We review the recent progress in the study of topological phases in systems with space-time inversion symmetry I_{ST}. I_{ST} is an anti-unitary symmetry which is local in momentum space and satisfies I_{ST}^{2}=1 such as PT in two dimensions (2D) and three dimensions (3D) without spin-orbit coupling and C_{2}T in 2D with or without spin-orbit coupling, where P, T, C_{2} indicate the inversion, time-reversal, and two-fold rotation symmetries, respectively. Under I_{ST}, the Hamiltonian and the periodic part of the Bloch wave function can be constrained to be real-valued, which makes the Berry curvature and the Chern number vanish. In this class of systems, gapped band structures of real wave functions can be topologically distinguished by the Stiefel-Whitney numbers instead. The first and second Stiefel-Whitney numbers w_{1} and w_{2}, respectively, are the corresponding invariants in 1D and 2D, which are equivalent to the quantized Berry phase and the Z_{2} monopole charge, respectively. We first describe the topological phases characterized by the first Stiefel-Whitney number, including 1D topological insulators with quantized charge polarization, 2D Dirac semimetals, and 3D nodal line semimetals. Next we review how the second Stiefel-Whitney class characterizes the 3D nodal line semimetals carrying a Z_{2} monopole charge. In particular, we explain how the second Stiefel-Whitney number w_{2}, the Z_{2} monopole charge, and the linking number between nodal lines are related. Finally, we review the properties of 2D and 3D topological insulators characterized by the nontrivial second Stiefel Whitney class.

Indium phosphide (InP) quantum dots (QDs) have shown great potential to replace the widely applied toxic cadmium-containing and lead perovskite QDs due to their similar emission wavelength range and emission peak width but without intrinsic toxicity. Recently, electrically driven red and green InP-based quantum-dot light-emitting diodes (QLEDs) have achieved great progress in external quantum efficiency (EQE), reaching up to 12.2% and 6.3%, respectively. Despite the relatively poor device performance comparing with cadmium selenide (CdSe)-and perovskite-based QLEDs, these encouraging facts with unique environmental friendliness and solution-processability foreshadow the enormous potential of InP-based QLEDs for energy-efficient, high-color-quality thin-film display and solid-state lighting applications. In this article, recent advances in the research of the InP-based QLEDs have been discussed, with the main focus on device structure selection and interface research, as well as our outlook for on-going strategies of high-efficiency InP-based QLEDs.

TOPICAL REVIEW—Fundamental research under high magnetic field

In the last few years, charge order and its entanglement with superconductivity are under hot debate in high-T_{c} community due to the new progress on charge order in high-T_{c} cuprate superconductors YBa_{2}Cu_{3}O_{6+x}. Here, we will briefly introduce the experimental status of this field and mainly focus on the experimental progress of high-field nuclear magnetic resonance (NMR) study on charge order in YBa_{2}Cu_{3}O_{6+x}. The pioneering high-field NMR work in YBa_{2}Cu_{3}O_{6+x} sets a new stage for studying charge order which has become a ubiquitous phenomenon in high-T_{c} cuprate superconductors.

TOPICAL REVIEW—Strong-field atomic and molecular physics

Air lasing is a concept that refers to remote no-cavity (mirrorless) optical amplification in ambient air with the air constituents as the gain media. Due to the high potential of air lasing in view of applications in atmospheric sensing, a variety of pumping schemes have been proposed so far for building up population-inverted gain media in air and producing forward and/or backward directional lasing emissions. This review paper presents an overview of recent advances in the experimental observations and physical understanding of air lasing in various pumping schemes of air molecules by intense laser fields. Special emphasis is given to the strong-field-induced N_{2}^{+} air lasing, the mechanism of which is currently still in a hot debate.

SPECIAL TOPIC—Strong-field atomic and molecular physics

We macroscopically investigate the effect of the laser intensity and gas density on quantum trajectories in the high-order harmonic generation of Ne atoms irradiated by few-cycle, 800-nm laser pulses. The time-frequency profile of the harmonics shows that the long quantum trajectory is dominant at both lower and higher gas densities for a low laser intensity. At high laser intensities, the long quantum trajectory plays an important role for lower gas densities, while the short quantum trajectory is dominant at higher gas densities. An analysis of the phase mismatch for high-order harmonic generation shows that the primary emission of the quantum trajectories is determined by dynamic changes in the laser electric field during the propagation process.

The high-order harmonic generation from a model solid structure driven by an intense laser pulse is investigated using the semiconductor Bloch equations (SBEs). The main features of harmonic spectrum from SBEs agree well with the result of the time-dependent Schrödinger equation (TDSE), and the cut-off energy can be precisely estimated by the recollision model. With increasing the field strength, the harmonic spectrum shows an extra plateau. Based on the temporal population of electron and the time-frequency analysis, the harmonics in the extra plateau are generated by the Bloch oscillation. Due to the ultrafast time response of the Bloch electron, the generated harmonics provide a potential source of shorter isolated attosecond pulse.

The quantum effect of nonlinear co-tunnelling process, which is dependent on atom-pair tunneling and asymmetry of an double-well trap, is studied by using an asymmetrical extended Bose-Hubbard model. Due to the existence of atom-pair tunneling that describes quantum phenomena of ultracold atom-gas clouds in an asymmetrical double-well trap, the asymmetrical extended Bose-Hubbard model is better than the previous Bose-Hubbard model model by comparing with the experimental data cited from the literature. The dependence of dynamics and quantum phase transition on atom-pair tunneling and asymmetry are investigated. Importantly, it shows that the asymmetry of the extended Bose-Hubbard model, corresponding to the bias between double wells, leads to a number of resonance tunneling processes, which tunneling is renamed conditional resonance tunneling, and corrects the atom-number parity effect by controlling the bias between double wells.

