Vol. 65, No. 22 (2016)
2016-11-20
GENERAL
2016, 65 (22): 220201.
doi: 10.7498/aps.65.220201
Abstract +
A Janus particle is a general term for a non-uniform particle that has different properties on different sides of particle. For a Pt-SiO2 type of Janus microsphere, Pt side serves as the catalysis surface to decompose H2O2 solution, leading to the self-propulsion motion of particle. In this paper, the relevant experimental phenomena in two driven modes are compared first. The results show that under the same concentration of solution, the microsphere with a diameter of about 1 m experiences self-diffusiophoresis propulsion; whereas, the one with an about 20 m diameter experiences bubble self-propulsion. Significant differences in motional trajectory and propulsion velocity are found between them. Then, the dominated physical factors are analyzed and the multi-field coupling numerical model is constructed based on the simplified force balance analysis. Subsequently, the velocity field distribution and O2 concentration distribution around Janus microsphere are also studied. According to these studies, we explain the position and size of the bubble generated. Further more, we infer that the wall slip coefficient is a key matching parameter in the numerical model, and two slip coefficients with a difference of an order of magnitude are given corresponding to the two types of self-propulsion modes. Then we explain the possible mechanism for the changes of wall slip coefficient under different particle sizes. The present study is beneficial to the in-depth exploration of the self-propulsion mechanism and also provides the theoretical foundation for improving the performance of self-propellant device.
2016, 65 (22): 220202.
doi: 10.7498/aps.65.220202
Abstract +
In this work, an improved smoothed particle hydrodynamics (SPH) method based on higher order Taylor expansion (CSPH_HT) is proposed and tentatively applied to the filling process of the viscoelastic fluid. Owing to the disadvantages of the traditional SPH method and the presented corrected SPH methods, the CSPH_HT method based on the higher order Taylor expansion is proposed and described in detail. In order to illustrate the validity and merits of the CSPH_HT method, two benchmark problems are simulated and discussed. The numerical results show that the proposed CSPH_HT method has higher numerical accuracy and better stability. Subsequently, the proposed improved SPH method is extended to simulate the filling processes of the viscoelastic fluid in the ring-shaped mold, for the purpose of exhibiting the capacity of the proposed method. The extended pom-pom (XPP) model and finitely extensible nonlinear elastic-Peterlin (FENE-P) model fluid are all considered in this case, in which the viscoelastic fluid flow and extra stress are shown. The differences in fluid flow between the XPP model and FENE-P model are also discussed. Finally, the filling process of the viscoelastic fluid in the square mold with single inlet or two inlets are tentatively simulated. The differences between the filling process of XPP fluid and the filling process of FENE-P fluid are shown, and the influences of the parameters of the mold on the flow are analyzed. Especially, the influences of locations and sizes of two inlets on the filling process of viscoelastic fluid are illustrated. The XPP model fluid and FENE-P model fluid show different characteristics in the filling process and small change of the size of the mold can lead to obvious change of the flow.
2016, 65 (22): 220301.
doi: 10.7498/aps.65.220301
Abstract +
We study the influences of the external magnetic field, temperature, and coupling parameter on the thermal quantum correlation in a two-qubit Heisenberg XY model, where the correlation is described in terms of two modified geometric discords and concurrence. It is shown that those three quantities behave similarly in the regions of lower temperature and weak magnetic field. However, the two geometric discords are more robust against temperature and magnetic field than the concurrence. Moreover, two geometric discords exhibit different behaviors under suitable conditions, that is, one discord increases as the magnetic field rises whereas the other discord decreases. In particular, the discord reveals the phenomenon of sudden change, while neither of other discord and concurrence displays such a phenomenon.
2016, 65 (22): 220302.
doi: 10.7498/aps.65.220302
Abstract +
Quantum teleportation plays an important role in quantum information science. In order to obtain the effect of quantum teleportation of a quantum state by using an entangled resource, the fidelity of teleporting the quantum state should be calculated. Braunstein and Kimble[Phys. Rev. Lett. 80 869 (1998)] derived a formula of calculating the fidelity of quantum teleportation for Gaussian entangled resource and any input state to be teleported. Then, the point is how to calculate the quantum teleportation fidelity for any entangled resource. In this paper, werealize this purpose by using the entangled state representation. First, we derive the Weyl expansion of any density operator by using the completeness relation between coherent state and P-representation. Then using the orthogonal property of entangled state representation and the traditional Kimble-Braunstein scheme of quantum teleportation, we further derive the mean density operator of the output state, which means that we establish the relation between the output density operator and the characteristic functions of the input state to be teleported and the entangled resources. The characteristic function of the output state is also derived which is in the concise form relating these two characteristic functions above. Then we further obtain a new formula for calculating the quantum teleportation fidelity for the coherent state input and any two-mode entangled resource. It is shown that the fidelity of teleportation can be easily calculated when the Q-function of the normally ordering form of entangled resource is known. This is a convenient way of obtaining the fidelity of teleportation. As its applications, some Gaussian and non-Gaussian entangled states are examined to teleport the coherent state, whose results are correct.
2016, 65 (22): 220501.
doi: 10.7498/aps.65.220501
Abstract +
In the past few decades, stochastic resonance (SR) has attracted considerable attention of researchers due to a curious phenomenon appearing in a nonlinear system:an input weak periodic signal can be amplified and optimized by the assistance of noise. It has been proved that the classical stochastic resonance (CSR) has the adiabatic limit, so the performance of CSR in high-frequency signal detection is restricted in practical engineering. To break the restriction, a number of methods have been suggested, such as re-scaling frequency stochastic resonance (RFSR), parameters normalized stochastic resonance, modulated stochastic resonance, etc. Although the high-frequency signal can be detected by the above methods in specific conditions, there are some problems that restrict their applications in different circumstances. In this paper, a new method, stochastic resonance based on frequency-information exchange (FIESR), is developed to deal with the adiabatic limit of CSR. The mechanism of FIESR is analyzed in detail by the theory of single-side band modulation (SSB) which is based on phase shift. The information in small-parameter frequency domain is swapped with the information of the high-frequency target signal. Then the amplitude and phase of the target signal are moved to the small-parameter frequency domain. Consequently the target signal can be enhanced and detected by CSR in small-parameter frequency domain. Besides, a necessary plan, narrow band spectrum exchange, is put forward to diminish the influence of the spectrum leakage of FIESR. It is well known that the RFSR is a method of detecting the practical signal with large-parameter frequency. Through rescaling the time interval of the signal and compressing its frequency according to the scale R, the large-parameter frequency is compressed into a small-parameter frequency. The RFSR has a good performance in mechanical incipient fault diagnosis. However, it has a high sampling ratio limitation. The ratio of sampling frequency to target signal frequency is more than 50. To overcome this weakness of RFSR, frequency-information exchange (FIE) is introduced into RFSR. A new signal detection method based on FIE and RFSR, named F-RFSR, is put forward simultaneously. The flow of F-RFSR consists of three steps. Firstly, the frequency of the original input signal is compressed linearly according to the estimated scale. Then, the frequency information is exchanged between the compressed target signal and the small-parameter signal in the frequency domain. Finally, the CSR is used to amplify and detect the weak target signal processed by re-scaling frequency and FIE. Performance analysis of signal detection and numerical simulation are carried out to demonstrate that F-RFSR has more efficient sampling ratio than RFSR for practical application.
