Volume 37 Issue 4
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GENERAL
Lax Pairs of Integrable Systems in Bidifferential Graded Algebras
Danda Zhang, Da-Jun Zhang, Sen-Yue Lou
Chin. Phys. Lett. 2020, 37 (4): 040201 .   DOI: 10.1088/0256-307X/37/4/040201
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Lax pairs regarded as foundations of the inverse scattering methods play an important role in integrable systems. In the framework of bidifferential graded algebras, we propose a straightforward approach to constructing the Lax pairs of integrable systems in functional environment. Some continuous equations and discrete equations are presented.
Superfluid-Mott-Insulator Transition in an Optical Lattice with Adjustable Ensemble-Averaged Filling Factors
Shifeng Yang, Tianwei Zhou, Chen Li, Kaixiang Yang, Yueyang Zhai, Xuguang Yue, Xuzong Chen
Chin. Phys. Lett. 2020, 37 (4): 040301 .   DOI: 10.1088/0256-307X/37/4/040301
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We study the quantum phase transition from a superfluid to a Mott insulator of ultracold atoms in a three-dimensional optical lattice with adjustable filling factors. Based on the density-adjustable Bose–Einstein condensate we prepared, the excitation spectrum in the superfluid and the Mott insulator regime is measured with different ensemble-averaged filling factors. We show that for the superfluid phase, the center of the excitation spectrum is positively correlated with the ensemble-averaged filling factor, indicating a higher sound speed of the system. For the Mott insulator phase, the discrete feature of the excitation spectrum becomes less pronounced as the ensemble-averaged filling factor increases, implying that it is harder for the system to enter the Mott insulator regime with higher filling factors. The ability to manipulate the filling factor affords further potential in performing quantum simulation with cold atoms trapped in optical lattices.
Breather Interaction Properties Induced by Self-Steepening and Space-Time Correction
Yu-Han Wu, Chong Liu, Zhan-Ying Yang, Wen-Li Yang
Chin. Phys. Lett. 2020, 37 (4): 040501 .   DOI: 10.1088/0256-307X/37/4/040501
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We study the properties of breather interactions in nonlinear Kerr media with self-steepening and space-time correction and with either self-focusing or self-defocusing nonlinearity, and present a new family of exact breather solutions via the Darboux transformation with a special-designed quadratic spectral parameter. In contrast to the previous results of the nonlinear Schrödinger equation (NLSE) hierarchy, we show that the relative phase of colliding breathers has a significant effect on the collision manifestation. In particular, only the out-of-phase interactions can generate small amplitude waves at the collision center, which are analogous to the NLSE super-regular breathers. Our results will deepen our understanding of the properties of breather interactions and they will offer the possibility of experimental observations of super-regular breather dynamics in systems with self-steepening and space-time correction.
Dark Soliton of Polariton Condensates under Nonresonant $\mathcal{P}\mathcal{T}$-Symmetric Pumping
Chun-Yu Jia, Zhao-Xin Liang
Chin. Phys. Lett. 2020, 37 (4): 040502 .   DOI: 10.1088/0256-307X/37/4/040502
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A quantum system in complex potentials obeying parity-time ($\mathcal{P}\mathcal{T}$) symmetry could exhibit all real spectra, starting out in non-Hermitian quantum mechanics. The key physics behind a $\mathcal{P}\mathcal{T}$-symmetric system consists of the balanced gain and loss of the complex potential. We plan to include the nonequilibrium nature (i.e., the intrinsic kinds of gain and loss of a system) to a $\mathcal{P}\mathcal{T}$-symmetric many-body quantum system, with an emphasis on the combined effects of non-Hermitian due to nonequilibrium nature and $\mathcal{P}\mathcal{T}$ symmetry in determining the properties of a system. To this end, we investigate the static and dynamical properties of a dark soliton of a polariton Bose–Einstein condensate under the $\mathcal{P}\mathcal{T}$-symmetric non-resonant pumping by solving the driven-dissipative Gross–Pitaevskii equation both analytically and numerically. We derive the equation of motion for the center of mass of the dark soliton's center analytically with the help of the Hamiltonian approach. The resulting equation captures how the combination of the open-dissipative character and $\mathcal{P}\mathcal{T}$-symmetry affects the properties of the dark soliton; i.e., the soliton relaxes by blending with the background at a finite time. Further numerical solutions are in excellent agreement with the analytical results.