We systematically studied surface plasmons reflection by graphene wrinkles with different heights on SiC substrate. Combined with numerical simulation, we found that the geometry corrugation of a few nanometer height wrinkle alone does not causes a reflection of graphene plasmons. Instead, the separated wrinkle from substrate exhibits a nonlinear spatial Fermi energy distribution along the wrinkle, which acts as a heterojunction. Therefor a higher graphene wrinkle induces a stronger damped region when propagating graphene surface plasmons encounter the wrinkle and get reflected.

Recently, the layered transition metal dichalcogenide 1T' MoTe_{2} has attracted considerable attention due to its non-saturating magnetoresistance, type-II Weyl semimetal properties, superconductivity, and potential candidate for two-dimensional (2D) topological insulator in the single-and few-layer limit. Here in this work, we perform systematic transport measurements on thin flakes of MoTe_{2} prepared by mechanical exfoliation. We find that MoTe_{2} flakes are superconducting and have an onset superconducting transition temperature ^{T}_{c} up to 5.3 K, which significantly exceeds that of its bulk counterpart. The in-plane upper critical field (^{H}_{c2||}) is much higher than the Pauli paramagnetic limit, implying that the MoTe_{2} flakes have Zeeman-protected Ising superconductivity. Furthermore, the ^{T}_{c} and ^{H}_{c2} can be tuned by up to 320 mK and 400 mT by applying a gate voltage. Our result indicates that MoTe_{2} flake is a good candidate for studying exotic superconductivity with nontrivial topological properties.

SPECIAL TOPIC—Recent advances in thermoelectric materials and devices

Based on the hybrid solutions to (2+1)-dimensional Kadomtsev-Petviashvili (KP) equation, the motion trajectory of the solutions to KP equation is further studied. We obtain trajectory equation of a single lump before and after collision with line, lump, and breather waves by approximating solutions of KP equation along some parallel orbits at infinity. We derive the mathematical expression of the phase change before and after the collision of a lump wave. At the same time, we give some collision plots to reveal the obvious phase change. Our method proposed to find the trajectory equation of a lump wave can be applied to other (2+1)-dimensional integrable equations. The results expand the understanding of lump, breather, and hybrid solutions in soliton theory.

To transform the exponential traveling wave solutions to bilinear differential equations, a sufficient and necessary condition is proposed. Motivated by the condition, we extend the results to the (2+1)-dimensional Kadomtsev-Petviashvili (KP) equation, the (3+1)-dimensional generalized Kadomtsev-Petviashvili (g-KP) equation, and the B-type Kadomtsev-Petviashvili (BKP) equation. Aa a result, we obtain some new resonant multiple wave solutions through the parameterization for wave numbers and frequencies via some linear combinations of exponential traveling waves. Finally, these new resonant type solutions can be displayed in graphs to illustrate the resonant behaviors of multiple wave solutions.

The dynamical behavior of a photon-added thermal state (PATS) in a thermal reservoir is investigated by virtue of Wigner function (WF) and Wigner logarithmic negativity (WLN), where this propagation model is abstracted as an input-output problem in a thermal-loss channel. The density operator of the output optical field at arbitrary time can be expressed in the integration form of the characteristics function of the input optical field. The exact analytical expression of WF is given, which is closely related to the Laguerre polynomial and is dependent on the evolution time and other interaction parameters (related with the initial field and the reservoir). Based on the WLN, we observe the dynamical evolution of the PATS in the thermal reservoir. It is shown that the thermal noise will make the PATS lose the non-Gaussianity.

There are some problems that quantum computers seem to be exponentially faster than classical computers, like factoring large numbers, machine learning, and simulation of quantum systems. Constructing an appropriate quantum algorithm becomes more important for solving these specific problems. In principle, any quantum algorithm can recast by a quantum random walk algorithm. Although quantum random walk with a few qubits has been implemented in a variety of systems, the experimental demonstration of solid-state quantum random walk remains elusive. Here we report the experimental implementation of the quantum continuous-time random walk algorithm by a two-qubit quantum processor in a nitrogen-vacancy center in diamond. We found that quantum random walk on a circle does not converge to any stationary distribution and exhibit a reversible property. Our results represent a further investigation of quantum walking dynamics in solid spin platforms, may also lead to other practical applications by the use of quantum continuous-time random walk for quantum algorithm design and quantum coherence transport.

We report a fabrication process and characterization of the Josephson parametric amplifier (JPA) for the single-shot quantum state measurement of superconducting multiqubit system. The device is prepared using Nb film as its base layer, which is convenient in the sample patterning process like e-beam lithography and film etching. Our results show that the JPA has a bandwidth up to 600 MHz with gain above 15 dB and noise temperature approaching the quantum limit. The qubit state differentiation measurements demonstrate the signal-to-noise ratio around 3 and the readout fidelity above 97% and 91% for the ground and first-excited states, respectively.

Effective Hamiltonian method is widely used in quantum information. We introduce a method to calculate effective Hamiltonians and give two examples in quantum information to demonstrate the method. We also give a relation between the effective Hamiltonian in the Shrödinger picture and the corresponding effective Hamiltonian in the interaction picture. Finally, we present a relation between our effective Hamiltonian method and the James-Jerke method which is currently used by many authors to calculate effective Hamiltonians in quantum information science.

The pre-Bötzinger complex (pre-BötC) in mammalian brainstem is essential for the generation of respiratory rhythms. Most dynamic studies on the pre-BötC neuron have been focused on its firing activities modulated by the ion conductances rather than that by the electromagnetic radiation or the external forcing current. In this paper, by adding the electromagnetic radiation and external forcing current to Park and Rubin's model, we mainly investigate the influences of those two factors on the mixed bursting (MB) of single pre-BötC neuron. First, we explore how the variation of external forcing current affects the MB patterns of the system with non-vanishing magnetic flux. We classify the MB patterns and show their dynamic mechanism through fast-slow decomposition and bifurcation analysis. Then, by modifying the feedback coefficient, we further analyze the sole effect of electromagnetic radiation on the firing activities of the system. Our results may be instructive in understanding the dynamical behavior of pre-BötC neuron.

