## Volume 38 Issue 6

#### Current Issue

GENERAL
 Select Angular Momentum Josephson Effect between Two Isolated Condensates Wei-Feng Zhuang, Yue-Xin Huang, and Ming Gong Chin. Phys. Lett. 2021, 38 (6): 060301 .   DOI: 10.1088/0256-307X/38/6/060301 Abstract   PDF(pc) (1155KB) ( 82 ) PDF(mobile)(1319KB) ( 8 ) HTML ( 54 )  We demonstrate that the two degenerate energy levels in spin–orbit coupled trapped Bose gases, coupled by a quenched Zeeman field, can be used for angular momentum Josephson effect. In a static quenched field, we can realize a Josephson oscillation with a period ranging from millisecond to hundreds of milliseconds. Moreover, by a driven Zeeman field, we realize a new Josephson oscillation, in which the population imbalance may have the same expression as the current in the direct-current Josephson effect. When the dynamics of the condensate cannot follow up the modulation frequency, it is in the self-trapping regime. This new dynamic is understood from the time-dependent evolution of the constant-energy trajectory in the phase space. This model has several salient advantages compared to the previous ones. The condensates are isolated from their excitations by a finite gap, thus can greatly suppress the damping effect induced by thermal atoms and Bogoliubov excitations. The oscillation period can be tuned by several orders of magnitude without influencing other parameters. In experiments, the dynamics can be mapped out from spin and momentum spaces, thus it is not limited by the spatial resolution in absorption imaging. This system can serve as a promising platform for matter wave interferometry and quantum metrology.
 Select Topological Knots in Quantum Spin Systems X. M. Yang , L. Jin, and Z. Song Chin. Phys. Lett. 2021, 38 (6): 060302 .   DOI: 10.1088/0256-307X/38/6/060302 Abstract   PDF(pc) (1433KB) ( 336 ) PDF(mobile)(1803KB) ( 87 ) HTML ( 142 )  Knot theory provides a powerful tool for understanding topological matters in biology, chemistry, and physics. Here knot theory is introduced to describe topological phases in a quantum spin system. Exactly solvable models with long-range interactions are investigated, and Majorana modes of the quantum spin system are mapped into different knots and links. The topological properties of ground states of the spin system are visualized and characterized using crossing and linking numbers, which capture the geometric topologies of knots and links. The interactivity of energy bands is highlighted. In gapped phases, eigenstate curves are tangled and braided around each other, forming links. In gapless phases, the tangled eigenstate curves may form knots. Our findings provide an alternative understanding of phases in the quantum spin system, and provide insights into one-dimension topological phases of matter.
 Select Space-Curved Resonant Line Solitons in a Generalized $(2+1)$-Dimensional Fifth-Order KdV System Zequn Qi , Zhao Zhang , and Biao Li Chin. Phys. Lett. 2021, 38 (6): 060501 .   DOI: 10.1088/0256-307X/38/6/060501 Abstract   PDF(pc) (604KB) ( 36 ) PDF(mobile)(652KB) ( 4 ) HTML ( 21 )  On the basis of $N$-soliton solutions, space-curved resonant line solitons are derived via a new constraint proposed here, for a generalized $(2+1)$-dimensional fifth-order KdV system. The dynamic properties of these new resonant line solitons are studied in detail. We then discuss the interaction between a resonance line soliton and a lump wave in greater detail. Our results highlight the distinctions between the generalized $(2+1)$-dimensional fifth-order KdV system and the classical type.
NUCLEAR PHYSICS
 Select Relativistic Chiral Description of the $^1\!S_0$ Nucleon–Nucleon Scattering Xiu-Lei Ren, Chun-Xuan Wang, Kai-Wen Li, Li-Sheng Geng, and Jie Meng Chin. Phys. Lett. 2021, 38 (6): 062101 .   DOI: 10.1088/0256-307X/38/6/062101 Abstract   PDF(pc) (489KB) ( 129 ) PDF(mobile)(664KB) ( 5 ) HTML ( 201 )  Recently, a relativistic chiral nucleon–nucleon interaction was formulated up to leading order, which provides a good description of the phase shifts of $J\leq1$ partial waves [Chin. Phys. C 42 (2018) 014103]. Nevertheless, a separable regulator function that is not manifestly covariant was used in solving the relativistic scattering equation. In the present work, we first explore a covariant and separable form factor to regularize the kernel potential and then apply it to study the simplest but most challenging $^1\!S_0$ channel which features several low-energy scales. In addition to being self-consistent, we show that the resulting relativistic potential can describe quite well the unique features of the $^1\!S_0$ channel at leading order, in particular the pole position of the virtual bound state and the zero amplitude at the scattering momentum $\sim$340 MeV, indicating that the relativistic formulation may be more natural from the viewpoint of effective field theories.
