, denoted as the y-z plane. The sp2-hybridized orbitals along y-axis (perpendicular to the zigzag chain) overlap with each other and form one πup-bonding band and one πup ^*-antibonding band. Likewise, the 2p_x orbitals of carbon atoms will provide also one πup-bonding band and one π*-antibonding band. These two π*-antibonding bands cross the Fermi level and contribute to the metallicity of Cmcm-Li2C2. The other two sp2-hybridized orbitals will give two σ-bonding bands, whose band tops are about 5 eV below the Fermi energy level. These two fully occupied σ bands are the framework of the zigzag carbon chains. The changes in electronic structure of Cmcm-Li2C2 under 5 GPa are negligible, compared with that in case of ambient pressure. To our best knowledge, there is no report upon the superconductivity for compounds containing one dimensional carbon chains. We choose Cmcm-Li2C2 as a model system to investigate its electron-phonon coupling and phonon-mediated superconductivity. To determine the phonon-mediated superconductivity, the electron-phonon coupling constant λ and logarithmic average frequency ωlog are calculated based on density functional perturbation theory and Eliashberg equations. We find that λ and ωlog are equal to 0.63 and 53.8 meV respectively at ambient pressure for Cmcm-Li2C2. In comparison, both the phonon density of states and the Eliashberg spectral function α2F(ω) are slightly blue-shifted at a pressure of 5 GPa. Correspondingly, λ and ωlog are calculated to be 0.56 and 58.2 meV at 5 GPa. Utilizing McMillian-Allen-Dynes formula, we find that the superconducting transition temperatures (Tc) for Cmcm-Li2C2 are 13.2 K and 9.8 K, respectively, at ambient pressure and 5 GPa. We also find that two phonon modes B1g and Ag at Γ point have strong coupling with π* electrons. Among lithium carbide compounds, the superconductivity is only observed in LiC2 below 1.9 K. Besides LiC2, theoretical calculations also predicted superconductivity in mono-layer LiC6, with Tc being 8.1 K. So if the superconductivity of Cmcm-Li2C2 is confirmed by experiment, it will be the first superconducting compound containing one dimensional carbon chains and its Tc will be the highest one among lithium carbide compounds. Thus experimental research to explore the possible superconductivity in Cmcm-Li2C2 is called for.

2015, 64 (21): 217101.
doi: 10.7498/aps.64.217101

Abstract +

The fermion sign plays a dominant role in Fermi liquid theory. However, in Mott insulators, the strong Coulomb interaction suppresses the charge fluctuations and eliminates the fermion signs due to electron permutation. In this article, we first review the phase string theory of the Hubbard model for a bipartite lattice, which unifies the Fermi liquid at weak coupling and the antiferromagnetic Mott insulator at strong coupling. We first derive the exact sign structure of the Hubbard model for an arbitrary Coulomb interaction U. In small U limit, the conventional fermion sign is restored, while at large U limit, it leads to the phase string sign structure of the t-J model. For half filling, we construct an electron fractionalization representation, in which chargons and spinons are coupled to each other via emergent mutual Chern-Simons gauge fields. The corresponding ground state ansatz and low energy effective theory capture the ground state phase diagram of the Hubbard model qualitatively. For weak coupling regime, the Fermi liquid quasiparticle is formed by the bound state of a chargon and a spinon, and the long range phase coherence is determined by the background spin correlation. The Mott transition can be realized either by forming the chargon gap or by condensing the background spinons.

2015, 64 (21): 217102.
doi: 10.7498/aps.64.217102

Abstract +

We review the recent theoretical progress of the multiorbital effects on the electron correlations in iron-based superconductors. Studying the metal-to-insulator transitions of the multiorbital Hubbard models for parent compounds of iron-based superconductors, a Mott transition is generally realized. The natures of both the Mott insulating and the metallic phases are affected by the Hund's rule coupling. In alkaline iron selenides, Hund's rule coupling stabilizes a novel orbital-selective Mott phase, in which the iron 3d xy orbital is Mott localized, while other 3d orbitals are still itinerant. We discuss the effects of the orbital selectivity on normal state properties and the superconductivity of the iron-based systems.

2015, 64 (21): 217401.
doi: 10.7498/aps.64.217401

Abstract +

One of the most salient features of heavy fermion superconductivity is its coexistence with various competing orders. Superconductivity often emerges near or at the border of these exotic orders and their interplay may give rise to many interesting quantum phenomena. In this paper, we give a brief review of the various heavy fermion superconductors discovered so far and show there may exist an intimate connection between their superconducting pairing and quantum critical spin/charge/orbital fluctuations. We classify these superconductors into three categories:(A) CeM2X2, CenMmIn3n+2m, CePt3Si, CeMX3, CeNiGe3, Ce2Ni3Ge5 and CePd5Al2, in which superconductivity emerges at the border of antiferromagnetic phase; YbRh2Si2, in which superconductivity was very recently found inside the antiferromagnetic phase at 2 mK; UX2Al3 and UPt3, in which superconductivity occurs inside the antiferromagnetic phase; and UBe13 and U6Fe, in which the connection between magnetism and superconductivity is not yet clear. Among them, CePt3Si and CeMX3 are noncentrosymmetric, while UPt3 exhibits spin triplet pairing inside an antiferromagnetic phase.(B) UGe2, URhGe, UCoGe, UIr and U2PtC2, are spin triplet superconductors under the influence of ferromagnetic order or fluctuations.(C) URu2Si2, PrOs4Sb12, PrT2X20, Pu-115, NpPd5Al2 and -YbAlB4, in which superconductivity may be related to other exotic quantum states or fluctuations such as hidden order, valence fluctuations and quadrupolar fluctuations.In these compounds, f-electrons may participate in both superconductivity and other competing orders and often behave simultaneously itinerant and localized. These could be described by a phenomenological two-fluid theory, in which two coexisting fluidsan itinerant heavy electron fluid (the Kondo liquid) and a spin liquid of unhybridized local f-momentscompete to give rise to the various low temperature orders as well as superconductivity. Combining the two-fluid picture and the idea of spin-fluctuation-induced superconducting pairing, a BCS-like formula is proposed for calculating the superconducting transition temperature, and the results are found to be in good agreement with the experimental data for Ce-115. This model can explain naturally the microscopic coexistence of superconductivity and antiferromagnetism in these materials, and provides a promising guidance to other heavy fermion superconductors to achieve a systematic examination of the interplay between superconductivity and other exotic orders.

2015, 64 (21): 217402.
doi: 10.7498/aps.64.217402

Abstract +

The high-Tc copper-oxide superconductors (cuprates) break the limit of superconducting transition temperature predicted by the BCS theory based on electron-phonon coupling, and thus it opens a new chapter in the superconductivity field. According to the valence of substitutents, the cuprates could be categorized into electron-and hole-doped types. So far, an enormous number of high-Tc cuprate superconductors have been intensively studied, most of them are hole-doped. In comparison with the hole-doped cuprates, the advantages of electron-doped cuprates (e.g. lower upper critical field, less-debated origin of “pseudogap”, etc.) make this family of compounds more suitable for unveiling the ground states. However, the difficulties in sample syntheses prevent a profound research in last several decades, in which the role of annealing process during sample preparation has been a big challenge. In this review article, a brief comparison between the electron-doped cuprates and the hole-doped counterparts is made from the aspect of electronic phase diagram, so as to point out the necessity of intensive work on the electron-doped cuprates. Since the electronic properties are highly sensitive to the oxygen content of the sample, the annealing process in sample preparation, which varies the oxygen content, turns out to be a key issue in constructing the phase diagram. Meanwhile, the distinction between electron-and hole-doped cuprates is also manifested in their lattice structures. It has been approved that the stability of the superconducting phase of electron-doped cuprates depends on the tolerance factor t (affected by dopants) doping concentration, temperature, and oxygen position. Yet it is known that the annealing process can vary the oxygen content as well as its position, the details how to adjust oxygen remain unclear. Recently, the experiment on Pr2-xCexCuO4-δ suggests that the oxygen position can be tuned by pressure. And, our new results on [La1.9Ce0.1CuO4-δ/SrCoO3-δ]N superlattices indicate that more factors, like strain, should be taken into account. In addition, the superconductivity in the parent compounds of electron-doped cuprates has emerged by employing a so-called “protective annealing” process. Compared to the traditional one-step annealing process, this new procedure contains an extra annealing step at higher temperature at partial oxygen pressure. In consideration of the new discoveries, as well as the Tc enhancement observed in multilayered structures of electron-doped cuprates by traditional annealing, a promising explanation based on the idea of repairing the oxygen defects in copper oxide planes is proposed for the superconductivity in parent compounds. Finally, we expect a comprehensive understanding of the annealing process, especially the factors such as atmosphere, temperature, and strain, which are not only related to the sample quality, but also to a precise phase diagram of the electron-doped cuprates.

2015, 64 (21): 217403.
doi: 10.7498/aps.64.217403

Abstract +

The non-centrosymmetric (NCS) superconductors (SCs), a class of novel superconducting materials, have recently attracted considerable interests. As a result of antisymmetric spin-orbital coupling (ASOC) arising from the absence of inversion symmetry, the superconducting pairing state of these compounds allows the admixture of spin-singlet and spin-triplet components. This is in contrast to other previously studied superconductors, which usually possess an inversion symmetry in their crystal structure, and therefore their pairing state is of either spin-singlet/even parity or the spin-triplet/odd parity due to the restrictions of the Pauli principles and parity conservation.#br#Determination of the gap structure is crucial for unveiling the pairing state of NCS SCs. In this article, we first describe a method of measuring the precise temperature dependence of the changes in the London penetration depth using the tunnel-diode-oscillator (TDO), which provides an important evidence for the superconducting gap structures. Then the pairing states of NCS SCs are briefly reviewed, putting the emphasis on a few compounds with different ASOC strengths. It is proposed that the ASOC may tune the ratio of the spin-triplet to the spin-singlet component and, therefore, the spin-triplet state may become dominant while the ASOC effect is sufficiently strong in NCS SCs. However, our investigations demonstrate that the actual case is more complicated and there is no simple correspondence between the ASOC size and the pairing states. Instead, it is found that the band splitting due to the ASOC effect divided by the superconducting transition temperature Tc may better characterize of the superconducting pairing states in NCS SCs.

2015, 64 (21): 217404.
doi: 10.7498/aps.64.217404

Abstract +

In the past several decades, quantum phase transition and the associated fluctuations have emerged as a major challenge to our understanding of condensed matter. Such transition is tuned by an external parameter such as pressure, chemical doping or magnetic field. The transition point, called quantum critical point (QCP), is only present at absolute zero temperature (T), but its influence (quantum criticality), is spread to nonzero temperature region. Quite often, new stable orders of matter, such as superconductivity, emerge around the QCP, whose relationship with the quantum fluctuations is one of the most important issues.
Iron-pnictide superconductors are the second class of high-temperature superconductor family whose phase diagram is very similar to the first class, the copper-oxides. Superconductivity emerges in the vicinity of exotic orders, such as antiferromagnetic, structural or nematic order. Therefore, iron-pnictides provide us a very good opportunity to study quantum criticality. Here we review nuclear magnetic resonance (NMR) study on the coexistence of states and quantum critical phenomena in both hole-doped system Ba1-xKxFe2As2 as well as electron-doped systems BaFe2-xNixAs2 and LaFeAsO1-xFx. Firstly, we found that the 75As NMR spectra split or are broadened for H//c-axis, and shift to a higher frequency for H//ab-plane below a certain temperature in the underdoped region of both hole-doped Ba1-xKxFe2As2 and electron-doped BaFe2-xNixAs2, which indicate that an internal magnetic field develops along the c-axis due to an antiferromagnetic order. Upon further cooling, the spin-lattice relaxation rate 1/T1 measured at the shifted peak shows a distinct decrease below the superconducting critical temperature Tc. These results show unambiguously that the antiferromagnetic order and superconductivity coexist microscopically, which is the essential condition of magnetic QCP. Moreover, the much weaker T-dependence of 1/T1 in the superconducting state compared with the optimal doping sample suggests that the coexisting region is an unusual state and deserves further investigation. Secondly, we conducted transport measurements in electron-doped BaFe2-xNixAs2 system, and found a T-linear resistivity at two critical points. One is at the optimal doping xc1 = 0.10, while the other is in the overdoped region xc2 = 0.14. We found that 1/T1 is nearly T-independent above Tc at xc1 where TN =0, which indicates that xc1 is a magnetic QCP and the observed T-linear resistivity is due to the quantum fluctuation. We find that 1/T1 close to the optimal doping in both Ba1-xKxFe2As2 and LaFeAsO1-xFx also shows a similar behavior as in BaFe2-xNixAs2. The results suggest that superconductivity in these compounds is strongly tied to the quantum antiferromagnetic spin fluctuation. We further studied the structural transition in BaFe2-xNixAs2 by NMR. Since the a-axis and b-axis are not identical below the nematic structural transition temperature Ts, the electric field-gradient becomes asymmetric. Therefore the NMR satellite peaks associated with nuclear spin I=3/2 of 75As split for a twinned single crystal, when the external magnetic field is applied along a- or b-axis. We were able to track the nematic structural transition up to x=0.12. The Ts extrapolates to zero at x=0.14 which suggests that xc2 is a QCP associated with a nematic structural phase transition and the T-linear resistivity at xc2 is therefore due to the QCP. No existing theories can explain such behavior of the resistivity and we call for theoretical investigations in this regard.