2016, 65 (22): 220502.
doi: 10.7498/aps.65.220502
Abstract +
Due to the limitation to the development level of modern science and technology, DC-DC (where DC stands for direct current) converters operating in chaotic state cannot be used to achieve some desired goals yet and the chaotic phenomena occurring in DC-DC converters are almost restrained. For DC-DC converter operating in continuous conduction mode (CCM), its characteristic has been widely studied, but DC-DC converter needs to operate in discontinuous conduction mode (DCM) at light load. Because if it always works in CCM, the inductor current will be less than zero when the load is light, which will increase conduction loss and reduce conversion efficiency. Moreover, DCM operation is frequently encountered, since power converters are usually required to operate with loads removed. For buck-boost converter, the obvious oscillation will appear when it works under the condition of varying operating point, so it is difficult to control. Considering the reasons above, the voltage mode controlled buck-boost converter operating in DCM is chosen to be studied to verify the validity of the two control methods presented in this paper. Under a certain condition, chaos and bifurcation will occur in the voltage mode controlled buck-boost converter operating in DCM. Having discussed its chaotic phenomenon, in this paper we present two ways to control the system to operate stably in one-cycle state. The first way is the self-controlling delayed feedback control method. The basic idea of this method is to use the difference between the delayed output signal and the output signal to form a feedback signal, and return it to the control circuit in a form of negative feedback to control the output signal. The simulation results show that the self-controlling delayed feedback control method can make the system which has already entered into chaos operate stably in one-cycle state. Besides, its dynamic response speed is fast and it does not change the system frequency. However, this method fails to work when the disturbance is too large. Therefore, the self-controlling delayed feedback control method is more suitable for small disturbance condition. The second way is the improved sliding mode control method. The basic idea of the sliding mode control is to design a switching function to determine a switching surface which represents a desired system dynamics, then, design a variable structure control law to drive any state to reach the switching surface, therefore, the sliding mode takes place and the system follows the desired dynamics. The simulation results show that the improved sliding mode control method can force the system which has already entered into chaos to operate stably in one-cycle state even when the system encounters large disturbance. In addition, although it is more complicated to design, it has great dynamic response characteristics and excellent robustness. Because the methods presented in this paper do not rely on the buck-boost converter itself, both methods can be used to control other DC-DC converters. When the disturbance is small, the self-controlling delayed feedback control method should be considered first, for it is easier to achieve. When the system encounters large disturbances the sliding mode control method has the priority, because this method is valid while the self-controlling delayed feedback control method may fails under such a condition.
SPECIAL TOPIC—Atomic and molecular processes driven by ultrafast intense laser fields
EDITOR'S SUGGESTION
2016, 65 (22): 223201.
doi: 10.7498/aps.65.223201
Abstract +
We experimentally investigate the Rydberg state excitations (RSEs) of noble gas atoms, He, Ar and Xe, in an 800-nm 50-fs strong laser field, by using the mass resolved pulsed electric field ionization method combined with the time-of-flight mass spectrometer. We measure the yields of the atomic RSE at different laser intensities and ellipticities, and compare the results with those of the nonsequential double ionization (NSDI) in strong laser fields. Our study shows that like that of NSDI, the yield of the atomic RSE increases as the atomic number increases, i.e., RSE yield trend is He Ar Xe. On the other hand, for any of the atoms, the probability of NSDI is lower than that of total RSE at the same laser intensity, which can be understood as that the yield of high energy electrons (for NSDI) is less than that of low energy electrons that can be captured into the Rydberg states. Additionally, our results show that the RSE yield strongly depends on the laser ellipticity, which is completely suppressed by a circularly polarized laser field. The dependence of RSE on laser ellipticity turns weaker as the atomic number increases, and is weaker than that of NSDI for any of the atoms. It is indicated that the atomic RSE in strong laser field can be attibuted to the capture of the low energy electrons after tunneling ionization into Rydberg states by the Coulomb potential at the end of the laser pulse.
2016, 65 (22): 220203.
doi: 10.7498/aps.65.220203
Abstract +
As the advances of laser technology, more and more nonlinear phenomena are observed in the atoms and molecules driven by strong laser pulses. Systematic investigations on these findings, such as above threshold ionization and high-order harmonic generation, will lead us to understanding the mechanisms in the microscopic world. The most exact way to simulate the experimental measurements is to solve the time-dependent Schrdinger equation (TDSE) numerically, in which the system is described by the wave function and thus one cannot have an intuitive insight into the underling process. Therefore, several semiclassical methods have been developed to understand the strong field ionization. In the classical point of view, the electrons tunnel out when the strong laser field suppresses the Coulomb potential. Then the electrons are driven by the laser electric field according to the Newtonian equations. Semiclassical methods take into account the tunnelling of the electron, the classical orbit of the electron, and the action as the phase of trajectory, which have successfully explained main structures in the ionization spectrum. Two of the most popular semiclassical methods are the quantum trajectory Monte Carlo method and the Coulomb-corrected strong field approximation method. In the present review, we will introduce these basic methods and show how they have been developed step by step, covering the most relevant and important works in the strong field physics. Finally we give two example of applications to show how these methods work. With the advantage of the classical picture, we can identify different kind of structures in the 2D photoelectron momentum distributions and tell how the structures are formed. Nonadiabatic effects can be studied by comparing the results of the two methods, together with accurate simulation from the numerical solution of TDSE. The current semiclassical methods can be further developed into advanced ones, which can be used in more complex molecular systems or multi-electron systems, and be widely used in the study of dynamics of molecule and atoms in strong laser fields.
2016, 65 (22): 223202.
doi: 10.7498/aps.65.223202
Abstract +
When an atom or a molecule interacts with an intense laser field, a coherent high-order harmonic emission is observed at a frequency that is an integer multiple magnitude of the initial frequency of the incident laser field. The harmonic emission has the characteristic of high emission efficiency at relatively high orders, and it also has a wide expansion in the frequency domain. Thus, the high-order harmonic generation can be utilized to generate coherent EUV or soft X-ray light sources as well as ultrashort at to second laser pulses. It is promising that the attosecond laser pulse will be an important tool for detecting and controlling the electron dynamics in atom and molecule systems.
The mechanisms of high-order harmonics especially the high energy part of the harmonic spectrum can be explained by the well-known three-step model. The three-step model assumes that the electron in the bound state firstly are ionized by the potential barrier formed by the laser electric field and the atomic potential, then the ionized electrons oscillate in the laser field, and finally the electron with high kinetic energy gained in the laser field has the possibility to return back to the parent ion and recombines with the ground state of the system with a high energy photon emitted. As for harmonics with low orders, especially those with single photon energy near the ionization threshold, the Coulomb potential of the atom has significant influences on them. However,the effect of the Coulomb potential of the atom are not included in the three-step model, so the mechanism of near-threshold harmonics (NTH) cannot be clearly interpreted with the three-step model alone. In this circumstance, the study of the mechanism of near-threshold harmonic emission attracted people's attention in general. One important application of NTH is that it can be utilized to generate optical comb with EUV frequencies.