ATOMIC AND MOLECULAR PHYSICS
 Select Multi-Electron Transfer of Ar$^{+}$ Colliding with Ne Atoms Based on a Time-Dependent Density-Functional Theory Shuai Qin, Cong-Zhang Gao, Wandong Yu, and Yi-Zhi Qu Chin. Phys. Lett. 2021, 38 (6): 063101 .   DOI: 10.1088/0256-307X/38/6/063101 Abstract   PDF(pc) (814KB) ( 80 ) PDF(mobile)(831KB) ( 11 ) HTML ( 205 )  The multi-electron capture and loss cross-sections of Ar$^{+}$–Ne collisions are calculated at absolute energies in the few-keV/a.u. regime. The calculations are performed using a novel inverse collision framework, in the context of a time-dependent density functional theory, combined with molecular dynamics. The extraction of the capture and loss probabilities is based on the particle-number projection technique, originating from nuclear physics, but validly extended to represent many-electron systems. Good agreement between experimental and theoretical data is found, which clearly reveals the non-negligible post-collision decay of the projectile's electrons, providing further evidence for the applicability of the approach to complex many-electron collision systems.
 Select Coherent Control of High Harmonic Generation Driven by Metal Nanotip Photoemission Hongdan Zhang, Xiwang Liu, Facheng Jin, Ming Zhu, Shidong Yang, Wenhui Dong, Xiaohong Song, and Weifeng Yang Chin. Phys. Lett. 2021, 38 (6): 063201 .   DOI: 10.1088/0256-307X/38/6/063201 Abstract   PDF(pc) (838KB) ( 71 ) PDF(mobile)(693KB) ( 4 ) HTML ( 27 )  Steering ultrafast electron dynamics with well-controlled laser fields is very important for generation of intense supercontinuum radiation. It can be achieved through coherent control of the symmetry of the interaction between strong-field laser fields and a metal nanotip. We employ a scheme of two-color laser pulses combined with a weak static field to realize the control of a single quantum path to generate high harmonic generation from a single solid-state nanoemitter. Moreover, a smooth and ultrabroad supercontinuum in the extreme ultraviolet region is obtained, which can produce a single attosecond pulse. Our findings are beneficial for efficient generation of isolated sub-100 as XUV pulses from solid-state sources.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)
 Select Broadband Sheet Parametric Oscillator for $\chi^{(2)}$ Optical Frequency Comb Generation via Cavity Phase Matching Xin Ni, Kunpeng Jia, Xiaohan Wang, Huaying Liu, Jian Guo, Shu-Wei Huang, Baicheng Yao, Nicolò Sernicola, Zhenlin Wang, Xinjie Lv, Gang Zhao, Zhenda Xie, and Shi-Ning Zhu Chin. Phys. Lett. 2021, 38 (6): 064201 .   DOI: 10.1088/0256-307X/38/6/064201 Abstract   PDF(pc) (1725KB) ( 63 ) PDF(mobile)(2879KB) ( 0 ) HTML ( 20 )  We demonstrate a broadband optical parametric oscillation, using a sheet cavity, via cavity phase-matching. A 21.2 THz broad comb-like spectrum is achieved, with a uniform line spacing of 133.0 GHz, despite a relatively large dispersion of 275.4 fs$^{2}$/mm around 1064 nm. With 22.6% high slope efficiency, and 14.9 kW peak power handling, this sheet optical parametric oscillator can be further developed for $\chi^{(2)}$ comb.
