## Vol. 64, No. 7 (2015)

##### 2015-04-05

###### SPECIAL ISSUE — Control of electronic states in surface low-dimensional structures

2015, 64 (7): 076802.
doi: 10.7498/aps.64.076802

Abstract +

Metal-phthalocyanines (MPcs) and their derivates have attracted increasing interest in recent years, due to their potential applications in molecular electronics, spintronics, sensors, and so on. To this end, it is essential to tune the structural, electronic and spin properties of MPcs. Using the low-temperature scanning tunneling microscopy (LT-STM), we demonstrate that the spin, chirality and adsorption site of MnPc on Au(111) surface can be tuned by hydrogen atoms. STM experiments and density functional theory (DFT) calculations reveal that the preferential adsorption sites for the MnPc molecules may switch from the fcc regions to the hcp regions on the Au(111) surface after a hydrogen atom is adsorbed on top of the central Mn ion of each MnPc molecule. Moreover, the molecular spin decreases from S=3/2 to S=1 and the molecule-substrate coupling is weakened after the adsorption of a hydrogen atom on a MnPc molecule, leading to the quenching of Kondo effect at 4.2 K. However, the molecular spin and Kondo effect can be recovered by local voltage pulse or sample heating. Adsorption of three hydrogen atoms on a MnPc molecule not merely lowers the molecular symmetry from 4-to 2-fold, but also breaks down the mirror symmetry of the entire adsorbate complex (molecule and surface), thus rendering it to become chiral without any realignment at the surface. Dehydrogenation of the adsorbate by means of inelastic electron tunneling can also restore the mirror symmetry of the adsorbate complex. STM experiments as well as DFT calculations show that the chirality is actually imprinted into the molecular electronic system by the surface, i.e., the lowest unoccupied orbital is devoid of mirror symmetry. Our novel reversible spin and hand control scheme can be easily realized at single-molecule level, thus opening up a new avenue to broader applications based on the molecular electronic and spin states.

2015, 64 (7): 076803.
doi: 10.7498/aps.64.076803

Abstract +

We demonstrate that the molecular ligands play important roles in controlling the electronic states and electron transport properties of single cobalt (Ⅱ) octaethylporphyrin (CoOEP) molecule adsorbed on Au(111) surface by using low-temperature scanning tunneling microscopy and spectroscopy. Single CoOEP molecule adsorbed on Au(111) surface has eight methyl groups pointing out of the surface plane. A peak located at -50 mV in dI/dV spectrum measured on the Co atom of CoOEP is identified as a d-orbital mediated resonance. We find that the methyl groups in CoOEP can be removed in a stepwise manner, and finally lead to a fully-demethylized CoOEP. The d-orbital mediated resonance gradually evolves into a sharp Kondo resonance located right at Fermi level in the demethylization process. Both experimental and theoretical results indicate that the chemical environment and magnetic moment of the central Co atom change slightly in the fully-demethylized CoOEP: the Co atom is slightly lifted by 0.15 and the magnetic moment increases from 0.5 B to 0.6 B. The emergence of Kondo effect is qualitatively explained with a simple model by consideringthe change in the tunneling parameters of the ligands upon demethylization. We also show that the transport properties of the CoOEP can be dramatically controlled by weak intramolecular van der Waals interaction. In CoOEP closely-packed dimers and trimers where CoOEP molecules are introduced close enough to each other, the ethyl groups in the neighboring area are found to be strongly lifted by 0.4 . More surprisingly, a pronounced resonance shows up at 00.8 V in the dI/dV spectra of the lifted ethyl groups. High resolution spectra show that the new resonance consists of multiple peaks with equal spacing of 137 8 mV. The spacing energy coincides with the vibrational energy of stretching mode of CC bond between ethyl group and the brim carbon atom of the porphyrin ring. Therefore the newly-emerged resonance is attributed to the vibronic states originating from the intramolecular CC bond stretching mode excited by tunneling electrons. A model considering the local formation of a barrier between the lifted part of the molecule and the substrate is employed to explain the experimental observations. Our findings show that the electron transport properties in single molecules can be intensely tuned by controlling the chemical properties of the molecular ligands.

2015, 64 (7): 077201.
doi: 10.7498/aps.64.077201

Abstract +

Linear magnetoresistance (LMR) observed in a topological insulator {(Bi0.5Sb0.5)2Te3} thin film is systematically studied. LMR exists in very large ranges of temperature and magnetic field. It shows no trend toward saturation in the magnetic field of up to 18 T nor temperature dependence. LMR can be changed effectively by tuning the chemical potential through gate voltage. LMR shows a largest value when the chemical potential approaches to the Dirac point. These phenomena indicate that charge inhomogeneity is the origin of the LMR in this material.

2015, 64 (7): 077303.
doi: 10.7498/aps.64.077303

Abstract +

Localized surface plasmon (LSP) of nanoparticles has become one of the world's research hotspots due to its novel optical properties. Based on the time-dependent density functional theory (TDDFT), this paper studies the physical nature of plasmon excitation which is modulated in metal clusters and graphene nanostructures. Compared with the plasmon in the macroscopic material, the plasmon in nanostructures has some different properties due to the effects of the size and the dimensional confinement. In lower-energy resonance zone, the spectral band is greatly broadened, and the photoabsorption strength line splits. Because of the electromagnetic coupling between the nano-monomers, aggregated nanostructures exhibit different optical properties. For plasmon regulation and control, these results provide a solid theoretical guidance.

2015, 64 (7): 077304.
doi: 10.7498/aps.64.077304

Abstract +

The surface morphology and molecular orientation of -conjugated polymers, along with the chemical interaction and electronic structure at the interface between metals and these polymers, strongly affect the performance of the polymer-based organic electronic and optoelectronic devices. In this study, atomic force microscopy (AFM), synchrotron radiation photoemission spectroscopy (SRPES), and near edge X-ray absorption fine structure (NEXAFS) have been used to in situ investigate the morphology, structure, and molecular orientation of spin-coated poly(9,9-dioctylfluorene-co-benzothiodiazole) (F8BT) films and their interaction with the vapor-deposited Al metal. F8BT films were prepared by spin-coating the F8BT chloroform solution onto clean gold-coated silicon wafer surfaces. The room temperature spin-coated F8BT film is rather flat, while mild annealing treatments (120 ℃) below the glass transition temperature (Tg=130 ℃) lead to an apparent increase of surface roughness of F8BT film, which is helpful to effectively increase the contact areas between metals and F8BT. After 70 ℃ annealing in vacuum, the aromatic rings of F8BT preferentially stand more edge-on, making an average tilt angle of approximately 49 with the substrate, while the 9,9-dioctylfluorene unit (F8) and the benzothiodiazole unit (BT) nearly lie in the same plane. Upon vapor-depositing Al metal onto F8BT at room temperature, strong chemical interactions occur between Al and F8BT, as evidenced by the distinct changes of the S 2p, N 1s and C 1s spectra. Al reacts with S atoms more strongly than with N and C atoms in F8BT. In addition, obvious structural changes in valence band of F8BT are also observed during the Al deposition. Furthermore, Al dopes electrons into F8BT, leading to downward band bending, formation of interfacial dipole at the Al/F8BT interface, and partial occupation of lowest unoccupied molecular orbits (LUMO). However, no doping-induced gap states can be observed during the formation of Al/F8BT interface. Through the investigation of the core-level and valence band spectra evolution of F8BT together with the shifts of secondary electron cutoff during Al deposition, an energy level alignment diagram at the Al/F8BT interface is derived. The information gained through this study will help better understand the correlation between the interface structures of metal electrodes on semiconducting, -conjugated polymer materials and the performances of real polymer-based electronic and optoelectronic devices, which will in turn help develop the more efficient polymer-based organic devices.

2015, 64 (7): 077305.
doi: 10.7498/aps.64.077305

Abstract +

Graphene Moiré superlattice, a unique 2D periodical structure originated from the interaction between graphene and its supporting substrate h-BN, has attracted great interest recently. Employing epitaxial graphene on h-BN single crystals, we have investigated systematically the physical properties related to the Moiré superlattice. From transport measurements, we can observe the superlattice Dirac points at both electron side and hole side. Similar to the Dirac point, the superlattice Dirac points have insulator behaviors. Under the action of magnetic field, the quantum Hall effects both in monolayer and bilayer graphenes are observed. Also, the Moiré superlattice can lead to the formation of self-similar mini-bands from the Landau fan diagram. According to the infrared optical spectroscopy measurements, the transitions between different Landau levels are characterized by massive Dirac fermions and thus reveal a band-gap of ～38 meV. Moreover, without magnetic fields, an optical conductivity peak related to the Moiré superlattice appears. We use three spinor potential components to explain the optical conductivity peak and demonstrate that the pseudospin-mixing component plays a dominant role in the spinor potential. In addition, the spinor potential depends sensitively on the gate voltage, indicating that the electron–electron interactions play an important part in the renormalization of the spinor potential.