ATOMIC AND MOLECULAR PHYSICS
Discriminating High-Pressure Water Phases Using Rare-Event Determined Ionic Dynamical Properties
Lin Zhuang, Qijun Ye, Ding Pan, Xin-Zheng Li
Chin. Phys. Lett. 2020, 37 (4): 043101 .   DOI: 10.1088/0256-307X/37/4/043101
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Recent discoveries of dynamic ice VII and superionic ice highlight the importance of ionic diffusions in discriminating high-pressure ($P$) water phases. The rare event nature and the chemical bond breaking associated with these diffusions, however, make extensive simulations of these processes unpractical to ab initio and inappropriate for force field based methods. Using a first-principles neural network potential, we performed a theoretical study of water at 5–70 GPa and 300–3000 K. Long-time dynamics of protons and oxygens were found indispensable in discriminating several subtle states of water, characterized by proton's and oxygen ion's diffusion coefficients and the distribution of proton's displacements. Within dynamic ice VII, two types of proton transfer mechanisms, i.e., translational and rotational transfers, were identified to discriminate this region further into dynamic ice VII T and dynamic ice VII R. The triple point between ice VII, superionic ice (SI), and liquid exists because the loosening of the bcc oxygen skeleton is prevented by the decrease of interatomic distances at high $P$'s. The melting of ice VII above $\sim$40 GPa can be understood as a process of two individual steps: the melting of protons and the retarded melting of oxygens, responsible for the forming of SI. The boundary of the dynamic ice VII and SI lies on the continuation line ice VII's melting curve at low $P$'s. Based on these, a detailed phase diagram is given, which may shed light on studies of water under $P$'s in a wide range of interdisciplinary sciences.
Nonadiabatic and Multielectron Effects in the Attoclock Experimental Scheme
Zhi-Lei Xiao, Wei Quan, Song-Po Xu, Shao-Gang Yu, Xuan-Yang Lai, Jing Chen, Xiao-Jun Liu
Chin. Phys. Lett. 2020, 37 (4): 043201 .   DOI: 10.1088/0256-307X/37/4/043201
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The problem of how long it takes for an electron to tunnel from one side of a barrier to the other has been debated for decades and the attoclock is a promising experimental procedure to address this problem. In the attoclock experiment, many physical effects will contribute to the experimental results and it is difficult to extract the tunneling time accurately. We numerically investigate a method of measuring the residual equivalent temporal offset (RETO) induced by the physical effects except for tunneling delay. The Coulomb potential effect, the nonadiabatic effect, the multielectron effect, and the Stark effect are considered in the theoretical model. It is shown that the ratio of the RETO of the target atoms to that of H is insensitive to the wavelength and is linearly proportional to (2$I_{\rm p}$)$^{-3/2}$. This work can help to improve the accuracy of the attoclock technique.
Ionic Angular Distributions Induced by Strong-Field Ionization of Tri-Atomic Molecules
Tian Sun, Shi-Wen Zhang, Rui Wang, Shuang Feng, Yang Liu, Hang Lv, Hai-Feng Xu
Chin. Phys. Lett. 2020, 37 (4): 043301 .   DOI: 10.1088/0256-307X/37/4/043301
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Angular distributions of fragment ions from ionization of several tri-atomic molecules (CO$_{2}$, OCS, N$_{2}$O and NO$_{2}$) by strong 800-nm laser fields are investigated via a time-of-flight mass spectrometer. Anisotropic angular distributions of fragment ions, especially those of atomic ions, are observed for all of the molecules studied. These anisotropic angular distributions are mainly due to the geometric alignment of molecules in the strong field ionization. Distinct different patterns in ionic angular distributions for different molecules are observed. It is indicated that both molecular geometric structure and ionization channels have effects on the angular distributions of strong field ionization/fragmentation.
Testing the Universality of Free Fall by Comparing the Atoms in Different Hyperfine States with Bragg Diffraction
Ke Zhang, Min-Kang Zhou, Yuan Cheng, Le-Le Chen, Qin Luo, Wen-Jie Xu, Lu-Shuai Cao, Xiao-Chun Duan, Zhong-Kun Hu
Chin. Phys. Lett. 2020, 37 (4): 043701 .   DOI: 10.1088/0256-307X/37/4/043701
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We perform a precision atom interferometry experiment to test the universality of free fall. Our experiment employs the Bragg atom interferometer with $^{87}$Rb atoms either in hyperfine state $\left| {F = 1,{m_F} = 0} \right\rangle $ or $\left| {F = 2,{m_F} = 0} \right\rangle $, and the wave packets in these two states are diffracted by one pair of Bragg beams alternatively, which is helpful for suppressing common-mode systematic errors. We obtain an Eötvös ratio ${\eta_{1 - 2}} = \left({ 0.9 \pm 2.7} \right) \times {10^{- 10}}$, and set a new record on the precision with improvement of nearly 5 times. This measurement also provides constraint on the difference of the diagonal terms of the mass-energy operator.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)
Scanning-Position Error-Correction Algorithm in Dual-Wavelength Ptychographic Microscopy
Rui Ma, Shu-Yuan Zhang, Tian-Hao Ruan, Ye Tao, Hua-Ying Wang, Yi-Shi Shi
Chin. Phys. Lett. 2020, 37 (4): 044201 .   DOI: 10.1088/0256-307X/37/4/044201
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We propose a new algorithm for the error correction of scanning positions in ptychographic microscopy. Since the scanning positions are varied mechanically by moving the illuminating probes laterally, the scanning errors will accumulate at multiple positions, greatly reducing the reconstruction quality of a sample. To correct the scanning errors, we use the correlation analysis for the diffractive data combining with the additional constraint of dual wavelengths. This significantly improves the quality of ptychographic microscopy. Optical experiments verify the proposed algorithm for two samples including a resolution target and a fibroblast.