 Select Bayesian Optimization for Wavefront Sensing and Error Correction Zhong-Hua Qian, Zi-Han Ding, Ming-Zhong Ai, Yong-Xiang Zheng, Jin-Ming Cui, Yun-Feng Huang, Chuan-Feng Li, and Guang-Can Guo Chin. Phys. Lett. 2021, 38 (6): 064202 .   DOI: 10.1088/0256-307X/38/6/064202 Abstract   PDF(pc) (835KB) ( 61 ) PDF(mobile)(934KB) ( 2 ) HTML ( 25 )  Algorithms for wavefront sensing and error correction from intensity attract great concern in many fields. Here we propose Bayesian optimization to retrieve phase and demonstrate its performance in simulation and experiment. For small aberration, this method demonstrates a convergence process with high accuracy of phase sensing, which is also verified experimentally. For large aberration, Bayesian optimization is shown to be insensitive to the initial phase while maintaining high accuracy. The approach's merits of high accuracy and robustness make it promising in being applied in optical systems with static aberration such as AMO experiments, optical testing shops, and electron or optical microscopes.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES
 Select Symmetry-Dependent Kinetics of Dislocation Reaction Hong Yu Chen, Lei Wang, and Tian Hui Zhang Chin. Phys. Lett. 2021, 38 (6): 066101 .   DOI: 10.1088/0256-307X/38/6/066101 Abstract   PDF(pc) (1339KB) ( 60 ) PDF(mobile)(1937KB) ( 2 ) HTML ( 18 )  Reactions between dislocations are investigated in two-dimensional colloidal crystals. It is found that, because of the conservation of total Burgers vectors, the kinetics of the reaction is dependent on the symmetry of the crystal lattice. Merging is possible only when the total Burgers vector of the reacting dislocations is in line with existing crystal lines. In non-merging reactions, the number of dislocations cannot be reduced but the interacting dislocations can exchange their Burgers vectors and migrate to different gliding lines. The changing of gliding lines promises additional annihilation in multi-dislocation reactions. The bonding of non-merging dislocations determines the configuration and the orientation of the grain boundaries. The findings in this study may shed new light on understanding of dislocations and have potential applications in fabrication of crystalline materials.
 Select New Members of High-Energy-Density Compounds: YN$_{5}$ and YN$_{8}$ Jun-Yi Miao, Zhan-Sheng Lu, Feng Peng, and Cheng Lu Chin. Phys. Lett. 2021, 38 (6): 066201 .   DOI: 10.1088/0256-307X/38/6/066201 Abstract   PDF(pc) (4452KB) ( 119 ) PDF(mobile)(5654KB) ( 7 ) HTML ( 103 )  Polymeric nitrogen is a promising candidate for a high-energy-density material. Synthesis of energetic compounds with high chemical stability under ambient conditions is still a challenging problem. Here we report a theoretical study on yttrium nitrides by first principles calculations combined with an effective crystal structure search method. It is found that many yttrium nitrides with high nitrogen content can be formed under relatively moderate pressures. The results indicate that the nitrogen-rich YN$_{5}$ and YN$_{8}$ compounds are recoverable as metastable high-energy materials under ambient conditions, and can release enormous energies (2.51 kJ$\cdot$g$^{-1}$ and 3.18 kJ$\cdot$g$^{-1}$) while decomposing to molecular nitrogen and YN. Our findings enrich the family of transition metal nitrides, and open avenues for design and synthesis of novel high-energy-density materials.
 Select Database Construction for Two-Dimensional Material-Substrate Interfaces Xian-Li Zhang, Jinbo Pan, Xin Jin, Yan-Fang Zhang, Jia-Tao Sun, Yu-Yang Zhang, and Shixuan Du Chin. Phys. Lett. 2021, 38 (6): 066801 .   DOI: 10.1088/0256-307X/38/6/066801 Abstract   PDF(pc) (1768KB) ( 209 ) PDF(mobile)(3419KB) ( 23 ) HTML ( 346 )  Interfacial structures and interactions of two-dimensional (2D) materials on solid substrates are of fundamental importance for fabrications and applications of 2D materials. However, selection of a suitable solid substrate to grow a 2D material, determination and control of 2D material-substrate interface remain a big challenge due to the large diversity of possible configurations. Here, we propose a computational framework to select an appropriate substrate for epitaxial growth of 2D material and to predict possible 2D material-substrate interface structures and orientations using density functional theory calculations performed for all non-equivalent atomic structures satisfying the symmetry constraints. The approach is validated by the correct prediction of three experimentally reported 2D material-substrate interface systems with only the given information of two parent materials. Several possible interface configurations are also proposed based on this approach. We therefore construct a database that contains these interface systems and has been continuously expanding. This database serves as preliminary guidance for epitaxial growth and stabilization of new materials in experiments.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