2015, 64 (7): 078101.
doi: 10.7498/aps.64.078101

Abstract +

In-plane heterostructure of hexagonal boron nitride and graphene (h-BN-G) has become a research focus of graphene due to its predicted fascinating properties such as bandgap opening and magnetism, which hence has ignited the attempt of experimentally growing such in-plane two-dimensional (2D) hybrid materials. Many previous researches demonstrated the synthesis of such heterostructures on Cu foils via chemical vapor deposition (CVD) process. The obtained 2D hybrid materials would offer a possibility for fabricating atomically thin electronic devices. However, many fundamental issues are still unclear, including the in-plane atomic continuity, the edge type, and the electronic properties at the boundary of hybridized h-BN and graphene domain. To clarify these issues, we report the syntheses of h-BN-G monolayer heterostructures on strongly coupled Rh(111) substrate and weakly coupled Ir(111) substrate via a two-step growth process in an ultrahigh vacuum (UHV) system, respectively. With the aid of scanning tunneling microscopy (STM), it is revealed that graphene and h-BN could be linked together seamlessly on an atomic scale at the linking boundaries. More importantly, we find that the atomically sharp zigzag-type boundaries dominate the patching interface between graphene and h-BN as demonstrated by atomic-scale STM images. To understand the physical origin of the atomic linking of the h-BN-G heterostructures, we also perform density functional theory (DFT) calculations, including geometry optimizations and binding energy calculations for different kinds of linking interfaces. The calculated results reconfirm that graphene prefers to grow on the h-BN domain edges and form zigzag linking boundaries. Besides the atomic structures on the linking interfaces, the electronic characteristics are also of particular importance. It is worth noting that the substrates coupled strongly with graphene by π-d orbital hybridization (such as Rh(111) and Ru(0001)), lead to downward shift of graphene π-bands away from the Fermi level, or decay of the intrinsic electronic structure of graphene. In this regard, the influence of h-BN on the electronic property of graphene is hard to identify on such h-BN-G heterostructures. The weakly coupled Ir(111) is chosen to be a perfect substrate to investigate the interface electronic properties of h-BN-G heterostructure due to the absence of substrate electronic doping effect. Scanning tunneling spectroscopy studies indicate that the graphene and h-BN tend to exhibit their own intrinsic electronic features near the linking boundaries on Ir(111). Therefore, the present work offers a deep insight into the h-BN-G boundary structures and the effect of adlayer-substrate coupling both geometrically and electronically.

2015, 64 (7): 078103.
doi: 10.7498/aps.64.078103

Abstract +

Transition metal oxides exhibit abundant physical properties due to the electronic interactions between charge, orbit and spin degrees of freedom. Lanthanum titanate, LaTiO3, a typical strongly correlated electron material, shows Mott-type metal-insulator and antiferromagnetic transitions at low temperature. And these interesting behaviors can be tuned by adjusting the occupation of the t2g orbit of Ti3+, or introducing symmetry breaking or lattice strain into the heterointerfaces. Especially on LaTiO3(110) surface, the anisotropic structure as well as the surface polarity allows the flexible control of artificial low-dimensional structure. However, the instability induced by surface polarity hinders the growth of high-quality LaTiO3(110) film. Here we show that by keeping the growing surface reconstructed in the molecular beam epitaxy (MBE) process, the surface polarity can be effectively compensated for, allowing the high-quality layer-by-layer film growth. Moreover, the intensity of reflective high-energy electron diffraction (RHEED) pattern sensitively changes with the surface cation concentration. Therefore the relative deposition rates of La and Ti sources can be monitored and further be precisely calibrated in situ and in real-time. We first prepare the (2× 16) reconstruction on SrTiO3(110) surface by depositing La and Ti (2 ML for each) metals. Further increasing the Ti concentration on (2×16), i. e., the [Ti]/[La] ratio, results in the significant decrease of RHEED “1×” intensity and the increase of “2×” intensity. And the change of RHEED intensity is quantitatively reversible through reducing the [Ti]/[La] ratio by the same amount. We set the evaporation rate of Ti source to be slightly higher than that of La for the MBE film growth. And the shutter state of Ti source is controlled to be open or close, which is determined by the change of RHEED intensity. Precise cation stoichiometry is achieved in the LaTiO3(110) film. X-ray diffraction confirms the single crystallinity of the film while scanning tunneling microscope images indicate the atomically flat surface with (2×16) reconstruction that is responsible for the stabilization of the polar surface. The annealing of the sample in oxygen at 700 ℃ will oxidize the LaTiO3 film into the thermodynamically stable phase, i. e. , La2Ti2O7, although the as-grown LaTiO3 phase can be stable at room temperature. The high-resolution STM images reveal the detailed structural information of the (2×16) film surface–along the [001] direction, the tilt of TiO6 octahedron in LaTiO3 lattice results in the “2×” periodicity modulation on the (110) surface. The “×16” periodicity along [110] might be related to the rotation of TiO6 octahedron in (001) plane or to the strain relief on the surface. Both of the RHEED and STM observations indicate that the film surface is terminated by the TiO6 octahedron, i. e., the (O2) atom layer. Indeed the LaTiO3(110) polar surface can be stabilized by making two holes on the (O2) layer by oxidizing Ti3+ into Ti4+. On the contrary, due to the Coulomb repulsion between electrons on Ti3+ 3d orbit, the (110) surface is difficult to reduce (to introduce extra electrons). Therefore the (LaTiO) termination layer cannot be stable.

###### GENERAL

2015, 64 (7): 070504.
doi: 10.7498/aps.64.070504

Abstract +

Multivariate chaotic time series is widely present in nature, such as in economy, society, industry and other fields. Modeling and predicting multivariate time series will help human to better manage, control, and make decision. A prediction method based on multiple kernel extreme learning machine is proposed in this paper to model the complex dynamics of multivariate chaotic time series. First, the multivariate chaotic time series is reconstructed in phase space, transforming the temporal correlation into spatial correlation. Then, a prediction model-multiple kernel extreme learning machine, which combines the multiple kernel learning and extreme learning machine with kernels, is proposed to approximate the nonlinear function of the input - output data in phase space. The proposed multiple kernel extreme learning machine could effectively combine the simple training of extreme learning machine with kernels and the data fusion capabilities of multiple kernel learning. Simulation results based on Lorenz chaotic time series prediction and San Francisco monthly runoff prediction demonstrate that, compared with other state-of-art chaotic time series prediction methods, the proposed multiple kernel extreme learning machine could get a better prediction accuracy.

2015, 64 (7): 070201.
doi: 10.7498/aps.64.070201

Abstract +

As a widely applied model for compressive sensing, the multitask compressive sensing can improve the performance of the inversion by appropriately exploiting the interrelationships of the tasks. The existing multitask compressive sensing recovery algorithms only utilize the statistical characteristics of a sparse signal, the structural characteristics of the sparse signal have not been taken into consideration. A multitask compressive sensing recovery algorithm is proposed in this paper based on the block sparse Bayesian learning. The block sparse single measurement vector model is applied to the multi-task problem. Both statistical and block structural characteristics of the sparse signal are used to build a mathematical model, and the sparse inverse problem is linked to the parameter iteration problems in the Bayesian framework. The proposed algorithm does not require the sparseness information and noise beforehand, which turns out to be an effective blind recovery algorithm. Extensive numerical experiments show that the proposed algorithm can exploit both statistical and structural characteristics of the signal, therefore it may reach a good trade-off between the recovery accuracy and the convergence rate.

2015, 64 (7): 070505.
doi: 10.7498/aps.64.070505

Abstract +

At present, commonly used methods of weak signal detection such as the wavelet threshold denoising method, digital filtering method, the Fourier frequency domain transformation etc. can achieve the lowest detection of signal-to-noise ratio (SNR) of -10 dB, and the bidirectional ring coupled Duffing oscillator can reach the lowest detected SNR of -20 dB. But the discharge pulse signal with a lower SNR often appears in on-site testing, so the existing detection methods are difficult to meet the practical requirements of weak signal detection. In order to effectively solve the problem, a new method for weak pulse signal detection is proposed based on an extended-Duffing oscillator. The main idea of this method is to make the Duffing oscillator model transform to an extended-Duffing oscillator model by using the general time scale transformation. This approach can effectively expand the frequency detection range for weak signal detection. In addition, because the critical amplitude of the Duffing system depends on various parameters, such as system parameters, initial values, driving signal frequency, and calculation step of Runge - Kutta method etc.. However, the Melnikov method is an approximate analytical method, which does not take into account the factors such as initial values and calculation step, therefore, the Melnikov method is not suitable for numerical simulations, and lack of practicality. For this, the critical amplitude of chaos with high accuracy is determined only through the simulation experiment. Experimental results show that the critical amplitude is equal to 0.825010 when the incentive angular frequency of the extended-Duffing oscillator equals 10000 rad/s, and the extended-Duffing oscillator changes from the critical chaotic state to the large scale cycle state for small changes (10-6) of the driving amplitude. The simulation results show that the extended-Duffing oscillator not only has a good noise immunity performance, but also can effectively detect weak partial discharge pulse signal so that the signal-to-noise ratio can be lower than -40 dB. This method further expands the detection range and application fields of weak signals.