A nuclear spin gyroscope based on an alkali-metal-noble-gas co-magnetometer operated in spin-exchange relaxation-free (SERF) regime is a promising atomic rotation sensor for its ultra-high fundamental sensitivity. However, the fluctuation of probe light intensity is one of the main technical error sources that limits the bias stability of the gyroscope. Here we propose a novel method to suppress the bias error induced by probe light intensity fluctuations. This method is based on the inherent magnetic field response characteristics of the gyroscope. By the application of a bias magnetic field, the gyroscope can be tuned to a working point where the output signal is insensitive to probe light intensity variation, referred to herein as ‘zero point’, thus the bias error induced by intensity fluctuations can be completely suppressed. The superiority of the method was verified on a K-Rb-^{21}Ne co-magnetometer, and a bias stability of approximately 0.01°/h was obtained. In addition, the method proposed here can remove the requirement of the closed-loop control of probe light intensity, thereby facilitating miniaturization of the gyroscope volume and improvement of reliability.

Magnetic coils for specific requirements are widely used in modern quantum physics. In this study, a general analytical method of designing the shielded coils for generating an arbitrary axial magnetic field is proposed. The theoretical formula for an axial magnetic field generated by a single shielded coil is obtained and used to construct specific coils. The structural parameters of these coils are determined by fitting the theoretical formula with their specific requirements. The feasibility of this method is proved by realizing four concrete kinds of coils:uniform magnetic field generating coils, gradient magnetic field generating coils, asymmetrical uniform magnetic field generating coils, and parabolic magnetic field generating coils. The correctness of these theoretical results is demonstrated by both the finite element simulations and the relevant experimental results. Furthermore, the application of this method is of great significance for developing the quantum physics and quantum devices in future.

It is important for environmental protection to search for catalysts with excellent performance and cost-effective to reduce SO_{2} by CO. In this work, using first-principles calculation, we have studied the catalytic performance of Au_{5}M^{n} (M=Ni, Pd, Pt, Cu, Ag, Au; n=1, 0, -1) clusters, and showed that, by giving a negative charge to the Au_{5}M (M=Cu, Ag, Au, Pd) clusters, we could improve the selectivity of SO_{2} and avoid effectively catalyst CO poisoning simultaneously. At the same time, the catalytic reaction rate for the reduction of SO_{2} by CO with Au_{5}M^{-} (M=Cu, Ag, Au, Pd) clusters is greatly improved when the Au_{5}M clusters are charged. These advantages can be well explained by the charge transfer between the clusters and adsorbed molecules, which means that we can effectively control the performance of the catalyst. The equilibrium structures of Au_{5}M^{n} (M=Ni, Pd, Pt, Cu, Ag, Au; n=1, 0, -1) clusters without or with adsorbed SO_{2} or CO molecule are also discussed, and the most stable geometrical structures of Au_{5}M^{n}-ML (ML=SO_{2}, CO, SO, and COS) can be explained very well by the match of orbitals symmetry and density of electron cloud through their frontier molecular orbitals. Considering the catalyst cost (Cu is much cheaper than Ag and Au), selectivity of SO_{2}, and effectively avoiding the catalyst CO poisoning, we propose that Au_{5}Cu^{-} is an ideal catalyst for getting rid of SO_{2} and CO simultaneously.

The molecular orientation created by laser fields is important for steering chemical reactions. In this paper, we propose a theoretical scheme to manipulate field-free molecular orientation by using an intense super-Gaussian laser pulse and a time-delayed terahertz half-cycle pulse (THz HCP). It is shown that the degree of field-free orientation can be doubled by the combined pulse with respect to the super-Gaussian pulse or THz HCP alone. Moreover, different laser intensities, carrier envelop phases, shape parameters, and time delays have great influence on the positive and negative orientations, with other conditions unchanged. Furthermore, it is indicated that the maximum degree and direction of molecular orientation can be precisely controlled by half of the duration of the super-Gaussian pulse. Finally, by adjusting the laser parameters of the super-Gaussian laser pulse and THz HCP, the optimal results of negative orientation and corresponding rotational populations are obtained at different temperatures of the molecular system.

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

A highly sensitive optical fiber temperature sensor based on a section of liquid-filled silica capillary tube (SCT) between single mode fibers is proposed. Two micro-holes are drilled on two sides of SCT directly by using femtosecond laser micromachining, and liquid polymer is filled into the SCT through the micro-holes without any air bubbles and then sealed by using ultra-violet (UV) cure adhesive. The sidewall of the SCT forms a Fabry-Perot resonator, and loss peaks are achieved in the transmission spectrum of the SCT at the resonant wavelength. The resonance condition can be influenced by the refractive index variation of the liquid polymer filled in SCT, which is sensitive to temperature due to its high thermo-optical coefficient (-2.98×10^{-4}℃^{-1}). The experimental result shows that the temperature sensitivity of the proposed fiber structure reaches 5.09 nm/℃ with a perfect linearity of 99.8%. In addition, it exhibits good repeatability and reliability in temperature sensing application.

We theoretically investigate the exact solutions for generalized parity-time(PT)-reversal-symmetric Rabi models driven by external fields with monochromatic periodic, linear, and parabolic forms, respectively. The corresponding exact solutions are presented in terms of the confluent Heun equations without any approximation. In principle, the analytic solutions derived here are valid in the whole parameter space. Such a kind of study may offer potential coherent control schemes of the PT-symmetric two-level systems.

