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 Select Experimental Realization of an Intrinsic Magnetic Topological Insulator Yan Gong, Jingwen Guo, Jiaheng Li, Kejing Zhu, Menghan Liao, Xiaozhi Liu, Qinghua Zhang, Lin Gu, Lin Tang, Xiao Feng, Ding Zhang, Wei Li, Canli Song, Lili Wang, Pu Yu, Xi Chen, Yayu Wang, Hong Yao, Wenhui Duan, Yong Xu, Shou-Cheng Zhang, Xucun Ma, Qi-Kun Xue, Ke He Chin. Phys. Lett.    2019, 36 (7): 076801 .   DOI: 10.1088/0256-307X/36/7/076801 Abstract   HTML   PDF (7918KB) An intrinsic magnetic topological insulator (TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi$_{2}$Te$_{4}$, by alternate growth of a Bi$_{2}$Te$_{3}$ quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.
 Select Discovery of Fractionalized Neutral Spin-1/2 Excitation of Topological Order Xiao-Gang Wen Chin. Phys. Lett.    2017, 34 (9): 090101 .   DOI: 10.1088/0256-307X/34/9/090101 Abstract   HTML   PDF (327KB)
 Select A High-Sensitivity Terahertz Detector Based on a Low-Barrier Schottky Diode Xiao-Yu Liu, Yong Zhang, De-Jiao Xia, Tian-Hao Ren, Jing-Tao Zhou, Dong Guo, Zhi Jin Chin. Phys. Lett.    2017, 34 (7): 070701 .   DOI: 10.1088/0256-307X/34/7/070701 Abstract   HTML   PDF (1269KB) A low-barrier Schottky barrier diode based on the InGaAs/InP material system is designed and fabricated with a new non-destructive dry over-etching process. By using this diode, a high-sensitivity waveguide detector is proposed. The measured maximum responsivity is over 2000 mV/mW at 630 GHz. The measured noise effective power (NEP) is less than 35 pW/Hz$^{0.5}$ at 570–630 GHz. The minimum NEP is 14 pW/Hz$^{0.5}$ at 630 GHz. The proposed high-sensitivity waveguide detector has the characteristics of simple structure, compact size, low cost and high performance, and can be used in a variety of applications such as imaging, molecular spectroscopy and atmospheric remote sensing.
 Select Rear-Surface Deformation of a Water Drop in Aero-Breakup of Shear Mode Xiang-Yu Yi, Yu-Jian Zhu, Ji-Ming Yang, Tun Wang, Ming-Yu Sun Chin. Phys. Lett.    2017, 34 (8): 084701 .   DOI: 10.1088/0256-307X/34/8/084701 Abstract   HTML   PDF (1990KB) Deformation of water drops in shock-induced high-speed flows is investigated with a focus to the influence of primitive flow parameters on the rear-surface deformation features. Two typical deformation patterns are discovered through high-speed photography. A simple equation to evaluate the radial acceleration of the drop surface is derived. The combined use of this equation and outer flow simulation makes it possible for us to reconstruct the profiles of the early deformed drops. The results agree well with the experiments. Further analysis shows that the duration of flow establishment with respect to the overall breakup time shapes the rear side profile of the drop. Thereby the ratio of the two times, expressed as the square root of the density ratio, appears to be an effective indicator of the deformation features.
 Select Erbium-Doped Zirconia-Alumina Silica Glass-Based Fiber as a Saturable Absorber for High Repetition Rate Q-Switched All-Fiber Laser Generation P. Harshavardhan Reddy, N. A. A. Kadir, M. C. Paul, S. Das, A. Dhar, E. I. Ismail, A. A. Latiff, S. W. Harun Chin. Phys. Lett.    2017, 34 (8): 084203 .   DOI: 10.1088/0256-307X/34/8/084203 Abstract   HTML   PDF (2477KB) We propose and demonstrate a Q-switched erbium-doped fiber laser (EDFL) using an erbium-doped zirconia-alumina silica glass-based fiber (Zr-EDF) as a saturable absorber. As a 16-cm-long Zr-EDF is incorporated into a ring EDFL cavity, a stable Q-switching pulse train operating at 1565 nm wavelength is successfully obtained. The repetition rate is tunable from 33.97 kHz to 71.23 kHz by increasing the pump power from the threshold of 26 mW to the maximum of 74 mW. The highest pulse energy of 26.67 nJ is obtained at the maximum pump power.