2015, 64 (7): 070202.
doi: 10.7498/aps.64.070202

Abstract +

Due to the characteristic of risk aversion, option has become one of the most fashionable derivatives in the financial field. More and more investigators are attracted to devote themselves to exploring the option pricing problem. In this paper, we are concerned with the valuation of American lookback options in terms of the Black-Scholes model. It is well known that the American lookback option satisfies a two-dimensional nonlinear partial differential equation in an unbounded domain, which couldn't be numerically solved directly. Based on the analysis of the issues for solving this problem, this paper introduces an approach to settle it. First, we transform the problem into a one-dimensional form by the numeraire transformation. And then, the Landau's transformation is applied to normalize the defined domain. For the nonlinear feature of the resulting problem, we propose a finite volume method coupled with Newton iterative method to obtain the optional value and the optimal exercise boundary simultaneously. We also give a proof on the nonnegativity of the numerical solutions under some appropriate assumptions. Finally, some numerical simulations are presented using the proposed method in this paper. Comparing with the binomial method, we can conclude that the proposed method is an effective one, which provides a theoretical basis for practical applications.

2015, 64 (7): 070506.
doi: 10.7498/aps.64.070506

Abstract +

It is well known that time delay is universal in complex networks. However, in most existing researches outer synchronization is realized between two networks with time delay by adding controllers to all nodes which may bring great economic costs and increase the difficulties in control in practice. In this paper, in order to deal with the problem of outer synchronization between two time-varying coupling networks with node delay and coupling delay, an adaptive pinning control scheme is proposed. First, a more realistic drive-response complex network model is constructed by introducing double delays and asymmetric coupling configuration matrices. Then, we design an adaptive pinning controller which is easy to implement, and choose an effective pinning strategy to control a crucial part of the nodes in the response network. Based on LaSalle' invariance principle and the linear matrix inequality, we may rigorously prove that the outer synchronization between the proposed drive-response networks can be achieved, and meanwhile some sufficient conditions are derived by adopting an appropriate Lyapunov-Krasovskii energy function. Finally, numerical simulation experiments are employed to verify the correctness and the effectiveness of the proposed method. Results indicate that the drive-response networks with double delays can indeed achieve outer synchronization by pinning control. Moreover, the synchronization is independent of coupling delay. And the remarkable influences of coupling delays on the synchronization speed are also revealed.

2015, 64 (7): 070301.
doi: 10.7498/aps.64.070301

Abstract +

In the experiment presented in the paper, HBT (Hanbury-Brown and Twiss) interferometer is used to measure the second-order correlation function (G2) for two entangled photons, denoted as signal and idle ones, generated by the spontaneous parametric down-conversion process. When an extra optical fiber is inserted in the signal's path, the arrival times of these two photons are different, so that the length of the fiber can be estimated by measuring the time offset in the G2 function.#br#As the laser light we used is not monochromatic, the experimental result is affected by the group velocity dispersion (GVD). Light of different wavelength has different velocity and leads to different travelling time in the fiber. Through calculation, we find that the G2 function of each wavelength has the same shape but different center time offset. Thus, the final G2 function, as a superimposition of the functions with different wavelengths, is broadened in a long-distance measurement, which seriously reduces the estimation accuracy. We analyze this phenomenon in both theory and experiment, and the experimental results fit our theoretical simulations quite well.#br#In the theoretical analysis, we find that the group velocity dispersion introduces an additional time offset to the final G2 function which is proportional to the total time offset. In our experiment, this additional time offset is about 1% of the total time offset. In particular, this effect results in a 63600 ps extra time offset, which can be compensated in post process, with a time accuracy of about 1 ps.#br#This paper focuses on the G2 function affected by the group velocity dispersion, and also calculates the additional time offset produced by group velocity dispersion, and thus gives a method to compensate the additional time offset through calculation.

2015, 64 (7): 070507.
doi: 10.7498/aps.64.070507

Abstract +

Vibrational resonance is a resonant dynamics induced by a high-frequency periodic force at the low-frequency of the input periodic signal, and the input periodic signal is enhanced by a high-frequency signal. In this paper, a linear time-delayed feedback bistable system with an asymmetric double-well potential driven by both low-frequency and high-frequency periodic forces is constructed. Based on this model, the vibrational resonance phenomenon is investigated. Making use of the method of separating slow motion from fast motion under the conditions of Ω>>ω (Ω is the frequency of the high-frequency signal and ω is the one of the low-frequency signal), equivalent equations to the slow motion and the fast motion are obtained. Neglecting the nonlinear factors, the analytical expression of the response amplitude Q can be obtained, and the effects of the time-delay parameter α and the asymmetric parameter r on the vibrational resonance are discussed in detail. Moreover, the locations at which the vibrational resonance occurs, are obtained by means of solving the condition for a resonance to occur. A major consequence of time-delayed feedback is that it gives rise to a periodic or quasiperiodic pattern of vibrational resonance profile with respect to the time-delayed parameter, i.e. in Q-α plot, α can induce the Q which is periodic with the periods of the high-frequency signal and the low-frequency signal. The locations at which the vibrational resonance occurs are not changed by the asymmetric parameter r. However, the resonance amplitude is enhanced with increasing r. Specifically, the resonance amplitude is greatly enhanced when r>0.15. On the other hand, in the symmetric case (r=0), BVR at which the vibrational resonance occurs is periodic with the periods of high-frequency signal and low-frequency signal as α increases, which is shown in BVR-α (B is the amplitude of the high-frequency signal) plot. In Q-Ω plot, Q is presented by multi-resonance at the small values of B and Ω, but Q tends to a fixed value at the small values of B and the large values of Ω. We believe that the above theoretical observations will stimulate the experimental study of vibrational resonance in nonlinear oscillators and electronic circuits with time-delayed feedback.

2015, 64 (7): 070302.
doi: 10.7498/aps.64.070302

Abstract +

Quantum correlations among different parts of a composite quantum system are the fundamental resource of several applications in quantum information. In general, quantum discord can measure quantum correlations. In that way, the quantum correlations in the Yang-Baxter spin-1/2 chain mode are investigated. In the second part of the paper, the Yang-Baxter spin-1/2 chain modes are constructed from the Yang-Baxter equation. First, we analyze the two matrix representations of Temperly-Lieb algebra. Second, the two solutions of the Yang-Baxter equation are generated using the Yang-Baxterization. Finally, we can change the usual two-particle spin-1/2 chain to the Yang-Baxter spin-1/2 chain modes by means of the unitary Yang-Baxter matrix-R. In the third part, the density matrices of the two chain modes are generated in the thermal equilibrium state in a canonical ensemble. According to the definition of the geometric measure of quantum discord, the analytical expressions of the geometric measure of quantum discord, in the temperature and the external magnetic field, are obtained for the Yang-Baxter spin-1/2 chain modes. When the temperature and the magnetic field intensity increase, the geometric measure of quantum discord decreases. Under the specific conditions, the result of the second chain mode is similar to that of the first one. Then we obtain the numerical results of quantum discord, the geometric measure of quantum discord, and concurrence. It is found that the concurrence can quickly decrease to the value of zero when the temperature is greater than the value of one. At the same time, quantum discord and the geometric measure of quantum discord are not of the value of zero. Thus the quantum discord and the geometric measure of quantum discord can go beyond the concept of entanglement and obtain the “quantumness” of the correlations between the two parts of a system for the Yang-Baxter spin-1/2 chain modes. They are very good quantum resources for quantum information and quantum computing.

## EDITOR'S SUGGESTION

2015, 64 (7): 070701.
doi: 10.7498/aps.64.070701

Abstract +

Sampling and reconstruction of short pulses based on Gabor frames have been proved to be effective, which overcome the difficulties that finite rate of innovation (FRI) sampling is unable to reconstruct the pulse streams without the prior information of waveforms. However, the windows sequences of sampling scheme based on Gabor frames proposed at present show complicated structure and are hard to realize physically. The exponential reproducing windows are then introduced in this paper and the windows sequences can be simplified as a first-order analog Butterworth filter. At the same time, the compressed sensing (CS) measurement matrix is constructed for the recovery of Gabor coefficients. In order to satisfy the restricted isometry property (RIP) of the measurement matrices for perfect signal reconstruction, we select appropriate windows for support according to the energy accumulation property. A restricted condition is deduced for perfecting the signal reconstruction and the system robustness is analyzed. By numerical simulations the above analysis is verified. This novel scheme can be used to implement short pulses sampling and reconstruction in the field of instrumentation, condition monitoring, radar and the communication.

2015, 64 (7): 070501.
doi: 10.7498/aps.64.070501

Abstract +

In the absence of external force and noise, a deterministic transport model for asymmetrically coupled nonlinear oscillators in a ratchet potential is established. By numerical simulation, both directed current and reversely directed current can be obtained by selecting appropriate parameters. The complex dependences of current velocity on the model parameters are discussed. It is observed that the average velocity of the particle chain varies non-monotonically with coupling strength and potential height, indicating a generalized resonance phenomenon. When the other parameters are fixed, the speed curve which is dependent on spring free length has a roughly inverse symmetry, and there also exists a generalized multi-peak resonance.