We report on the generation of optical pulses with a nearly one octave-spanning spectrum ranging from 1300 nm to 2500 nm at 1 kHz repetition rate, which are based on intra-pulse difference frequency generation (DFG) in β-barium borate crystal (β-BBO) and passively carrier-envelope-phase (CEP) stabilized. The DFG is induced by few-cycle pulses initiated from spectral broadening in multiple thin plates driven by a Ti:sapphire chirped-pulse amplifier. Furthermore, a numerical simulation is developed to estimate the conversion efficiency and output spectrum of the DFG. Our results show that the pulses from the DFG have the potential for seeding intense mid-infrared (MIR) laser generation and amplification to study strong-field physics and attosecond science.

We consider a three-mode optomechanical system where two cavity modes are coupled to a common mechanical oscillator. We focus on the resolved sideband limit and illustrate the relation between the significant parameters of the system and the instantaneous-state mean phonon number of the oscillator cooled to the ground state, particularly at the early stage of the evolution. It is worth noting that the optical coupling sets up a correlation between the two cavity modes, which has significant effect on the cooling process. Using numerical solutions, we find that the inter-cavity coupling will decrease the cooling effect when both cavities have the same effective optomechanical coupling. However, when the effective optomechanical couplings are different, the cooling effect will be strongly improved by selecting appropriate range of inter-cavity coupling.

Atom-nanowire coupling system is a promising platform for optical quantum information processing. Unlike the previous designing of optical switch and transistor requiring a dedicated multi-level emitter and high fineness microcavity, a new proposal is put forward which contains a single two-level atom asymmetrically coupled with two nanowires. Single-emitter manipulation of photonic signals for bilateral coherent incident is clear now, since we specify atomic saturation nonlinearity into three contributions which brings us a new approach to realizing light-controlled-light at weak light and single-atom levels. An efficient optically controllable switch based on self-matching-induced-block and a concise optical transistor are proposed. Our findings show potential applications in full-optical devices.

A comparative study on the laser performance between bonding and non-bonding Er,Pr:GYSGG rods side-pumped by 970-nm laser diodes (LDs) is conducted for the thermal lensing compensation. The analyses of the thermal distribution and thermal focal length show that the bonding rod possesses a high cooling efficiency and weak thermal lensing effect compared with the conventional Er,Pr:GYSGG rod. Moreover, the laser characteristics of maximum output power, slope efficiency, and laser beam quality of the bonding rod with concave end-faces operated at 2.79 μm are improved under the high-repetition-rate operation. A maximum output power of 13.96 W is achieved at 150-Hz and 200-μs pulse width, corresponding to a slope efficiency of 17.7% and an electrical-to-optical efficiency of 12.9%. All results suggest that the combination of thermal bonding and concave end-face is a suitable structure for thermal lensing compensation.

We investigate the tunable bistable behavior of a hybrid nano-electro-optomechanical system (NEOMS) composed of S-shaped in the presence of two-level atoms, trapped inside a Fabry-Pérot cavity, and driven by a strong driving field and a weak probe field. The bistable behavior of the steady-state photon number and the mechanical steady-state positions are discussed. Further, we tune bistability by tuning all the coupling frequencies involved in the system and amplitude of the driving field. The present study provides the possibility of realization of a controllable optical switch depending on atom-field coupling, optomechanical coupling, electrostatic Coulomb coupling, and threshold power. In addition, we discuss that the non-linear effect of the hybrid NEOMS generates the four-wave mixing (FWM) process. Moreover, we show that the FWM process can be suppressed by the atom-field detuning and cavity-field detuning, which exhibits low photon transmission.

The goal of this article is to establish the conditions of excitation where one has to deal with ultrasound contrast agent (UCA) microbubbles pulsating near biological tissues with spherical boundary in ultrasound field for targeted drug delivery and cavitation-enhanced thrombolysis, etc., and contributes to understanding of mechanisms at play in such an interaction. A modified model is presented for describing microbubble dynamics near a spherical boundary (including convex boundary and concave boundary) with an arbitrary-sized aperture angle. The novelty of the model is such that an oscillating microbubble is influenced by an additional pressure produced by the sound reflection from the boundary wall. It is found that the amplitude of microbubble oscillation is positively correlated to the curve radius of the wall and negatively correlated to the aperture angle of the wall and the sound reflection coefficient. Moreover, the natural frequency of the microbubble oscillation for such a compliable wall increases with the wall compliance, but decreases with the reduction of the wall size, indicating distinct increase of the natural frequency compared to a common rigid wall. The proposed model may allow obtaining accurate information on the radiation force and signals that may be used to advantage in related as drug delivery and contrast agent imaging.

Ultrasonic inspection of austenitic steel weld is a great challenge due to skewed and distorted beam in such a highly anisotropic and inhomogeneous material. To improve the ultrasonic measurement in this situation, it is essential to have an in-depth understanding of ultrasound characteristics in austenitic steel weld. To meet such a need, in the present study we propose a method which combines the weld model, Dijkstra's path-finding algorithm and Gaussian beam equivalent point source model to calculate the acoustic fields from ultrasonic phased array in such a weld. With this method, the acoustic field in a steel-austenitic weld-steel three-layered structure for a linear phase array transducer is calculated and the propagation characteristics of ultrasound in weld are studied. The research results show that the method proposed here is capable of calculating the acoustic field in austenitic weld. Additionally, beam steering and focusing can be still realized in the austenitic steel weld and the beam distortion is more severe in the middle of weld than at other positions.