 Select Numerical Analysis of Magnetic-Shielding Effectiveness for Magnetic Resonant Wireless Power Transfer System Wei-Guo Lu, Hui-Rong Li, Wei-Ming Chen, Li-Hui Liu Chin. Phys. Lett.    2017, 34 (8): 088801 .   DOI: 10.1088/0256-307X/34/8/088801 Abstract   HTML   PDF (1676KB) Magnetic radiation phenomena appear inevitably in the magnetic-resonance wireless power transfer (MR-WPT) system, and regarding this problem the magnetic-shielding scheme is applied to improve the electromagnetic performance in engineering. In this study, the shielding effectiveness of a two-coil MR-WPT system for different material shields is analyzed in theory using Moser's formula and Schelkunoff's formula. On this basis a candidate magnetic-shielding scheme with a double-layer structure is determined, which has better shielding effectiveness and coils coupling coefficient. Finally, some finite element simulation results validate the correctness of the theoretical analysis, and the shielding effectiveness with the double-layer shield in maximum is 30 dB larger than the one with the single-layer case.
 Select Electronic, Elastic and Piezoelectric Properties of Two-Dimensional Group-IV Buckled Monolayers Jing Shi, Yong Gao, Xiao-Li Wang, Si-Ning Yun Chin. Phys. Lett.    2017, 34 (8): 087701 .   DOI: 10.1088/0256-307X/34/8/087701 Abstract   HTML   PDF (791KB) Electronic, elastic and piezoelectric properties of two-dimensional (2D) group-IV buckled monolayers (GeSi, SnSi and SnGe) are studied by first principle calculations. According to our calculations, SnSi and SnGe are good 2D piezoelectric materials with large piezoelectric coefficients. The values of $d_{11}$ of SnSi and SnGe are 5.04 pm/V and 5.42 pm/V, respectively, which are much larger than 2D MoS$_{2}$ (3.6 pm/V) and are comparable with some frequently used bulk materials (e.g., wurtzite AlN 5.1 pm/V). Charge transfer is calculated by the Löwdin analysis and we find that the piezoelectric coefficients ($d_{11}$ and $d_{31}$) are highly dependent on the polarizabilities of the anions and cations in group-IV monolayers.
 Select High Refractive Index Ti$_3$O$_5$ Films for Dielectric Metasurfaces Sohail Abdul Jalil, Mahreen Akram, Gwanho Yoon, Ayesha Khalid, Dasol Lee, Niloufar Raeis-Hosseini, Sunae So, Inki Kim, Qazi Salman Ahmed, Junsuk Rho, Muhammad Qasim Mehmood Chin. Phys. Lett.    2017, 34 (8): 088102 .   DOI: 10.1088/0256-307X/34/8/088102 Abstract   HTML   PDF (1298KB) Ti$_3$O$_5$ films are deposited with the help of an electron beam evaporator for their applications in metasurfaces. The film of subwavelength (632 nm) thickness is deposited on a silicon substrate and annealed at 400$^{\circ}\!$C. The ellipsometry result shows a high refractive index above 2.5 with the minimum absorption coefficient in the visible region, which is necessary for high efficiency of transparent metasurfaces. Atomic force microscopy analysis is employed to measure the roughness of the as-deposited films. It is seen from micrographs that the deposited films are very smooth with the minimum roughness to prevent scattering and absorption losses for metasurface devices. The absence of grains and cracks can be seen by scanning electron microscope analysis, which is favorable for electron beam lithography. Fourier transform infrared spectroscopy reveals the transmission and reflection obtained from the film deposited on glass substrates. The as-deposited film shows high transmission above 60%, which is in good agreement with metasurfaces.
 Select Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu$_3$Zn(OH)$_6$FBr Zili Feng, Zheng Li, Xin Meng, Wei Yi, Yuan Wei, Jun Zhang, Yan-Cheng Wang, Wei Jiang, Zheng Liu, Shiyan Li, Feng Liu, Jianlin Luo, Shiliang Li, Guo-qing Zheng, Zi Yang Meng, Jia-Wei Mei, Youguo Shi Chin. Phys. Lett.    2017, 34 (7): 077502 .   DOI: 10.1088/0256-307X/34/7/077502 Abstract   HTML   PDF (3207KB) We report a new kagome quantum spin liquid candidate Cu$_3$Zn(OH)$_6$FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature ($\sim$200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from $^{19}$F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.