2015, 64 (7): 070702.
doi: 10.7498/aps.64.070702

Abstract +

This paper presents a relativistic coaxial overmoded surface wave oscillator (SWO) working at the terahertz band in the double-ring metamaterial slow wave structure (SWS). A relativistic electron beam passes through the SWS between the inner and outer rings. A coaxial overmoded SWS made up of metal metamaterial is designed to generate the high-power terahertz wave by increasing the beam-wave interaction efficiency and enlarging the transverse size of the terahertz device. It consists of double rings periodically arrayed along the z-direction, and a coaxial conductor with a radius of 2.4 mm. By its dispersive relation the proposed device is studied, from which we choose the 0.14 THz as the operating frequency of the device. Then the parameters of the geometric structure and the electron beam are optimized; the transitional section for extracting the terahertz signal is designed of the largest propagation coefficient. Particle simulation code UNIPIC is employed to verify the initial expectation and potential advantages. When the beam voltage and current are increasing, the operating frequency of the device remains almost constant, and this is the typical characteristic of the SWO. Particle simulation results show that the coaxial inner conductor has a stable operating mode of double-ring metamaterial SWS and can increase the beam-wave interaction efficiency of the SWO at the terahertz band. For a guiding magnetic field of 2.0 T, with the electron beam of 600 kV and a current of 1.0 kA, a 0.141 THz wave output power of 316.8 MW is obtained.

2015, 64 (7): 070502.
doi: 10.7498/aps.64.070502

Abstract +

In this paper, a sliding mode control based on an online error correction adaptive SVR is put forward for a class of fractional order chaotic system with nonlinear uncertainty. In order to solve the problem that the uncertainty of the fractional order chaotic system model is difficult to predict, so the nonlinear function of the system is estimated by the offline SVR and the state trace error is forecasted by using incremental learning adaptive online SVR. In addition, the adaptive parameter adjustment law is selected by using the Lyapunov stability theory. Result of simulation of the fractional order Arneodo system shows that the sliding mode control based on the online error correction adaptive SVR can stabilize the nonlinear uncertain fractional order chaotic system with external noise disturbance to an expected state within a limited time. At the same time, both the control performance and the prediction precision of the system's nonlinear function can be improved.

2015, 64 (7): 070703.
doi: 10.7498/aps.64.070703

Abstract +

When the working frequencies of vacuum electronic devices reach the terahertz frequency (0.1–10 THz), the Ohmic loss has a great impact on the vacuum electronic devices. To study the effect of the Ohmic loss on the working characteristic of the vacuum electronic devices in the terahertz band, this paper presents the boundary condition of surface impedance used in the 2.5-dimensional fully electromagnetic particle simulation code UNIPIC, which is verified by simulating the terahertz wave in the circular copper waveguide; the simulation result indicates that the code can correctly simulate the propagation of terahertz waves in the waveguide with an Ohmic loss. Then, the coaxial surface wave oscillators (SWO) with slow wave structures (SWS) made of different metals are numerically studied by using the above code, and the dependences of output power on the SWOs with different metal SWSs are analyzed. Numerical results show that the metal conductivity has a considerable effect on the output power of the device: When the conductance of the metal decreases, the quality factor of the device becomes smaller, the start-up time becomes longer, also the output power of the device decreases also. For the coaxial SWOs operating at 0.14 THz, the output powers from the copper and stainless steel SWSs are reduced by 13.4% and 63.9%, the start-up times of the devices are delayed by 0.4 ns and 15 ns, respectively. Meanwhile, the working frequencies of the devices with the SWSs made of different metals keep unchanged.

2015, 64 (7): 070503.
doi: 10.7498/aps.64.070503

Abstract +

In this paper the synchronization problem for the uncertain fractional-order chaotic systems with unknown non-symmetrical control gain matrices is investigated by means of adaptive fuzzy control. Fuzzy logic systems are employed to approximate the unknown nonlinear functions. We decompose the control gain matrix into a positive definite matrix, a unity upper triangular matrix, and a diagonal matrix with diagonal entries +1 or -1. The positive matrix is used to construct the Lyapunov function; the diagonal matrix is employed to design the controller. Based on the fractional Lyapunov stability theorem, an adaptive fuzzy controller, which is accompanied by fractional adaptation laws, is established. The proposed methods can guarantee the boundedness of the involved signals as well as the asymptotical convergence of the synchronization errors. It should be pointed out that the methods for using quadratic Lyapunov function in the stability analysis of the fractional-order chaotic systems are developed in this paper. Based on the results of this paper, many control methods which are valid for integer-order nonlinear systems can be extended to control fractional-order nonlinear systems. Finally, the effectiveness of the proposed methods is shown by simulation studies.

2015, 64 (7): 070704.
doi: 10.7498/aps.64.070704

Abstract +

Concentration of CH4 in ambient atmosphere is quite low (～1.8 ppmv) and mixed well. So observation of CH4 variation with high precision has to rely on high-precision of inversion technique. This paper demonstrates the observation of CH4 variations using high resolution (0.02 cm-1) Fourier transform solar spectrometry, in which a high precision and large scale of retrieval algorithm is investigated. In the CH4VCD (vertical column density) retrieval, both a nonlinear least-square method and an iterative scheme are applied after the measured spectrum is modeled with various a-priori parameters. The XCH4 (column-averaged dry air mixing ratio) is calculated by using the O2 VCD derived from the 7885 cm-1 O2-A band. Fitting errors for both CH4 VCD and XCH4 are less than 1%, and most XCH4 retrievals are within the 4 variations are also investigated based on one typical daily observation. It shows that the daily CH4 VCD decreases with time and the variations of the XCH4 is within 0.02 ppmv.

###### NUCLEAR PHYSICS

2015, 64 (7): 072801.
doi: 10.7498/aps.64.072801

Abstract +

Simulation of thermal desorption spectroscopy (TDS) of the hydrogen isotope-deuterium in tungsten has been investigated in this paper based on rate theory. Data are obtained using polycrystalline tungsten, which is under the irradiation of a plasma with an energy of 40 eV and a dose of 1× 1026 D/m2 at 520 K. By adjusting the trapping energy, trapping rate, and other parameters in the rate theory, we can obtain the TDS simulation spectrum, which coincides with the experimental results. It is found that there mainly exist three trapping states for deuterium in tungsten, whose trapping energies are 1.14, 1.40 and 1.70 eV, and the temperature peaks of them is 500, 600 and 730 K, respectively. These three trapping energies correspond to the energy for trapping the 3rd-5th hydrogen by vacancy (the zero point energy correction has been taken into account), the energy for trapping the 1st-2nd hydrogen by vacancy, and the energy for trapping the hydrogen by vacancy cluster, obtained from first-principle calculation, respectively. It is suggested that the vacancy and vacancy cluster are the main trapping objects for deuterium in tungsten, under the experimental condition mentioned above.

###### ATOMIC AND MOLECULAR PHYSICS

2015, 64 (7): 073101.
doi: 10.7498/aps.64.073101

Abstract +

Dibutyl phthalate (DBP) is the main component of the plasticizers. In order to study the influence of an external electrical field on the molecular structure and spectra of DBP, the method B3LYP of the density functional theory (DFT) at 6-311++G(d, p) level has been used in this paper to calculate its geometrical parameters and infrared (IR) spectra, in the ground state of DBP molecule under different external electric fields (from 0 to 0.020 a.u.). On this basis, the UV-Vis absorption spectra of DBP are studied using the time-dependent density functional theory (TDDFT) in the same fundamental groups and compared with the ultraviolet(UV) absorption peak of the molecules measured by UNICO ultraviolet and visible spectrophotometer. The rule of variation of wavelength and oscillator strength of the first twenty-six excited states of a DBP molecule under the influence of the external electric fields are studied. Results show that the molecular geometric parameter is strongly dependent on the external field intensity, and the dipole moment of DBP is proved to be sharply increased, and the total energy first decreases with the increases of the external field intensity; the significant negative (“red”) and positive (“blue”) frequency shifts are observed, i.e., vibrational Stark effect (VSE) is shown obviously; the ultraviolet absorption peaks of the excited states of DBP show an observable red shift, and the oscillator strength decreases sharply with increasing field intensity.

2015, 64 (7): 073301.
doi: 10.7498/aps.64.073301

Abstract +

The smooth, uniform, and transparent solid nitrogen-molecular film was grown by applying the slow thermal cycles near the triple-point temperature to the growth process in our home-made liquid/solid preparation apparatus. The infrared absorption spectra of solid nitrogen at near-triple point temperature are measured by the infrared spectroscopy system. A broad absorption band can be observed from 2222 to 2439 cm-1 with the strongest peak at 2288 cm-1. This has been well explained theoretically on the basis of the ground-state vibration and the coupling between the ground-state vibration and rotation at low temperatures within the framework of anharmonic approximation.