Liquid metal alloys (LMAs) are the potential candidates of thermal interface materials (TIMs) for electronics cooling. In the present work, buffer layers of Ag, Ti, Cu, Ni, Mo, and W were deposited on polished Cu plates by DC magnetron sputtering, the contact angles of de-ionized water and diiodomethane on the buffer layers were measured by an easy drop shape analyzer and the surface free energies (SFEs) of the buffer layers were calculated by the Owens-Wendt-Kaelble equation. Samples were prepared by sandwiching the filmed Cu plates and LMAs. The thermal properties of the samples were measured by laser flash analysis method. The SFE of the buffer layer has a strong influence on the interface heat transfer, whereas the measurement temperature has no obvious effect on the thermal properties of the samples. As the SFE of the buffer layer increases, the wettability, thermal diffusivity, and thermal conductivity are enhanced, and the thermal contact resistance is decreased.

A relaxation-rate formula is presented for the entropic lattice Boltzmann model (ELBM) – a discrete kinetic theory for hydrodynamics. The simple formula not only guarantees the discrete time H-theorem but also gives full consideration to the consistency with hydrodynamics. The relaxation rate calculated with the formula effectively characterizes the drastic changes of the flow fields. By using this formula, the computational cost of the ELBM is significantly reduced and the model now can be efficiently used for a broad range of applications including high Reynolds number flows.

At present, aero-engines face a major need to widen the ignition envelope. In order to provide a technical support to expand the high altitude ignition envelope of aero-engines, in this article we propose a novel ignition technology, i.e., “pre-combustion plasma jet ignition technology”. In this paper, we also design a pre-combustion plasma jet igniter. Its discharge characteristics, jet characteristics, and ignition effects are studied. The results show that increasing the equivalent ratio of jet gas can enhance the discharge stability and increase the duty cycle. At the same time, it can reduce working power and energy consumption. The increase of equivalent ratio in jet gas can enhance the length and ignition area of plasma jet. In the process of ignition, the pre-combustion plasma jet igniter has obvious advantages, suchn as shortening the ignition delay time and enlarging the ignition boundary. When the airflow velocity is 39.11 m/s and the inlet air temperature is 80℃, compared with the spark igniter and the air plasma jet igniter, the pre-combustion plasma jet igniter has an ignition boundary that is expanded by 319.8% and 55.7% respectively.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

In this paper, we present a highly efficient structure determination pipeline software suite (X^{2}DF) that is based on the “Parameter space screening” method, by combining the popular crystallographic structure determination programs and high-performance parallel computing. The phasing method employed in X^{2}DF is based on the single-wavelength anomalous diffraction (SAD) theory. In the X^{2}DF, the choice of crystallographic software, the input parameters to this software and the results display layout, are all parameters which users can select and screen automatically. Users may submit multiple structure determination jobs each time, and each job uses a slightly different set of input parameters or programs. Upon completion, the results of the calculation performed can be displayed, harvested, and analyzed by using the graphical user interface (GUI) of the system. We have applied the X^{2}DF successfully to many cases including the cases that the structure solutions fail to be yielded by using manual approaches.

Polycrystalline samples of La_{2}Zr_{2}O_{7} pyrochlore are irradiated by different energetic heavy ions to investigate the dependence of the vibrational mode variations on the irradiation parameters. The applied electronic energy loss (dE/dx)_{e} increases from about 5.2 keV/nm to 39.6 keV/nm. The ion fluence ranges from 1×10^{11} ions/cm^{2} to 6×10^{15} ions/cm^{2}. Vibrational modes of irradiated pyrochlore are analyzed by using Raman spectrum. Infrared active modes F_{1u} at 192, 308, and 651 cm^{-1} appear in Raman spectra, and the F_{2g} band at 265 cm^{-1} rises up due to the irradiation by 200-MeV Kr ions with (dE/dx)_{e} of 16.0 keV/nm. Differently, for the pyrochlore irradiated by 1750-MeV Bi ions with (dE/dx)_{e} of 39.6 keV/nm, in spite of the appearance of infrared active mode F_{1u} 651 cm^{-1}, the amorphous structure occurs according to the vibrational mode variations of pyrochlore irradiated at higher ion fluences. Amorphous tracks are observed in the samples, which confirm the occurrence of pyrochlore-amorphous transition in pyrochlore irradiated with (dE/dx)_{e} of 39.6 keV/nm.

A one-dimensional (1D) self-organized array composed of dislocation and anti-dislocation is analytically investigated in the frame of Peierls theory. From the exact solution of the Peierls equation, it is found that there exists strong neutralizing effect that makes the Burgers vector of each individual dislocation in the equilibrium array smaller than that of an isolated dislocation. This neutralizing effect is not negligible even though dislocations are well separated. For example, when the distance between the dislocation and the anti-dislocation is as large as ten times of the dislocation width, the actual Burgers vector is only about 80% of an isolated dislocation. The neutralizing effect originates physically from the power-law asymptotic behavior that enables two dislocations interfere even though they are well separated.

We employ multiple order parameters to analyze the local structure of liquid water obtained from all-atom simulations, and accordingly identify three types of molecules in water. In addition to the well-known low-density-liquid and high-density-liquid molecules, the newly identified third type possesses an ultra-high density and over-coordinated H-bonds. The existence of this third type decreases the probability of transition of high-density-liquid molecules to low-density-liquid molecules and increases the probability of the reverse one.

Order-disorder phase transitions for CH_{3}NH_{3}PbCl_{3} are studied with density functional theory. Our calculations show that the disorder is manifested in two aspects in the cubic phase, namely, the disorder of orientation and rotation of organic groups. Organic groups of[CH_{3}] and[NH_{3}] in cubic crystals can easily rotate around its C_{3} axis. At the same time,[CH_{3}NH_{3}]^{+} organic groups can also orient to different spatial directions due to the weak interactions between organic group and inorganic frame. Our results show that its possible phase transition path starts from the deviation of organic groups from the crystal c-axis. Its structural transition changes from disordered cubic phase to hydrogen-only disordered tetragonal structure in the process of decreasing symmetry. The disordered high temperature cubic phase can be expressed as a statistical average of substructures we rebuilt. The electrostatic repulsive force between adjacent organic groups triggers out the formation of low temperature phase on cooling.