Coaxial Multi-Wavelength Generation in YVO$_{4}$ Crystal with Stimulated Raman Scattering Excited by a Picosecond-Pulsed 1064 Laser
Jing-Jie Hao, Wei Tu, Nan Zong, Yu Shen, Shen-Jin Zhang, Yong Bo, Qin-Jun Peng, Zu-Yan Xu
Chin. Phys. Lett. 2020, 37 (4): 044202 .   DOI: 10.1088/0256-307X/37/4/044202
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The multiwavelength characteristics of stimulated Raman scattering (SRS) in YVO$_{4}$ crystal excited by a picosecond laser at 1064 nm are investigated theoretically and experimentally. Laser output with seven wavelengths is achieved coaxially and synchronously at 894, 972, 1175, 1312, 1486, 1713 and 2022 nm in a YVO$_{4}$ crystal. The maximum total Raman output energy is as high as 2.77 mJ under the pump energy of 7.75 mJ. A maximum total Raman conversion efficiency of 47.8% is obtained when the pump energy is 6.54 mJ. This is the highest order of Stokes components and the highest output energy generated by YVO$_{4}$ reported up to date. This work expands the Raman spectrum of YVO$_{4}$ crystal to the near-IR regime, with seven wavelengths covered at the same time, paving the way for new wavelength generation in the near-IR regime and its multiwavelength application.
Influence of Hot-Carriers on the On-State Resistance in Si and GaAs Photoconductive Semiconductor Switches Working at Long Pulse Width
Chong-Biao Luan, Hong-Tao Li
Chin. Phys. Lett. 2020, 37 (4): 044203 .   DOI: 10.1088/0256-307X/37/4/044203
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We demonstrate that the transport of hot carriers may result in the phenomenon where an oscillated output current appears at the waveforms in a high-power photoconductive semiconductor switch (PCSS) working at long pulse width when the laser disappears or the electric field changes. The variational laser and electric field will affect the scattering rates of hot carriers and crystal lattice in high-power PCSS, and the drift velocity of hot carriers and also the on-state resistance will be changed. The present result is important for reducing the on-state resistance and improving the output characteristics of high-power Si/GaAs PCSS.
Optical Properties of Atomic Defects in Hexagonal Boron Nitride Flakes under High Pressure
Xiao-Yu Zhao, Jun-Hui Huang, Zhi-Yao Zhuo, Yong-Zhou Xue, Kun Ding, Xiu-Ming Dou, Jian Liu, Bao-Quan Sun
Chin. Phys. Lett. 2020, 37 (4): 044204 .   DOI: 10.1088/0256-307X/37/4/044204
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We investigate the pressure spectral characteristics and the effective tuning of defect emissions in hexagonal boron nitride (hBN) at low temperatures using a diamond anvil cell (DAC). It is found that the redshift rate of emission energy is up to 10 meV/GPa, demonstrating a controllable tuning of single photon emitters through pressure. Based on the distribution character of pressure coefficients as a function of wavelength, different kinds of atomic defect states should be responsible for the observed defect emissions.