2015, 64 (7): 073302.
doi: 10.7498/aps.64.073302

Abstract +

1, 1-diamino-2, 2-dintroethylene (FOX-7) is a novel explosive of high energy and low sensibility. In order to study the effect of temperature changes on the molecular structural characteristics of the explosive, its absorption spectra in the frequency range of 0.2–2.5 THz at a constant rate of heating from 298 K to 393 K are detected by terahertz time-domain spectroscopy (THz-TDS). Results show that a number of characteristic absorption peaks with different intensities appear at 1.59–2.13 THz when the temperature is 298 K, while the absorption spectra change with the increase of temperature of the explosive sample; a new characteristic absorption peak located at 1.12 THz appears at 384 K, and its absorption peak intensity gradually increases, but disappears when the temperature drops to 298 K. The absorption spectra of FOX-7 molecular crystal at 298 and 393 K within the 0.2–2.5 THz region based on density functional theory (DFT) are also simulated by using Materials Studio 6.0 software in this article, and the simulated results agree well with the experimental data. In addition, the vibrational modes of the characteristic peaks of two kinds of crystalline in the experimental absorption spectra are analyzed and identified, showing that the formation of the characteristic absorption peaks is closely related to the molecular vibration, and the molecular structure may change under the influence of temperature, and the tautomeric polymorphism of the crystalline has different vibrational modes. This article indicates that the process of phase transformation of FOX-7 starts from 384 K, and this process is reversible; the characteristic absorption peak at 1.12 THz is composed of two kinds of vibrations (the swinging and torsional vibrations of the nitro and amido groups).

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

2015, 64 (7): 074101.
doi: 10.7498/aps.64.074101

Abstract +

We introduce graphene into conventional photonic crystals to build new photonic crystal structures, and strictly derive the dispersion relations of the structures based on the electromagnetic boundary conditions and the Maxwell's equations required. The dispersion relations are different from that of the conventional photonic crystals, and the optical properties of the structures may also differ from that of the conventional photonic crystals because of the presence of graphene conductivity in the dispersion relations. By changing the Fermi energy of graphene, the conductivity of it can be changed, the dispersion relations adjusted, the energy band structure altered, and its light propagation manipulated as well. With increasing Fermi energy, the energy band can be transformed from the allowed bands to the prohibited bands and then transformed along the opposite direction to the allowed bands. Because the conductivity changes rapidly in low frequency range, while changes slowly in high frequency range, as the Fermi energy increases, the energy band in the low frequency region will move quickly to higher frequency region, and the energy band in the high frequency region moves slowly, leading to the band compression and mutual conversion between the allowed and the prohibited bands. The larger the Fermi energy, the more obvious the band compression, and the more easy the mutual conversion.

## EDITOR'S SUGGESTION

2015, 64 (7): 074201.
doi: 10.7498/aps.64.074201

Abstract +

Support-mesh structures of a self-supporting transmission grating may decrease the transmission area and diffraction efficiency, thus exert a ill effect on the original diffraction pattern of the grating and make it complicated to apply, especially in soft X-ray region. To solve this problem, we put forward a method to realize a kind of totally self-supporting transmission grating by arranging the rectangular transmission holes in a high Z foil quasi-randomly. Based on Fraunhofer's diffraction theory, we make a theoretical analysis on the diffraction pattern of this new kind of grating. We also calculate the diffraction pattern of this new kind of grating based on the Kirchhoff's diffraction theory. What's more, we fabricated a sample grating of 1000 lines/mm using focused ion beam (FIB) technology and then tested it on beam line 4B7B on the Beijing synchrotron radiation facility. Theatrical analysis and experimental result both proved that the diffraction of the transmission grating can get rid of the bad influence of the support-mesh structures by using this new designed method. Such a methodology can profit applications of this kind of X-ray dispersive element in a wide field.

2015, 64 (7): 074301.
doi: 10.7498/aps.64.074301

Abstract +

Based on the relationship between the horizontal wavenumber difference of two modes and the waveguide invariant in a range-independent shallow water waveguide, a frequency-warping operator is proposed for the autocorrelation function of the received signal. This operator can transform the cross-correlation functions of two different modes in the signal autocorrelation function into separable impulsive sequences. With a guide source providing the dispersive characteristics of the waveguide, the source range can be extracted from the relative delay time of the impulsive sequence using a single hydrophone. The availability of the waveguide-invariant-warping operator in the passive source range estimation is verified by simulated and experimental data.

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

2015, 64 (7): 075201.
doi: 10.7498/aps.64.075201

Abstract +

The blow-by which occurs in a coaxial plasma gun is the result of reinforcing feedback caused by the gradient of magnetic field and the component of axial current due to the canting of current sheath. The blow-by has become a serious negative effect which limits the effective use of the coaxial plasma gun, so it is necessary to study by experiment the parameters that influence the degree of blow-by. This will not only contribute to the study of the theory and mode about blow-by but also give advices to the weakening or eliminating blow-by by choosing suitable parameters in engineering field. The degree of blow-by can be observed directly by photomultiplier, and the influence of voltage of capacitance, capacitance, and the pressure of gas on blow-by have also been studied. It is shown that the blow-by would become more serious with the increase of capacitance or the voltage of capacitance while it becomes weaker with the increase of gas pressure. These phenomena can be explained based on the snowplow model. We consider that the increase of capacitance or the voltage of capacitance can make the current sheath canting more serious, however it would reduce the degree of current sheath canting with the increase of gas pressure. So the blow-by can be controlled by the parameters which influence current sheath canting.

2015, 64 (7): 075202.
doi: 10.7498/aps.64.075202

Abstract +

Extreme ultraviolet lithography (EUVL), which uses the extreme ultraviolet radiation at a wavelength of 13.5 nm, is the leading candidate of next generation lithography addressing not only the 10 nm half-pitch nodes, but several nodes beyond that. Among all the methods for getting EUV radiation, laser-produced plasma (LPP) light source is the most promising EUV light source because of its high conversion efficiency (CE), large collect angle and low debris output. In this paper, pulsed TEA-CO2 laser and Nd:YAG laser are used to irradiate tin droplets to obtain plasma EUV emission, and the properties of EUV radiation from the plasma are studied. Results show that the EUV emission spectra induced by Nd:YAG laser have an obvious blueshift as compared with those by CO2 laser. In addition, the LPP sources are point light sources, so that the angular distribution of EUV emission from LPP can be described by Lambertian distribution.

2015, 64 (7): 075203.
doi: 10.7498/aps.64.075203

Abstract +

To analyze the influence of environmental media on nanopowders prepared by electrical explosion method, electrical explosion experiment is carried out using copper wire under water and different air pressures. Voltage and current waveforms are measured by Rogowski coil and high-voltage probe. The basic process of electrical explosion and the function of environmental media in the course of electrical explosion are analyzed by combined characters of the voltage, current, and energy deposition. The particle size of explosive products is analyzed by transmission electron microscopy. Results show that it is mainly after the vaporization stage of the copper wires that the medium may affect the formation of the products. Effects of the medium on the electric explosion of metal wires include the restriction of the medium on the expansion of the metal vapor, the influence of the medium ionization on breakdown of the copper surface as well as the cooling of the copper vapor and the plasma. For the water medium, the diameter of the products distributes in a wide range but is mainly in the range about 10 to 20 nm, while in the air medium, the average particle size is about 40 nm, basically ranging from 20 to 100 nm.

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

2015, 64 (7): 076101.
doi: 10.7498/aps.64.076101

Abstract +

Granular materials posses disorder structures which are the origin of dynamical heterogeneity. On the basis of non-equilibrium thermodynamics, the structure characteristics, complex deformations, and energy dissipations are analysed. Based on the photoelastic tests, the granular elasticity is discussed. The strain increments are classified into three categories. By means of the non-equilibrium thermodynamics, two granular temperatures, Tk, Tc, are introduced as the state variables, which denote the fluctuations of the kinetic energy and the elastic energy, respectively. Further, a two-granular-temperature thermodynamics (i.e. TGT theory) are developed for granular materials. The thermodynamic forces and fluxes are particularly analyzed. TGT theory is also compared with the previous internal variable thermodynamics for sands (IVT theory) developed a few decades ago. It is found that from TGT the Gibbs free energy in the IVT theory can be deduced, and the energy dissipation function can be apparently expressed from TGT theory.

2015, 64 (7): 076102.
doi: 10.7498/aps.64.076102

Abstract +

Porous anodic alumina thin films with rainbow rings were fabricated in oxalic acid electrolyte by one-step electrochemical oxidation of Al sheet. The hole depth and aperture diminish symmetrically outward from the center of the films. The relationship between the density of rainbow rings to the oxidation voltage and the time in oxalic acid solution are discussed. Theoretical study on the formation mechanism of porous anodic alumina thin films with rainbow rings indicates that the effect of carbon rod is similar to carbon dot electrode in the process of anode electrochemical reaction. Result of the theoretical study is consistent with the experimental phenomenology.