2015, 64 (21): 217405.
doi: 10.7498/aps.64.217405

Abstract +

Superconductivity is one of the most important research fields in condensed matter physics. The rapid development of material preparation technology in last few years has made the experimental study of low-dimensional physical superconducting properties feasible. This article gives a brief introduction on superconductivity and technology of low-dimensional material fabrication, and mainly focuses on the experimental progress in electrical transport studies on one-and two-dimensional superconductors, especially the results from our group. As for one-dimensional superconductivity, we review the superconductivities in single crystal Bi nanowires, crystalline Pb nano-belts, and amorphous W nanobelts, and the proximity effects in superconducting nanowires, metallic nanowires, and ferromagnetic nanowires. Surface superconductivity is revealed for crystalline Bi nanowire. The step-like voltage platforms in V-I curves are observed in Pb nano-belts and may be attributed to phase slip centers. Besides, vortex glass (VG) phase transition is discovered in amorphous W nano-belts. Inverse proximity effect is detected in crystalline Pb nanowires with normal electrodes, and proximity induced mini-gap is found in crystalline Au nanowire with superconducting electrodes. Furthermore, in crystalline ferromagnetic Co nanowire contacted by superconducting electrodes, unconventional long range proximity effect is observed. As for two-dimensional superconductivity, we review the superconductivities in Pb thin films on Si substrates, 2 atomic layer Ga films on GaN substrates, and one-unit-cell thick FeSe film on STO substrates grown by molecular beam epitaxy (MBE) method. By both in situ scanning tunneling microscopy/spectroscopy and ex situ transport and magnetization measurements, the two-atomic-layer Ga film with graphene-like structure on wide band-gap semiconductor GaN is found to be superconducting with Tc up to 5.4 K. By direct transport and magnetic measurements, the strong evidences for high temperature superconductivities in the 1-UC FeSe films on insulating STO substrates with the onset Tc and critical current density much higher than those for bulk FeSe are revealed. Finally, we give a summary and present a perspective on the future of low dimensional superconductors.

2015, 64 (21): 217406.
doi: 10.7498/aps.64.217406

Abstract +

In the frontiers of condensed matter physics, pressure is widely adopted as an independent control parameter for tuning states of matters and plays an important role in finding new phenomena and corresponding physics, as well as in testing the relevant theories. Remarkably, a great deal of success has been achieved in searching for new superconductors and uncovering the microphysics for known superconductors. In this brief review, we attempt to describe the progress in high pressure studies of alkaline selenide superconductors AxFe2-ySe2 (A=K, Rb, Tl/Rb).#br#The high-pressure studies of Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2 and K0.8Fe1.78Se2 superconductors show that after the ambient-pressure superconducting phase is completely suppressed under about 9 GPa, the reemergence of a pressure-induced superconductivity with a maximum Tc of 48.7 K is observed at ～11 GPa, which is the highest Tc in this kind of superconductor. The systematic investigations on transport and structural properties for K0.8FeySe2 (y=1.7 and 1.78) reveal that a pressure-induced quantum phase transition occurs at pressure between 9.2 GPa and 10.3 GPa, where the antiferromagnetic state with Fermi liquid behavior converts into the paramagnetic state with non-Fermi liquid behavior. Therefore, it is proposed that the observed reemergence of superconductivity at high pressure is probably driven by the quantum critical transition.#br#In addition, some intriguing puzzles on these superconductors and corresponding possible answers are also reviewed from the perspective of high-pressure studies, including the roles of the insulating magnetic phase in developing/stabilizing ambient-pressure and high-pressure superconducting phases and the significance of the pressure-induced antiferromagnetic fluctuation state for the emergency of superconductivity in the high-pressure superconducting phase.

2015, 64 (21): 217502.
doi: 10.7498/aps.64.217502

Abstract +

Sr2CrO4 with a K2NiF4 structure can be synthesized by different methods under ambient pressure and high pressure, while the properties reported are quite different. In this paper, pure phase Sr2CrO4 with K2NiF4 structure is obtained by one-step solid state reaction under high pressure and high temperature. Powders of SrO and CrO2 are used as the starting materials. The Sr2CrO4 structure at room temperature is determined by powder XRD measurement and GSAS Rietveld refinement. Sr2CrO4 crystal is of tetragonal symmetry with space group I4/mmm and its lattice parameters a = 3.8191 Å and c=12.5046 Å. There are two oxygen sites, apical O1 and equatorial O2. The CrO6 octahedron is slightly elongated along the c-axis, forming a longer bond Cr–O1=1.9180 Å and a shorter bond Cr–O2=1.9096 Å. Temperature dependence of the magnetic susceptibility is measured in the temperature range of 2-300 K under the magnetic field 1 T. A weak antiferromagnetic transition can be seen at TN=95 K. Above TN, the susceptibility obeys Curie-Weiss law. The Curie-Weiss fitting gives the Weiss constant θ =-364 K and the effective magnetic moment μeff=2.9 μB, in good agreement with the theoretical value of localized Cr4+, indicating the localized electronic state. Field dependence of susceptibility has been measured at different temperatures. The magnetic properties here are different from those in the previous reports, and this discrepancy is attributed to the quite different sample synthesis methods. Temperature dependence of electrical resistivity of Sr2CrO4 shows insulating behavior. Activation energy Δ is estimated by the relation ρ ∝ exp(Δ/kBT) at temperature range 150-300 K. In the temperature range 150-200 K and 200-300 K the activation energies are ΔL=0.134 eV and ΔH=0.168 eV, respectively. The insulating behavior is consistent with the previous experiment reports and the theoretical calculation, which is possibly attributed to the suppression of orbital degree of freedom, resulting from the elongation of CrO6 octahedron and the narrow band width induced by the two-dimensional crystal structure.

###### REVIEW

## EDITOR'S SUGGESTION

2015, 64 (21): 210202.
doi: 10.7498/aps.64.210202

Abstract +

Lithium ion battery is nowadays one of the most popular energy storage devices due to its high energy, power density and cycle life characteristics. It has been known that the overall performance of battery depends on not only electrolyte and electrode materials, but also operation condition and choice of physical parameters. Designers need to understand the thermodynamic and kinetic characteristics of battery, which is costly and time-consuming by experimental methods. However, lithium ion battery is a complicated electrochemical system with multi physicochemical processes including the mass, charge, and energy conservations as well as the electrochemical kinetics. It not only has a typical multiple level arrangement: across the electrode level, cell level, and extending to the battery module level, which is different from the basic active material particle level arrangement, but also confronts the challenges to meeting the requirements for sorting and consistency method for battery. These facts increase the difficulties in designing the battery and evaluating the overall performance. Owing to the rapid development of multi-scale numerical simulation technology, the multi-scale mathematical models for lithium ion battery are developed to help battery designer comprehensively and systematically gain the interaction mechanisms between different physicochemical fields in the battery working process and analyze the regulations of these interaction mechanisms, which is significant in providing theoretical supports for designing and optimizing the battery systems. At present, multi-type lithium ion battery models coupled with many physicochemical processes have been developed on different scales to study different issues, such as thermal behavior, inner polarization, micro structure, inner stress and capacitance fading, etc. In this paper, we review the research statuses and development trends of multi-scale mathematical models for lithium ion battery. The primary theoretical models for lithium ion battery are systemized and their features, application ranges and limitations are also summarized. Furthermore, the future research area and the difficulty in industry application are discussed. All of these are helpful for the theoretic research and engineering application of the multi-scale numerical models for lithium ion battery.

###### GENERAL

2015, 64 (21): 210201.
doi: 10.7498/aps.64.210201

Abstract +

Magnetic refrigeration is a cooling method based on the magnetocaloric effect, which uses solid magnetocaloric materials as refrigerant, and helium, water or other fluid as heat transfer fluids. Stirling refrigeration is a kind of mature gas regenerative cooling method, using helium gas as the refrigerant. These refrigerations have similar cycling characteristics, and are both safe, environmantal-friendly and high efficient cooling methods. Therefore, a hybrid magnetic refrigerator combined with Stirling gas refrigeration effect is proposed and designed. In our previous works for hybrid magnetic refrigeration, numerical simulation and experimental performance of the low-pressure hybrid magnetic refrigerator was carried out, and the cycling mechanism of hybrid magnetic refrigeration was also figured out. In this study, a numerical model for the high-pressure hybrid magnetic refrigeration cycle is established. The magnetic refrigeration materials are utilized as the regenerator matrix for both gas Stirling and active magnetic regenerative refrigeration in this model. Effects of gas Stirling and active magnetic regenerative refrigeration are combined to build a kind of high efficient refrigeration cycle. Ansys Fluent software is applied in this paper. Based on the physical model of hybrid refrigerator and the theories of magnetocaloric effect and numerical calculation of regenerator, computational fluid dynamics (CFD) model of high-pressure hybrid magnetic refrigerator is established. This paper describes the internal heat transfer mechanism of Stirling and magnetic refrigeration effect in an active regenerator. Some parameters of the model such as working frequency and utilization are analyzed and the best phase angle is figured out in order to couple these two cooling effects positively. Simulation results show that Stirling and magnetic cooling effects can be coupled positively at phase angle of 60o. Results also show that with increasing system pressure, which means to increase the utilization of the system, the system frequency can enhance the cooling performance of the system as well as improve the coefficient of performance (COP) of it. The results and analysis of the numerical model will be helpful for the construction of experimental prototype in our future work.

2015, 64 (21): 210301.
doi: 10.7498/aps.64.210301

Abstract +

The micro-motion Doppler echo simulation and characteristic parameter extraction of the extended micro-motion target are carried out. For the extended micro-motion target, the echo from the target cannot be regarded as several points' echo. Based on the connections between the scattering field and Doppler echo, an echo simulation method for micro-motion target (based on physical optics) and a method of equivalent current are proposed. At the moment, the micro-motion target can be taken as a static target, so the back scattering field series can be calculated by physical optics and the method of equivalent current. The back scattering field series calculated in the target coordinate system is transformed into the echo of radar coordinate system by the conversion of coordinates, and the Doppler echo is obtained. By comparing with the analytic signal model, the method is validated. The precession characteristics of a cone and warhead with fins are analyzed. Echoes come from every part of the extended micro-motion target and contain the motion characteristics of that part. So the traditional time-frequency analytical methods are not appropriate. In order to achieve better time frequency concentration and avoid the cross terms, the S-method is used to get the time-frequency distributions. The time-frequency characteristics at different radar waves' incidence angles, target different motion states and different geometries are analyzed. From the time-frequency distribution map, the micro-motion of the cone behaves as the micro-motion of two strong scattering points at the bottom of the cone. Because of the shielding effect, the time-frequency curves are not integrated when the radar waves are incident from the cone's bottom. The sinusoidal curve can be mapped to a point in the parameter space based on the inverse radon transform, and the target micro-motion parameters can be obtained. Results of inverse radon transform also show that the precession of the cone behaves as the precession of the two strong scattering points, and the two points' phase difference is equal. For warhead with fins, the time frequency distribution of spin behaves as four sinusoidal curves whose phase differences are equal, implying that the micro-motion of the target behaves like the four fins' micro-motion. However, the sinusoidal curves of precession of the warhead with fins are very different, i.e. their phase differences are not equal. This is because the precession consists of spinning and coning, and the coning has a modulation effect on the spinning. These phase information and the number of strong scattering points can be directly and easily obtained through inverse radon transform. This study combines the electromagnetic scattering with the signal procession. And some results are different from that of traditional micro-motion models through the simulation of typical ballistic targets. Results are explained and analyzed by combining scattering theory. This research has important theoretical and application values in the ballistic target detection and recognition.