Theoretically, Xiong et al. studied the mechanism of below-threshold harmonic (BTH) emission and found that the mechanism of this part of harmonics include the effect of the quantum-path interference and the Coulomb potential. He et al. analyzed the emission of BTH in various laser intensity regions and found that the harmonic spectrum exhibits a periodic structure as a function of the harmonic frequency when the incident laser intensity is about 1013 W/cm2. Utilizing the quantum-path and time-frequency analyses of the harmonic emission, He et al. indicated that this periodic structure can be attributed to the interference effect between two specific quantum paths. Li et al. adopted the synchrosqueezing scheme to study the near-and below-threshold harmonic emission of Cs atoms in an intense mid-infrared laser field and they showed that the multiphoton and the multiple rescattering trajectories have an effect on the NTH and BTH generation processes. Shafir et al. found that the ionic potential plays an critical role in NTH emission. Under the interaction between the atom and the intense laser field, electron in the ground state not only can be ionized but also be pumped into excited state, and these excitation processes also affect the harmonic emission.
We studied the harmonic emission process near the ionization threshold by solving the time-dependent Schrdinger equation of an atom interacting with a strong laser field. Utilizing the obtained wavefunction, we systematically studied the high-order harmonic emission with the variation of the incident laser intensity. Meanwhile, through solving the TDSE with the momentum-space method, the excited-state population is precisely calculated and achieved. We show that the ninth harmonic exhibits a periodic oscillation structure with the intensity of the incident laser field increasing, and we reveals that there is a synchronous variation between the harmonic intensity and the relatively high bound state population.Within a certain range of laser intensity, the increase of the total population of the excited states corresponds to the low efficiency of harmonic emission, and this competition relationship is quite clear. Therefore, when the wavelength of the driving laser pulse is fixed, we can optimize the driving laser intensity to achieve the near-threshold harmonic emission with high efficiency.
2016, 65 (22): 223203.
doi: 10.7498/aps.65.223203
Abstract +
A Wigner-distribution-like function is proposed to obtain various distributions of photoelectron emitted from H atoms in few-cycle laser pulses with different frequencies:time-energy distribution, time ionization distribution for linearly polarized laser field and time-emission angle distribution, angular distribution, and time ionization distribution for elliptically polarized laser field. With decreasing frequency, all the distributions clearly show a transition of ionization process from the multi-photon regime to the tunneling regime. For the case of linearly polarized laser pulse, accompanying this transition, the semiclassical relationship between the ionization moment and the final drift energy is becoming more and more close to the time-energy distribution. Meanwhile, the time-energy distribution clearly shows the interference structures in the tunneling regime, which can be attributed to the interference between the electrons with the same energy ejected at different times. For the case of elliptically polarized laser pulse, both the angular offset in the angular distribution and the time offset in the time ionization distribution are obtained by comparing the quantum calculation with the semi-classical result. The results show that the time offset is much smaller than the angular offset. This indicates that the attoclock technique which is based on the correspondence between two offsets is in principle inaccurate. Furthermore, the time offset can be both positive and negative. So this time offset cannot be interpreted as the tunneling time.
2016, 65 (22): 223204.
doi: 10.7498/aps.65.223204
Abstract +
The semi-classical method based on the recently developed analytical R-matrix theory is reviewed in this work. The method is described with the application to ultra-fast strong-field direct ionization of atoms with one active electron in a linearly polarized field[Torlina L, Smirnova O 2012 Phys. Rev. A 86 043408]. The analytical R-matrix theory separates the space into inner and outer regions, naturally allowing the possibility of an analytical or semi-analytical description of wave function in the outer region, which can be approximated by Eikonal-Volkov solutions while the inner region provides well-defined boundary conditions. Applying the stationary phase method, the calculation of the ionization amplitude is cast into a superposition of components from trajectories and their associated phase factors. The shape of the tunneling wave packets associated with different instants of ionization is presented. It shows the exponential cost of deviating from the optimal tunneling trajectory renders the tunneling wave packet a Gaussian shape surrounding the semi-classical trajectory. The intrinsically non-adiabatic corrections to the sub-cycle ionization amplitude in the presence of both the Coulomb potential and the laser field is shown to have different influences on the probability of ionization. As a specific study case, soft recollisions of the released electron near the ionic core is investigated by using pure light-driven trajectories with Coulomb-corrected phase factor[Pisanty E, Ivanov M 2016 Phys. Rev. A 93 043408]. Incorporating the Coulomb potential, it is found problematic to use the conventional integration contour as chosen in other methods with trajectory-based Coulomb corrections, because the integration contour may run into the Coulomb-induced branch cuts and hence the analyticity of the integrand fails. In order to overcome the problem, the evolution time of the post-tunneling electron is extended into the complex domain which allows a trajectory to have an imaginary component. As the soft recollision occurs, the calculation of the ionization amplitude requires navigating the branch cuts cautiously. The navigating scheme is found based on closest-approach times which are the roots of closest-approach times equations. The appropriately selected closest-approach times that always present in the middle of branch-cut gate may serve to circumvent these branch cuts. The distribution of the closest-approach times presents rich geometrical structures in both the classical and quantum domains, and intriguing features of complex trajectories emerge as the electron returns near the core. Soft recollisions responsible for the low-energy structures are embedded in the geometry, and the underlying emergence of near-zero energy structures is discussed with the prediction of possible observations in experiments.
2016, 65 (22): 223205.
doi: 10.7498/aps.65.223205
Abstract +
The time-dependent Schrodinger equation of alkali metal Na atom in an infrared laser field is solved numerically by using the pseudo-spectral method. In the calculation, an accurate model potential of Na atom is used. The bound state energy levels, which are consistent with experimental data, are obtained with the potential, so that we can study the characteristics of high-order harmonic generation for emission of the exited stated of Na atom. Our results show that the high-order generation spectrum of emission of 4s, and 5s excited states of Na atom is super-continuum in the over-barrier ionization regime. By superposed certain orders harmonics below threshold, a single pulse can be obtained with the central frequency from high frequency of visible light to the ultraviolet band. Through the calculated ionization probability of Na atom and the time-frequency analysis by wavelet transform of the superposed harmonics, it reveals that the emission process of low-order harmonic generation in over-barrier ionization regime is different from in the tunnel ionization regime.