Giant Broadband One Way Transmission Based on Directional Mie Scattering and Asymmetric Grating Diffraction Effects
Xuannan Wu, Guanwen Yuan, Rui Zhu, Jicheng Wang, Fuhua Gao, Feiliang Chen, Yidong Hou
Chin. Phys. Lett. 2020, 37 (4): 044205 .   DOI: 10.1088/0256-307X/37/4/044205
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High performance optical diode-like devices are highly desired in future practical nano-photonic devices with strong directional selectivity. We demonstrate a kind of giant broadband reciprocity optical diode-like devices by simultaneously using the directional Mie scattering effect and the asymmetric grating diffraction effect. The maximum asymmetric subtraction and the asymmetric transmission ratio can reach nearly 100% and 40 dB at specified wavelength, respectively. In a wide waveband from 500 nm to 800 nm, the asymmetric subtraction and the ratio keep larger than 80% and 3.5 dB, respectively, even under oblique incidence. To the best of our knowledge, this is the best one-way-transmission effect observed in the reciprocity optical diode-like devices. In addition, we further demonstrate that this one-way-transmission effect can bring an effective absorption enhancement on gold films. The giant, broadband and angle-insensitive one-way-transmission effect demonstrated here is far beyond the well-known anti-reflection effect in the light-trapping devices and will bring new design philosophy for nano-photonic devices.
Pulse Characteristics of Cavityless Solid-State Laser
Rui Guo, Ye-Wen Jiang, Ting-Hao Liu, Qiang Liu, Ma-Li Gong
Chin. Phys. Lett. 2020, 37 (4): 044206 .   DOI: 10.1088/0256-307X/37/4/044206
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We propose a theoretical model (cavityless pulsed solid-state-laser theory) to analyze the pulse characteristics of cavity-less solid-state lasers. A high gain Nd:YVO$_{4}$ end-pumped cavityless laser system is adopted to verify the theoretical model. It shows that the performance of output energy and pulse width achieved in cavityless configuration is better than that in resonator configuration when the small-signal gain reaches the saturated level. The simulation results calculated by our theoretical model agree very well with the experimental results. This agreement proves the validity of our theoretical model, which has great importance for theoretical analyses of high gain pulsed laser.
Mode Control of Quasi-PT Symmetry in Laterally Multi-Mode Double Ridge Semiconductor Laser
Ting Fu, Yu-Fei Wang, Xue-You Wang, Xu-Yan Zhou, Wan-Hua Zheng
Chin. Phys. Lett. 2020, 37 (4): 044207 .   DOI: 10.1088/0256-307X/37/4/044207
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In traditional semiconductor lasers, it is usual to obtain single lateral mode operation by narrowing the ridge of waveguide, which is sensitive to fabrication inaccuracies. To overcome this shortcoming, a quasi-PT (parity-time) symmetric double ridge semiconductor laser is proposed to reach single lateral mode operation for an intrinsic multi-mode stripe laser. The coupled mode theory is used to analyze the non-Hermitian modulation of the gain (or loss) of the PT symmetric double ridge laser to obtain the coupling coefficient between the two ridge waveguides. Finally, the mode field distributions of the quasi-PT symmetric double ridge laser are simulated before and after the spontaneous PT symmetry breaking, which keep the laser operating in single lateral mode.
Energy-Selective X-Ray Ghost Imaging
Yu-Hang He, Ai-Xin Zhang, Wen-Kai Yu, Li-Ming Chen, Ling-An Wu
Chin. Phys. Lett. 2020, 37 (4): 044208 .   DOI: 10.1088/0256-307X/37/4/044208
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X-ray ghost imaging (XGI) has opened up a new avenue for damage-free medical imaging. Here energy-selective spectroscopic XGI under poor illumination is demonstrated with a single-pixel detector for the first time. The key device was a specially fabricated Au mask incorporating a new modulation pattern design, by which means images of a real object were obtained with a spatial resolution of 10 μm and a spectral energy resolution of about 1.5 keV. Compressed sensing was also introduced to improve the image quality. Our proof-of-principle experiment extends the methodology of XGI to make possible the retrieval of spectral images with only a single-pixel detector, and paves the way for potential applications in many fields such as biology, material science and environmental sensing.