2015, 64 (7): 076201.
doi: 10.7498/aps.64.076201

Abstract +

Ni-based single crystal line alloy is constituted with γ phase and γ' phase in the form of coherency. Since an indenter for two-phase coherent structure is bigger than the usual nano-scale indenter, the press location of indenter may be unclear in nanoindentation simulation. Both γ phase and γ' phase may be pressed initially, and the mechanical properties shown are different because of the initial press locations. The nanoindentation of Ni-based single crystal line alloys is simulated by molecular dynamics method. Two models are used to study about the hardness in [001] crystal orientation, one is the model γ /γ' with the initial indentation on γ phase, and the other is the model γ'/γ with the initial indentation on γ' phase. The influence of misfit dislocation at (001) interface on nanoindentation of the two models is analyzed using a center-symmetry parameter. Results show that the misfit dislocation shape of the two models are different after relaxation. Lomer-Cottrell dislocation occurs on (001) interface in the γ'/γ model. Before 0.930 nm press depth is reached, there is little change in the (001) interface misfit dislocation of the two models. Relationship between press load and press depth is similar for the two models, and it is the same in the relationship between hardness and press depth. After press depth reaches 0.930 nm, the misfit dislocation at (001) interface for model γ'/γ grows big, which results in a smaller press load and a smaller hardness computation in the model γ'/γ than that in model γ /γ'. When the press depth reach 2.055 nm, we find only a small amount of dislocations in γ phase that can go into γ' phase since the misfit dislocation at (001) interface in model γ /γ' hinders the process. However, none of dislocations can go into γ phase because of the prevention caused by Lomer-Cottrell dislocation at the (001) interface in the model γ'/γ . That means the Lomer-Cottrell dislocation reinforces the material obviously. So the press load in model γ'/γ grows faster than that in model γ /γ'.

2015, 64 (7): 076801.
doi: 10.7498/aps.64.076801

Abstract +

Using the density functional theory (DFT) with both the generalized gradient approximation (GGA) and HSE06 hybrid functional calculation, we have investigated the structural and electronic properties of hydrogenated bilayer silicene. Results show that the hydrogenated bilayer silicene may have three configurations: AA-chair-like, AB-chair-like and AA-boat-like forms; after hydrogenation the material properties change from zero band gap semimetal into an indirect band gap semiconductor with forbidden band widths of 1.208, 1.437, and 1.111 eV. We have performed a hybrid HSB06 functional calculation and the correction for the band gaps: 1.595, 1.785, and 1.592 eV. Further analysis of the hydrogenated bilayer silicene with a strained band gap, the relationship between strain and the band gap can be continuously adjusted. Possible applications may be found in future nano-electronic devices.

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

2015, 64 (7): 077101.
doi: 10.7498/aps.64.077101

Abstract +

Recently, newly created unnatural fluorescent nucleobase analogs have gained increasing attention. In the present work, a comprehensive theoretical study on the structural, electronic, and excited-state properties of y-guanine (yG-t1) and its five possible tautomers (yG-t2, yG-t3, yG-t4, yG-t5 and yG-t6) is performed. Tautomerization analysis reveals that the canonical form of yG is not the most stable tautomer in the gas phase since it has three tautomers with the same stabilities. The spectroscopic properties are investigated: It is found that these tautomers have different absorption spectra, and so we can distinguish them by their spectroscopic signatures. In addition, effects of methanol solution and hydrogen bonding with cytosine on the absorption and emission spectra are examined. The methanol solution is found to red-shift both the absorption and emission maxima of the studied bases except for yG-t1, for which the absorption and emission maxima have blue-shifts after solvation. On the other hand, hydrogen bonding with cytosine is found to are blue-shifted both the absorption and emission maxima of yG-t1, yG-t2, yG-t5, and yG-t6. Theoretical predictions here are helpful for the investigation of the tautomerism of yG and the optical properties of yDNA.

2015, 64 (7): 077102.
doi: 10.7498/aps.64.077102

Abstract +

In this paper, we have designed a new (TiO2)12 quantum ring structure and studied its geometry, average binding energy, and the electron density distributions using the generalized gradient approximation (GGA), which is based on the density functional theory (DFT) with the first-principles calculations. This new quantum ring structure is doped with transition metal compounds MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2 respectively, to modify its properties. Thus we can calculate and analyze their geometrics and electronic properties (such as average binding energies, energy levels, electronic density of states and the HOMO-LUMO electron density distributionsatc). We find that the (TiO2)12 quantum ring with a diameter of 1.059 nm seems to be of a two-dimensional structure with a center symmety which ensurs it a stable structure. In addition, the HOMO-LUMO orbital electron density in the quantum ring distributes evenly, and its energy gap is 3.17 eV which is very close to the experimental value of TiO2 semiconductor materials (3.2 eV). The energy gaps decrease substantially after introducing the transition metal compounds into the quantum ring. Among these results, the ring doped with WTe2 has the smallest energy gap (0.61 eV), and that with MoTe2 has the biggest energy gap (1.16 eV), but it is still smaller by about 2 eV than that of the (TiO2)12 quantum ring. Furthermore, other doping results have energy gap variation around 1 eV. The TiO2 clusters with this energy gap could make use most of the solar energy and so expand applications of TiO2.

2015, 64 (7): 077103.
doi: 10.7498/aps.64.077103

Abstract +

The electronic and thermoelectric properties of Mg2Si under hydrostatic pressures have been investigated using the first principles calculations with general potential linearized augmented plane-wave method and the semiclassical Boltzmann theory with the rigid band approach and the constant scattering time relaxation approximation. In this work, the hydrostatic pressure is simulated by applying equiaxial strain method for the cubic anti-fluorite structure of Mg2Si in space group Fm3m. The strain values ranging from -0.03 to 0.03 describe the compressive and tensile Processes under pressure. The band structure, electrical conductivity, Seebeck coefficient and power factor have been calculated and analyzed in detail.#br#From the band structure in Mg2Si one can see that the bottom of the conduction band shows significant changes under strains. Especially, when the strain is up to 0.02, there are two twofold-degeneracy states occurring at the center of the Brillouin zone. The top of the valence band shows a slight change due to the strain effect. For the unstrained structure, our calculated thermoelectric data are in accordance with other reports. Moreover, the results indicate that when the value of strain is up to 0.02, the transport properties get an optimal functioning of Mg2Si due to electron doping. At 300 K, the Seebeck coefficient improves obviously and comes up to 126%. And the power factor is up to 47% (45%) at T=300 K (700 K). Consequently, the thermoelectric properties can be improved through applying negative pressures to the Mg2Si crystal. For the case of hole doping, the transport parameters change obviously at a small strain value, and change gently at a high strain values. When the strain is up to 0.01, the Seebeck coefficient reaches the maximum value 439 μV/K-1. But, the power factor only increases 0.9%–2%. Hence, we can conclude that the hydrostatic pressures have a slight influence on the thermoelectric properties of hole-doped materials.

2015, 64 (7): 077104.
doi: 10.7498/aps.64.077104

Abstract +

In Ga2-based Heusler alloys Ga2XCr (X = Mn, Fe, Co, Ni, Cu) the tetragonal distortion, electronic structure, magnetism and phonon dispersion have been studied by first-principles calculations based on the density functional theory. The volume-conserving tetragonal distortions of the cubic Ga2XCr show that Cr atom makes the greatest contribution to the total magnetic moment. No martensitic transformation has been found in Ga2FeCr, Ga2CoCr and Ga2CuCr. For both Ga2MnCr and Ga2NiCr, the tetragonal phase is lower in energy as compared with the cubic phase. Ga2MnCr and Ga2NiCr have the lowest total energy at c/a = 1.28 and 1.11, respectively. Correspondingly, the energy difference ΔE between the cubic and the tetragonal phase is -8.26 meV in Ga2MnCr and -6.14 meV in Ga2NiCr. For Ga2MnCr and Ga2NiCr, calculations of electronic structure and phonon dispersion reveal that a sharp peak near the Fermi level will lead to a structural instability by increasing the energy of the system, which can result in a broadening in the energy range due to hybridizations between 3d electrons as well as the potential structural transformation. With proper c/a and ΔE a potential tetragonal martensitic transformation can be expected in Ga2MnCr, the phonon dispersion of which further shows that the acoustic modes tend to be softened.

2015, 64 (7): 077301.
doi: 10.7498/aps.64.077301

Abstract +

A quantum-dot system is a typical low-dimensional system, and previous researches showed that its thermoelectric conversion efficiency can be markedy improved due to its unique physical properties. In this poper, we choose the parallel double-quantum-dot structure and discuss the influence of the electron-phonon interaction on the thermoelectric-related parameters, i.e., the electric conductance, thermopower, the figure of merit, and thermal conductance, by using the nonequilibrium Green's function method. Our theoretical calculation results show that under the condition of low temperature, the occurrence of the Fano interference can assist to enhance the thermoelectric effect. When the electron-phonon interaction is taken into account, it can suppress the electric and thermal conductances to a certain extent because of its negative effect on the Fano interterence. However, we readily find that apparently the strengthening of the electron-phonon interaction cannot suppress the maximum of the thermopower. Instead, in some regions, the thermopower has an opportunity to enhance due to the appearance of a new channel caused by the electron-phonon interaction. Meanwhile, the figure of merit is found to cause similar effects to the thermopower. Therefore, in the case of low temperature, the electron-phonon interaction contributes little to the destruction of the thermoelectric effect, namely, it is not the necessary condition for the suppression of the thermoelectric effect. With the increase of temperature, the negative effect of the electron-phonon interaction on the Fano interference becomes relatively distinct, which inevitably weakens the thermoelectric effect. Results of this paper will help to clarify the influence of electron-phonon interaction on the thermoelectric effect.