2015, 64 (21): 210302.
doi: 10.7498/aps.64.210302

Abstract +

Understanding the non-Markovian dynamics of dissipative processes induced by memory effects of the environment is a fundamental subject of open quantum systems. Because of the complexity of open quantum systems, e.g., the multiple energy scales involving that of the system, the environment, and their mutual coupling, it is generally a challenging task to characterize the relationship among the parameters of the system dynamics and the reservoir spectra. For the two-level spontaneous emission model within structured environments, it was shown in a recent literature (Opt. Lett. 38, 3650) that a functional relation could be established between the asymptotically non-decaying population and the spectral density of the reservoir as the system undergoes a long-time evolution. It hence renders a distinct perspective to look into the character of long-lived quantum coherence in the corresponding non-Markovian process. This article is devoted to further investigate the phenomena of limit cycle oscillations possibly occurring in such non-Markovian dissipative systems in a long-time evolution. For a two-level system subjected to an environment with Ohmic class spectra, due to the presence of a unique bound-state mode of the system, the evolution trajectory of the given initial states will converge to a limit cycle in the Bloch space. The dependence of the radius and the location of the limit cycle on the spectral density function of the reservoir are manifested by virtue of the described functional relation. For the model subjected to a photonic crystal environment with multiple bands, our studies reveal that, owing to the presence of two or more bound states, the evolution trajectory of the system will converge to a toric curve of a paraboloid in the Bloch space and the phenomena of periodic or quasi-periodic oscillations could exhibit. While the equation of the parabolic curve is fully determined by the initial values of the state vector in the Bloch space, our results reveal that the scope of the evolution trajectory inside the toric curve is related to the spectral density of the reservoir and their quantified relation is distinctly characterized. Finally, the asymptotic dynamics of the correlations of a two-qubit system is discussed when it is subjected locally to the non-Markovian dissipative process.

2015, 64 (21): 210303.
doi: 10.7498/aps.64.210303

Abstract +

Quantum entanglement swapping can be used to establish reliable quantum remote transmission channel so as to realize transmission of quantum states. However, the highly stable quantum network is required in the quantum channels when using quantum entanglement swapping, otherwise it will waste a lot of entanglement resources. In order to save entanglement resources, we have to put forward a kind of quantum communication network transmission protocol based on packet switching, according to the theory of quantum teleportation. Firstly, the principle of packet switching in computer network is introduced. Next we describe the implementation process of quantum network transmission protocols which are based on entanglement swapping and packet switching. We then analyze the reliability, security and utilization rate of entanglement of the protocol we have proposed. And after that the quantitative relationship about the number of entanglement quantum states, the number of routers and link error rates are calculated. Finally, we compare these two transmission protocols. Simulation results show that the number of entanglement for these two protocols is equal without consideration of the link errors. When taking them into account, the packet switching transmission protocol can save numbers of entanglement resources obviously. In addition, with the increase of number of routers and the rise in link error rates, the quantum communication network transmission protocol based on packet switching will need less entanglement resources than that based on entanglement swapping. Therefore, when the quantum transmission network is not stable, the packet switching transmission protocol has a better transmission performance, and it can be applied to the future construction of quantum network.

2015, 64 (21): 210501.
doi: 10.7498/aps.64.210501

Abstract +

Random vibration energy is widely existing in the environment. To efficiently harvest it, many researchers have designed lots of harvesters up till now. A lot of research works have found that when a harvester with bistable piezoelectric energy is excited by stochastic forces, if the intensity of them is low, the system's motion will be trapped in a single potential well. This will result in a low output voltage. In order to overcome the difficult of it and improve the harvesting efficiency, we develop an impact facility with two stops and incorporate it to a bi-stable energy harvester. This design can improve the harvesting efficiency greatly. First the electromechanical coupling equations are derived based on the Euler-Bernoulli beam theory and Kirchhoff's law. Then, we analyze the symmetric stops' effect on the potential function and the elastic restoring force of the system. Results show that both the potential energy and the magnitude of restoring force will be enhanced when collision takes place. Furthermore, we investigate the impact's effect on the system's dynamic responses and efficiency at harmonic excitation. Results reveal that a well designed impact can transform an intrawell motion into an interwell, and then increase the output voltage. And the chaotic motion can be changed into the large amplitude periodic one. Then, the harvester's dynamic responses under random excitations at a low intensity are obtained by using Euler-Maruyama method. Results indicate that the collision gaps can greatly influence the efficiency of the energy harvester. Collisions between the beam and the stops can force the system to oscillate between two potential wells more frequently. According to the relationship between the gap and the standard deviation of output voltage, we know that there exists an optimal collision gap for a definite intensity of stochastic excitation. The bistable energy harvester with this optimal gap will oscillate between the two wells frequently, and output a large voltage. Moreover, the collision stiffness can influence the system's performance as well. With the increase of collision stiffness, the system will exhibit a more frequently jumping between the two potential wells, but the stiffness has a limitation, exceeding which it cannot increase the frequency of jumping and improve the output power any more. So by properly designing the collision gap and stiffness, the system can most frequently jump between the two wells with a large amplitude of displacement, hence can attain the highest harvesting efficiency.

2015, 64 (21): 210502.
doi: 10.7498/aps.64.210502

Abstract +

According to the attitude estimation and three-axis magnetometer on-line calibration, a real time moving horizon estimation algorithm is presented in this paper. First, moving horizon estimation filter is designed since system constraints existing in most practical cases cannot be solved analytically in the framework of Kalman filter. Taking advantage of the optimal problem in dealing with constraints, the presented method converts the attitude estimation problem into an optimal one by which the quaternion normalization property can be solved analytically in smaller searching space with better efficiency and accuracy. Second, through a series of linearization of system equations, Gauss-Newton iterative method is applied in the horizon window composed of finite history information to obtain the best state estimation and meet the real time requirement at the same time. Once the newest best state estimation value is obtained, it will be sacked into the horizon window and the oldest one discarded. By this way, the filter is moving forward. Finally, based on the proposed method, the three-axis magnetometer parameter on-line calibration combined with attitude estimation is solved without adding any system state dimension, which can also make sure that the measurements with three-axis magnetometer are in the form of vector as its obvious benefits in the sense of ensuring information quantity. On considering the extreme environment such as great temperature gradient, mechanical pressure and complex electromagnetic fields, different from that of the off-line calibration, the calibration parameter is changed definitely. So the on-line calibration is necessary though neglected by most papers. Simulation results show that under the condition of small initial errors, the difference of accuracy among EKF, UKF and moving horizon estimation is small. But the computational burden of the last one is relatively large. The advantage of the described method is not so obvious in this case. But when the initial errors are large, the moving horizon estimation still can get the precise results no matter how great are the EKF (extended Kalman filter) and UKF (unscented Kalman filter) deviated from the true values. Thus the proposed method has its high accuracy and poor sensitivity of the initial and systematic errors along with fast convergence, all of which are vital in most actual environments.

2015, 64 (21): 210503.
doi: 10.7498/aps.64.210503

Abstract +

The potential function of a mono-stable system is studied in this paper. The response of pulse series with different half-peak width via the mono-stable system is analysed. Our conclusion is that the larger the half-peak width of the input pulse series, the higher the height of the output pulse series . Distortion of the pulse response wave appears. This is the reason that the potential function of the mono-stable system is similar to the horizontal line around zero point.#br#In view of the difficulties of adjusting parameter of the mono-stable system to reach stochastic resonance, a method of moving pulse series is put forward. Adjusting the system parameter is not considered but the stochastic resonance of the pulse signal is realized by setting an offset in the method. To reduce the response pulse wave distortion, a mechanism for the proposed method to reduce the distortion is discussed in detail. When noise exists, the mechanism reveals that the moving pulse series method can adjust the distribution of the noise power spectrum and improve the mono-stable stochastic resonance. Therefore, the method proposed in this paper is helpful to detect pulse signal masked by noise.

2015, 64 (21): 210504.
doi: 10.7498/aps.64.210504

Abstract +

A meminductor is a new type of nonlinear inductor with memory, which is generalized from the concept of a memristor and defined by current-flux. This paper presents a flux-controlled meminductor model with a smooth quadratic function and designs its corresponding equivalent circuit, which can be used as an emulator to imitate the behavior of a meminductor when actual solid-state meminductor has not yet appeared. Furthermore, a new chaotic oscillator is designed based on this meminductor model, and the dynamical behaviors of the oscillator are investigated, such as chaotic attractors, equilibrium points, Lyapunov exponent spectrum, bifurcations and dynamical map of the system, etc. Bifurcation analysis shows that the meminductor can make the oscillator produce periodic and chaotic oscillations. Moreover, an analog circuit is designed to confirm the correction of the proposed oscillator using the proposed equivalent circuit model of meminductor. It is shown that the experimental results are in good agreement with that of the numerical simulations and the theoretical analysis.

2015, 64 (21): 210505.
doi: 10.7498/aps.64.210505

Abstract +

The four hexagonal grid state patterns and a variety of non-grid states are obtained by changing the values of intensity ratio between two Turing modes in the two-layer coupled Lengel-Epstein model system. Results of numerical investigation show that those grid states in reaction diffusion are interleaving structures of three sets of different sublattices, which result from the interaction of both the wave number ratio and intensity ratio between Turing modes in the two subsystems; and the specific expressions of three-wave resonance in physical space are governed by the mode intensity ratio. Furthermore, the value of intensity ratio between the two Turing modes in the grid state patterns is greater than that of non-grid state structures, and the type of pattern selected by the system changes from complex to simple pattern with the increase of mode intensity ratio. Finally, it is found that these four hexagonal grid states correspond to different number pair (a, b) having different stability, and the grid state with the number pair (3, 2) is the most stable structure.

2015, 64 (21): 210506.
doi: 10.7498/aps.64.210506

Abstract +

Positive output super-lift (POSL) Luo converter, which has some particular good features: such as its power switch being grounded, high voltage gain and positive polarity output, is a good topology for overcoming the drawbacks of the conventional Buck and Boost converters to obtain high output voltage and power for satisfying the requirements in practical engineering. In this paper, based on the averaging method and taking into account the abrupt changing of the voltage across the energy-transferring capacitor, the improved reduced order averaged model and the corresponding small signal model of the POSL Luo converter are established, and its transfer function from the duty cycle to the output voltage is derived and analyzed. By combining the derived transfer function from the duty cycle to the output voltage of the POSL Luo converter, with that for the voltage compensator and that for the pulse width modalation (PWM) generator, the transfer function from the reference voltage to the output voltage of the voltage-mode controlled POSL Luo converter is also derived. And then, the stability of the voltage-mode controlled POSL Luo converter is identified by calculating the poles of its transfer function from the reference voltage to the output voltage, so the corresponding stability boundaries are obtained. The power electronic simulator (PSIM) software is applied to simulate the POSL Luo converter in time domain and frequency domain to preliminarily confirm the effectiveness of the established transfer function from the duty cycle to the output voltage of the POSL Luo converter, and to simulate the voltage-mode controlled POSL Luo converter to preliminarily verify the theoretical analysis about its stability. Finally, the hardware circuits for the POSL Luo converter and the voltage-mode controlled POSL Luo converter are designed, and the circuit experimental results in time domain from the digital oscilloscope and in frequency domain from the impedance/gain-phase analyzer are presented for further validation. Theoretical analysis, PSIM simulations and circuit experiments are in basic agreement with one other, and all of them demonstrate that it is effective to use the improved average model to analyze the performance of the POSL Luo converter and the stability of the voltage-mode controlled POSL Luo converter.