High-Fidelity Manipulation of the Quantized Motion of a Single Atom via Stern–Gerlach Splitting
Kun-Peng Wang, Jun Zhuang, Xiao-Dong He, Rui-Jun Guo, Cheng Sheng, Peng Xu, Min Liu, Jin Wang, Ming-Sheng Zhan
Chin. Phys. Lett. 2020, 37 (4): 044209 .   DOI: 10.1088/0256-307X/37/4/044209
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We demonstrate high-fidelity manipulation of the quantized motion of a single $^{87}$Rb atom in an optical tweezer via microwave couplings induced by Stern–Gerlach splitting. The Stern–Gerlach splitting is mediated by polarization gradient of a strongly focused tweezer beam that functions as fictitious magnetic field gradient. The spatial splitting removes the orthogonality of the atomic spatial wavefunctions, thus enables the microwave couplings between the motional states. We obtain coherent Rabi oscillations for up to third-order sideband transitions, in which a high fidelity of larger than $0.99$ is obtained for the spin-flip transition on the first order sideband after subtraction of the state preparation and detection error. The Stern–Gerlach splitting is measured at a precision of better than $0.05$ nm. This work paves the way for quantum engineering of motional states of single atoms, and may have wide applications in few body physics and ultracold chemistry.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Tunable Dielectric Properties of Carbon Nanotube@Polypyrrole Core-Shell Hybrids by the Shell Thickness for Electromagnetic Wave Absorption
De-Ting Wang, Xian-Chao Wang, Xiao Zhang, Hao-Ran Yuan, Yu-Jin Chen
Chin. Phys. Lett. 2020, 37 (4): 045201 .   DOI: 10.1088/0256-307X/37/4/045201
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Carbon nanotube@polypyrrole (CNT@PPy) hybrids have been successfully fabricated via a simple in situ chemical oxidation polymerization. The thickness of the PPy shell can be finely controlled in the range of 3.0–6.4 nm. The dielectric loss of core-shell hybrids can be tuned by the shell thickness, resulting in a well-matched characteristic impedance that can enhance electromagnetic wave (EMW) absorption performance. Minimum reflection loss of the hybrid with moderate PPy shell thickness can reach $-51.4$ dB at 11.8 GHz with a matching thickness of merely 2 mm. Furthermore, the minimum reflection loss values of the hybrid are below $-30$ dB even at thickness in the range of 1.4–1.9 mm, endowing the possibility of practical application of the hybrids in electromagnetic wave absorption field.
A New Path to Improve High $\beta_{\rm p}$ Plasma Performance on EAST for Steady-State Tokamak Fusion Reactor
Baonian Wan and the EAST team
Chin. Phys. Lett. 2020, 37 (4): 045202 .   DOI: 10.1088/0256-307X/37/4/045202
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High $\beta_{\rm p}$ scenario is foreseen to be a promising candidate operational mode for steady-state tokamak fusion reactors. Dedicated experiments on EAST and data analysis find that density gradient $\nabla n$ is a control knob to improve energy confinement in high $\beta_{\rm p}$ plasmas at low toroidal rotation as projected for a fusion reactor. Different from previously known turbulent stabilization mechanisms such as ${\boldsymbol E} \times {\boldsymbol B}$ shear and Shafranov shift, high density gradient can enhance the Shafranov shift stabilizing effect significantly in high $\beta_{\rm p}$ regime, giving that a higher density gradient is readily accessible in future fusion reactors with lower collisionality. This new finding is of great importance for the next-step fusion development because it may open a new path towards even higher energy confinement in the high $\beta_{\rm p}$ scenario. It has been demonstrated in the recent EAST experiments, i.e., a fully non-inductive high $\beta_{\rm p}$ ($\sim $2) H-mode plasma ($H_{98y2}\ge 1.3$) has been obtained for a duration over 100 current diffusion times, which sets another new world record of long-pulse high-performance tokamak plasma operation with the normalized performance approaching the ITER and CFETR regimes.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES
Effects of Total-Ionizing-Dose Irradiation on Single-Event Burnout for Commercial Enhancement-Mode AlGaN/GaN High-Electron Mobility Transistors
Si-Yuan Chen, Xin Yu, Wu Lu, Shuai Yao, Xiao-Long Li, Xin Wang, Mo-Han Liu, Shan-Xue Xi, Li-Bin Wang, Jing Sun, Cheng-Fa He, Qi Guo
Chin. Phys. Lett. 2020, 37 (4): 046101 .   DOI: 10.1088/0256-307X/37/4/046101
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We investigate the synergism effect of total ionizing dose (TID) on single-event burnout (SEB) for commercial enhancement-mode AlGaN/GaN high-electron mobility transistors. Our experimental results show that the slight degradation of devices caused by gamma rays can affect the stability of the devices during the impact of high energy particles. During heavy ion irradiation, the safe working values of drain voltage are significantly reduced for devices which have already been irradiated by $^{60}$Co gamma rays before. This could be attributed to more charges trapped caused by $^{60}$Co gamma rays, which make GaN devices more vulnerable to SEB. Moreover, the electrical parameters of GaN devices after $^{60}$Co gamma and heavy-ion irradiations are presented, such as the output characteristic curve, effective threshold voltages, and leakage current of drain. These results demonstrate that the synergistic effect of TID on SEB for GaN power devices does in fact exist.