2016, 65 (22): 223206.
doi: 10.7498/aps.65.223206
Abstract +
The advent of the ultrafast laser pulse provides the powerful and efficient tool for probing the ultrafast electron dynamics in atoms and molecules. The various nonlinear process induced by the laser-matter interaction allows one to obtain the electron motion information on the sub-femtosecond time scale. A series of the ultrafast spectroscopic technique, such as attosecond streak camera, attosecond transient absorption spectrum, and etc., have been successfully applied to the probe of electron dynamics in atoms, molecules, and solids. Using two-color field is one of the significant methods to achieve the coherent control and exploring of the electron motion. This paper summarizes recent research activities in the field of the atomic and molecular ultrafast process investigated in State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, including the detection of the electron dynamics of the multi-bound states, measurement of the carrier envelope phase (CEP) and the phase of the attosecond pulse, and the ultrafast electron control with the THz/UV and MIR/IR field. To measure the dynamics of the multi-bound states, a broadband attosecond pulse can be used to ionize the electrons after it is excited by the pump laser. By changing the delay between the pump laser and the attosecond pulse, the measured electrons ionized by the broadband xuv attosecond pulse can present the multi-bound states dynamics simultaneously. The XUV/IR scheme is popularly used in attosecond dynamics measurement. But usually, the IR field is not very strong. We find that, if the IR field is strong enough to induce the above threshold ionization (ATI), the interference between the ATI electron and the electron from XUV pulse can be used to measure the CEP of the attosecond XUV pulse. Besides, if the electron ionized by attosecond pulse can be pushed back to the nuclei, the emission from the recombination can be used to determine the spectral phase of the attosecond pulse, which is an all-optical measurement. We also investigate the two color scheme of THz/UV and MIR/IR fields. With THz/UV two color scheme, very high electron localization can be achieved duration molecular dissociation when we use the UV pulse to excite the electron and the THz pulse to control the following electron movement. When we use the MIR/IR field to control the electron motion during the high harmonic generation, the recollision can be greatly decreased and the single attosecond pulse can be produced with multi-cycle MIR laser field.
EDITOR'S SUGGESTION
Strong field photoelectron holography studied by a generalized quantum-trajectory Monte Carlo method
2016, 65 (22): 223207.
doi: 10.7498/aps.65.223207
Abstract +
Strong-field photoelectron holography encodes detailed temporal and spatial information about both theelectron and ion dynamics. Here, we review a series of numerical studies of strong-field photoelectron holographyin atoms and molecules by a generalized quantum-trajectory Monte Carlo method. By comparingthe generalized quantum-trajectory Monte Carlo simulationwiththe numerical solution of thetime-dependent Schrdinger equation, we demonstrate that, in the nonadiabatic tunneling regime, pronounced nonadiabatic effects occur which manifest in the energy cutoff of the holographic interference structure. Moreover, we found that a profound ring-like pattern can be observed in the deep tunneling ionization regime. Theappearance of the ring-like interference pattern masks the holographic interference structure. In contrast to the tunneling regime, the long-range Coulomb potential is found to play an essential role in the formation of the photoelectron holography in the nonadiabatic tunneling regime.
2016, 65 (22): 224205.
doi: 10.7498/aps.65.224205
Abstract +
The research of laser-matter interaction has become a major direction in the field of laser physics since the invention of laser in 1960. Based on the development of the laser technique in the recent several decades, the ranges of the laser's frequency, intensity and pulse width have been explored widely. Therefore, the excitation, emission and ionization dynamic processes of a complex system in intense laser fields have been studied deeply. Especially, the nonsequential double ionization (NSDI) process has continuously attracted much attention from both experimental and theoretical sides. So far, the recollision picture is widely accepted as a dominating mechanism accounting for the NSDI process under an infrared (IR) laser field condition. This recollision picture can be classified into two mechanisms:the collision-ionization (CI) mechanism and the collision-excitation-ionization (CEI) mechanism. Recently, it is found that the NSDI process can take place in an extreme ultraviolet (XUV) laser field, and thus few-photon double ionization has been extensive studied by solving the full-dimensional time-dependent Schrdinger equation (TDSE) and the conventional nonstationary perturbation theory. This article reviews the frequency-domain theory of the NSDI processes of an atom in a monochromatic IR and IR+XUV two-color laser fields. In contrast with other approaches, such as the TDSE calculation and S-matrix method, the frequency-domain theory based on the nonperturbative quantum electrodynamics is involved in some advantages:(i) all the recollision processes, including high-order above-threshold ionization (HATI), high-order harmonic generation (HHG) and NSDI, can be dealt under the unified theoretical frame and can be decoupled into two processesa direct above-threshold ionization (ATI) followed by a laser-assisted collision (LAC) or by a laser-assisted recombination process, where these subprocesses can be investigated separately; (ii) the approach can save a lot of computation time because of its nature of time-independent. In this review, we show the different momentum spectral distributions under the CI and CEI mechanisms in the IR and IR+XUV laser fields. With the help of the channel analysis, we compare the contributions of the forward and backward collisions to the NSDI under two conditions of the monochromic IR and IR+XUV two-color laser fields. It is found that, in the CI mechanism, the backward collision makes major contribution to the NSDI in the IR laser field, while the forward collision plays a crucial role in the NSDI when the energy of the recolliding electron is very large in the IR+XUV two-color laser fields. Furthermore, by employing the saddle-point approximation, it is found that the momentum spectrum, whether in the monochromic IR or the IR+XUV two-color laser fields, is attributed to the interference between two trajectories at different saddle-point t0 and 2/1-t0 (1 is the frequency of an IR laser field) when the collision happens in each channel. On the other hand, in the CEI mechanism, the momentum spectra in the monochromic IR or the IR+XUV two-color laser fields present a distinct difference. It is further found that the momentum spectrum in the IR+XUV two-color laser fields is involved in the much more channels than that in the monochromic IR laser field, and thus the complex interference patterns in the momentum spectrum in the two-color laser fields are shown. Moreover, it is found that, in both the CI and CEI mechanisms, the XUV laser field in the NSDI not only can enhance the ionization probability of the first electron, but also can accelerate the first ionized electron so that the bound electron can gain much energy by collision, which is in favor of significant boost of the NSDI probability. This work can help people understand more deeply about the NSDI, and also may pave a way for us to continue investigating the NSDI process of complex system in intense laser fields.
2016, 65 (22): 224206.
doi: 10.7498/aps.65.224206
Abstract +
High harmonic generation (HHG) is one of the most fundamental processes in the interaction of strong laser fields with atoms and molecules. Because of wide applications of HHG, for example, imaging atomic or molecular orbitals, visualizing chemical reactions, synthesizing a single attosecond pulse, the HHG attracts huge attentions in both theories and experiments. The HHG can be explained by the famous three-step model:first, the laser field bends the Coulomb potential and the electron tunnels out; second, the electron is accelerated in the laser field and gains kinetic energy; Third, the energetic electron recombines with the parent ion and release its energy as high energetic photons. The HHG can be tailored by controlling the each step. In this paper, we conceive a strategy to control the third step. We simulate the HHG when He+ is exposed to the combined few-cycle Ti-Sapphire (800 nm) IR femtosecond laser pulse and XUV laser pulse by numerically solving the time dependent Schrdinger equation. The simulation shows that after the electron tunnels out and gains energies from the infrared laser field, extra XUV photons may be absorbed during the electron and parent ion recombination, contributing multiple cutoffs separated by XUV photon energies in the high harmonic spectrum. This scenario is confirmed by time-delay-dependent HHG in the time-frequency representation, and by the power scaling of the cutoffs' intensities as a function of the XUV intensity.