 Select Lorentz Force Electrical Impedance Detection Using Step Frequency Technique Zhi-Shen Sun, Guo-Qiang Liu, Hui Xia Chin. Phys. Lett.    2018, 35 (1): 014301 .   DOI: 10.1088/0256-307X/35/1/014301 Abstract   HTML   PDF (520KB) Lorentz force electrical impedance tomography (LFEIT) inherits the merit of high resolution by ultrasound stimulation and the merit of high contrast through electromagnetic field detection. To reduce the instantaneous peak power of the stimulating signal to the transducer, the sinusoidal pulse and step-frequency technique is investigated in LFEIT. The theory of application of step-frequency technique in LFEIT is formulated with the direct demodulation method and the in-phase quadrature demodulation method. Compared with the in-phase quadrature demodulation method, the direct demodulation method has simple experimental setup but could only detect half of the range. Experiments carried out with copper foils confirmed that LFEIT using the step-frequency technique could detect the electrical conductivity variations precisely, which suggests an alternative method of realization of LFEIT.
 Select Characterization of Interface State Density of Ni/p-GaN Structures by Capacitance/Conductance-Voltage-Frequency Measurements Zhi-Fu Zhu, He-Qiu Zhang, Hong-Wei Liang, Xin-Cun Peng, Ji-Jun Zou, Bin Tang, Guo-Tong Du Chin. Phys. Lett.    2017, 34 (9): 097301 .   DOI: 10.1088/0256-307X/34/9/097301 Abstract   HTML   PDF (703KB) For the frequency range of 1 kHz–10 MHz, the interface state density of Ni contacts on p-GaN is studied using capacitance-voltage ($C$–$V$) and conductance-frequency-voltage ($G$–$f$–$V$) measurements at room temperature. To obtain the real capacitance and interface state density of the Ni/p-GaN structures, the effects of the series resistance ($R_{\rm s}$) on high-frequency (5 MHz) capacitance values measured at a reverse and a forward bias are investigated. The mean interface state densities obtained from the $C_{\rm HF}$–$C_{\rm LF}$ capacitance and the conductance method are $2\times10^{12}$ eV$^{-1}$cm$^{-2}$ and $0.94\times10^{12}$ eV$^{-1}$cm$^{-2}$, respectively. Furthermore, the interface state density derived from the conductance method is higher than that reported from the Ni/n-GaN in the literature, which is ascribed to a poor crystal quality and to a large defect density of the Mg-doped p-GaN.
 Select A 526mJ Subnanosecond Pulsed Hybrid-Pumped Nd:YAG Laser Jiang-Peng Shi, Jian-Guo Xin, Jun Liu, Jia-Bin Chen, Sher Zaman Chin. Phys. Lett.    2017, 34 (7): 074208 .   DOI: 10.1088/0256-307X/34/7/074208 Abstract   HTML   PDF (724KB) A hybrid-pumped Nd:YAG pulse laser with a double-pass two-rod configuration is presented. The focal length of offset lens is particularly studied to compensate for the thermal lens effect and depolarization. For input pulse energy of 141 $\mu$J with pulse duration of 754 ps, the pulse laser system delivers 526 mJ pulse energy and 728 ps pulse width output at 10 Hz with pulse profile shape preservation. The energy stability of the laser pulse is less than 3%, and the beam quality factor $M^2$ is less than 2.26.
 Select Magnetic Sensing inside a Diamond Anvil Cell via Nitrogen-Vacancy Center Spins Yan-Xing Shang, Fang Hong, Jian-Hong Dai, Hui-Yu, Ya-Nan Lu, En-Ke Liu, Xiao-Hui Yu, Gang-Qin Liu, Xin-Yu Pan Chin. Phys. Lett.    2019, 36 (8): 086201 .   DOI: 10.1088/0256-307X/36/8/086201 Abstract   HTML   PDF (1001KB) The diamond anvil cell-based high-pressure technique is a unique tool for creating new states of matter and for understanding the physics underlying some exotic phenomena. In situ sensing of spin and charge properties under high pressure is crucially important but remains technically challenging. While the nitrogen-vacancy (NV) center in diamond is a promising quantum sensor under extreme conditions, its spin dynamics and the quantum control of its spin states under high pressure remain elusive. In this study, we demonstrate coherent control, spin relaxation, and spin dephasing measurements for ensemble NV centers up to 32.8 GPa. With this in situ quantum sensor, we investigate the pressure-induced magnetic phase transition of a micron-size permanent magnet Nd$_{2}$Fe$_{14}$B sample in a diamond anvil cell, with a spatial resolution of $\sim$2 μm, and sensitivity of $\sim$20 $\mu$T/Hz$^{1/2}$. This scheme could be generalized to measure other parameters such as temperature, pressure and their gradients under extreme conditions. This will be beneficial for frontier research of condensed matter physics and geophysics.