Molecular Dynamics Simulation of Effects of Stretching and Compressing on Thermal Conductivity of Aligned Silicon Oxygen Chains
Wen-Xue Xu, Xin-Gang Liang
Chin. Phys. Lett. 2020, 37 (4): 046601 .   DOI: 10.1088/0256-307X/37/4/046601
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The effects of stretching and compressing on the thermal conductivity (TC) of silicon oxygen chain are studied by means of non-equilibrium molecular dynamics simulation. It is found that stretching can improve TC, and compressing may reduce the TC and can also increase the TC. This mechanism is explained based on the variation of phonon group velocity and the specific heat per volume with stretching and compressing. The distributions of bond angle and bond length under different normalized chain lengths are given. It is found that the bond length and bond angle in the skeleton chain would deviate from their original position. In addition, the phonon density of states (PDOSs) of silicon and oxygen atoms in the chains under different normalized chain lengths are analyzed. The overall trend is that the TC increases and the peaks of PDOSs move towards higher frequency with increasing stretch strain.
Wettability and Surface Energy of Hydrogen- and Oxygen-Terminated Diamond Films
Zi-Cheng Ma, Nan Gao, Shao-Heng Cheng, Jun-Song Liu, Ming-Chao Yang, Peng Wang, Zhi-Yuan Feng, Qi-Liang Wang, Hong-Dong Li
Chin. Phys. Lett. 2020, 37 (4): 046801 .   DOI: 10.1088/0256-307X/37/4/046801
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The contact angle and surface energy values of diamond are systemically investigated in terms of surface treatments (hydrogen- and oxygen-terminations), structure feature (single crystal diamonds and polycrystalline diamond films), crystal orientation ((100), (111) and mixed (100)/(111) orientations), different fluids (probes of polar deionized water and nonpolar di-iodomethane). It is found that the hydrophobic/hydrophilic characteristic and surface energy values of diamond are mainly determined by the surface hydrogen/oxygen termination, and less related to the structural features and crystal orientation. Based on the contact angle values with polar water and nonpolar di-iodomethane, the surface energies of diamond are estimated to be about 43 mJ/m$^{2}$ for hydrogen-termination and about 60 mJ/m$^{2}$ for oxygen-termination. Furthermore, the varying surface roughness of diamond and fluids with different polarities examined determine the variation of contact angles as well as the surface energy values. These results would be helpful for a more detailed understanding of the surface properties of diamond films for further applications in a broad number of fields, such as optical and microwave windows, biosensors, and optoelectronic devices, etc.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Pressure-Stabilized New Phase of CaN$_{4}$
Xu-Han Shi, Bo Liu, Zhen Yao, Bing-Bing Liu
Chin. Phys. Lett. 2020, 37 (4): 047101 .   DOI: 10.1088/0256-307X/37/4/047101
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We propose a new CaN$_{4}$ high pressure structure with the $P2_{1}/m$ space group. The $P2_{1}/m$-CaN$_{4}$ structure is constituted by the infinite armchair N-chain. The dynamical stability and mechanical stability are verified by the calculations of phonon dispersion curves and elastic constants. The enthalpy difference calculation shows that the $P2_{1}/m$ phase is more stable than the reported P4$_{1}2_{1}$2 phase. The advantaged properties of $P2_{1}/m$-CaN$_{4}$, such as high nitrogen content (58.3%) and low polymerization pressure (18.3 GPa), allow it to be a potential high energy material. Band structure calculation shows that the $P2_{1}/m$-CaN$_{4}$ structure is a metallic phase. The nonpolar covalent single N–N bond is a sigma bond. The charge transfer between the Ca and N atoms results in an ionic bond interaction.
Pressure-Induced Metallization and Structural Phase Transition in the Quasi-One-Dimensional TlFeSe$_{2}$
Zi-Yi Liu, Qing-Xin Dong, Peng-Fei Shan, Yi-Yan Wang, Jian-Hong Dai, Rajesh Jana, Ke-Yu Chen, Jian-Ping Sun, Bo-Sen Wang, Xiao-Hui Yu, Guang-Tong Liu, Yoshiya Uwatoko, Yu Sui, Huai-Xin Yang, Gen-Fu Chen, Jin-Guang Cheng
Chin. Phys. Lett. 2020, 37 (4): 047102 .   DOI: 10.1088/0256-307X/37/4/047102
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We report a comprehensive high-pressure study on the monoclinic TlFeSe$_{2}$ single crystal, which is an antiferromagnetic insulator with quasi-one-dimensional crystal structure at ambient pressure. It is found that TlFeSe$_{2}$ undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above $P_{\rm c} \approx 13$ GPa, accompanied with a large volume collapse of $\Delta V/V_{0}=8.3{\%}$. In the low-pressure monoclinic phase, the insulating state is easily metallized at pressures above 2 GPa; while possible superconductivity with $T_{\rm c}^{\rm onset} \sim 2$ K is found to emerge above 30 GPa in the high-pressure phase. Such a great tunability of TlFeSe$_{2}$ under pressure indicates that the ternary $A$FeSe$_{2}$ system ($A$ = Tl, K, Cs, Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition, dimensionality crossover, and superconductivity.