EDITOR'S SUGGESTION
2016, 65 (22): 224207.
doi: 10.7498/aps.65.224207
Abstract +
When atoms and molecules are excited by ultrashort laser pulses, highly nonlinear strong-field processes like above-threshold ionization and high harmonic generation occur. By analyzing the emitted light and electron signals, the atomic and molecular structures and ultrafast dynamics can be detected with a combination of Angstrom spatial resolution and sub-femtosecond temporal resolution, which provides a powerful tool to study the basic structures and physical processes in the microscopic world. The molecular orbital tomography (MOT) developed since 2004 enables one to image the wavefunction of the molecular orbital itself, which will help people gain deeper insight into the chemical reactions. In this paper, the theory of MOT will be introduced, and the progresses of MOT in the past ten years will be reviewed.
2016, 65 (22): 224208.
doi: 10.7498/aps.65.224208
Abstract +
Recently, three major types of minima (i.e., Cooper-like minimum, two-center interference minimum and multi-channel interference minimum) have been observed in high-order harmonic generation (HHG) spectra. Identification of the origin of the minimum in a HHG spectrum is critical for self-probing of the molecular structures and dynamics, which has been an important subject in attosecond physics. In this paper, we report the investigation of the multi-electron dynamics in HHG from N2 molecules driven by intense mid-infrared laser pulses. Based on a pump-probe experimental setup, clear spectral minima in the cutoff region of high harmonic spectra from N2 molecules are observed in measurements with mid-infrared laser pulses at three wavelengths (i.e., 1300, 1400 and 1500 nm). A systematic investigation has been carried out for clarifying the origin of these minima. We carefully measured the spectral minima under three different experimental conditions:1) different alignment angles of molecules; 2) various peak laser intensities; 3) tunable driving laser wavelengths. Experimental results show that the positions of the spectral minima do not depend on the alignment angles of molecules. In addition, the measured spectral minima shift almost linearly with the laser intensity for all three wavelengths, and the positions of the spectral minima strongly depend on the wavelengths of the driven field. These findings are in conflict with the Cooper-like and two-center interference minima predictions, providing strong evidences on the dynamic multi-channel interference origin of these minima. Besides, we theoretically calculated the positions of multi-channel interference minima by using a classical three-step model and found out perfect agreements between the experimental results and theoretical calculations, which again strongly support the multi-channel interference picture. Moreover, the advantages of the observed dynamic multi-channel interference based on HHG driven by long wavelength lasers are discussed. The long wavelength driver lasers are attractive for not only generating coherent XUV radiation and attosecond pulses, but also investigating structures and dynamics of molecules in strong laser fields.
2016, 65 (22): 224209.
doi: 10.7498/aps.65.224209
Abstract +
We experimentally studied the dissociative single and double ionization of CO molecules by counter-rotating circularly polarized two-color (CRTC) laser fields. By coincidently measuring the electrons and the fragmented ions, trefoil asymmetric momentum distributions of C+ in the polarization plane were observed, which are mainly determined by the selective ionization of CO with asymmetric orbitals. The threefold pattern could rotate continuously in the two-dimensional space by finely tuning the relative phase of the CRTC fields, providing a new method to manipulate the directional bond breaking of molecules by strong laser fields.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (22): 224201.
doi: 10.7498/aps.65.224201
Abstract +
Taking into account the gradient change of the near-surface viscous property, we develop a finite element model of laser-generated surface acoustic wave in composite plate. The propagation characteristics of the surface acoustic wave in the composite plate are studied in detail, and the influences of the near-surface viscous modulus, thickness, and Lam constant on the attenuation characteristics of the surface acoustic wave are discussed. In addition, the propagation characteristics of the surface acoustic wave are verified by the theoretical calculations of the dispersion and attenuation curves. The results show that the near-surface viscous modulus and thickness are related to the attenuations of the surface shear wave and the Rayleigh wave, but have no influence on the propagation velocity. Furthermore, the imaginary part of the Lam constant has great influence on the attenuations of the surface shear wave and the Rayleigh wave, whereas the imaginary part of has no effect on the attenuation characteristics of the two waves, which indicates that the attenuation of the surface acoustic wave is related to the near-surface shear viscous modulus. The study gives theoretical basis for evaluating the near-surface mechanical properties of the composite plates by the laser ultrasound technique. Besides, it provides a feasible way to study the surface micro-cracks on the composite plates.
2016, 65 (22): 224202.
doi: 10.7498/aps.65.224202
Abstract +
An experimental setup used to measure the important optical properties of electrowetting liquid lens is proposed. The simple and precise method of measuring dynamic responses and focal lengths of liquid lens under different excitation signals is based on Gaussian beam transmission theory. The measurement method can be widely used in all kinds of zoom lens systems. The device is simple and economical, and also has the advantages of convenient operation, high measurement precision and wide range measurement. This work provides a new way to study the dynamic response of electrowetting liquid lens and the the mechanism of electrowetting liquid lens.
The fabrication process and some relevant noticeable points for the homemade liquid lens are introduced. The testing device of dynamic process of lens consists of a He-Ne laser, an electrowetting lens, a circular diaphragm, a phototube, a digital storage oscilloscope and a computer. The change of the focal length of liquid lens due to the applied voltage will affect the flux detected by the photoelectric receivers. It is proved according to Gaussian beam transmission theory that the light flux received by the phototube changes with time, which represents the relationship between the focal length and time and the dynamic characteristics of the liquid lens. Therefore, the intensity of output signal of photoelectric receiver reflects the focal length of liquid lens.
A dynamic changing process of the focal length of a self-regulating varifocal liquid lens based on electrowetting technology is tested under alternating current signal. It shows that the focal length of the liquid lens changes with the corresponding amplitude and polarity of the sine voltage. In one cycle, 4 peak signals of 50 Hz appear in turn, and the peak amplitude increases with the increase of voltage. Peaks 1 and 2 are caused by the voltage polarity, while peaks 3 and 4 by the oscillation modes. This is due to the fact that the liquid surface changes with time in the spherical shape under low voltage, but it will generate new oscillation mode when the amplitude is high.
2016, 65 (22): 224203.
doi: 10.7498/aps.65.224203
Abstract +
Techniques for generating microwave waveform such as square or triangle waveform have been a topic of general interest recently. Because they are important for applications in radar, measurement, instrument technology, medical imaging, etc. Usually, high quality microwave waveform is preferred as it largely determines the performance of the microwave system. In recent years, photonic microwave generation has aroused the great interest of both the research community and commercial sector all over the world, as it has the advantages of high frequency and large bandwidth. A variety of techniques for generating arbitrary waveforms have been demonstrated. In this paper, a new approach to photonic microwave waveform generation based on polarization delay interference is proposed and experimentally demonstrated, in which only one laser and Mach-Zehnder modulator (MZM) are used. In our scheme, odd-order optical sidebands can be generated by setting the bias of the MZM at the quadrature point. Thus, square waveform and triangular waveform can be generated, with a sinusoidal microwave signal applied to MZM and carefully controlling the modulation depth of MZM as well as the amount of delay on the differential delay line. A theoretical analysis of the principle of the proposed approach is performed. And optimum parameters of the photonic waveform generation are derived from numerical simulation. By simulation, the fourth order of approximation of the square waveform and triangular waveform is generated. In addition, it is proved that the scheme can generate the stable square waveform and triangular waveform by analyzing the stability of the bias and the modulation depth of the MZM. Experimentally, square waveform and triangular waveform with a repetition rate of 5 GHz are generated by carefully setting the bias and the modulation depth of the MZM, which are consistent with theoretical analyses. The scheme has the advantages of the low cost, simplicity, easy-to-tune, and it adopts the innovative technology of polarization delay interference. Hence the scheme is effective and worth spreading.