The carbon diffusivity in tungsten is one fundamental and essential factor in the application of tungsten as plasma-facing materials for fusion reactors and substrates for diamond growth. However, data on this are quite scarce and largely scattered. We perform a series of first-principles calculations to predict the diffusion parameters of carbon in tungsten, and evaluate the effect of temperature on them by introducing lattice expansion and phonon vibration. The carbon atom prefers to occupy octahedral interstitial site rather than tetrahedral interstitial site, and the minimum energy path for its diffusion goes through a tetrahedral site. The temperature has little effect on the pre-exponential factor but a marked effect on the activation energy, which linearly increases with the temperature. Our predicted results are well consistent with the experimental data obtained at high temperature (>1800 K) but significantly larger than the experimental results at low temperature (<1800 K).

Density functional theory calculations are carried out to identify various configurations of oxygen molecules adsorbed on the Au-doped RuO_{2} (110) surface. The binding energy calculations indicate that O_{2} molecules are chemically adsorbed on the coordinatively unsaturated Ru (Ru_{cus}) sites and the bridge oxygen vacancies on the Au sites. Transition state calculations show that O^{*} can exist on the Ru_{cus} site by O_{2}^{*} dissociation and diffusion. The calculations of the reaction path of CO indicate that the reaction energy barrier of CO adsorbed on Au with lattice oxygen decreases to 0.28 eV and requires less energy than that on the undoped structure.

Using first-principles calculations in the generalized gradient approximation plus on-site Coulomb interaction (GGA+U) scheme, the effects of internal structural parameters x and U on the electronic structure of YbB_{6} are investigated. The results show that the band gap of YbB_{6} increases with x increasing, and does not change with U. It not only illustrates the influence of internal structural parameter x on band gap, but also explains the discrepancy between the previous experimental result and the theoretical prediction. In addition, the electronic structure and density of states reveal that there exist the interactions between B atoms in different cages, and that a small band gap can form around the Fermi level (E_{F}). The present work plays a leading role in ascertaining the relation between crystal structure and electronic property for the further analysis of its topological properties.

The band structure, magnetism, charge distribution, and optics parameters of TMO_{3}-h-BN hybrid systems are investigated by adopting first-principles study (FPS) calculations. It is observed that the TMO_{3} clusters add finite magnetic moments to bilayer h-BN (BL/h-BN), thereby making it a magnetic two-dimensional (2D) material. Spin-polarized band structures for various TMO_{3}-BL/h-BN hybrid models are calculated. After the incorporation of TMO_{3}, BL/h-BN is converted into semimetal or conducting material in spin up/down bands, depending on the type of impurity cluster present in BL/h-BN lattice. Optics parameters are also investigated for the TMO_{3}-BL/h-BN complex systems. The incorporation of TMO_{3} clusters modifies the absorption and extinction coefficient in visible range, while static reflectivity and refraction parameter increase. It can be surmised that the TMO_{3} substitution in BL/h-BN is a suitable technique to modify its physical parameters thus making it functional for nano/opto-electronic applications, and an experimental approach can be adapted to reinforce the outcomes of this study.

Reflection and transmission are two behaviors of wave propagating to an interface. The immiscible binary mixtures of Bose-Einstein condensates can form the symmetry-breaking state, in which the domain wall on the center can serve as the interface. In this study, we explore in detail the propagation of a dark soliton interacting with the domain wall in the harmonic trap. We find that the low-energy dark soliton is easy to form the transmission and the high-energy dark soliton trends to reflect from the domain wall. Both reflection and transmission of dark soliton on the domain wall induce the sound radiation. But the sound radiation in the reflection derives from the collective oscillation of condensates, and it in the transmission comes not only from the collective oscillation, but also from the condensate filling in the dark soliton.

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

The polarization effect introduced by electric field deformation is the most important bottleneck of CdZnTe detector in x-ray imaging. Currently, most of studies focus on electric field deformation caused by trapped carriers; the perturbation of electric field due to drifting carriers has been rarely reported. In this study, the effect of transient space-charge perturbation on carrier transport in a CdZnTe semiconductor is evaluated by using the laser-beam-induced current (LBIC) technique. Cusps appear in the current curves of CdZnTe detectors with different carrier transport performances under intense excitation, indicating the deformation of electric field. The current signals under different excitations are compared. The results suggest that with the increase of excitation, the amplitude of cusp increases and the electron transient time gradually decreases. The distortion in electric field is independent of carrier transport performance of detector. Transient space-charge perturbation is responsible for the pulse shape and affects the carrier transport process.

Previous studies presented the phase diagram induced by the disorder existing separately either in the higher-order topological states or in the topological trivial states, respectively. However, the influence of disorder on the system with the coexistence of the higher-order topological states and other traditional topological states has not been investigated. In this paper, we investigate the disorder induced phase transition in the magnetic higher-order topological insulator. By using the convolutional neural network and non-commutative geometry methods, two independent phase diagrams are calculated. With the comparison between these two diagrams, a topological transition from the normal insulator to the Chern insulator is confirmed. Furthermore, the network based on eigenstate wavefunction studies also presents a transition between the higher-order topological insulator and the Chern insulator.

Hysteresis current-voltage (I-V) characteristics are often observed in a highly non-ideal (n>2) as-deposited nickel (Ni)/4H-SiC Schottky contact. However, we find that this kind of hysteresis effect also exists in an as-deposited Ni/n-type 4H-SiC Schottky structure even if the ideality factor (n) is less than 1.2. The hysteresis I-V characteristics is studied in detail in this paper by using the various measurements including the hysteresis I-V, sequential I-V sweeping, cycle I-V, constant reverse voltage stress (CRVS). The results show that the hysteresis I-V characteristics are strongly dependent on the sweeping voltage and post-deposition annealing (PDA). The high temperature PDA (800℃) can completely eliminate this hysteresis. Meanwhile, the magnitude of the hysteresis effect is shown to decrease in the sequential I-V sweeping measurement, which is attributed to the fact that the electrons tunnel from the 4H-SiC to the localized states at the Ni/n-type 4H-SiC interface. It is found that the application of the reverse bias stress has little effect on the emission of those trapped electrons. And a fraction of the trapped electrons will be gradually released with the time under the condition of air and with no bias. The possible physical charging mechanism of the interface traps is discussed on the basis of the experimental findings.