2015, 64 (21): 210507.
doi: 10.7498/aps.64.210507

Abstract +

A memristor is a nonlinear element of nanoscale size with memory function and when it works as the nonlinear part in a chaotic system, the physical size of the system will be greatly reduced, rich nonlinear curve will be produced, and at the same time, the complexity of the chaotic systems and the randomness of signals will be enhanced. So in this paper, a new chaotic system is designed based on an ion migration memristor. The complex dynamic characteristics of the memristive system are investigated by means of theoretical derivation, numerical simulation, Lyapunov exponent spectrum, power spectrum, and Poincaré map. In addition, the change of system dynamic behaviors with the different parameters are analyzed. Then, a SPICE-based analog circuit is presented. The SPICE simulation results are in conformity with the numerical analysis, and thus verify that the chaotic systems can produce chaos. The linear feedback control structure is simple, economic and easy to realize in engineering practice, so the linear feedback control method has a high application value. At present, most studies focus on memristors' applications in memory and analog neural networks, but little research work is for voice security transmission. Therefore, by using the method of linear feedback control of chaotic synchronization, this paper proves the effectiveness of this method by numerical simulation experiments. As a result, it can achieve secure communication of voice signals. Finally, we conclude that the linear synchronous control method based on memristive chaotic system when applied to the secure communications can achieve the purpose of covering a specific speech. In addition, this method is able to restore the specific speech signal without distortion, which is very meaningful for the promotion of applications of memristor.

2015, 64 (21): 210508.
doi: 10.7498/aps.64.210508

Abstract +

Many biomedical engineering fields are studied by combining with nonlinear science which has major advances in theoretical curing related diseases. The coronary artery system is chosen as a muscular blood vessel model. With the change of vessel diameter, some chaotic behaviors will occur which may cause complex diseases such as myocardial infarction.#br#In order to avoid the undesired chaotic motion, this paper investigates the finite-time chaos synchronization problem for a coronary artery system by utilizting high-order sliding mode adaptive control method. First, the error chaos synchronization system is obtained using the master and slave systems. Second, the error chaos synchronization system can be transformed into an integrator chain system by coordinate transformation, which is equivalent to an error chaos synchronization system. Considering that the sliding mode control has main obstacle (the control high activity and chattering phenomenon), a high-order sliding mode adaptive controller is designed for a coronary artery system with unknown disturbances at geometric homogeneity and integral sliding mode surface. The proposed method shows that the drive and response systems are synchronized and the states of the response system track the states of the drive system in finite-time. This approach does not require any information about the bound of disturbances in advance. Theoretic analysis based on Lyapunov theory proves that the systems with the proposed controller could be stabilized in finite-time. The convergence time of the system states is estimated. In order to alleviate the chattering effect, we use tanh(·) function in place of sign(·) function to design an improved high-order sliding mode adaptive controller. Simulation results show that the proposed adaptive sliding mode controller can achieve better robustness and adaptation against disturbances, which offer the theoretic basis for curing myocardial infarction.

###### NUCLEAR PHYSICS

2015, 64 (21): 212801.
doi: 10.7498/aps.64.212801

Abstract +

In the magnetic confinement fusion device, the first wall as plasma facing components will directly affect the performance of high temperature plasma. And the interaction of plasma and materials also affect the life of the first wall. Liquid lithium first wall receives more and more attention due to the properties of repairing itself and effectively inhibiting boundary particle recycling. So the research of the interaction between liquid lithium wall and plasma is particularly important. Erosion and deposition characteristics of lithium and its influence on the performance of plasma during lithium limiter experiment in HT-7 device are studied in-depth in this paper. Experimental results show that when the interaction between Li and plasma is weak, Li enters into the plasma mainly by weak surface evaporation and sputtering. During this process, Li line emission is strengthened, impurity and hydrogen recycling is decreased resulting in the improvement of plasma performance. When the interaction between Li and plasma becomes extremely strong, it is found so many big scale Li droplets ejected from liquid lithium surface to cause intense Li efflux into plasma, leading to plasma discharge disruption. Li atoms coming from Li limiter are ionized in the scrape-off layer (SOL), and entered into hot plasma column as ions (Li+, Li2+, Li3+) and transported in plasma. After the experiment, it can be found that a lot of white spots distributed in the vacuum chamber wall, with its main composition being Li2CO3 by XPS analysis. Through observing Li spot distribution and analyzing the lithium film thickness by scanning electron microscopy (SEM) in different samples, it is observed that the lithium is primarily deposited around the limiter, but the number of Li spots is more at the low field side than that at the high field side of the device, and the Li film gradually becomes thinner along the toroidal direction of the HT-7 device, leading to the non-uniformity of impurity and hydrogen recycling. The experiment may provide a reference for studying the interaction of plasma and liquid lithium first wall and the application of liquid lithium first wall in future tokamak device.

###### ATOMIC AND MOLECULAR PHYSICS

2015, 64 (21): 213101.
doi: 10.7498/aps.64.213101

Abstract +

The α-cyclohexanedione (α-CHD) molecule is an important structural unit in the six-membered ring systems with a large number ofbiologically meaningfulmoleculeswhich have been found. It has important applications in synthetic science also. It is found that some fragments can be obtained through vacuum ultraviolet absorption spectrum and induction photolysis experiments for α-CHD molecules. In order to understand the dissociation reaction mechanism of α-CHD and reveal the resource of those fragments, the potential energy surface of the dissociation reaction for α-CHD molecules in ground state is studied by B3LYP and CCSD(T) methods. The reaction paths of the products are obtained, such as P1(c-C5H8O+ CO), P2(2 C2H4+ 2 CO), P3 (CH2CHCH2CH2CHO+ CO), P4(2 C2H2O+ C2H4), P5(CH3CHCO+ CH2CHCHO). And the structure parameters of the reactant, products, intermediates and transition states in the reaction processes are also obtained. Their reaction mechanisms can be summarized as the isomerization and dissociation processes, and these processes mainly involve the hydrogen atom transfer, ring-opening and C–C bond cleavages. A reactionchannel in which α-CHD dissociates into cyclopentanone and CO needs lower energy, so it is more advantage our to make dissociation study than other studies. In addition, we think that α-dissociationreaction cannotoccur directly in ground state from our calculations. Based on the UV photolysis experiment of α-CHD with a wavelength of 253.7 nm (112.7 kcal/mol) and the theoretical calculation of potential energy surface in ground state, we obtain that Path 1 (α-CHD→ c-C5H8O+ CO) is the most possible channel, Path 3 (α-CHD→ CH2CHCH2CH2CHO+ CO) is the next, and Path 5(α-CHD→ CH3CHCO+ CH2CHCHO) is the third, while Path 2 (α-CHD→ 2 C2H4+ 2 CO) and Path 4 (α-CHD→ 2 CH2CO+ C2H4) are difficult to be achieved. So c-C5H8O and CO are the major fragment products, CH2CHCH2CH2CHO is the subsidiary one, maybe a minor distribution of CH3CHCO and CH2CHCHO, but the fragments C2H4 and CH2CO are difficult to obtain. This agrees well with the analysis using mass spectrometry in experiment. Results can clarify the microcosmic reaction mechanism of the photodissociation reaction for α-CHD molecule in ground state. It may provide an important reference for realizing its spectrum in-depth. The obtained results are informative for future studies on α-CHD relative.

2015, 64 (21): 213102.
doi: 10.7498/aps.64.213102

Abstract +

In this paper, the influence of charge trapping memory storage feature is studied by doping the substitutional impurity Al and introducing oxygen vacancy within HfO2. HfO2 is widely used in trapping layer of charge trapping memory, for it belongs to high dielectric constant materials with the abilities to shrink the device size and improve the device performance. Materials studio and Vienna Ab-initio Simulation Package are used to investigate the influence of doping Al on the formation of the oxygen vacancy in HfO2 as a trapping layer. At the same time, the interaction energy of two defects at different distances is calculated. Results show that doping the substitutional impurity Al reduces the formation energy of oxygen vacancies in HfO2, and the reduced formation energy of the three-fold-coordinated O vacancy is larger than that of the four-fold-coordinated O vacancy. After having studied three different defect distances between the substitutional impurity Al and the three-fold-coordinated O vacancy, the results indicate that the system acquires the largest charge trapping energy, the most of quantum states, the smallest population number, and the longest Al–O bond length when the distance between the defects is 2.107 Å. Studying the bond length changes of the three systems after writing a hole, we obtain a result that the change of Al–O bond length is the smallest when the distance between defects is 2.107 Å. In conclusion, the data retention in the trapping layer of monoclinic HfO2 can be improved by doping the substitutional impurity Al. This work will provide a theoretical guidance for the performance improvement in the data retention of charge trapping memory.

2015, 64 (21): 213301.
doi: 10.7498/aps.64.213301

Abstract +

Accurate spectroscopic parameters of probed species, especially the line strengths at high temperatures, are important for combustion diagnosis based on tunable diode laser absorption spectroscopy (TDLAS). However, most of the line strengths in databases are measured at normal atmospheric temperature and calculated at high temperatures. For example, the mostly used HITRAN database focuses on atmospheric conditions where the temperature ranging from 200-350 K. The high-temperature parameters in HITRAN database are obtained by calculation and the temperatures are limited to 3000 K. In this paper the line strengths of 30012-00001 transition band of CO2 and 3-0 transition band of CO at normal temperature (300 K) and some high temperatures (400-6000 K) are calculated using our calculated partition function and experimental transition moment squared and Herman-Waills factor coefficients. The total internal partition sums (TIPS) are calculated for CO2 and CO with the product approximation of the vibrational and the rotational partition functions. The vibrational partition function is calculated in harmonic oscillator approximation. For rotational partition sums, the centrifugal distortion corrections are taken into consideration. In order to validate the calculation, a high-temperature measurement system based on TDLAS is developed using a DFB diode laser near 1.573 μm. High-resolution absorption spectra of CO2 and CO can be recorded in a heated cell as a function of temperature and pressure. The 9 lines of CO2 30012-00001 band and 2 lines of CO 3-0 band have been measured by means of direct absorption spectroscopy in the temperature range of 300-800 K. The corresponding line strengths are inferred from the measured direct absorption spectrum. The calculated result and experimental data are compared with the HITRAN values. The calculated and measured data agree well with the existing databases (HITRAN 2012), the discrepancies being less than 3% for most of the probed transitions. All the results confirm the validity of the calculation of total partition function and the line strengths calculated. The variation of the line strength as a function of temperature for a certain transition is also discussed. While the lower state energy determines the equilibrium molecular population in the unabsorbing state as a function of temperature, how the line strength of a particular transition varies with temperature can also be controlled.

###### ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2015, 64 (21): 214201.
doi: 10.7498/aps.64.214201

Abstract +

In this paper ultrashort laser pulses with different fluences (18 J/cm2-115 J/cm2) and pulse widths (50 fs-4 ps) are employed to ablate highly oriented pyrolytic graphite in vacuum (4×10-4 Pa). By recording the time-resolved emission spectra of the ablated plume, the ultrafast time evolution of the ablation process is investigated. The Swan bands of C2 radicals, the spectral band near 416 nm which may be assigned to the electronic transition from 1Σu+ to X1Σg+ of C15 clusters, and the emission continuum ranging from 370-700 nm are observed. From the recorded time-resolved emission spectra of the ablated plume, it is seen that at larger time delays only the emission continuum is observed. The decay process of the emission continuum of the plume generated by 50 fs, 115 J/cm2 laser pulses can be divided into a fast decreasing stage (before 20 ns time delay) and a slow decreasing stage (after 20 ns time delay), indicating that the emission continuum may come from two different compositions. During the fast decreasing process, the bremsstrahlung of the ablation-generated carbon plasma contributes to the major part of the continuum; while during the slow decreasing process, the thermal radiation of carbon clusters generated at a later stage of ablation mainly contributes to the continuum. In addition, the existence time of the continuum generated by 50 fs laser pulses increases with the decrease of laser fluence, indicating that laser pulses with lower fluences can generate more carbon clusters at later stages of ablation. It is also found that for the 50 fs pulses, when the laser fluence increases at the early stage of ablation, the quantities of carbon plasma and excited C2 radicals in the plume increase significantly, but the quantity of excited C15 radicals with larger mass only increases slightly. Therefore the laser fluence has a great impact on the concentrations of different compositions in the ejected plume, implying that different material removal mechanisms exist for ablation induced by laser pulses with different laser fluences. Finally, pulse width plays an important role in the time evolution manner of the emission continuum. As the laser pulse width increases, the two-stage decay process of the emission continuum gradually changes into one-stage process, indicating that the existence time intervals of carbon plasma and carbon clusters overlap each other for longer laser pulse width. And the whole evolution process of the emission continuum induced by 4 ps laser pulses is much slower than that induced by 50 fs laser pulses. Longer laser pulse width also causes the decrease of the spectral intensity of C2 radicals, and thus higher laser intensity favors the generation of excited C2 radicals.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Numerical simulation of heat transfer and natural convection of the indirect-driven cryogenic target

2015, 64 (21): 215201.
doi: 10.7498/aps.64.215201

Abstract +

ICF design requires smooth and uniform deuterium-tritium (D-T) ice layers in a spherical shell. Thermal environment around the capsule is the key to reach the low-mode ice layer roughness requirement and obtain a high quality ice layer. In this paper, we present the results of three-dimensional simulation for an indirect-driven cryogenic target, focusing on the issues of heat transfer and natural convection flow inside the hohlraum. A thermal and hydrodynamic calculation is first proposed to investigate the convection heat transfer effect on the D-T ice layer. Comparing the two cases with gravity considered or neglected, we find that the temperature variation at the ice layer inner surface caused by the natural convection flow and the hohlraum's structure are of the same order of magnitude. Then the parameters study on Rayleigh number, which is a dimensionless number associated with free convection, is carried out. Thermal simulations on different Rayleigh number are provided. Temperature variation at the D-T ice layer inner surface is to increase as soon as the Rayleigh number reaches 60. Comparisons among different gases under different operating pressure conditions are made. In order to avoid the convection heat transfer effect in a wide range of pressure, it is necessary to take pure helium or mixture gas with a small amount of hydrogen as the tamping gas. The influence of hohlraum's orientation on the natural convection is also studied. It is found that the convective heat transfer effect in a horizontally orientated hohlraum is stronger than that in a vertical one. Based on these, we discuss the possibility to eliminate the convection flow by partitioning the hohlraum into several regions. The calculated results for several cases of different gas-region models indicate that the convection flow can be eliminated with an appropriate division in a vertically orientated hohlaum but cannot in a horizontally orientated one. The conclusions in this paper have certain guiding significance for further design and experiments of cryogenic target.

2015, 64 (21): 215202.
doi: 10.7498/aps.64.215202

Abstract +

A miniature ion thruster has been proposed in recent years for a small propulsion system applied in space missions such as deep space exploration, precise high-stability attitude and position control. An electron cyclotron resonance (ECR) ion thruster is free from contamination and degradation of electron emission capacity and will offer a potentially longer thruster lifetime than that in the electron bombardment type. The microwave ECR ion source with a 20-mm diameter designed here consists of two annular permanent magnets (SmCo), ring coupling antenna and a grid system including screen and acceleration. For the ion source performance optimization, with a fixed magnetic structure, the antenna position and cavity length in the discharge chamber can be adjusted to strengthen electron ECR heating and increase ion beam extraction. According to the distribution of static magnetic field and the ECR layer measured by Gauss meter, three possible sizes of antenna position (L1) are set; depending on the cut-off characteristics of the discharge chamber and the distribution of microwave electric field calculated by finite element method, six candidate sizes of cavity length (L2) are set. By comparing the difference in plasma discharge and ion beam extraction, the optimal structure of ion source can be obtained. Experimental results show that for a given antenna position, there is a cavity length not too long or too short to extract the maximum ion beam. And the launch of microwave from strong magnetic field near ECR layer is conductive to lossless wave propagation in plasma and highly efficient electron ECR heating. To maintain a plasma in very low power and flow, the size combination of 0.6-mm in L1 and 5-mm in L2 is selected as the preferred structure. The performances of miniature ECR ion source, that is, ion beam current, discharge loss, propellant utilization efficiency, thrust and specific impulse are 5.4 mA, 389 W/A, 15%, 163 μup N and 1051 s, respectively, at an incident power of 2.1 W and argon flow of 14.9 μg/s.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2015, 64 (21): 216101.
doi: 10.7498/aps.64.216101

Abstract +

As a kind of clean and high efficient energy conversion devices, the proton exchange membrane fuel cell (PEMFC) is a promising technology for clean and sustainable power generation. Metal-coordinated nitrogen-doped graphene is attractive since its use as a cathode material for the PEMFC. The mechanism of O2 activation and hydrogenation on TiN4 embedded graphene has been investigated in terms of the dispersion-corrected density functional theory (DFT-D) method. It is found that: 1) O2 prefers to stay on top of the Ti atom with the side-on configuration, forming the O-Ti-O three-member ring with an adsorption energy of 4.96 eV. 2) According to the Mulliken atomic charges analysis, the absorbed O2 molecule are negatively charged by 0.60 e in the side-on configuration. 3) Upon the chemisorption of the O2 on TiN4-graphene, there are two possible pathways during the activation of the O2 molecule: dissociation and hydrogenation. In the dissociation pathway, the adsorbed O2 molecule is first dissociated into two O atoms, with a fairly big reaction barrier of 0.95 eV and an endothermic reaction energy of 0.20 eV. Subsequently, the two O atoms are hydrogenated into O+OH with a reaction barrier of 0.40 eV and an exothermic reaction energy of 2.46 eV. In the hydrogenation pathway, the reaction barrier of the hydrogenation of the adsorbed O2 is 0.52 eV. The OOH formed subsequently is dissociated into O+OH with a small reaction barrier of 0.04 eV and an exothermic reaction of 2.14 eV. The hydrogenation pathways of the adsorbed O2 is more preferable, and the corresponding rate-limiting step of this pathway is the hydrogenation of the O2 with a reaction barrier of 0.52 eV and an exothermic reaction energy of 0.64 eV.#br#In summary, the preferable path of the hydrogenation reactions of O2 on TiN4-Graphene is O2(ads)+H(ads) → OOH(ads)→O+OH(ads). Current results may be benefitial to the design of new electrocatalyst materials based on graphene.

2015, 64 (21): 216102.
doi: 10.7498/aps.64.216102

Abstract +

Based on the review of previous experimental and theoretical studies on the surface processing by a pulsed intense electron beam, the induced temperature field in aluminum and 304 stainless steel is simulated by the finite element method (FEM) to estimate the existing time and depth of molten metal flow field on the irradiated surface. The generation of craters is attributed to the thermal resistance formed by the grain boundaries, and the influence of material properties on the mechanism of crater evolution is also discussed. Two-phase flow field simulation on molten metal is carried out with a combination of level-set method and FEM to estimate the mass transfer behavior at the craters and surface protuberance. It is revealed that the mass transfer effect driven by surface tension is an important factor for the formation and evolution of round-shaped craters on the surface of metals with high melting point, viscosity and surface tension coefficient. However, for metals with low melting point, due to the strong disturbance by ablating gas plume and low surface tension effect, the craters are more likely to have irregular splashing edges.

2015, 64 (21): 216103.
doi: 10.7498/aps.64.216103

Abstract +

Single event multiple-cell upsets (MCU) increase sharply as the feature size of semiconductor devices shrinks. MCU poses a large challenge on present radiation hardening technology and modeling test technique. Experimental study of the influence of proton incidence angle on single event multiple-cell upsets in 90 nm static random access memory (SRAM) for middle and high energy proton is carried out. The result shows that MCU percentage and multiplicity increase with increasing proton energy, and the MCU topological pattern presents a certain track-orientation characteristic along the trajectories of the incidence ion when the incidence proton is tilted along the X-direction or Y-direction. Single event upset (SEU) cross section has no evident angular dependence. There is some difference in proton MCU cross section between normal incidence and tilt angle incidence only for 30 MeV proton. Angular effect of proton MCU is associated with proton energy. Due to the lower efficiency of Monte-Carlo method in calculating proton MCU, a fast calculation method for cross section, which aims at single event MCU induced by proton nuclear reaction, is adopted. The binary cascade model in Geant4 toolkit serves as event generators in middle on high proton nuclear reaction. In terms of double differential scattering cross section of secondary particle from proton-material spallation reaction, proton MCU cross section is calculated through integration over the entire space of memory cells array. Based on the distribution of secondary particles, those spallation products with the highest linear energy transfer (LET) and longest range are revealed to emit preferentially in the forward direction, which is the root cause why the angular effect of proton-induced MCU exists. The angular dependence of single event MCU in nanometer SRAM depends strongly on proton energy and critical charge. The higher the proton energy is, the wider the angular distribution of secondary particle is, the greater the energy and LET value of the lateral scattered secondary particle is; and so the angular enhancement effect in MCU cross section for lower energy protons is greater than the higher energy protons. MCU cross section is more isotropic with the increase of the proton energy. Angular effect in MCU cross section becomes stronger with the increase of the critical charge for the same energy proton.

## EDITOR'S SUGGESTION

2015, 64 (21): 216201.
doi: 10.7498/aps.64.216201

Abstract +

Dynamic damage of material is a complex process that is dependent on lots of effects on a mesoscale, including grain size, morphology and micro-voids. In order to study the shocked lead micro-damage characteristics in oxygen-free high-purity copper, the variational thickness values of flyers and samples are designed to vary pulse duration and strain rate in plate-impact experiment, and the special recovery chamber and surface profile measurement system are used for soft-recovery and cross-section measure respectively. Based on the reconstruction, quantitative and statistical analysis, it is found that the longer pulse duration and higher shock loading stress bring about more serious local damage in oxygen-free high-purity copper. The mensurable damage width of sample cross-section results from the damage evolution on a sub-micron scale. Critical evolution time of sub-micron is observed to decrease with strain rate increasing, suggesting that damage evolution speed of sub-micron becomes faster as strain rate increases. The void size distribution of recovered sample is presented, and the topological characteristic transition accompanied with nucleation, growth, and coalescence processes of microscopic voids is also discussed. Through a comparison of difference between this work and the literature of previous research, a physical explanation of voids size distribution characteristics of oxygen-free high-purity copper is presented.

2015, 64 (21): 216501.
doi: 10.7498/aps.64.216501

Abstract +

Using non-equilibrium molecular dynamics method, we have studied the thermal rectification of heterojunction nanotubes (HCNTs). All of these HCNTs, composed of two 4 nm long carbon nanotubes (CNTs), only have a pentagon-heptagon defects pair. Here the positive direction is defined as the direction where the heat flux flows from the large diameter CNTs to the small diameter CNTs. We have found that the thermal rectification depends on the diameter, the chirality and the temperature.#br#Diameter effect: We fix the diameter on one side and changed it on another side, i.e., the left side of the HCNTs is (3, 3) while the right side of the HCNTs is (n, n), in which n changes from 4 to 9. It is found that the thermal rectification efficiency of HCNTs increases with n (also with the diameter difference). If considering the temperature field of (3, 3)-(4, 4) HCNTs, one can find that there exists a region near the HCNT where the temperature changes sharply. This region when the flux is positive is similar to that when the flux is negative. However, if taking into consideration the (3, 3)-(9, 9) HCNTs, we find that the distribution of temperature field shows different behaviors when the directions of the heat flux are different, and the length of this region becomes longer than (3, 3)-(4, 4). It can be explained that the thermal rectification is caused by different temperature distributions in HCNTs.#br#Chirality effect: We keep the chirality unchanged on one side of HCNTs and change the chirality of the other side, namely, the chirality of the left side of HCNTs are (3, 3) and the right side are (9, 9), (11, 7), (13, 4) and (15, 1), all of their diameters are close to 4.1 Å. We can find that the intersection angle between two CNTs decreases when the right side of HCNTs changes from (9, 9) to (15, 1), and the thermal rectification efficiency will be enhanced. It can be explained that the phonon is scattered and absorbed more effectively at the hetero-junction as the intersection angle decreases.#br#Temperature effect: We have constructed a HCNT (3, 3)-(9, 9) and changed its average temperature from 200 to 400 K. Our results show that the thermal rectification efficiency will be weakened with the rise in average temperature because of increasing heat flux in the negative direction.#br#This research may be helpful to the research in nanoscale thermal diodes, thermal logical gates and controlling heat flux.

