We study the solution to the Fokker–Planck equation with piecewise-constant drift, taking the case with two jumps in the drift as an example. The solution in Laplace space can be expressed in closed analytic form, and its inverse can be obtained conveniently using some numerical inversion methods. The results obtained by numerical inversion can be regarded as exact solutions, enabling us to demonstrate the validity of some numerical methods for solving the Fokker–Planck equation. In particular, we use the solved problem as a benchmark example for demonstrating the fifth-order convergence rate of the finite difference scheme proposed previously [Chen Y and Deng X Phys. Rev. E 100 (2019) 053303].

One of the appealing features of topological systems is the presence of robust edge modes. Under a sudden quantum quench, the edge modes survive for a characteristic time that scales with the system size, during which the nontrivial topology continues to manifest in entanglement properties, even though the post-quench Hamiltonian belongs to a trivial phase. We exemplify this in the quench dynamics of a two-dimensional Haldane model with the help of one-particle entanglement spectrum and the probability density of its mid-states. We find that, beyond our knowledge in one-dimensional models, the momentum dependence of the transverse velocity plays a crucial role in the out-of-equilibrium evolution of the entanglement properties.

Excitation energy transfer (EET) plays a vital role in many areas of physics and biology processes. Here we address the role of quantum-jump-based feedback control in the efficiency of EET through a chain model. Usually, the decoherence caused by dissipative noise is detrimental to the transfer efficiency. We demonstrate that feedback control can always enhance the efficiency of EET and the dependence of different feedback controls is also discussed in detail. In addition, we investigate the strategy to enhance the efficiency of EET in the Fenna–Matthews–Olson complex as a prototype for larger photosynthetic energy transfer systems.

We numerically investigate the effects of ion-to-electron temperature ratio $T_{\rm i}/T_{\rm e}$ and temperature gradient ratio $\eta_{\rm i}/\eta_{\rm e}$ on resistive ballooning modes (RBMs) under tokamak edge plasma conditions. The results show that the growth rates of the RBMs exhibit the characteristic of a quite broad poloidal wavenumber spectrum in the cold ion limit. The growth rate spectrum becomes narrower and the peak of the spectrum shifts from the short to long wavelength side with increasing $T_{\rm i}/T_{\rm e}$ and $\eta_{\rm i}/\eta_{\rm e}$. The electron temperature gradient has a very weak effect on the stability of RBMs. However, the ion-to-electron temperature ratio and the temperature gradient ratio have strong stabilizing effects on short-wavelength RBMs, while they have relatively weak effects on long-wavelength RBMs.

As an important electromagnetic field in experiment, Gaussian beams have non-vanishing longitudinal electric and magnetic components that generate significant energy fluxes on transverse directions. We focus on the transverse energy flux and derive the theoretical propagation properties. Unlike the longitudinal energy flux, the transverse energy flux has many unique physical behaviors, such as the odd symmetry on propagation, slower decay rate on resonant condition. By means of the characteristics of transverse energy flux, it is feasible to find the suitable regions where the information of coherent lights could be extracted exactly. With the typical laser parameters, we simulate the energy fluxes on receiver surface and analyze the corresponding distribution for the coherent light beams. Especially for coherent lights, the transverse energy flux on the $y$–$z$ plane with $x=0$ and $x$–$z$ plane with $y=0$, contains pure coherent information. Meanwhile, in the transverse distance $|y| < 2W_{0}$ ($W_{0}$ is the waist radius) and $|x| < W_{0}/3$ the coherent information could also be extracted appropriately.

We report 10 Gb/s data transmissions using a packaged two-section InGaAsP/InP distributed Bragg reflector (DBR) laser. The tunable DBR laser has a wavelength tuning range of 12.12 nm. The DBR laser has greater than 10.84 GHz 3-dB direct modulation bandwidth within the wavelength tuning range. The 10 Gb/s data transmissions are performed at up to a distance of 30-km.