2016, 65 (22): 224204.
doi: 10.7498/aps.65.224204
Abstract +
High energy and high repetition rate femtosecond Ti:sapphire lasers are widely used in isolated attosecond pulses and high-order harmonic generation. Enhancing the driving laser energy is a convenient and effective way to improve attosecond pulse energy. A 1 kHz or higher repetition rate millijoule level femtosecond Ti:sapphire amplifier is generally used to generate isolated attosecond. However, due to the limitation of its green pump laser energy, the energy of femtosecond Ti:sapphire laser is limited to several millijoules. Appropriately reducing the requirements for repetition rate, realizing high energy driving laser will significantly improve attosecond pulse energy and extend its application scope. Meanwhile, a 532 nm pump laser from frequency doubled 1064 nm Nd:YAG flash lamp pumped laser at 100 Hz repetition rate can achieve high pump energy with lower cost. Accordingly, we develope a 100 Hz repetition rate high energy amplifier based on Ti:sapphire crystal.
The femtosecond amplifier system consists of oscillator, stretcher, ring cavity regenerative amplifier, four-pass amplifier and grating compressor. The ring cavity regenerative amplifier is the first amplifier as pre-amplifier, and the four-pass amplifier is the booster amplified-stage. 80 MHz seed pulse from the oscillator has a full width at half maximum bandwidth of 61 nm with a 20 fs duration. Then the seed pulses are stretched to 200 ps with a Martinez grating stretcher, rotated to vertical polarization and injected into the regenerative amplifier. The amplified uncompressed 1 kHz repetition rate laser pulses with 3 mJ pulse energy are selected to be 100 Hz and input into the four-pass amplifier. With a pulse energy of 75.1 mJ, wavelength at 532 nm flash lamp pumped pump laser at 100 Hz repetition rate, single pulse energy up to 25.4 mJ is obtained from a Ti:sapphire crystal, corresponding to a high energy conversion efficiency of 33.8%. We believe that higher energy should be possible if the pump energy can be further increased. After expanding the beam to 10 mm in diameter, the amplified chirped pulse is compressed using a four-pass, single grating compressor, with an overall efficiency of 72%. The highest pulse energy after compression is 18.3 mJ. For a fluctuation of the 100 Hz pump laser is as high as 3.4% for over 10000 shots, the 3.6% energy stability of the amplifier has a room to be improved. The typical spectrum bandwidth after the compressor is 39 nm, which can support transform-limited pulse duration of 32.8 fs. After fine dispersion compensation by the compressor, A pulse duration of 37.8 fs is measured using a single shot autocorrelator (Minioptic Technology, Inc). In addition, the spatial profile of the output beam from the compressor is measured using a commercial laser beam analyzer (Spiricon, Inc). The beam quality M2 factor are 1.8 and 1.6 in X and Y directions, respectively.
In summary, a peak power of 0.48 TW compact 100 Hz femtosecond laser with pulse duration of 37.8 fs, pulse energy of 18.3 mJ is achieved from a two-stage amplifier system based on Ti:sapphire crystal. We believe that with a more stable and better spatial profile pump source, even better performance can be obtained by this scheme. Nevertheless, the current results show that this system should be favorable for high energy attosecond pulse generation and further amplification such as Terawatt system.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2016, 65 (22): 225201.
doi: 10.7498/aps.65.225201
Abstract +
The influences of safety factor q profile and poloidal rotation profile on the q=1 tearing and Kelvin-Helmholtz (K-H) instabilities are investigated numerically by using a magnetohydrodynamic model in cylindrical geometry. With increasing the poloidal rotation, the m/n=1/1 mode is suppressed, while four domains exist for the high-order harmonic modes (such as m/n=2/2, m/n=3/3):the destabilized tearing mode domain, stabilized tearing mode domain, stable-window domain, and unstable K-H mode domain. Further, we find that the growth rate of the m/n=1/1 mode is related to the location of shear layer. Roles of shear flow in the m/n=1/1 mode for the shear layer located t on both the inner and outer sides of rational surface are almost the same, which is different from the scenarios of high-order harmonic modes. In addition, the smaller the magnetic shear on the rational surface, the smaller the growth rate of tearing mode is, and the more easily the K-H instability is excited.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (22): 226401.
doi: 10.7498/aps.65.226401
Abstract +
The confined environment plays a very important role in the phase separation of copolymers, which can change bulk phase behaviors of copolymers. The different confinement conditions can induce the formations of various interesting and novel morphologies, which can be used in a variety of nanotechnology applications such as high-density medium storage, nanolithography and photonic crystals. The grafting of polymers to confined surfaces is an efficient means for tailoring surface properties. In this work, we investigate the effect on architecture of the AB diblock copolymer confined between mixed brush-grafted surfaces by using self-consistent field theory. The brush contains two types of homopolymers. We study the effects of the fraction of A block, grafted period and the volume fraction of the polymer brush, the distance between two surfaces and the interaction strength between two blocks on the morphology. 1) With the increase of the fraction of A block (fA), the phase morphology changes from the A-block hexagonal cylinder to the parallel lamellae, to the curving lamellae, and then to the B-block hexagonal cylinder. The period of hexagonal cylinder and curving lamellae is equal to the grafted period of the polymer brush due to the influence of the polymer brush. 2) The grafted period of polymer brush is a very important factor for the morphology of diblock copolymer. When fA=0.3, we change the grafted period of the polymer brush. We obtain the phase transition from the hexagonal cylinder to the alternating phase of tetragonal and hexagonal cylinder, then to the alternating phase of tetragonal and octagonal cylinder. When fA=0.4, the structure changes from the hexagonal cylinder to the order phase of the waving lamellae and cylinder with the increase of the grafted period of the polymer brush. Compared with the single homopolymer brush system, the mixed brush enlarges the range of ordered phase and reduces the range of disordered phase. Block copolymers are prone to forming cylinder in mixed brush system and tending to form lamellae in single homopolymer brush system. 3) When fA=0.3, we obtain the phase transition from the hexagonal cylinder to the one-layered cylinder phase by increasing the volume fraction of the polymer brush. This transition is different from that of the single homopolymer brush system. Interestingly, when fA=0.45, the structure of AB block copolymer changes from the parallel lamellae to the perpendicular lamellae with the increase of the volume fraction of the polymer brush. The entropic energy plays an important role in this transition process. Similarly, we also observe the phase transition from the parallel lamellae to the perpendicular lamellae by decrease the distance between two surfaces. 4) We construct the phase diagram for a range of the fraction of A block and the interaction strength. The results provide an effective approach to obtaining the desired microstructures for fabricating nanomaterials.