2015, 64 (21): 216701.
doi: 10.7498/aps.64.216701

Abstract +

Diamond coating has many excellent properties as the same as those of the natural diamond, such as extreme hardness, high thermal conductivity, low thermal expansion coefficient, high chemical stability, and good abrasive resistance, which is considered as the best tool coating material applied to the high-silicon aluminum alloy cutting. We can use the hot filament chemical vapor deposition method (HFCVD) to deposit a 2–20 μm diamond coating on the cemented carbide tool to improve the cutting performance and increase the tool life significantly. Many experiments have proved that the existence of cobalt phase can weaken the adhesive strength of diamond coating. However, we still lack a perfect theory to explain why the Co element can reduce the adhesive strength of diamond coating is still lacking. What we can do now is only to improve the adhesive strength of diamond coating by doing testing many times in experiments. Compared with these traditional experiments, the first principles simulation based on quantum mechanics can describe the microstructure property and electron density of materials. It is successfully used to investigate the surface, interface, electron component, and so on etc. We can also use this method to study the interface problem at an atomic level. So the first principles based upon density functional theory (DFT) is used to investigate the influence of cobalt binding phase in cemented carbide substrate on adhesive strength of diamond coating. In this article, we uses Material Studios software to build WC/diamond and WC-Co/diamond interface models to evaluate the influence of cobalt phase on the adhesive strength of diamond coating with CASTEP program which can calculate the most stablest structure of film-substrate interface. We use PBE functional form to obtain the exchange potential and relevant potential, and to solve the self-consistent Kohn-Sham equations. We calculate the interfacial bonding energy, analyse the electron density of diamond coating and the bond Mulliken population of diamond film-substrate interface. The results show that the interfacial bonding energy of WC/diamond is 6.74 J/m2 and that of WC-Co/diamond is 5.94 J/m2, which implies that the adhesive strength of WC/diamond is better than that of WC-Co/diamond. We also find that Co element can transfer the charges near the interface of WC/diamond model when the magnetic Co element exists at the WC/diamond interface. As a result, the polarity of tungsten element in tungsten carbide and the polarity of carbon element in diamond coating near the interface turn to be identical polarity, and then the charge density of tungsten in cemented carbide changes from 0.430 e/A3 to 0.201 e/A3 and the charge density of Carbon in diamond changes from-0.045 e/A3 to 0.037 e/A3, and they exclude to each other, so the distance of interface becomes larger than that from the WC/diamond model, which changes from 2.069 Å to 3.649 Å. This can explain why the existence of Co element can weaken the adhesive strength of diamond coating. Meanwhile, Mulliken population analyses show that the bond strength of WC-Co /diamond at the interface is smaller than that of WC/diamond. So this can prove that the cobalt binding phase in cemented carbide substrate can weaken the adhesive strength of diamond coating, and then we need to do some pretreatments in order to reduce the cobalt binding phase in the cemented carbide substrate before depositing diamond coating.

2015, 64 (21): 216801.
doi: 10.7498/aps.64.216801

Abstract +

The wetting characteristic of micro-droplets on surfaces with different free energies is crucial to heterogeneous nucleation theory and the growth mechanism of micro-droplets during vapor condensation. In this paper, the spreading processes and wetting characteristics of nanoscale water droplets on various surfaces are explored by molecular dynamics simulation method. The surfaces are constructed from face centered cubic copper-like atoms with different Lennard-Jones potential parameters. The Lennard-Jones interaction energy well-depth of the surface atoms is adjusted to acquire different surface free energies, and the ratio of surface-water interaction energy well-depth to the water-water interaction energy well-depth is defined as the interaction intensity. In the present study, the relationship between interfacial free energies and solid-liquid interaction intensities is evaluated using molecular dynamics simulations. The wetting characteristics of TIP4P/2005 water droplets on surfaces with various free energies are simulated and analyzed as well, using molecular dynamics simulations under an NVT ensemble. Results indicate that the solid-liquid interfacial free energy increases as the solid-liquid interaction intensity increases, with different spreading processes and wetting characteristics achieved for the water droplets on these surfaces. For the surfaces with lower interaction intensities, water cannot spread on the solid surfaces and hydrophobic surfaces are obtained when the interaction intensity ratio between surface atoms and water molecules is lower than 1.6. As the interaction intensity increases, the surface translates from hydrophobic into hydrophilic, and finally into a complete wetting state as the interaction intensity reaches up to 3.5. Due to the limitation of nanoscale dimensions, the forces that exert on droplet surface are non-continuous and asymmetric. As a result, significant fluctuations of liquid-vapor interface and local solid-liquid contact line can be observed for the droplet in nanoscale. The transient contact angle of nano-droplets is also fluctuating within a certain range, which is different from that observed for macro-droplets. From the viewpoint of statistics, an apparent contact angle can be obtained for the droplet on each surface. The contact angle decreases with solid-liquid interaction intensities linearly, which is in accordance with the calculated results of classic Young's theory using the interfacial free energies obtained from molecular dynamics simulations. The fact that an apparent contact angle is already established for a droplet in nanoscale, supporting the capillary assumption that is widely adopted in classic nucleation theory. The fluctuation of liquid-vapor interface and contact angle also provides a qualitative explanation for the discrepancy between experimental nucleation rates and predictions in classic nucleation theory.

2015, 64 (21): 216802.
doi: 10.7498/aps.64.216802

Abstract +

Micro-eletromechanical system (MEMS) thermal-control shutters for spacecraft are fabricated by using the flexible Al/PI film, because of its light mass, no brittleness and withstanding severe mechanical environment (mechanical environment adaptability) in space. But the stress in the film would be able to bend the shutters too much to fabricate shutter array. Therefore, how to control the thin film stress is an important problem and it is necessary for flat shutters to take some measure to remove or reduce the thin film stress. This internal stress in the thin film formed intricately during the deposition process would make the film exhibit macroscopic compressive stress. So it is difficult to control the thin film stress micro-mechanically, but macro-mechanically. According to the results of the current study, the controlling technology of thin stress is commonly applicable to rigid substrates. In this paper, the flexible Al/PI film may be controlled by interface alloying. We put forward a way of adding Sn layer to the flexible Al/PI film, which makes Al/Sn interface to be alloyed as a measure to control the stress. In the alloy phase, lattice expansion and distortion results in the emergence of transverse shearing stress. The intrinsic compressive stress can be canceled out by the transverse shearing stress and the apparent stress in the films decreases consequently. The Sn atoms diffusion behaviour is proved to form Al-Sn alloying layer by SEM and EDS. This method can be used as a new technology of controlling thin film stress.

2015, 64 (21): 216803.
doi: 10.7498/aps.64.216803

Abstract +

In the low As beam equivalent pressure condition, the in-situ annealing treatment is carried out for the previously atomically flat GaAs(001) βup 2(2×4) reconstruction surface. Utilizing scanning tunneling microscopy, the surface is found to change its morphology simultaneously with the surface reconstruction during the increase of low As beam equivalent pressure annealing time. The surface morphology undergos from ordered flat to disordered flat and then gradually returns to the ordered flat state again. The surface reconstruction turns from βup 2(2×4) to (2×6) and then changes to “zig-zag” (2×6) state. And there is a correlation between the evolution of the surface morphology and surface reconstruction.

## EDITOR'S SUGGESTION

2015, 64 (21): 216804.
doi: 10.7498/aps.64.216804

Abstract +

According to Moore's Law, as the feature size of semiconductor devices becoming smaller and smaller, the chip integration degree keeps increasing. In particular, accompanying with the development of high chip integration and unit size reduction, the metal interconnects, i. e. the wire bonding, are becoming a challenging problem. Copper wire is believed to be an excellent metal for wire bonding, instead of gold wire, due to its attractive advantages such as low cost, favorable electrical and thermal conductivities etc. However, the excess Cu/Al intermetallic compounds (IMC) at the interface of copper wire and aluminum pad will increase the contact resistance and reduce bonding strength. This can affect the properties and reliability of devices. Currently, the evolutions of the interfacial microstructures as well as the growth mechanism of Cu/Al IMC at the bonding interface under thermal condition are still unclear.In-situ transmission electron microscope (TEM) has high spatial resolution and strong analysis ability. With fast CCD cameras, TEM can also record the dynamic structure evolution of the sample in real time. Combined with multi-function holders, TEM can also exert diverse fields and loads on the sample and synchronously monitor their structures and component evolutions. Hence, in situ TEM provides an advanced technique to explore the structural evolution and growth mechanism of Cu/Al IMC.In this paper, the growth mechanism of Cu/Al IMC is investigated during the annealing temperature from 50-220 ℃ based on the in-situ high resolution transmission electron microscopy (in-situ HRTEM). Specifically, the dynamic growth and structural evolution of Cu/Al IMC during annealing are recorded in real time. Results show that the isolated Cu/Al IMC is distributed in the bonding interface before annealing. The main component of IMC is Cu9Al4, whereas the minor one of IMC is CuAl2. After annealing at 50-220 ℃ for 24 h, Cu/Al IMC near the Cu layer is Cu9Al4, while Cu-Al IMC apart from the Cu layer is CuAl2. Meanwhile, the reaction rates and the activation energy of Cu/Al IMC at different temperatures are calculated. Furthermore, the more accurate growth equation of Cu/Al IMC is also proposed based on the in-situ experimental results, which will benefit the optimization of bonding process and the reliability of Cu/Al wire bonding.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2015, 64 (21): 217501.
doi: 10.7498/aps.64.217501

Abstract +

In comparison with uniformly magnetized states, vortex structures demonstrate a rich frequency spectrum of spin-wave (SW) excitations. However, a detailed theoretical description of the magnetic modes is generally still a challenge due to the difficulty of analytic calculation, except for the well-defined symmetric circular states. In contrast, the method of micromagnetic simulations combined with Fourier analysis is shown to be very powerful for gaining insight into the nature of magnetic excitation modes. Vortex excitation modes have been reported to be directly influenced by the geometric symmetry of the elements and/or the nature of the initial perturbation of pulse field. In order to understand how the reduced symmetry affects the vortex SW modes, we perform the micromagnetic simulations on vortex modes excited in a submicron-sized thin ellipse. In order to excite the spin-wave modes, a short in-plane Gaussian field pulse is applied along the short axis direction. After the pulse, the off-centered vortex core moves following an elliptical trajectory around its equilibrium position. Simulations provide the time evolution of the local magnetizations (at each discretization point) and dynamics of the spatially averaged magnetization. To determine the mode frequencies, the spectrum is obtained from the average magnetization through Fourier transformation from time domain the frequency domain. By means of Fourier analysis, a variety of azimuthal SW modes can be observed in the excitation spectrum. The ellipse in single vortex state has a twofold rotational symmetry with a rotation of πup around the z-axis (out-of plane) and can be described by the C2 group. The observed azimuthal modes can be divided into two categories according to their symmetry. Two modes occur alternately with increasing azimuthal number, indicating that the magnetic excitation modes remain to keep the symmetry of the ellipse structure. Their frequencies are found to increase linearly with the azimuthal index number. An increase of the SW frequency with increasing number of nodal planes is rather well known, which results from the competition between exchange and dipolar energy terms. According to the temporal evolution of the ellipse's spatially averaged energy densities, our micromagnetic simulation shows that the average exchange energy is significantly higher than the magnetostatic energy, suggesting that the exchange interaction plays a more important role in the excitation modes. The exchange energy density is mainly focused on the core origin while the largest contribution of the magnetostatic energy is distributed near the long axis. Thus, we can conclude that the exchange interaction provides the principal contribution to the vortex energy in such small ellipses with a single vortex state, resulting in the increasing frequency versus the azimuthal number, that is observed.