 Select NMR Evidence of Antiferromagnetic Spin Fluctuations in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ Yi Cui, Cong Li, Qing Li, Xiyu Zhu, Ze Hu, Yi-feng Yang, Jinshan Zhang, Rong Yu, Hai-Hu Wen, and Weiqiang Yu Chin. Phys. Lett. 2021, 38 (6): 067401 .   DOI: 10.1088/0256-307X/38/6/067401 Abstract   PDF(pc) (689KB) ( 182 ) PDF(mobile)(815KB) ( 6 ) HTML ( 268 )  Despite the recent discovery of superconductivity in Nd$_{1-x}$Sr$_{x}$NiO$_2$ thin films, the absence of superconductivity and antiferromagnetism in their bulk materials remains a puzzle. Here we report the $^{1}$H NMR measurements on powdered Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ samples by taking advantage of the enriched proton concentration after hydrogen annealing. We find a large full width at half maximum of the spectrum, which keeps increasing with decreasing the temperature $T$ and exhibits an upturn behavior at low temperatures. The spin-lattice relaxation rate $^{1}T_1^{-1}$ is strongly enhanced when lowering the temperature, developing a broad peak at about 40 K, then decreases following a spin-wave-like behavior $^{1}T_1^{-1}\propto T^2$ at lower temperatures. These results evidence a short-range glassy antiferromagnetic ordering of magnetic moments below 40 K and dominant antiferromagnetic fluctuations extending to much higher temperatures. Our findings reveal the strong electron correlations in bulk Nd$_{0.85}$Sr$_{0.15}$NiO$_2$, and shed light on the mechanism of superconductivity observed in films of nickelates.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
 Select Bufferless Epitaxial Growth of GaAs on Step-Free Ge (001) Mesa Ding-Ming Huang, Jie-Yin Zhang, Jian-Huan Wang, Wen-Qi Wei, Zi-Hao Wang, Ting Wang, and Jian-Jun Zhang Chin. Phys. Lett. 2021, 38 (6): 068101 .   DOI: 10.1088/0256-307X/38/6/068101 Abstract   PDF(pc) (2784KB) ( 83 ) PDF(mobile)(3706KB) ( 4 ) HTML ( 96 )  GaAs/Ge heterostructures have been employed in various semiconductor devices such as solar cells, high-performance CMOS transistors, and III–V/IV heterogeneous optoelectronic devices. The performance of these devices is directly dependent on the material quality of the GaAs/Ge heterostructure, while the material quality of the epitaxial GaAs layer on the Ge is limited by issues such as the antiphase domain (APD), and stacking-fault pyramids (SFP). We investigate the epitaxial growth of high-quality GaAs on a Ge (001) mesa array, via molecular beam epitaxy. Following a systematic study of the Ge terrace via an in situ scanning tunneling microscope, an atomically step-free terrace on the Ge mesa measuring up to $5 \times 5$ µm$^{2}$ is obtained, under optimized growth conditions. The step-free terrace has a single-phase $c$ ($4\times 2$) surface reconstruction. The deposition of a high-quality GaAs layer with no APD and SFP is then achieved on this step-free Ge terrace. High-resolution transmission electron microscopy and electron channel contrast image characterizations reveal the defect-free growth of the GaAs layer on the step-free Ge mesa. Furthermore, InAs quantum dots on this GaAs/Ge mesa reveal photoluminescent intensity comparable to that achieved on a GaAs substrate, which further confirms the high quality of the GaAs layer on Ge.
 Select Effect of Pt Interlayer on Low Resistivity Ohmic Contact to p-InP Layer and Its Optimization Lili Han, Chunhua Du, Ziguang Ma, Yang Jiang, Kanglin Xiong, Wenxin Wang, Hong Chen, Zhen Deng, and Haiqiang Jia Chin. Phys. Lett. 2021, 38 (6): 068102 .   DOI: 10.1088/0256-307X/38/6/068102 Abstract   PDF(pc) (1027KB) ( 59 ) PDF(mobile)(2582KB) ( 4 ) HTML ( 71 )  The contact characteristic between p-InP and metal plays an important role in InP-related optoelectronic and microelectronic device applications. We investigate the low-resistance Au/Pt/Ni and Au/Ni ohmic contacts to p-InP based on the solid phase regrowth principle. The lowest specific contact resistivity of Au(100 nm)/Pt(115 nm)/Ni (50 nm) can reach $2.64 \times 10^{-6}\,\Omega \cdot$cm$^{2}$ after annealing at 380 ℃ for 1 min, while the contact characteristics of Au/Ni deteriorated after annealing from 340 ℃ to 480 ℃ for 1 min. The results of scanning electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy show that the Pt layer is an important factor in improving the contact characteristics. The Pt layer prevents the diffusion of In and Au, inhibits the formation of Au$_{3}$In metal compounds, and prevents the deterioration of the ohmic contact. The metal structures and optimized annealing process is expected to be helpful for obtaining high-performance InP-related devices.