We study ABA trilayer graphene under irradiation of a circularly polarized light. In high-frequency regime, the effective low-energy Hamiltonian is obtained based on the Floquet theory. With increasing circularly polarized light intensity, the band structure shows a band gap closing and reopening, which happen twice. The process of the band gap closing and reopening is accompanied with a topological phase transition. We investigate the Chern numbers and the anomalous Hall conductivities to confirm the topological phase transition. The interplay between light-induced circularity-dependent effective potential and effective sublattice potential is discussed.

We consider a highly unconventional superconducting state with chiral d-wave symmetry in doped graphene under strain with the Gutzwiller-RVB method in the momentum space. It is shown that flat bands emerge in the normal state for reasonable strain. As a result, the superconducting critical temperature is found to be linearly proportional to the strength of the electron-electron interaction. Furthermore, the chiral d-wave superconducting state is shown with coexistence of the charge density wave and the pair density wave. There are different coexisting states with those orders under different doping levels.

The crystal structures, magnetization, and spontaneous magnetostriction of ferromagnetic Laves phase Pr_{1-x}Tb_{x}Fe_{1.9} compounds are investigated in a temperature range between 5 K and 300 K. High resolution synchrotron x-ray diffraction (XRD) analysis shows that different proportions of Tb in Pr_{1-x}Tb_{x}Fe_{1.9} alloys can result in different easy magnetization directions (EMD) below 70 K, i.e.,[100] with x=0.0, and[111] with x ≥ 0.1. This indicates Tb substitution can lead the EMD to change from[100] to[111] with x rising from 0.0 up to 0.1. The Tb substitution for Pr reduces the saturation magnetization M_{s} and the magnetostriction to their minimum value when x=0.6, but it can increase low-field (0 ≤ H ≤ 9 kOe, the unit 1 Oe=79.5775 A·m^{-1}) magnetostriction when x=0.8 and 1.0 at 5 K. This can be attributed to the larger magnetostriction of PrFe_{1.9} than that of TbFe_{1.9}, as well as the decrease of the resulting anisotropy due to Tb substitution at low temperatures.

Atomic spin relaxation in a vapor cell, which can be characterized by the magnetic resonance linewidth (MRL), is an important parameter that eventually determines the sensitivity of an atomic magnetometer. In this paper, we have extensively studied how the pump intensity affects the spin relaxation. The experiment is performed with a cesium vapor cell, and the influence of the pump intensity on MRL is measured at room temperature at zero-field resonance. A simple model with five atomic levels of a Λ-like configuration is discussed theoretically, which can be used to represent the experimental process approximately, and the experimental results can be explained to some extent. Both the experimental and the theoretical results show a nonlinear broadening of the MRL when the pump intensity is increasing. The work helps to understand the mechanism of pump induced atomic spin relaxation in the atomic magnetometers.

The organic-inorganic hybrid (C_{2}H_{5}NH_{3})_{2}CuCl_{4} (EA_{2}CuCl_{4}) single crystals are prepared by the solvothermal condition method. The x-ray diffraction, scanning electron microscopy, dielectric permittivity, pyroelectric current, and heat capacity are used to systematically investigate the electrocaloric performances of EA_{2}CuCl_{4}. The pyroelectric currents are measured under various voltages, and the electrocaloric effect (ECE) is calculated. Its ECE exhibits an isothermal entropy change of 0.0028 J/kg·K under an electric field of 30 kV/cm associated with a relatively broad temperature span. Further, the maximum pyroelectric coefficient (p) is 4×10^{-3} C/m^{2}·K and the coefficient β for generating ECE from electric displacement D is 1.068×10^{8} J·cm·K^{-1}·C^{-2} at 240 K. Our results indicate that the ECE behavior of organic-inorganic hybrid EA_{2}CuCl_{4} is in accordance with Jona and Shirane's opinion in which the ECE should occur both below and above the Curie temperature T_{c}.

Both long-term environmental durability and high reflectance of protected-Al mirrors are of great importance for developing the optical instruments in the vacuum ultraviolet (VUV) applications. In this paper, the dependence of spectral property and environmental durability of MgF_{2} over-coated Al mirrors using a 3-step method on deposition temperature of the outermost MgF_{2} layer are investigated in detail. Optics (reflectance), structure (surface morphology and crystalline), and environmental durability (humidity test) are characterized and discussed. The results show that both optical and moisture-resistant properties of MgF_{2} over-coated Al mirrors are dependent on MgF_{2} deposition temperature, and the optimal deposition temperature for the outermost MgF_{2} layer should be between 250℃ and 300℃ for MgF_{2} over-coated Al mirrors to have both reasonably high reflectance in the VUV spectral range and high moisture resistance for long lifetime applications.