2016, 65 (22): 226801.
doi: 10.7498/aps.65.226801
Abstract +
The adsorption and dissociation of water on the oxygen pre-covered Cu(110) surface are studied with scanning tunneling microscopy (STM). At room temperature, oxygen atoms are adsorbed on the Cu(110) surface and self-assembled into ordered (21) Cu-O chains along the[001] direction. The relative proportion of clean and (21) O-strips can be tuned by the sample exposure time to oxygen gas. When the oxygen pre-covered Cu(110) sample is exposed to water molecules at 77 K, the water molecules are adsorbed at the edges and on the top of the Cu-O chains. On the bare Cu(110) surface, we observe the formation of a hexagonal structure right next to the Cu-O stripes at 77 K. This is different from the water molecule adsorption on the clean Cu(110) surface, in which water molecules are adsorbed and self-assembled into ordered zig-zag chains along the[001] direction. While on oxygen pre-covered Cu(110) surface, water molecules prefer to hydrogen bond with oxygen atoms inside the Cu-O chains and then bond with the other water molecules, forming stable hexagonal network. From our earlier STM results, we find that water forms zig-zag chains only when oxygen pre-coverage is lower than 0.125 ML. On the top of hexagonal network, we observe the bright spots and attribute them to the 2nd layer water clusters. The fact that the 2nd layer clusters form on the top of the hexagonal water-hydroxyl regions rather than at the other locations on the Cu(110) surface indicates that the mixed hexagonal network may have more H-dangling bonds that facilitate the 2nd layer growth. In order to remove the upper layer water molecules, we apply a 5 V bias voltage for scanning, for which the tunneling electrons provide enough energy for overcoming the water desorption and dissociation barrier (0.5-0.55 eV at UHV and low temperature). With the excitation of tunneling electrons from the tip, the water molecules in the hexagonal network react with oxygen atoms inside the Cu-O chains (H2O+O2OH). According to Forster proposed Bjerrum defect model, the hexagonal network is formed by water donating hydrogen to hydroxyl, in which two hydrogen atoms are located between two adjacent oxygen atoms. Our results demonstrate that the oxygen atoms pre-adsorbed on the Cu(110) surface act as nucleation centers for water adsorption and catalyze its dissociation, which is important in water gas shift reaction study. However, we still need more X-ray photoelectron spectroscopy experiments to certify whether the water molecules react with the pre-covered oxygen atoms at low temperature (below 100 K).
CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (22): 227301.
doi: 10.7498/aps.65.227301
Abstract +
Quantum metrological standards based on the fundamental physical constants are the trend of modern metrology because of their attributes such as high accuracy, high stability, and good reproducibility. The quantum Hall effect (QHE), which refers to the electronic charge e and the Planck constant h, is used to define the quantum resistance standard. The quantum Hall resistance (RH) of h/2e2 at the filling factori=2 is used as the standard. It is obvious that the RH is non-integral. However the resistors that need to be calibrated each have a decimal value, such as 1 k, 10 k, 100 k, etc. The calibration chain from the non-integral RH to the real resistor is long. The quantum Hall array resistance standards (QHARSs) are invented to solve this problem. The QHARS which are based on the decimal resistance values can shorten the calibration chain, improve the resistance transfer accuracy, and finally realize the quantization of the whole resistance calibration chain. The QHARS can also replace the traditional physical transfer standard resistor and realize the quantization of the transfer standard resistor. The decimal QHARS devices can be realized by connecting single QHE devices in series or parallel with the multiple link technology.
In this paper we report the design, fabrication and characterization of a 1 k QHARS device based on the GaAs/AlxGa1-xAs heterostructures. In our design, the 1 k array device consists of only 29 Hall bars. The nominal value of the device is 999.9999658 with a relative deviation of -3.4210-8 from 1 k. The ratio between the maximum and minimum current flowing through the Hall bars is as small as 14.5.
The 1 k QHARS devices are measured in the national resistance standard system at a temperature of 1.5 K. The measurement is taken at the central magnetic field of the 2nd quantum Hall plateau. We compare our 1 k QHARS resistor with a 1 k transfer standard resistor using the direct current comparator. The 1 k transfer standard resistor has already been calibrated in advance with our single QHR standard by cryogenic current comparator. Therefore the resistance of our 1 k QHARS resistor can be obtained. The relative deviation between the measured resistance value and the designed value is -1.9610-7with a standard uncertainty of 2.0710-7. The results show that we have realized the 1 k quantum Hall resistance standard device which can be used for the resistance calibration.
2016, 65 (22): 227302.
doi: 10.7498/aps.65.227302
Abstract +
Based on the free electronic model and Winful's theory about tunneling times, the dwell times and the phase times in NM/SF/I/SF/NM double spin filter junctions are investigated, where the NM denotes the normal metal, SF the insulator barrier with spin filter effects and I the nonmagnetic insulator barrier. There are three different cases which are analyzed in detail:1) the dependences of dwell time and phase time on the energy of the incident electron; 2) the dependences of dwell time and phase time on the heights of the barrier; 3) the dependences of dwell time and phase time on the width of the barrier and the molecular field in the spin filter layer. The numerical results show that for the first case, when the electrons have low incident energy (smaller than the barrier height), as the influence of the spin-dependent self-interfere term, the phase times are always larger than the dwell times for electrons with different spinorientations. But when the electrons have high incident energy (higher than the barrier heights), the influence of the self-interfere term disappears and the differences between the phase time and dwell time for electrons with different spin orientations disappear also. For case 2, the numerical results show that the variation of nonmagnetic insulator barrier height has little influence on the dwell time and phase time in NM/SF/I/SF/NM double spin filter junctions. But when the nonmagnetic insulator barrier height is lower than the barrier height of spin filter layer, the quantum well will appear and the resonant tunneling can be induced to lead to the peaks in the dependences of dwell and phase times on the insulator barrier height. The variation of spin-filter barrier height has obvious influence on the dwell time and phase time in NM/SF/I/SF/NM double spin filter junction. With increasing the height of spin-filter barrier, the dwell times and phase time both first increase and then decrease. For case 3, the influences of the widths of the nonmagnetic insulator barrier layer and spin filter layer on the dwell time and phase time are little. But when the barrier height of nonmagnetic insulator barrier is lower than that of spin-filter layer, the variation of width of insulator barrier can lead to the resonant tunneling and the peaks in dwell and phase times. Unlike the influence of width of barrier, the influences of molecular field in the spin filter layer on the dwell time and phase time are obvious. For the up-spin electrons, dwell time and phase time decrease with increasing the molecular fields, which is contrary to the scenario for the down-spin electrons.