2015, 64 (7): 077302.
doi: 10.7498/aps.64.077302

Abstract +

SiO2-based solid state electrolyte films are deposited at room temperature by using the plasma-enhanced chemical vapor deposition (PECVD) technique. An electric-double-layer (EDL) effect has been observed. Then, indium-zinc-oxide thin-film transistors (IZO TFTs) are fabricated by using such SiO2 films as dielectrics in a self-assembling process through a shadow mask. The IZO films for source/drain electrodes and channel are deposited on the nanogranular SiO2 film by RF sputtering the IZO target in an Ar ambient. Such TFTs exhibit a good performance at an ultralow operation voltage of 1.5 V, with a high field-effect mobility of 11.9 cm2/Vs, a small subthreshold swing of 94.5 mV/decade, and a large current on-off ratio of 7.14×106. Effects of protons in the SiO2-based solid state electrolyte films on the electrical performances of the IZO TFTs are also studied. It is observed that a big EDL capacitance can be obtained for SiO2 films dipped in pure water, as a result of the fact that there are more protons in such SiO2 films. Because of the migration of protons in SiO2 electrolytes, an anti-clockwise hysteresis is observed on the transfer curve. Moreover, a bigger hysteresis is observed at a higher gate voltage scan rate. Gate bias stressing stabilities are also studied the shifts in threshold voltage are observed to obey a stretched exponential function.

2015, 64 (7): 077401.
doi: 10.7498/aps.64.077401

Abstract +

Single-domain GdBCO bulk superconductor (20 mm in diameter) has been fabricated by a top-seeding infiltration and growth (TSIG) mathod, it has a new solid phase of [(1-x)(Gd2O3+1.2BaCuO2) + x Ni2O3] (where x =0, 0.02, 0.06, 0.10, 0.14, 0.18, 0.30, 0.50 wt%). Effect of Ni2O3 additions on the growth morphology, microstructure, critical temperature Tc, magnetic levitation force, and trapped flux of single-domain GdBCO bulks have been investigated. Results show that the single-domain GdBCO bulk can be gained when x is in the range of 0-0.50 wt%; and the Gd211 particles are not affected by the Ni2O3 doping in the samples. The Tc of the samples decrease from 92.5 K (x=0 wt%) to 86.5 K (x=0.50 wt%) when x increases from 0 to 0.50 wt%, which is caused by the substitution of Ni3+ for Cu2 +. Both of the levitation force and trapped field of the samples increase first and then decrease with the increase of x; the largest levitation force of 34.2 N is obtained for the samples with x=0.14 wt%, and the largest trapped field of 0.354 T is obtained for the samples with x=0.10 wt%. The change of the levitation force and trapped field of the samples is closely related to the doping content x. As is known, the doping of Ni2O3 can result in substitution of Ni3+ for Cu2+ at its site in GdBCO crystals, which can reduce the critical temperature Tc of the samples; although Tc and the physical properties of the samples is reduced with the increase in the doping amount of Ni2O3, but at the same time, the substitutions of Ni3 + for Cu2 + in GdBCO crystals can produce local lattice distortions, which can act as magnetic flux pinning centers to improve the properties of the samples. The highest Tc is obtained in the samples without any Ni2O3 additions (x=0), but the magnetic flux pinning force of the samples is weak, so both of the levitation force and trapped field of the samples are relatively lower. When the doping content x ≤ 0.14 wt%, although the Tc is reduced slightly, it still has a value higher than 90 K; and the magnetic flux pinning force in the samples, due to the substitutions of Ni3+ for Cu2 +, would increase with the increase of doping content x, and result in an enhancement of levitation force and trapped field. When the doping content x is greater than 0.14 wt%, the magnetic flux pinning force of the samples is still increasing with the increase of x, but the Tc of the sample is significantly reduced and even less than 90 K, and finally result in an decrease of levitation force and trapped field. Only when the doping amount of Ni2O3 is appropriate, both of Tc and magnetic flux pinning force are of a relative optimal value, and lead to an enhancement of levitation force and trapped field.

## EDITOR'S SUGGESTION

2015, 64 (7): 077501.
doi: 10.7498/aps.64.077501

Abstract +

Diluted magnetic semiconductor (Ga, Mn)Sb and its related hetero-structures have attracted much attention in recent years since they are predicted to have some novel properties, such as the quantum anomalous Hall effect etc. However, it is not easy to grow high-quality (Ga, Mn)Sb films due to their narrow growth window. In this article, a series of 10 nm thick (Ga, Mn)Sb films with different Mn contents from 0.016 to 0.039 have been grown by molecular-beam epitaxy at low temperaturs (～230 ℃). The films have high crystalline quality as confirmed by in situ reflection high-energy electron diffraction and ex situ atomic force microscopy, and no MnSb phase could be observed. Curie temperature up to 30 K has been obtained in one (Ga, Mn)Sb film after post-growth thermal annealing. The magneto-resistance and anomalous Hall effect of this film have also been investigated at different temperatures.

2015, 64 (7): 077801.
doi: 10.7498/aps.64.077801

Abstract +

In this paper, a broadband metamaterial absorber is designed based on a hybrid substrate consisting of the dielectric and magnetic materials. The absorber is composed of the resistance film, dielectric layer, magnetic layer, and metal backboard. Numerical simulation of the absorbing properties is performed by means of the finite-difference time-domain method, and the bandwidth of the reflectivity below -10 dB is optimized by the genetic algorithm. Simulated results indicate that a bandwidth of reflectivity below -10 dB can be achieved over the frequency range from 7.8 to 18 GHz when the thickness of the absorber is only 2.5 mm. The proposed metamaterial absorber has many advantages, such as thin thickness, broadband, and polarization insensitivity. The operation mechanism of the absorber has also been analyzed and discussed within the model of equivalent circuit. In the end, an absorber sample is fabricated based on the design. It is found that the experimental result is well consistent with the design requirements.

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

2015, 64 (7): 078102.
doi: 10.7498/aps.64.078102

Abstract +

With SnCl4·5H2O and graphene oxide as raw materials and aqueous solution of ethanol as the solvent, we have prepared SnO2 quantum dots (diameter about 3-5 nm)/graphene nanocomposites using a facile hydrothermal method in one step, and solved the reunion of quantum dots successfully. The visible-light-driven photocatalytic efficiency of SnO2 quantum dots depends to a great extent on their dispersity. Because of the large-sized two-dimensional surface, the graphene sheet could behave as a solid support for quantum dots through interfacial interaction to avoid particle aggregation. Composites of SnO2 quantum dot/graphene show a great photocatalytic performance in visible light, and the morphology and structure of the product are characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared Spectrometer (FT-IR) and other techniques. The optical properties are investigated by using UV-visible (UV-vis) absorption spectrum. Additionally, the photocatalytic activity of the product is measured by the degradation of rhodamine-B dye solution in visible light. Results show that the preparation of samples with high catalytic activity in visible light, the shift in the optical response of composites may produce a positive effect on the improvement of photocatalytic efficiency in UV to visible spectral range Moreover, owing to its special π-conjugation structure, large specific surface area as well as high conductivity, graphene can enhance the photocatalytic activity. Compared with the pure SnO2, pure graphene catalytic performance is greatly improved in visible light, its excellent photocatalytic activity is due to the combination of strong absorption and effective separation of photogenerated carriers in the samples. Finally, the formation mechanism of the composite and its photocatalytic mechanism are studied.

2015, 64 (7): 078401.
doi: 10.7498/aps.64.078401

Abstract +

Nano titanium oxide memristor is expected to be the basic cell of a new generation of resistive memory and applied in the control and data storage systems of spacecrafts that work in a radiation environment. The changes of radiation key factors, such as energy, intensity, direction, and duration etc. probably have an influence on the radiation damage of the titanium oxide memristor. However, there has been no relatively detailed research of it. Based on the SRIM simulation, with the Monte Carlo method used as its core, the main part of cosmic rays——proton and alpha rays and the relevance between the key factors and radiation damage in titanium oxide memristor are quantitatively studied. According to the experimental data, the relations between key factors and R_{ON}, R_{OFF}, the mobility of oxygen vacancies are analyzed. We find that the mobility of oxygen vacancies increases abruptly when the ratio between oxygen vacancies and titanium oxide molecules is greater than 0.16. Moreover, compared with proton radiation, the alpha particle radiation going into the active region in titanium oxide memristor, especially at an oblique incidence angle may cause a greater damage to the device and should be strictly avoided, and the radiation damage increases as the intensity and duration of the radiation are raised. SPICE simulations are further utilized to show the influence of radiation on the characteristics of the coexistence of dopant drift and the tunnel barrier. We also find that the titanium oxide memristor device will gradually turn into a normal resistor with a low resistance and lose its charge-memory ability after persistent radiations. This work provides support for evaluating and reducing radiation damage for titanium oxide memristors, so as to improve the reliability of the device in radiation environment.