 Select High-Performance Visible Light Photodetector Based on BiSeI Single Crystal Xiu Yan, Wei-Li Zhen, Hui-Jie Hu, Li Pi, Chang-Jin Zhang, and Wen-Ka Zhu Chin. Phys. Lett. 2021, 38 (6): 068103 .   DOI: 10.1088/0256-307X/38/6/068103 Abstract   PDF(pc) (2389KB) ( 80 ) PDF(mobile)(2572KB) ( 3 ) HTML ( 43 )  The continuing demand for new optoelectronic devices drives researchers to seek new materials suitable for photodetector applications. Recently, ternary compound semiconductors have entered researchers' field of vision, among which chalcohalides have attracted special interest because of their rich properties and unique crystal structure consisting of atom chains and inter-chain van der Waals gaps. We have synthesized high-quality BiSeI single crystals with [110]-plane orientation and fabricated a photodetector. The optoelectronic measurements show a pronounced photocurrent signal with outstanding technical parameters, namely high responsivity (3.2 A/W), specific detectivity ($7 \times 10^{10}$ Jones) and external quantum efficiency (622%) for $\lambda = 635$ nm, $V_{\rm ds} = 0.1$ V and $P_{\rm opt} = 0.23$ mW/cm$^{2}$. The high performance of BiSeI photodetector and its layer structure make it a promising candidate for low-dimensional optoelectronic applications.
 Select FeSO$_{4}$ as a Novel Li-Ion Battery Cathode Jiachao Yang, Jian Zou, Chun Luo, Qiwen Ran, Xin Wang, Pengyu Chen, Chuan Hu, Xiaobin Niu, Haining Ji, and Liping Wang Chin. Phys. Lett. 2021, 38 (6): 068201 .   DOI: 10.1088/0256-307X/38/6/068201 Abstract   PDF(pc) (1466KB) ( 42 ) PDF(mobile)(1400KB) ( 1 ) HTML ( 40 )  FeSO$_{4}$ has the characteristics of low cost and theoretical high energy density (799 W$\cdot$h$\cdot$kg$^{-1}$ with a two-electron reaction), which can meet the demand for next-generation lithium-ion batteries. Herein, FeSO$_{4}$ as a novel high-performance conversion-reaction type cathode is investigated. We use dopamine as a carbon coating source to increase its electronic conductivity. FeSO$_{4}$@C demonstrates a high reversible specific capacity (512 mA$\cdot$h$\cdot$g$^{-1}$) and a superior cycling performance (482 mA$\cdot$h$\cdot$g$^{-1}$ after 250 cycles). In addition, we further study its reaction mechanism. The FeSO$_{4}$ is converted to Fe and Li$_{2}$SO$_{4}$ during lithium ion insertion and the Fe|Li$_{2}$SO$_{4}$ grain boundaries further store additional lithium ions. Our findings are valuable in exploring other new conversion-type lithium ion battery cathodes.
 Select LiCoO$_{2}$ Epitaxial Film Enabling Precise Analysis of Interfacial Degradations Changdong Qin, Le Wang, Pengfei Yan, Yingge Du, and Manling Sui Chin. Phys. Lett. 2021, 38 (6): 068202 .   DOI: 10.1088/0256-307X/38/6/068202 Abstract   PDF(pc) (3045KB) ( 134 ) PDF(mobile)(2271KB) ( 11 ) HTML ( 184 )  Interfacial structure evolution and degradation are critical to the electrochemical performance of LiCoO$_{2}$ (LCO), the most widely studied and used cathode material in lithium ion batteries. To understand such processes requires precise and quantitative measurements. Herein, we use well-defined epitaxial LCO thin films to reveal the interfacial degradation mechanisms. Through our systematical investigations, we find that surface corrosion is significant after forming the surface phase transition layer, and the cathode electrolyte interphase (CEI) has a double layer structure, an inorganic inner layer containing CoO, LiF, LiOH/Li$_{2}$O and Li$_{x}$PF$_{y}$O$_{z}$, and an outmost layer containing Li$_{2}$CO$_{3}$ and organic carbonaceous components. Furthermore, surface cracks are found to be pronounced due to mechanical failures and chemical etching. This work demonstrates a model material to realize the precise measurements of LCO interfacial degradations, which deepens our understanding on the interfacial degradation mechanisms.
17 articles