We demonstrate a multi-branch all polarization-maintaining Er:fiber frequency comb with five application ports for precise measurement of atomic/molecular transition frequencies in the near-infrared region. A fully stabilized Er:fiber frequency comb with a nonlinear amplifying loop mirror is achieved. The in-loop relative instability of stabilized carrier-envelope-offset frequency is $5.6\times 10^{-18}$ at 1 s integration time, while that of the repetition rate is well below $1.8\times 10^{-12}$ limited by the measurement noise floor of the commercial frequency counter. Five application ports are individually optimized for applications with different wavelengths (1064 nm, 1083 nm, 1380 nm, 1637 nm and 1750 nm). The beat note between the optical frequency comb and continuous laser exhibits the signal-to-noise ratio of at least 30 dB at a resolution bandwidth of 100 kHz. The in-loop frequency instability of the comb is evaluated to be good enough for measurement of rotation-resolved transitions of molecules below 1 kHz resolution.

We report a continuous-wave end-pumped Nd:YVO$_{4}$ zigzag slab laser with multilayer amplified spontaneous emission (ASE) absorbing coatings. The coatings are deposited on the slab faces. A five-layer structure consists of SiO$_{2}$-Ti-SiO$_{2}$-Ti-Au, and the thicknesses are 2520 nm, 10 nm, 160 nm, 24 nm and 200 nm, respectively. The designed coatings show good performance for the ASE control in the experimental tests. A stable-unstable hybrid laser oscillator along orthogonal directions in the slab aperture is further configured, achieving the 117 W output at a pump of 328 W. The beam quality factors $M^{2}$ in the unstable direction and stable direction are 1.57 and 1.66, respectively.

We propose a switchable THz metamaterial that can be switched between two functions of half-wave plate and quarter-wave plate. The two switchable functions can be simply achieved by inserting a VO$_{2}$ film in the metamaterial design. Finite-difference time-domain (FDTD) simulation results show that the proposed metamaterial can convert $x$-polarized incident wave to $y$-polarized reflected wave when VO$_{2}$ is at metal phase, and convert $x$-polarized wave to circularly polarized wave when VO$_{2}$ is at insulator phase. The metamaterial performs well in the two functions, i.e., the same broad working frequency band and near perfect polarization conversion. The switching effect originates from the switchable Fabry–Pérot cavity length induced by the phase change of VO$_{2}$. We believe that our findings provide a reference in designing switchable metamaterials.

Without prior knowledge of medium parameters, a method is proposed to detect and locate a target in layered media. Experiments were carried out for liquid/liquid and solid/liquid layered media, and the location of a target in them was obtained using three methods combined, i.e., the least-square method, the method of finding minimum dispersal degree of target distribution, and the snapshot time reversal and reverse time migration mixed method. The medium parameters, i.e., the acoustic velocities of upper and lower media as well as the thickness of the upper medium, were inversed simultaneously. The results show that the position of target is consistent with its actual position. Thus, the detection and location of a target in layered media are achieved without prior knowledge of medium parameters, and it overcomes the difficulty that the common time reversal method only detects the target, but cannot locate it.

The terahertz (THz) temporal waveform and spectrum from a longitudinal electrically biased femtosecond filament is studied experimentally. The initial direction of the electron motion inside the unbiased filament plasma is deduced from the transformation of the THz temporal waveform with applied fields of opposite polarities. Furthermore, a spectrum shift to lower frequency of the THz spectrum is observed in the presence of a biased field. It agrees well with theoretical predictions.

Silane (SiH$_{4}$) is a promising hydrogen-rich compound for pursing high temperature superconducting. Previous high pressure measurements of Raman, x-ray diffraction and theoretical studies on SiH$_{4}$ mainly focused on its polymorphic structures above 50 GPa, while the structure and the stability under lower pressure range are still unclear. Here we report an investigation of condensed SiH$_{4}$ by Brillouin scattering measurements at high temperature up to 407 K and high pressure up to 18 GPa. Brillouin scattering frequencies of fluid SiH$_{4}$ under compression are obtained under isothermal conditions of 300 K, 359 K and 407 K. The SiH$_{4}$ becomes unstable with increasing temperature and subsequently decomposes into silicon and H$_{2}$. We find that compression at room temperature induces two velocity anomalies corresponding to a fluid-solid state transition and a phase IV to phase V transition, respectively. Brillouin scattering spectra has been a powerful tool to investigate the fruitful phases and structure transitions in the hydrogen-rich compound under extreme conditions.