2015, 64 (21): 217701.
doi: 10.7498/aps.64.217701

Abstract +

Multiferroic materials have drawn increasing interest due to the coexistence of ferromagnetism (FM) and ferroelectricity (FE), which provides significant potentials for applications in spintronics, information storage, and sensors, etc. In this paper, the multiferroic Bi6Fe2-xCoxTi3O18 (BFCT-x,x=0-2.0) ceramics are prepared by the solid-state reaction. The BFCT-x samples belong to Aurivillius structure containing five perovskite layers clapped between two Bi-O layers. The lattice constants a, b, and c of BFCT-x samples increase simultaneously with increasing cobalt content up to 0.6 and then decrease with further addition of cobalt. The magnetic and ferroelectric properties, and their corresponding Curie temperatures are measured. At room temperature (RT), the magnetism of the BFCT-0, BFCT-1.8 and BFCT-2.0 samples can be understood by the presence of the antiferromagnetic (AFM) interaction with the dominant paramagnetism (PM) state, which is consistent with the linear behavior of the M-H plot. The Fe3+-O-Fe3+ and Co3+-O-Co3+ interactions present in the BFCT-x samples lead to AFM. The BFCT-0.21.0 samples show saturated magnetic loops, while the BFCT-1.2 sample is far from saturation even under an applied magnetic field of 10 kOe. The M-H curve of BFCT-1.6 sample shows a weak ferromagnetism. The Co content (x=0.2-1.6) dependences of 2Ms and 2Mr have been recorded. Both the 2Ms and 2Mr experience first-increase-then-decrease variation tendency with their maximal values of ～ 4.49 emu/g and ～ 0.89 emu/g located at x =0.6 and x =1.0, respectively. As the cobalt content varies from x=0.2 to x=1.2, the paramagnetic-ferromagnetic phase transition temperature (TMC) decreases from 752 to 372 K. At RT, the BFCT-x samples are ferroelectric, and the maximum and minimum values of remnant polarization (2Pr) are about 8.0 up C/cm2 (x=0.6) and 1.1 up C/cm2(x=1.2), respectively. The 2Pr of the BFCT-0.6 is about three times larger than that of Bi5Fe2Ti3O18 (x=0) sample. Furthermore, the dependence of 2Pr on Co content first increases with Co doping when x qslant 0.6, and decreases from x=0.8 to x=1.2, and then increases again. The ferroelectric Curie temperature Tc of the BFCT-x samples increases with increasing x up to 0.8 and then decreases with further increasing cobalt content. It is noteworthy that the Tc of BFCT-1.0 is 2 K lower than that of BFCT-0.6, while the 2Pr decreases by 63%. It is seen that the 2Pr and 2Mr increase simultaneously with increasing Co content (below 0.6). When 0.8 x qslant 1.0, the 2Mr increases while 2Pr decreases with increasing Co content. After x1.2, the 2Mr decreases while 2Pr increases with increasing Co content. The repelling between the FE and FM as discussed above may result from the magnetic-crystalline and ferroelectric-crystalline anisotropy. The mechanism of this phenomenon is not quite clear and needs further investigation.

2015, 64 (21): 217801.
doi: 10.7498/aps.64.217801

Abstract +

Transparent ceramics have been widely researched for their broad range of applications, e.g. from optical windows to laser and optoelectronic switches. However, the challenge is to obtain the optical materials with high refractive index to miniaturize optical functional elements, such as lens for optical information storage and waveguides for flat optical components. The hexagonal complex perovskite Ba(Mg1/3Nb2/3)O3(BMN) ceramic, being widely researched as a type of microwave dielectric ceramics, presents the excellent dielectric properties such as high dielectric constant and high Q value, which indicate its potential application as optical materials. In this paper, the electronic structure of BMN is calculated by using the first principle method, to analyze and predict its intrinsic optical properties. The hexagonal complex perovskite BMN ceramic is synthesized using conventional solid-state reaction at 1600 ℃ for 24 h. The structure parameters are obtained through Rietveld refinement of X-ray diffraction data. The crystal model is established, based on the Rietveld refinement result of the XRD test on synthesized BMN (with the weighted profile R-factor Rwp=6.73%, the profile R-factor Rp=5.05%), and then the crystal geometry optimized. With the optimized crystal model, the energy band structure, density of states and optical properties of BMN are calculated using the first principle method based on density functional theory (DFT) with local density approximation (LDA). Results show that BMN has an indirect band gap of 2.728 eV. There are the strong ionic interactions between Mg and O as well as Ba and O, while there is covalent interaction between Nb and O. The energy band near the Fermi level is mainly occupied by O-2p and Nb-4d electrons, which forms the d-p hybrid orbits. With real band gap correction, the optical properties of BMN are obtained from the definition of direct transition probability and the Kramers-Kronig dispersion relations along the polarization directions [100] and [001], including the complex dielectric function, absorption coefficients and reflectivity, respectively. It is shown that the optical properties of BMN are nearly isotropic. According to the Lambert-Beer's law, the intrinsic transmittance of BMN ranges from 77% to 83% in the visible region, and its refractive index is dispersive, ranging from 1.91 to 2.14. Experimental test results are consistent with the theoretical calculation results.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2015, 64 (21): 218101.
doi: 10.7498/aps.64.218101

Abstract +

Oxide thermoelectric materials have been considered to be potential candidates in high-temperature thermoelectric power generation, however, their high thermal conductivity renders them inferior to the conventional thermoelectric materials and limit their practical application. In this paper, we successfully reduce the thermal conductivity of CdO polycrystals through Ca2+ doping, and the improvement in ZT is also obtained due to the low thermal conductivity. Cd1-xCaxO (x=0, 0.01, 0.03, 0.08) polycrystals are synthesized by adding CaCO3 into CdO via conventional solid-state reaction method and their high-temperature thermoelectric properties are studied. XRD results reveal that all samples are composed of CdO polycrystals, and the lattice parameters increase with Ca2+ content due to the larger radius of Ca2+ as compared with that of Cd2+. Addition of CaCO3 can induce the formation of point defects as well as pores in the CdO polycrystals, thus inhibits the grain growth of CdO and induces the increase of grain boundaries. The main electron carriers in CdO are reported to be shallow level donor impurities formed by oxygen vacancies; as the Ca2+ concentration in Cd1-xCaxO increases, the conduction band minimum of the samples shifts upward and the level of donor impurity becomes deeper, finally resulting in the decrease of electron carrier concentration. Meanwhile, the reduced carrier concentration in the doped samples leads to the increase of both the electrical resistivity ρ and the absolute Seebeck coefficient |S|, while the electrical thermal conductivity κ e will decrease with increasing Ca content. Investigations on the thermal properties of the obtained samples demonstrate that the introduction of Ca2+ is effective to suppress the thermal conductivity. The increment of pores and grain boundaries in the doped samples will enhance the long-wavelength phonon scattering, resulting in the decrease of phonon thermal conductivity κ p. Furthermore, the point defects, which come from the mass and size differences between Ca and Cd atoms, also act as scattering centers and lead to a considerable decrease in phonon thermal conductivity. Due to the simultaneous reduction of both electrical and phonon thermal conductivity, the total thermal conductivity κ may substantially be suppressed, for example, the total thermal conductivity of Cd0.95Ca0.05O reaches 2.2 W·m-1·K-1 at 1000 K, a remarkable decrease as compared with pristine CdO, which is 3.6 W·m-1·K-1 measured at the same temperature. Benefiting from the drastically reduced thermal conductivity, Cd0.99Ca0.01O polycrystals can achieve a high ZT of 0.42 at 1000 K, 27% higher than the pure CdO, which is one of the best n-type oxide TE materials reported so far.

2015, 64 (21): 218102.
doi: 10.7498/aps.64.218102

Abstract +

In this paper, a kind of composite radar absorption materials, consisting of polygonal and seamed resistor with frequency selective surface (FSS) and traditional magnetic radar absorption materials (RAM), is presented. After analyzing such a material and its topological structure, we obtain the equivalent circuit model of this structure, and acquire the reflectivity and input impedance of such models on the basis of transmission line theory. By the application of CST (computer simulation technology), we have made a comparison between structures with nonresistor and resistor FSS. The structure with resistor FSS has a dual-band whose bandwidths are 0.8 GHz from 8.4 to 9.2 GHz and 0.22 GHz from 11.5 GHz to 11.72 GHz with the reflectivity below-10 dB, respectively. The simulated reflection coefficient for the resistor FSS shows two resonant frequencies at 8.7 and 11.5 GHz which respectively make contribute to a higher absorbing peak reaching-24 dB and-23 dB. However, the nonresistor FSS does not have the absorption peak at 8.7 GHz, and the absorption peak at 11.5 GHz reaches-20 dB, confirming the importance of resistors in improving absorption performance. We have observed which part of such a structure influences amost the bsorption by ascertaining power loss density in the absorbing structure. Based on the current distribution of the FSS, two different schemes of LC equivalent circuits can be modeled, at 8.7 GHz and 11.5 GHz, which can explain the anti-resonance and higher absorbing peak of resistor FSS. Moreover, due to the fact that the induced current increases significantly after adding resistors, we could see that the losses happen when the induced current flows through the resistors, Finally the usage of resistor could improve the absorptive performance of FSS at around 8.7 GHz, this result is coincident with that of simulation. In addition, the combination of resistor FSS and RAM can lead to a frequency-doubling effect, meaning that it has remarkable absorptive performance in the range of 8–15 GHz.

2015, 64 (21): 218201.
doi: 10.7498/aps.64.218201

Abstract +

Reactive molecular dynamics (MD) is used to simulate the equilibrium process of water confined between two fully hydroxylated α-quartz (001) surfaces with separation distances from 7 to 20 Å. Effect of different patterns of interfacial hydrogen bonds on the structure and dynamics of confined water is investigated. Density profiles, radial distribution functions, number of interfacial hydrogen bonds, and mean square displacements are calculated. The α-quartz (001) surface is cut from an α-quartz crystal at a certain depth to construct a surface with geminal silanols after being fully hydroxylated. The silanol groups on the surface are treated in two different ways in the MD simulations. One of the silanol groups are treated as to be fixed, and the other one is treated as no constraint for the movement of surface silanols. Our results show that different patterns of hydrogen bonds are formed at the interface between SiO2 surface and water. For the fixed silanol surface there is one type of strong hydrogen bonds interacting between the oxygen atoms of water and the hydrogen atoms of surface silanols, leading to the dipole moment of water molecules pointing out from the surface. For the movable silanol surface there are two types of strong hydrogen bonds formed at the interface. One is between the oxygen atoms of water and the hydrogen atoms of surface silanols, and the other is between the oxygen atoms of surface silanols and the hydrogen atoms of water. The number of hydrogen bonds of the first type is much less than those of the second type, leading to the dipole moment of water molecules pointing to the surface. Moreover, the total number of interfacial hydrogen bonds formed on the fixed silanol surfaces is larger than that on the movable silanol surfaces. The density profiles of the confined water indicate the formation of a strong layering of water in the vicinity of the fixed silanol surface, and the water layer is also more ordered with an ice-like structure, as compared with a dense water layer with a liquid-like structure in the case of movable silanol surfaces. Thus the mean square displacements of confined water show that, as compared with interfacial hydrogen bonds formed on the fixed silanol surfaces, the weaker and the lesser interfacial hydrogen bonds formed on the movable silanol surfaces may be responsible for more intense movement of confined water between the movable silanol surfaces. Our simulation suggests that the different pattern of interfacial hydrogen bonds could signifiantly affect the structure and dynamic behaviors of the confined water between two fully hydroxylated silica surfaces.