Metal to Orthogonal Metal Transition
Chuang Chen, Xiao Yan Xu, Yang Qi, Zi Yang Meng
Chin. Phys. Lett. 2020, 37 (4): 047103 .   DOI: 10.1088/0256-307X/37/4/047103
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Orthogonal metal is a new quantum metallic state that conducts electricity but acquires no Fermi surface (FS) or quasiparticles, and hence orthogonal to the established paradigm of Landau's Fermi-liquid (FL). Such a state may hold the key of understanding the perplexing experimental observations of quantum metals that are beyond FL, i.e., dubbed non-Fermi-liquid (nFL), ranging from the Cu- and Fe-based oxides, heavy fermion compounds to the recently discovered twisted graphene heterostructures. However, to fully understand such an exotic state of matter, at least theoretically, one would like to construct a lattice model and to solve it with unbiased quantum many-body machinery. Here we achieve this goal by designing a 2D lattice model comprised of fermionic and bosonic matter fields coupled with dynamic $\mathbb{Z}_2$ gauge fields, and obtain its exact properties with sign-free quantum Monte Carlo simulations. We find that as the bosonic matter fields become disordered, with the help of deconfinement of the $\mathbb{Z}_2$ gauge fields, the system reacts with changing its nature from the conventional normal metal with an FS to an orthogonal metal of nFL without FS and quasiparticles and yet still responds to magnetic probe like an FL. Such a quantum phase transition from a normal metal to an orthogonal metal, with its electronic and magnetic spectral properties revealed, is calling for the establishment of new paradigm of quantum metals and their transition with conventional ones.
Negative Magnetoresistance in Antiferromagnetic Topological Insulator EuSn$_2$As$_2$$^{*}$
Huan-Cheng Chen, Zhe-Feng Lou, Yu-Xing Zhou, Qin Chen, Bin-Jie Xu, Shui-Jin Chen, Jian-Hua Du, Jin-Hu Yang, Hang-Dong Wang, Ming-Hu Fang
Chin. Phys. Lett. 2020, 37 (4): 047201 .   DOI: 10.1088/0256-307X/37/4/047201
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The measurements of magnetization, longitudinal and Hall resistivities are carried out on the intrinsic antiferromagnetic (AFM) topological insulator EuSn$_2$As$_2$. It is confirmed that our EuSn$_2$As$_2$ crystal is a heavily hole doping A-type AFM metal with the Néel temperature $T_{\rm N}$ = 24 K, with a metamagnetic transition from an AFM to a ferromagnetic (FM) phase occurring at a certain critical magnetic field for the different field orientations. Meanwhile, we also find that the carrier concentration does not change with the evolution of magnetic order, indicating that the weak interaction between the localized magnetic moments from Eu$^{2+}$ $4f^7$ orbits and the electronic states near the Fermi level. Although the quantum anomalous Hall effect (AHE) is not observed in our crystals, it is found that a relatively large negative magnetoresistance ($-$13%) emerges in the AFM phase, and exhibits an exponential dependence upon magnetic field, whose microscopic origin is waiting to be clarified in future research.
Anomalous Hall Effect in Layered Ferrimagnet MnSb$_{2}$Te$_{4}$
Gang Shi, Mingjie Zhang, Dayu Yan, Honglei Feng, Meng Yang, Youguo Shi, Yongqing Li
Chin. Phys. Lett. 2020, 37 (4): 047301 .   DOI: 10.1088/0256-307X/37/4/047301
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We report on low-temperature electron transport properties of MnSb$_{2}$Te$_{4}$, a candidate of ferrimagnetic Weyl semimetal. Long-range magnetic order is manifested as a nearly square-shaped hysteresis loop in the anomalous Hall resistance, as well as sharp jumps in the magnetoresistance. At temperatures below 4 K, a ${\rm ln}T$-type upturn appears in the temperature dependence of longitudinal resistance, which can be attributed to the electron-electron interaction (EEI), since the weak localization can be excluded by the temperature dependence of magnetoresistance. Although the anomalous Hall resistance exhibits a similar ${\rm ln}T$-type upturn in the same temperature range, such correction is absent in the anomalous Hall conductivity. Our work demonstrates that MnSb$_{2}$Te$_{4}$ microflakes provide an ideal system to test the theory of EEI correction to the anomalous Hall effect.