 Select Superconducting (Li,Fe)OHFeSe Film of High Quality and High Critical Parameters Yulong Huang, Zhongpei Feng, Shunli Ni, Jun Li, Wei Hu, Shaobo Liu, Yiyuan Mao, Huaxue Zhou, Fang Zhou, Kui Jin, Huabing Wang, Jie Yuan, Xiaoli Dong, Zhongxian Zhao Chin. Phys. Lett.    2017, 34 (7): 077404 .   DOI: 10.1088/0256-307X/34/7/077404 Abstract   HTML   PDF (989KB) A superconducting film of (Li$_{1-x}$Fe$_{x})$OHFeSe is reported for the first time. The thin film exhibits a small in-plane crystal mosaic of 0.22$^{\circ}$, in terms of the full width at half maximum of the x-ray rocking curve, and an excellent out-of-plane orientation by x-ray $\varphi$-scan. Its bulk superconducting transition temperature $T_{\rm c}$ of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. The upper critical field $H_{\rm c2}$ is estimated to be 79.5 T and 443 T for the magnetic field perpendicular and parallel to the $ab$ plane, respectively. Moreover, a large critical current density $J_{\rm c}$ of a value over 0.5 MA/cm$^{2}$ is achieved at $\sim$20 K. Such a (Li$_{1-x}$Fe$_{x})$OHFeSe film is therefore not only important to the fundamental research for understanding the high-$T_{\rm c}$ mechanism, but also promising in the field of high-$T_{\rm c}$ superconductivity application, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.
 Select Electrically Tunable Energy Bandgap in Dual-Gated Ultra-Thin Black Phosphorus Field Effect Transistors Shi-Li Yan, Zhi-Jian Xie, Jian-Hao Chen, Takashi Taniguchi, Kenji Watanabe Chin. Phys. Lett.    2017, 34 (4): 047304 .   DOI: 10.1088/0256-307X/34/4/047304 Abstract   HTML   PDF (1435KB) The energy bandgap is an intrinsic character of semiconductors, which largely determines their properties. The ability to continuously and reversibly tune the bandgap of a single device during real time operation is of great importance not only to device physics but also to technological applications. Here we demonstrate a widely tunable bandgap of few-layer black phosphorus (BP) by the application of vertical electric field in dual-gated BP field-effect transistors. A total bandgap reduction of 124 meV is observed when the electrical displacement field is increased from 0.10 V/nm to 0.83 V/nm. Our results suggest appealing potential for few-layer BP as a tunable bandgap material in infrared optoelectronics, thermoelectric power generation and thermal imaging.
 Select Two-Dimensional Node-Line Semimetals in a Honeycomb-Kagome Lattice Jin-Lian Lu, Wei Luo, Xue-Yang Li, Sheng-Qi Yang, Jue-Xian Cao, Xin-Gao Gong, Hong-Jun Xiang Chin. Phys. Lett.    2017, 34 (5): 057302 .   DOI: 10.1088/0256-307X/34/5/057302 Abstract   HTML   PDF (1478KB) Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional (3D) Weyl semimetals, 3D Dirac semimetals, and 3D node-line semimetals (NLSs). In particular, several compounds (e.g., certain 3D graphene networks, Cu$_{3}$PdN, Ca$_{3}$P$_{2}$) were discovered to be 3D NLSs, in which the conduction and valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional (2D) Dirac semimetal (e.g., graphene), 2D topological semimetals are much less investigated. Here we propose a new concept of a 2D NLS and suggest that this state could be realized in a new mixed lattice (named as HK lattice) composed by Kagome and honeycomb lattices. It is found that A$_{3}$B$_{2}$ (A is a group-IIB cation and B is a group-VA anion) compounds (such as Hg$_{3}$As$_{2})$ with the HK lattice are 2D NLSs due to the band inversion between the cation Hg-$s$ orbital and the anion As-$p_{z}$ orbital with respect to the mirror symmetry. Since the band inversion occurs between two bands with the same parity, this peculiar 2D NLS could be used as transparent conductors. In the presence of buckling or spin-orbit coupling, the 2D NLS state may turn into a 2D Dirac semimetal state or a 2D topological crystalline insulating state. Since the band gap opening due to buckling or spin-orbit coupling is small, Hg$_{3}$As$_{2}$ with the HK lattice can still be regarded as a 2D NLS at room temperature. Our work suggests a new route to design topological materials without involving states with opposite parities.