2016, 65 (22): 227401.
doi: 10.7498/aps.65.227401
Abstract +
In this work, a series of single domain GdBCO bulk superconductors with different ratios of BaO addition in the solid phase pellet, is successfully fabricated to inhibit the Gd/Ba substitution in the growth process by the modified top seeded infiltration growth (M-TSIG) technique. The reaction of the precursor powders, the growth morphology, the magnetic levitation force (F), the trapped magnetic flux (Btr) and critical temperature (Tc) of the single domain GdBCO bulk superconductors are investigated in detail. First, the differential thermal analysis is performed on the precursor powders of 10 mg solid phase pellet (containing various amounts of BaO) and 15 mg liquid phase pellet in order to investigate the melting temperature (Tm) and the peritectic temperature (Tp) of the GdBCO superconductor system. The results show that the melting point of the precursor powder decreases by nearly 8℃ as the BaO composition increases from 0 wt% to 4 wt%, which leads to Tp decreasing with BaO content increasing. Second, the top view morphologies of the GdBCO samples are also discussed. All of the samples exhibit clear, fourfold growth sector boundaries on their top surfaces, and spontaneous satellite grains are observed in none of these samples. It can be seen that the different ratio of BaO addition cannot affect the growth morphology of the single domain GdBCO bulk superconductor. And thirdly, the levitation force and trapped field of each of the samples are measured under a zero field cooling state at 77 K by the three-dimensional magnetic force and field device. The values of the levitation force for the samples are slightly different for different ratios of BaO additions. The largest levitation force is 35 N, which is obtained in the sample with 2.5 wt%, and the smallest one is 28 N in the sample with 1 wt% BaO addition. And also, the trapped field of the sample can be attributed simply to the variation in the pinning strength. It can be obviously seen that these values fluctuate between 0.28 T and 0.32 T for these samples. Finally, the critical temperatures of the samples are measured by the vibrating sample magnetometer with an external magnetic field of 0.01 T. The samples exhibit outstanding features of high Tc, which indicates that these samples are of good quality and the Gd/Ba substitution is inhibited by the BaO addition. The above results show that the values of melting temperature (Tm) and peritectic temperature (Tp) of the samples have the decrease tendencies, and the superconducting properties of the samples (such as F, Btr and Tc) can be improved to a certain extent when the amount of BaO added ranges from 2 wt% to 4 wt%, which are very helpful in inhibiting the Gd/Ba substitution and fabricating the high-quality single domain GdBCO bulk superconductors.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (22): 228101.
doi: 10.7498/aps.65.228101
Abstract +
The thermophysical properties and liquid-solid phase transition characteristics of ternary (Co0.5Cu0.5)100-xSnx(x=10, 20, 30, 40 and 50 at%) alloys are systematically investigated. The liquidus temperature and latent heat of fusion, as well as the undercooling are determined by differential scanning calorimetry (DSC) method. Based on the measured data, their relationships with Sn content are fitted by polynomial functions. The liquidus temperature shows a decreasing tendency with the increase of Sn content. The undercooling of liquid (Co0.5Cu0.5)100-xSnx alloys significantly increases with increasing Sn amount, indicating that the addition of Sn element enhances the undercoolability. By using the laser-flash and DSC methods, the thermal diffusion coefficients and specific heats of solid ternary (Co0.5Cu0.5)100-xSnx alloys are respectively measured in a temperature range from 293 to 473 K. The thermal diffusion coefficients increase linearly as temperature rises. The thermal diffusion coefficient varies from 1.0610-5 to 1.1210-5 m2s-1 for ternary Co45Cu45Sn10 alloy, which is close to that of Co element but much lower than those of Cu and Sn elements in the same temperature range. However, the thermal diffusion coefficients of other (Co0.5Cu0.5)100-xSnx alloys are far less than that of ternary Co45Cu45Sn10 alloy. The specific heat shows an increasing trend with temperature, and drops apparently with increasing Sn amount. From the measured thermal diffusion coefficients, specific heats and densities, the thermal conductivities of ternary (Co0.5Cu0.5)100-xSnx alloys at 293 K are derived. With the Sn content increasing up to 40 at%, the thermal conductivities for (Co0.5Cu0.5)100-xSnx alloys monotonically decrease from 33.83 to 7.90 Wm-1K-1, and subsequently increases slightly when the Sn content further increases up to 50 at%. In addition, on the basis of the DSC curves and solidification microstructures, the liquid-solid phase transitions are also explored. When the Sn content is less than 30 at%, the primary (Co) phase appears as coarse dendrites, whose volume fraction decreases as Sn content increases. Once Sn content exceeds 30 at%, the Co3Sn2 phase preferentially nucleates and grows during solidification, which occupies about 89% volume in the solidified Co30Cu30Sn40 alloy. The phase constitution investigation indicates that with the increase of the Sn content, the (Cu) solid solution phase disappears, whereas intermetallic compounds, including Cu41Sn11, Cu3Sn, and Cu6Sn5 phases successively precipitate from the alloy melts. The (Sn) solid solution phase even appears when Sn amount reaches 50 at%.
2016, 65 (22): 228501.
doi: 10.7498/aps.65.228501
Abstract +
Photomultiplier tubes (PMTs) widely used in neutrino detectors are critical to reconstructing the direction of the neutrino accurately. Large photocathode coverage, compact design and good time properties for single-photoelectron light are essential performances to meet the requirements for the next generation detectors. Therefore, a novel digital optical module housing 31 3-inch. diameter PMTs is developed. In order to maximize the effective photocathode area and improve the time performance, a modified PMT with a larger photocathode area and 10 dynodes is optimized with the aid of the CST Particle Studio in this paper. Based on the Monte Carlo method and finite integration theory, the main characteristics of the modified PMT, such as uniformity, collection efficiency, gain and transit-time spread, are investigated. As the earlier stages of the PMT contribute the greatest weight to the total transit time spread, the transit time spread of single-photoelectron from photocathode to the first dynode (TTSCD1) is discussed mainly in this paper. The influences of the dynodes position on collection efficiency and TTSCD1 are analyzed. The voltage ratio scheme is also optimized slightly to obtain better collection efficiency and minimum TTSCD1. By tracing the trajectories of secondary electrons from the first to the second dynode stage, dynodes are optimized for improving timing performance and secondary electrons collection efficiency. Direct collection efficiency of secondary electrons from the first dynode to the second is improved from 56.38% to 61.01%. The effective photocathode diameter of the modified PMT is increased from traditional 72 mm to 77.5 mm and the effective area of photocathode is increased by 30.87% compared with the traditional one. What is more, the length of the new PMT is reduced to 103 mm so that the available space of the multi-PMT digital optical module is increased by 63.09% compared with the traditional one containing the high-voltage power supplies, front-end and readout electronics, refrigerating equipment, etc. The simulation results show that the mean collection efficiency of the modified PMT is ~96.40% with the supply voltage of 1000 V and it changes little by changing the supply voltage from 900 V to 1300 V. The mean transit time spread from photocathode to the first dynode is ~1 ns which is better than the transit time spread of the traditional model. And the gain can reach above 106 with a supply voltage of above 1100 V.
2016, 65 (22): 228901.
doi: 10.7498/aps.65.228901
Abstract +
In this paper, we use the civil aviation passenger reservation record data to analyze the characteristics of air passenger group mobility behaviors. We find that the airline network has restriction on passengers traveling, which causes the group travel length distribution to be more consistent with the stretched exponential distribution and to have no scale-free property. The passenger travel interval time satisfies a truncated power law distribution rather than the power law distribution. Meanwhile, holidays have a great influence on passengers traveling. In particular, during the Spring Festival, the summer vacation, and National Day, the number of traveling passengers is far more than at any other time. And passengers who will travel during the holidays prefer to book tickets in advance. By analyzing the throughput, we find that it has a periodicity and is easy to be affected by festival and holidays. Moreover, the throughput of the airport is greatly related to the number of cities which are navigable to each other.