2015, 64 (21): 218202.
doi: 10.7498/aps.64.218202

Abstract +

To cause the sodium ion activation gate of cardiomyocyte delay to open, the ability of excitation delay should be given to the medium. The time of excitation delay of the medium increases as the control voltage and frequency of stimulation increase. When the control voltage exceeds a threshold value, the medium with excitation delay has the property of low-pass filtering: low-frequency waves can continuously pass through the medium, whereas the high-frequency wave does not pass consecutively. In this paper, the effect of excitation delay of the medium on spiral waves and spatiotemporal chaos is investigated by using Luo-Rudy phase I model. Numerical simulation results show that when the control voltage exceeds the threshold value, the excitation delay of the medium can effectively eliminate the spiral wave and spatiotemporal chaos. When the control voltage gradually increases from a small value, at a small maximal conductance of calcium channel, the excitation delay could reduce the excitability of the medium, making the amplitude of the spiral wave meander increase until conduction failure results in the disappearance of the spiral wave. Under a large maximal conductance of calcium channel, the excitation delay can reduce the unstability of the spiral wave so that spatiotemporal chaos evolve into meandering spiral waves when the control voltage is large enough. The phenomenon that the spiral wave with a large meandering motion of its tip moves out of the system is observed when the control voltage is properly chosen. Further increase of the control voltage leads to the disappearance of spatiotemporal chaos.

2015, 64 (21): 218401.
doi: 10.7498/aps.64.218401

Abstract +

Target tracking has been introduced as a key point in the physical applications, such as passive sonar and chaotic communication etc. It is typically a nonlinear filtering problem to estimate the position and the velocity of a target from noise-corrupted measurements. Some approaches have been proposed for the problem, such as the extended Kalman filter, the unscented Kalman filter, and the cubature Kalman filter (CKF). However, they are effective only in the Gaussian and white assumption for the measurements. Actually, the measurements are easily polluted by the measurement outliers in practice. The measurement outliers may lead to inaccurate performance due to non-symmetrical or non-Gaussian property. In order to cope with the measurement outliers in nonlinear target tracking system, a robust filtering algorithm called the M-estimation based robust cubature Kalman filter (MR-CKF) is proposed for the target tracking problem. Firstly, the nonlinear measurement equation is transformed into an equivalently linear form according to the orthogonal vector, and then the Gaussian extremal function of the target tracking can be obtained by the constrained total least square (CTLS) criterion. By employing the Huber's robust score function, the Gaussian extremal function is further rendered into a robust extremal function, thus the generalized M-estimation can be introduced to the CKF without linearization approximation. The only difference between the Gaussian extremal function and the robust extremal function is the weight matrix, implying that the CKF solution framework does not change and the virtues of both the CKF and M-estimation can be fully utilized such as derivative-free, high accuracy and robust performance. Furthermore, an improved Huber equivalent weight function is designed for the MR-CKF based on the Mahalanobis distance. The outliers' judge threshold is determined according to the confidence level of Chi-square distribution and improper empirical value of the Huber's method can be avoided. In addition, the improved Huber weight function reduces weights of small outliers and removes large outliers, and this is more robust and reasonable than the Huber's method. Moreover, the statistical information of outliers is also not required. Theoretical analysis and numerical results show that the proposed filtering algorithm can improve the accuracy and robustness than the conventional robust algorithms.

2015, 64 (21): 218901.
doi: 10.7498/aps.64.218901

Abstract +

An important problem in the area of social networking is the community detection. In the problem of community detection, the goal is to partition the network into dense regions of the graph. Such dense regions typically correspond to entities which are closely related with each other, and can hence be said to belong to a community. Detecting communities is of great importance in computing biology and sociology networks. There have been lots of methods to detect community. When detecting communities in social media networks, there are two possible sources of information one can use: the network link structure, and the features and attributes of nodes. Nodes in social media networks have plenty of attributes information, which presents unprecedented opportunities and flexibility for the community detection process. Some community detection algorithms only use the links between the nodes in order to determine the dense regions in the graph. Such methods are typically based purely on the linkage structure of the underlying social media network. Some other community detection algorithms may utilize the nodes' attributes to cluster the nodes, i.e. which nodes with the same attributes would be put into the same cluster. While traditional methods only use one of the two sources or simple linearly combine the results of community detection based on different sources, they cannot detect community with node attributes effectively. In recent years, matrix factorization (MF) has received considerable interest from the data mining and information retrieval fields. MF has been successfully applied in document clustering, image representation, and other domains. In this paper, we use nodes attributes as a better supervision to the community detection process, and propose an algorithm based on joint matrix factorization (CDJMF). Our method is based on the assumption that the two different information sources of linkage and node attributes can get an identical nodes' affiliation matrix. This assumption is reasonable and can interpret the inner relationship between the two different information sources, based on which the performance of community detection can be greatly improved. We also conduct some experiments on three different real social networks; theoretical analysis and numerical simulation results show that our approach can get a superior performance than some classical algorithms, so our method is an effective way to explore community structure of social networks.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

High-resolution laser frequency scanning interferometer based on fiber dispersion phase compensation

2015, 64 (21): 219501.
doi: 10.7498/aps.64.219501

Abstract +

The laser frequency scanning interferometer has several advantages, such as non-contact, high accuracy and low signal to noise ratio in detection. In order to achieve higher resolution of the laser frequency scanning interferometer, increasing the tuning range of the light source and reducing the tuning non-linearity have become the key factors. The commonly used method is to correct the non-linearity of the wide bandwidth external cavity tuning laser by a fiber optical auxiliary interferometer constructed external frequency sampling clock. When using the broadband external cavity tuning laser and the auxiliary interferometer with an optical path difference of 220 m, it is found experimentally that the single-mode fiber dispersion makes the frequency of sampled signals change over time, causing the spectrum to broaden and resolution to decline. This paper has established the dispersion mismatch model which shows that the fiber dispersion of the auxiliary interferometer causes linear chirp frequency changes during the measurement of signals. The linear chirp frequency is proportional to the tuning bandwidth and measured distance. The phenomenon and theoretical model of dispersion mismatch is verified by experiments. The results for targets in the air are shown to linearly decrease as the tuning range increases with the maximum offset of 156.3 m for the 20 nm tuning bandwidth. The experiment also proves the peak broadening intensifies with increasing distance measured, and thus verifies as the time delay of free space increase, and the peak broadening and distortion also increases. This result means that it will limit the ranging distance and make large errors in measurement result for long distance targets. The dispersion of the auxiliary interferometer should be compensated in the laser frequency scanning interferometer for large-sized high resolution measurements. In this paper, phase dispersion compensation method based on the evolution of peak variation distortion elimination is proposed, by taking the peak amplitude variation as the criterion; the phase compensation can offset the dispersion and improve the resolution. The original signal is multiplied by the complex phase compensation term, then regulating the phase compensation factor, the chirp becomes smaller as the phase compensation factor is approaching the distortion factor. Under the condition that the phase compensation factor is equal to the distortion factor, the chirp is offset. Then, the relationship between the amplitude and the peak FWHM is studied. It is found that the peak FWHM decreases while the amplitude shows a gradually increasing trend. Therefore, the amplitude can be referred to in order to determine whether the peak FWHM reaches the minimum. The resolution for target's peak can be improved by searching for the maximum amplitude of the spectrum and adjusting the phase distortion coefficient. The experiment shows that the peak FWHM of the target is obviously narrowed after dispersion compensation. The peak value becomes close to the theoretical resolution, and the static target at a distance of 975.216254 mm from the laser frequency scanning interferometer is measured. Results show the measurement accuracy of the interferometer is 584 nm. To further verify the accuracy of the laser frequency scanning interferometer, the laser frequency scanning interferometer is compared with the Renishaw laser interferometer in the measurement range of 0692 mm. The standard deviation between them is 4.5 m. The proposed method is put forward to provide basis for future studies on the large size high resolution laser frequency scanning interferometer.

2015, 64 (21): 219701.
doi: 10.7498/aps.64.219701

Abstract +

Since X-ray pulsar signals cannot be directly detected on the ground, and the space flight detection is both time-consuming and costly, simulation of X-ray pulsar signals with true physical characteristics is of great importance to the validation of various X-ray pulsar signal processing algorithms and X-ray pulsar-based navigation strategies. In this paper, a new simulation method of X-ray pulsar signals is proposed, in which according to the pulsar signal model at the solar system Barycenter (SSB) and the trajectory information of the spacecraft, the real-time photon arrival rate function at the spacecraft is established, then based on this, a scale transforming method is employed to directly generate the photon event time stamps at the spacecraft which follow a non-homogeneous Poisson process. The proposed simulation method takes into account the pulsar spin down law and the influences of the largescale time-space effects introduced in the process of dynamic detection, and thus avoids the complicated iteration procedure involved in the state of the art simulation methods. Finally, a series of simulations are designed to evaluate the performance of the proposed simulation method. The main results can be concluded as follows: 1) The simulated photon event timestamps have a slowly changing period, which are consistent with the pulsar spin down law. 2) The observed pulsar profile accurately reflects how the radiation intensity of pulsars changes over time within a phase cycle, and it has a Pearson correlation coefficient of up to 0.99 with a standard profile. 3) The simulated average fluxes of the pulsars are very close to the true values, and thereby verifies the correctness of the proposed simulation method from an overall point of view. 4) The simulated photon series are very similar to the real data detected by the RXTE explorer, and when the simulation time is longer than 50 s, the relevancy between the simulated profile and the profile obtained from the real data is higher than 0.9. 5) The computational cost of the scale transforming method is much less than that for the commonly used Poisson sifting method and the inverse mapping method. The above results show the validity and high efficiency of the proposed method in terms of the period property, the profile and flux accuracy, the similarity to the RXTE real data and the computational cost.

2015, 64 (21): 219702.
doi: 10.7498/aps.64.219702

Abstract +

Pulsars, a small portion of celestial sources that emit radiation at varying intensity, provide new possible navigation algorithms which are different from steady point sources. Time-delay estimation is one of the key aspects of pulsar-based navigation technology. Previous work for pulse phase estimation uses a maximum likelihood estimator (MLE) for the phase-in time domain, which is seen as one of the most useful phase estimators. However, the analytic solution for phase cannot be found using MLE. As a result, a brute-force method involving nested, iterative grid-searches is needed to solve this MLE issue, which leads to lots of computations. In order to solve this problem, a multiple harmonic X-ray pulsar signal phase estimation (MHSPE) method is proposed. This method uses the times of arrivals (TOAs) measured pulsar signal to estimate the time-delay in the frequency domain. In this paper, firstly we use the arrival time to derive the maximum-likehood (ML) estimation of phase-delay by fundamental frequency, then an analytic expression for the fundamental frequency phase is obtained. The MHSPE method based on the fundamental frequency phase equation, calculates different harmonic phases by generalizing the analytic expression of fundamental frequency phase, and the normalized amplitude of each harmonic in the spectrum is used as the weight of each harmonic phase. Finally, the weighted average of harmonic phases, which is given by the final analytic expression, is used as the estimation of the moment. To evaluate the MHSPE method, the error and variance equations are calculated and the MHSPE method is demonstrated to be unbiased and consistent. Moreover, by comparing with the ML estimation of the first harmonic, if the amplitudes of harmonic in the spectrum are almost the same, the signal-to-noise ratio (SNR) of MHSPE improves m1/2 times when the number of harmonic waves is m. Compared with the Cramer-Rao bound of pulsar time-delay estimation, the derivative of pulsar signal in the time domain reflects the number of harmonic waves in the frequency domain. Hence, the MHSPE can greatly approximate to the Cramer-Rao bound for the estimation of pulsar signal timedelay when the harmonic number increases. Finally, we utilize the TOAs of Crab pulsar, observed by Rossi X-ray timing explorer (RXTE) spacecraft, to verify the performance of MHSPE. The results show that for low SNR of pulsar signal, MHSPE can obtain high precision phase estimations. When the amplitude of the harmonic in the spectrum is larger, the estimation variance of the harmonic phase tends to be smaller. The projection orbit determined by MHSPE method can match the projection of RXTE in the direction of Crab pulsar, with the observed time increasing, the estimation accuracy converges rapidly to Cramer-Rao bound.