Polycrystalline BAs$_{x}$ ($x = 0.80$–1.10) compounds with different boron-to-arsenic elemental molar ratios were synthesized by a high-pressure and high-temperature sintering method. Compared with other ambient-pressure synthesis methods, high pressure can significantly promote the reaction speed as well as the reaction yield. As the content of arsenic increases from $x = 0.91$ to 1.10, the thermal conductivity of BAs$_{x}$ gradually increases from 53 to 65 W$\cdot$m$^{-1}\cdot$K$^{-1}$. Furthermore, the temperature dependence of thermal conductivities of these samples reveals an Umklapp scattering due to the increasing phonon population. This work provides a highly efficient method for polycrystalline BAs synthesis.

Recently, natural van der Waals heterostructures of (MnBi$_{2}$Te$_{4}$)$_{m}$(Bi$_{2}$Te$_{3}$)$_{n}$ have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states. We systematically investigate both the structural and electronic responses of MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ to external pressure. In addition to the suppression of antiferromagnetic order, MnBi$_{2}$Te$_{4}$ is found to undergo a metal–semiconductor–metal transition upon compression. The resistivity of MnBi$_{4}$Te$_{7}$ changes dramatically under high pressure and a non-monotonic evolution of $\rho (T)$ is observed. The nontrivial topology is proved to persist before the structural phase transition observed in the high-pressure regime. We find that the bulk and surface states respond differently to pressure, which is consistent with the non-monotonic change of the resistivity. Interestingly, a pressure-induced amorphous state is observed in MnBi$_{2}$Te$_{4}$, while two high-pressure phase transitions are revealed in MnBi$_{4}$Te$_{7}$. Our combined theoretical and experimental research establishes MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ as highly tunable magnetic topological insulators, in which phase transitions and new ground states emerge upon compression.

Surface terminations of diamond play an important role in determining the electric properties of diamond-based electronic devices. We report an ultraviolet/ozone (UV/ozone) treatment process on hydrogen-terminated single crystal diamond (H-diamond) to modulate the carrier behavior related to varying oxygen adsorption on surfaces. By UV/ozone treatments, the induced oxygen radicals are chemically adsorbed on the H-terminated diamond and replace the original adsorbed H, which is analyzed by x-ray photoelectron spectroscopy. The concentration of oxygen adsorbed on surface increases from $\sim$3% to $\sim$8% with increasing the ozone treatment time from 20 s to 600 s. It is further confirmed by examining the wettability properties of the varying diamond surfaces, where the hydrophobic for H-termination transfers to hydrophilic for partly O-termination. Hall effect measurements show that the resistance (hole mobility) of the UV/ozone-treated H-diamond continuously increases (decrease) by two orders of magnitude with increasing UV/ozone treatment time from 20 s to 600 s. The results reveal that UV/ozone treatment becomes an efficient method to modulate the surface electrical properties of H-diamonds for further investigating the oxygenation effect on two-dimensional hole gas based diamond devices applied in some extreme environments.

The hydrogenation of two-dimensional (2D) systems can efficiently modify the physical and chemical properties of materials. Here we report a systematic study on the hydrogenation of 2D semiconductor Sn$_{2}$Bi on Si(111) by scanning tunneling microscopy experiments and first principle calculations. The unique butterfly-like and trench-like features were observed for single H adsorption sites and hydrogen-saturated surfaces respectively, from which the bridge-site adsorption geometry can be unambiguously determined. The structural model was further confirmed by the theoretical calculations, which is in good agreement with the experimental observation. In addition, the hydrogenation is found to vanish the flat band of Sn$_{2}$Bi and increase the band gap obviously.

We theoretically and experimentally show that, with water being adsorbed, the graphene oxide (GO) is converted to a spontaneously dynamic covalent material under ambient conditions, where the dominated epoxy and hydroxyl groups are mediated by water molecules to spontaneously break/reform their C–O bonds to achieve dynamic oxygen migration. This dynamic material presents structural adaptivity for response to biomolecule adsorption. Both density functional theory calculations and ab initio molecular dynamics simulations demonstrate that this spontaneously dynamic characteristics is attributed to the adsorption of water molecules, which sharply reduces the barriers of these oxygen migration reactions on GO to the level less than or comparable to the hydrogen bonding energy in liquid water.