2015, 64 (7): 078402.
doi: 10.7498/aps.64.078402

Abstract +

Flexible control of terahertz waves is now a research hotspot. Based on the electromagnetic theory the dispersion relation and field distributions in a plasmon-assisted parallel-plated waveguide are deduced. The transmission property of such a waveguide is obtained and confirmed by the full-wave simulation. Results show that the plasmon-assisted parallel-plated waveguide shows a band gap characteristic, and the cutoff frequency of the upper sideband is equal to the plasmon frequency; generally, the thinner the plasmon layer, the higher the cutoff frequency will be, and the narrower the bandwidth will become. Emergence of the band gap is due to the excited surface plasmon polaritons, and the coupling between surface plasmon and the medium in the waveguide. Besides, the influence of plasmon frequency and collision frequency on the transmission properties is investigated, and a method for adjusting the filter characteristic of the waveguide by tuning the plasmon frequency is proposed. Moreover, the plasmon layer is realized by a textured metallic structure, and a sensing model based on the parallel-plated waveguide is designed. Simulation results show that a 0.1 percent change in permittivity of the sample materials filling in the groove will give rise to a significant change of the cutoff frequency, which is 1.8 GHz in average; interestingly, different liquid samples such as nitrogen, gasoline, paraffin, glycerine and water can be identified through detecting the change of cutoff frequency, which further confirms the excellent terahertz sensing characteristic of the proposed sensor. This work may be helpful for the study of terahertz wave transmission, and may have potential applications in the design of terahertz devices.

2015, 64 (7): 078403.
doi: 10.7498/aps.64.078403

Abstract +

In the joint estimation for time of arrival (TOA) and direction of arrival (DOA) in the narrow-band orthogonal frequency division multiplexing (OFDM) system with antenna arrays, the estimation accuracy is not high in the situation of few numbers of arrays. Especially, DOA cannot be estimated if the number of multiple paths is more than that of the arrays. For these problems, a joint estimation algorithm for TOA and DOA based on the subspace of the extended hadamard product is proposed. First of all, the algorithm constructs an extended channel response in frequency domain via channel estimation for each array in the frequency domain. Then, auto-correlation matrix of extended channel response in the frequency domain is estimated by sampling many times. This estimation method of channel response in the frequency domain can use the fast Fourier transform algorithm. And the hadamard product in the extended noise subspace is obtained by eigenvalue decomposition. Finally, the pseudo-spectral function is constructed and used to search for spectrum peaks, so as to realize the joint estimation of TOA and DOA. The proposed algorithm requires no parameter paring but needs a two-dimensional searching. Monte Carlo algorithm can be used to reduce computational complexity. Simulation results show that the root mean square error of the joint TOA and DOA estimation which can be matched automatically is closer to the Cramer-Rao bound than that using present algorithms. And the proposed algorithm can be still applied when the number of multiple paths is more than number of arrays.

2015, 64 (7): 078501.
doi: 10.7498/aps.64.078501

Abstract +

The transfer characteristics and low-frequency noise behavior of partially depleted silicon on insulator n-channel metal-oxide-semiconductor transistors after γ-ray irradiation up to a total dose of 1M rad (Si) have been investigated in this paper. Due to the radiation-induced positive buried-oxide trapped charges and the interface traps, the back gate threshold voltage decreases from 44.72 to 12.88 V, and the electron field effect on mobility decreases from 473.7 to 419.8 cm2/V·s; while the sub-threshold swing increases from 2.47 to 3.93 V/dec. Based on the measurements of sub-threshold swing and the back gate threshold voltage, the variations of extracted radiation-induced buried oxide trapped charge and interface trap densities, are about 2.36×1012 cm-2 and 5.33×1011 cm-2 respectively. In addition, the normalized back gate flat-band voltage noise power spectral density is a sensitive function of radiation-induced buried oxide trapped charges and interface traps, which increases from 7×10-10 V2·Hz-1 to 1.8×10- 9 V2·Hz-1. According to the carrier number fluctuation model, the extracted trap density near the interface between channel and buried oxide increases from 1.42×1017 to 3.66×1017 cm- 3·eV-1. By considering the tunneling attenuation coefficient of the electron wave function and the tunneling depth of the electron in the buried oxide, the spatial distribution of trapped charges in the buried oxide before and after radiation are calculated and discussed.

2015, 64 (7): 078701.
doi: 10.7498/aps.64.078701

Abstract +

Ensemble empirical mode decomposition (EEMD) method eliminates mode mixing phenomenon which is an inherent problem in empirical mode decomposition (EMD), and decomposes signals according to their intrinsic characteristics. It is suitable for analyzing nonlinear and non-stationary signals. Electrocardiogram (ECG) energy distribution exhibits a certain regularity which may vary with heart diseases. Researches on ECG energy distribution change are important for heart disease clinical diagnosis. In this paper, we use EEMD method to analyze ECG and find out how ECG energy distribution varies with age and heart diseases. We decompose the ECG signal into several intrinsic mode function (IMF) components by EEMD, and find that these IMFs can reveal the fluctuation rhythm and physical significance of ECG on different time scales. After IMFs have been decomposed, we calculate their energy and obtain an energy vector. By comparing the energy vectors among healthy young subjects, healthy old subjects, and three types of patients suffering from different heart diseases, we find that there is a significant decrease of high-frequency components of energy vector in heart disease patients as compared to healthy subjects, and a slight decrease of healthy old subjects as compared to healthy young subjects. T-test is performed to compare heart disease subjects with healthy subjects. Results show that there are significant differences between certain energy vector components, especially the first component p1 which could be used as heart disease auxiliary diagnosis. Compared to traditional frequency-domain analysis methods which simply concern about the frequency of a signal and ignore its own characteristics and interactions between signal components, EEMD method depends on ECG signal itself, therefore can reflect its real characteristics, and reveals the way how age and illness influence ECG energy distribution accurately.

2015, 64 (7): 078901.
doi: 10.7498/aps.64.078901

Abstract +

In this paper, the regularized finite difference contrast source inversion algorithm based on inverse scattering theory is utilized to invert the seismic velocity distribution in the underground medium, and this algorithm is a waveform inversion method in frequency domain based on wave equation. Velocity distribution is updated iteratively by minimizing the cost function using the nonlinear conjugate gradient method. The geophysical inversion problem has the characters of ill-posedness and instability, we handle this problem by adding an additional regularization item based on total variation of inversion parameter, making the inversion problem become a well-posed problem, and the algorithm has the ability of anti-noise interference. In the inversion process we use the frequency-space domain 9-point difference propagation operator under PML absorption boundary condition. Compared with other inversion algorithm, the construction of forward modeling operator and other matrix factorization calculation process can be avoided in each iterative step because the background medium remains unchanged throughout the inversion process, this makes the algorithm more suitable for large-scale three-dimensional inversion. In addition, the MPI parallel computation is applied, by which the efficiency of inversion process is improved greatly. The two-dimensional CSEG model inversion results show that this algorithm can obtain high resolution seismic velocity reconstruction, and provide an accurate velocity information for seismic data processing and interpretation.

2015, 64 (7): 078902.
doi: 10.7498/aps.64.078902

Abstract +

The path of vehicle movement in a complex geographical environment has the 3D space feature, which is evidently the constraint for vehicular movement behaviors and cannot be described in one-dimensional or two-dimensional space. But the path of vehicle movement in a complex geographical environment can be abstracted into a space curve. By introducing theories in differential geometry, we can build a Serret-Frenet frame moving along this space curve with the geometric invariants of arc length, curvature, and torsion. And then, we can give a mathematical description to the dynamic behavior of the Serret-Frenet frame with the time-varying property at an arbitrary point of space curve. Finally, the spatiotemporal evolution model of the vehicle movement behaviors under the path constraint conditions is established and is rigidly proven in mathematics to be suitable for the longitudinal movement and uniform circular motion of a vehicle (in Serret-Frenet frame). It will lay the theoretical foundation for the future study of vehicular movement behaviors on the transport line in a complex geographical environment, including vehicular microscopic behaviors such as the vehicle following operation, lane changing, as well as the vehicular macroscopic behavior in traffic flows.

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

2015, 64 (7): 079501.
doi: 10.7498/aps.64.079501

Abstract +

Atmospheric correction is very important to the accurate retrieval of geophysical parameters from spaceborne L-band radiometers. In this paper, the L-band upwelling and downwelling radiation brightness temperature and transmittance above sea surface are calculated using the atmospheric radiation transfer model based on NCEP temperature and humidity profile data. A regression model, i.e., radiation-vapor model, is established to describe the relationship between the three atmospheric radiation parameters and the atmospheric water vapor content as well as the sea surface pressure. Using this model, the atmospheric radiation parameters can be calculated and used to correct the atmospheric effects in L-band microwave radiometer observation. In order to test the proposed model, the atmospheric radiation parameters are calculated by this model and compared with the SSM/I water vapor content data and the NCEP surface pressure data. Finally, the model outputs are compared with the Aquarius satellite data. Results indicate that the radiation brightness temperature calculated by the proposed model is lower than the Aquarius data about 0.335 K and the root-mean-square error between them is about 0.086 K after correcting the systematic errors. The atmospheric transmittance calculated by the proposed model agrees well with the Aquarius data. Besides, the proposed model uses fewer input data and is faster and more stable than other existing models.