High-Pressure Ultrafast Dynamics in Sr$_{2}$IrO$_{4}$: Pressure-Induced Phonon Bottleneck Effect
Yanling Wu, Xia Yin, Jiazila Hasaien, Yang Ding, Jimin Zhao
Chin. Phys. Lett. 2020, 37 (4): 047801 .   DOI: 10.1088/0256-307X/37/4/047801
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By integrating pump-probe ultrafast spectroscopy with diamond anvil cell (DAC) technique, we demonstrate a time-resolved ultrafast dynamics study on non-equilibrium quasiparticle (QP) states in Sr$_{2}$IrO$_{4}$ under high pressure. On-site in situ condition is realized, where both the sample and DAC have fixed position during the experiment. The QP dynamics exhibits a salient pressure-induced phonon bottleneck feature at 20 GPa, which corresponds to a gap shrinkage in the electronic structure. A structural transition is also observed at 32 GPa. In addition, the slowest relaxation component reveals possible heat diffusion or pressure-controlled local spin fluctuation associated with the gap shrinkage. Our work enables precise pressure dependence investigations of ultrafast dynamics, paving the way for reliable studies of high-pressure excited state physics.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Chemical Vapor Deposition of Two-Dimensional PbS Nanoplates for Photodetection
Yi-Yi Gu, Yi-Fan Wang, Jing Xia, Xiang-Min Meng
Chin. Phys. Lett. 2020, 37 (4): 048101 .   DOI: 10.1088/0256-307X/37/4/048101
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Non-layered two-dimensional (2D) lead sulfide (PbS) has attracted growing interest recently due to its direct narrow bandgap (0.4 eV) and broad spectral detection from visible to mid-IR region, which lead to remarkable electronic and optoelectronic properties promising for real applications. We report the chemical vapor deposition growth of highly crystalline 2D PbS crystals on mica substrates. The high quality and uniformity of 2D PbS nanoplates are confirmed by atomic force microscopy, x-ray powder diffraction, transmission electron microscopy and x-ray photoelectron spectroscopy. The morphology and lateral size are controllable by different growth temperatures. Photodetectors made from 2D PbS nanoplates reveal good stability, high photoresponsivity, and fast response time, which indicates their promising applications for ultrathin optoelectronics.
CH$_{4}$ Gas Extraction by CO$_{2}$: Substitution in Clathrate Hydrate through Bimolecular Iteration
Xiao-Hui Yu, Yuan Liu, San-Ya Du, Xu Zheng, Jin-Long Zhu, Hong-Wu Xu, Jian-Zhong Zhang, Shi-Yu Du, Xiao-Cheng Zeng, J. S. Francisco, Chang-Qing Jin, Yu-Sheng Zhao, Hui Li
Chin. Phys. Lett. 2020, 37 (4): 048201 .   DOI: 10.1088/0256-307X/37/4/048201
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Methane clathrate hydrate (MCH) is a promising energy resource, but controllable extraction of CH$_{4}$ from MCH remains a challenge. Gradually replacing CH$_{4}$ in MCH with CO$_{2}$ is an attractive scheme, as it is cost efficient and mitigates the environmentally harmful effects of CO$_{2}$ by sequestration. However, the practicable implementation of this method has not yet been achieved. In this study, using in situ neutron diffraction, we confirm that CH$_{4}$ in the 5$^{12}6^{2}$ cages of bulk structure-I (sI) MCH can be substituted by gaseous CO$_{2}$ under high pressure and low temperature with a high substitution ratio ($\sim $44%) while conserving the structure of the hydrate framework. First-principles calculations indicate that CO$_{2}$ binds more strongly to the 5$^{12}6^{2}$ cages than methane does, and that the diffusion barrier for CH$_{4}$ is significantly lowered by an intermediate state in which one hydrate cage is doubly occupied by CH$_{4}$ and CO$_{2}$. Therefore, exchange of CO$_{2}$ for CH$_{4}$ in MCH is not only energetically favorable but also kinetically feasible. Experimental and theoretical studies of CH$_{4}$/CO$_{2}$ substitution elucidate a method to harness energy from these combustible ice resources.
Analogue Soliton with Variable Mass in Super-Conducting Quantum Interference Devices
Ying Yang, Ze-Hua Tian, Ji-Liang Jing
Chin. Phys. Lett. 2020, 37 (4): 048501 .   DOI: 10.1088/0256-307X/37/4/048501
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It is difficult to investigate the behavior of solitons in realistic inhomogeneity media in experiment due to inhomogeneity of the media and noise from the unwanted coupling. We propose to use a waveguide-like transmission line which is based on direct-current super-conducting quantum interference devices to simulate behavior of solitons because we find that the behavior of the node flux in this transmission is similar to that of solitons with variable mass.
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