Time-periodic laser driving can create nonequilibrium states not accessible in equilibrium, opening new regimes in materials engineering and topological phase transitions. We report that black phosphorus (BP) exhibits spatially nonuniform topological Floquet–Dirac states under laser illumination, mimicking the "gravity" felt by fermionic quasiparticles in the same way as that for a Schwarzschild black hole (SBH). Quantum tunneling of electrons from a type-II Dirac cone (inside BH) to a type-I Dirac cone (outside BH) emits an SBH-like Planck radiation spectrum. The Hawking temperature $T_{\rm H}$ obtained for a fermionic analog of BH in the bilayer BP is approximately 3 K, which is several orders of magnitude higher than that in previous works. Our work sheds light on increasing $T_{\rm H}$ from the perspective of engineering 2D materials by time-periodic light illumination. The predicted SBH-like Hawking radiation, accessible in BP thin films, provides clues to probe analogous astrophysical phenomena in solids.

We use femtosecond time-resolved optical reflectivity to study the photoexcited quasiparticle (QP) dynamics in the iron-based 112 type superconducting (SC) samples Ca$_{0.82}$La$_{0.18}$Fe$_{1-x}$Ni$_{x}$As$_{2}$, with $x = 0$ and 0.024. In the parent sample, a fast and a slow relaxation emerge at temperatures below the magnetic-structure (MS) transition $T_{\rm ms} \approx 50$ K and the SC transition $T_{\rm c} \approx 33$ K, respectively. The latter obviously corresponds to an SC QP dynamics, which is further confirmed in the $x = 0.024$ sample with $T_{\rm c} \approx 25$ K. The former suggests that a partial of photoexcited QP relaxation through a pesudogap (PG) channel, which is absent in the doped $x = 0.024$ sample without the MS transition.

We study the influence of Cr doping on magnetic properties of $\alpha$-RuCl$_{3}$ single crystals in detail. With increasing Cr content, the $c$-axial lattice parameter increases gradually, implying that the Cr doping may weaken the interlayer interactions. The magnetism of Ru$_{1-x}$Cr$_{x}$Cl$_{3}$ single crystals evolves from a long-range AFM order to a possible spin-glass state with Cr doping. The appearance of a possible spin-glass state can be explained by the introduction of FM interaction by Cr$^{3+}$ ions, which competes with the AFM interaction between Ru$^{3+}$ ions. Moreover, the larger magnetic moment of Cr$^{3+}$ ion with $S= 3/2$ than Ru$^{3+}$ ion with $J_{\rm eff}= 1/2$ also results in a monotonic increase of the effective moment of Ru$_{1-x}$Cr$_{x}$Cl$_{3}$ single crystal.

Equipped with multiple and unique features, a terahertz absorber exhibits great potential for use in the development of communication, military, and other fields where achieving perfect broadband absorption has always been a challenge. We present a metamaterial terahertz (THz) absorber comprising a cross-dipole patch, four symmetric square patches and an asymmetric open-loop patch with a good perfect absorption rate for TE and TM polarizations. The average absorption of more than 96% occurs in the frequency range from 2.4 THz to 3.8 THz, in which the absorptance peak can reach 99.9%, as indicated by simulated results. Our design has broad potential applications in THz couplers, as well as in fields like biology and security.

A simple one-dimensional subwavelength plasmonic grating can support symmetry protected bound states in the continuum (BICs), but not necessarily for the non-symmetry protected BICs. By dimerizing the lattice, non-symmetry protected BIC can be supported on the dimerized grating and can be tuned readily. The mechanism for the BICs in the dimerized grating is interpreted in the viewpoint of interference between the electromagnetic multipoles.

A new extended solid nitrogen, referred to as post-layered-polymeric nitrogen (PLP-N, or Panda-N), was observed by further heating the layered-polymeric nitrogen (LP-N) to above 2300 K at 161 GPa. The new phase is found to be very optically transparent and exhibits ultra-large $d$-spacings ranging from 2.8 to 4.9 Å at 172 GPa, suggesting a lower-symmetry large-unit-cell 2D chain-like or 0D cluster-type structure with wide bandgap. However, the observed x-ray diffraction pattern and Raman scattering data cannot match any predicted structures in the published literature. This finding further complicates the phase diagram of nitrogen and also highlights the path dependence of the high-pressure dissociative transition in nitrogen. In addition, the phase transition from cubic gauche nitrogen (cg-N) to LP-N is observed at 157 GPa and 2000 K.