Graphene and transition metal dichalcogenides (TMDs), two-dimensional materials, have been investigated wildely in recent years. As a member of the TMD family, MoTe_{2} possesses a suitable bandgap of~1.0 eV for near infrared (NIR) photodetection. Here we stack the MoTe_{2} flake with two graphene flakes of high carrier mobility to form a graphene-MoTe_{2}-graphene heterostructure. It exhibits high photo-response to a broad spectrum range from 500 nm to 1300 nm. The photoresponsivity is calculated to be 1.6 A/W for the 750-nm light under 2 V/0 V drain-source/gate bias, and 154 mA/W for the 1100-nm light under 0.5 V/60 V drain-source/gate bias. Besides, the polarity of the photocurrent under zero V_{ds} can be efficiently tuned by the back gate voltage to satisfy different applications. Finally, we fabricate a vertical graphene-MoTe_{2}-graphene heterostructure which shows improved photoresponsivity of 3.3 A/W to visible light.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

As an important member of the two-dimensional layers of metal dichalcogenides family, the two-dimensional (2D) group IV metal chalcogenides (GIVMCs) have been attracting intensive attention. However, the growth of monolayer tin disulfide (SnS_{2}) remains a great challenge contrasted to transition metal dichalcogenides, which have been studied quite maturely. Till date, there have been scant reports on the growth of large-scale and large-size monolayer SnS_{2}. Here, we successfully synthesized monolayer SnS_{2} crystal on SiO_{2}/Si substrates via NaCl-assisted CVD and the edge can be as long as 80 μm. Optical microscope, Raman spectroscopy, x-ray diffraction, atomic force microscopy (AFM), and energy-dispersion x-ray (EDX) were performed respectively to investigate the morphology, crystallographic structure, and optical property of the 2D SnS_{2} nanosheets. In addition, we discussed the growing mechanism of the NaCl-assisted CVD method.

The InAs/AlSb heterostructures with step-graded GaAs_{x}Sb_{1-x} metamorphic buffer layers grown on Si substrates by molecular beam epitaxy are studied. The step-graded GaAs_{x}Sb_{1-x} metamorphic buffer layers are used to relax the strain and block defects at each interface of the layers. Meanwhile, adding Sb to GaAs is also beneficial to suppressing the formation of dislocations in the subsequent materials. The influences of the growth temperature of the step-graded GaAs_{x}Sb_{1-x} metamorphic buffer layer on the electron mobility and surface topography are investigated for a series of samples. Based on the atomic force microscopy (AFM), high resolution x-ray diffraction (HRXRD), reciprocal space map (RSM), and Hall measurements, the crystal quality and composition of GaAs_{x}Sb_{1-x} layer are seen to strongly depend on growth temperature while keeping the Ga growth rate and V/III ratio constant. The results show that the highest electron mobility is 10270 cm^{2}/V·s and the roughness is 4.3 nm for the step-graded GaAs_{x}Sb_{1-x} metamorphic buffer layer grown at a temperature of 410℃.

This study presents a systematic review of the literature pertaining to dye-sensitized solar cells (DSSCs), in order to anticipate the direction and speed of change in technology trend. To study the general progression in DSSC research, we have assessed the evolution in annual DSSCs publications and their citations. Further, in order to identify the intellectual bases, we have also classified the journals, authors, institutes, and countries according to their scientific productivity in the field of DSSCs research during the period of 2007-2017.

The applications of magnetrons are greatly limited because of the poor output spectrum of the free-running magnetron. Currently, one of the best ways to solve this problem is injection locking. However, the injection locking theory which is widely used nowadays is based on the simplified oscillator, which does not include the frequency pushing effect of the magnetron. In this paper, the theory of injection locking magnetrons with frequency pushing effect is systematically studied. Analytical analysis shows that the locking bandwidth turns larger with the consideration of the pushing parameter (α), and the increase of locking bandwidth is expanded with α increasing. Experimental results show that the locking bandwidth is expanded by 0.3 MHz, 1 MHz, and 1.6 MHz compared with the locking bandwidth from the conventional locking theory under an injection ratio (ρ) of 0.05, 0.075, and 0.1, respectively. This research provides a more accurate prediction of the properties of the injection-locked magnetron.

We report properties of contact resistances observed on pentacene organic field-effect transistors (OFET) with four different source/drain electrodes, namely, copper (Cu), gold (Au), silver (Ag), and germanium (Ge). The metals were selected to provide a wide range of energy barriers for charge injection, from blocking contact to smooth injection. All OFETs exhibited strong voltage dependence of the contact resistance, even for devices with smooth injection, which is in strong disagreement with the definition of ohmic contacts. A comparison with current crowding, resistive network, Fowler-Nordheim tunneling, and electric field enhanced thermionic injection (Schottky emission) pointed to importance of local electric fields and/or electrostatic field charges.

Antenna-coupled field-effect-transistors (FETs) offer high sensitivity for terahertz detection. Both the magnitude and the polarity of the response signal are sensitive to the localized terahertz field under the gate. The ability of accurate sensing the intensity pattern is required for terahertz imaging systems. Here, we report artefacts in the intensity pattern of a focused terahertz beam around 1 THz by scanning a silicon-lens and antenna coupled AlGaN/GaN high-electron-mobility-transistor (HEMT) detector. The origin of the image distortion is found to be connected with one of the antenna blocks by probing the localized photocurrents as a function of the beam location and the frequency. Although the exact distortion is found with our specific antenna design, we believe similar artefacts could be commonplace in antenna-coupled FET terahertz detectors when the beam spot becomes comparable with the antenna size. To eliminate such artefacts, new antenna designs are welcomed to achieve strong asymmetry in the terahertz field distribution under the gate while maintaining a more symmetric radiation pattern for the whole antenna.

A type of photonic crystal fiber based on Kagome lattice cladding and slot air holes in a rectangular core is investigated. Full vector finite element method is used to evaluate the modal and propagation properties of the designed fiber. High birefringence of 0.089 and low effective material loss of 0.055 cm^{-1} are obtained at 1 THz. The y-polarized fundamental mode of the designed fiber shows a flattened and near-zero dispersion of 0±0.45 ps·THz^{-1}·cm^{-1} within a broad frequency range (0.5 THz-1.5 THz). Our results provide the theory basis for applications of the designed fiber in terahertz polarization maintaining systems.

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