By vacuum sputtering and annealing processes of gold (Au) films on boron-doped diamond (BDD) surfaces, Au-nanoparticles/BDD (AuNP/BDD) composite substrates were prepared as surface-enhanced Raman scattering (SERS) substrates. The SERS performances of the substrate were investigated using methylene blue molecule as a probe. With the AuNPs having an average diameter of 20 nm, high performance of SERS was achieved at an enhancement factor of $9\times 10^{5}$, arising from the synergistic effect of electromagnetic enhancement from AuNPs and chemical enhancement from diamond. The AuNP/BDD substrate is demonstrated to be highly sensitive, reproducible, stable, and reusable for the SERS examination. Due to the facile preparation process and controllable surface morphology, the AuNP/BDD substrates are favorable as a high performance SERS platform performed in practical applications.

Two-dimensional (2D) materials are playing more and more important roles in both basic sciences and industrial applications. For 2D materials, strain could tune the properties and enlarge applications. Since the growth of 2D materials on substrates is often accompanied by strain, the interaction between 2D materials and substrates is worthy of careful attention. Here we demonstrate the fabrication of strained monolayer silver arsenide (AgAs) on Ag(111) by molecular beam epitaxy, which shows one-dimensional stripe structures arising from uniaxial strain. The atomic geometric structure and electronic band structure are investigated by low energy electron diffraction, scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and first-principle calculations. Monolayer AgAs synthesized on Ag(111) provides a platform to study the physical properties of strained 2D materials.

Mid-wavelength infrared planar photodiodes were demonstrated, in which both the epitaxy growth of InAs/GaSb superlattices and the thermal diffusion of p-type dopant were performed in production-scale metal–organic chemical vapor deposition reactors. The formation of a planar homojunction was confirmed by secondary ion mass spectroscopy and its $I$–$V$ characteristics. A cut-off wavelength around 5 μm was determined in 77 K optical characterization, and photo-current as high as 600 nA was collected from a reverse-biased planar diode of 640 μm diameter. These preliminary results were obtained despite the structural degradation revealed by x-ray diffraction, and we attribute the degradation to the concert of thermal annealing and high Zn concentration behind the diffusion front.

We fabricated 4H-SiC ultraviolet avalanche photodiode (APD) arrays and systematically investigated the effect of threading dislocations on electrical and single photon detection characteristics of 4H-SiC APDs. Based on a statistical correlation study of individual device performance and structural defect mapping revealed by molten KOH etching, it is determined with high confidence level that even a single threading dislocation within APD active region would lead to apparent device performance degradation, including increase of dark current near breakdown voltage, premature breakdown and reduction of single photon detection efficiency at fixed dark count rate.

A novel O$_{2}$ plasma-based digital etching technology for p-GaN/AlGaN structures without any etch-stop layer was investigated using an inductively coupled plasma (ICP) etcher, with 100 W ICP power and 40 W rf bias power. Under 40 sccm O$_{2}$ flow and 3 min oxidation time, the p-GaN etch depth was 3.62 nm per circle. The surface roughness improved from 0.499 to 0.452 nm after digital etching, meaning that no observable damages were caused by this process. Compared to the dry etch only methods with Cl$_{2}$/Ar/O$_{2}$ or BCl$_{3}$/SF$_{6}$ plasma, this technique smoothed the surface and could efficiently control the etch depth due to its self-limiting characteristic. Furthermore, compared to other digital etching processes with an etch-stop layer, this approach was performed using ICP etcher and less demanding on the epitaxial growth. It was proved to be effective in precisely controlling p-GaN etch depth and surface damages required for high performance p-GaN gate high electron mobility transistors.

The knowledge of sequence and structural properties of residues in the catalytic sites of enzymes is important for understanding the physiochemical basis of enzymatic catalysis. We reveal new features of the catalytic sites by analyzing the coevolutionary behavior of amino acid sequences. By performing direct coupling analysis of the sequences of homologous proteins, we construct the coevolution networks at the residue level. Based on the analysis of the topological features of the coevolution networks for a dataset including 20 enzymes, we show that there is significant correlation between the catalytic sites and topological features of protein coevolution networks. Residues at the catalytic center often correspond to the nodes with high values of centralities in the networks as characterized by the degree, betweenness, closeness, and Laplacian centrality. The results of this work provide a possible way to extract key coevolutionary information from the sequences of enzymes, which is useful in the prediction of catalytic sites of enzymes.