 Select Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS$_{2}$ Hao Wu, Yong-Hui Zhou, Yi-Fang Yuan, Chun-Hua Chen, Ying Zhou, Bo-Wen Zhang, Xu-Liang Chen, Chuan-Chuan Gu, Chao An, Shu-Yang Wang, Meng-Yao Qi, Ran-Ran Zhang, Li-Li Zhang, Xin-Jian Li, Zhao-Rong Yang Chin. Phys. Lett.    2019, 36 (10): 107101 .   DOI: 10.1088/0256-307X/36/10/107101 Abstract   HTML   PDF (1071KB) The insulator-metal transition triggered by pressure in charge transfer insulator NiS$_{2}$ is investigated by combining high-pressure electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements up to 40–50 GPa. Upon compression, we show that the metallization firstly appears in the low temperature region at $\sim$3.2 GPa and then extends to room temperature at $\sim$8.0 GPa. During the insulator-metal transition, the bond length of S–S dimer extracted from the synchrotron x-ray diffraction increases with pressure, which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa. Considering the decreasing bonding-antibonding splitting due to the expansion of S–S dimer, the charge gap between the S-$pp\pi^*$ band and the upper Hubbard band of Ni-3$d$ $e_{\rm g}$ state is remarkably decreased. These results consistently indicate that the elongated S–S dimer plays a predominant role in the insulator-metal transition under high pressure, even though the $p$-$d$ hybridization is enhanced simultaneously, in accordance with a scenario of charge-gap-controlled type.
 Select Simulation and Experiments on the Capillary Force between a Circular Disk and a Parallel Substrate Le-Feng Wang, Ben-Song Huang, Yuan-Zhe He, Wei-Bin Rong, Li-Ning Sun Chin. Phys. Lett.    2017, 34 (5): 056801 .   DOI: 10.1088/0256-307X/34/5/056801 Abstract   HTML   PDF (421KB) The capillary force of a liquid bridge with a pinned contact line between a small disk and a parallel plate is investigated by simulation and experiments. The numerical minimization simulation method is utilized to calculate the capillary force. The results show excellent agreement with the Young–Laplace equation method. An experimental setup is built to measure the capillary force. The experimental results indicate that the simulation results agree well with the measured forces at large separation distances, while some deviation may occur due to the transition from the advancing contact angle to the receding one at small distances. It is also found that the measured rupture distance is slightly larger than the simulation value due to the effect of the viscous interaction inside the liquid bridge.
 Select Structural, Optical and Luminescence Properties of ZnO Thin Films Prepared by Sol-Gel Spin-Coating Method: Effect of Precursor Concentration R. Amari, A. Mahroug, A. Boukhari, B. Deghfel, N. Selmi Chin. Phys. Lett.    2018, 35 (1): 016801 .   DOI: 10.1088/0256-307X/35/1/016801 Abstract   HTML   PDF (644KB) Transparent zinc oxide (ZnO) thin films are fabricated by a simple sol-gel spin-coating technique on glass substrates with different solution concentrations (0.3–1.2 M) using zinc acetate dehydrate [Zn(CH$_{3}$COO)$_{2}\cdot$2H$_{2}$O] as precursor and isopropanol and monoethanolamine (MEA) as solvent and stabilizer, respectively. The molar ratio of zinc acetate dehydrate to MEA is 1.0. X-ray diffraction, ultraviolet-visible spectroscopy and photoluminescence spectroscopy are employed to investigate the effect of solution concentration on the structural and optical properties of the ZnO thin films. The obtained results of all thin films are discussed in detail and are compared with other experimental data.
 Select Strain Effects on Properties of Phosphorene and Phosphorene Nanoribbons: a DFT and Tight Binding Study Ruo-Yu Zhang, Ji-Ming Zheng, Zhen-Yi Jiang Chin. Phys. Lett.    2018, 35 (1): 017302 .   DOI: 10.1088/0256-307X/35/1/017302 Abstract   HTML   PDF (1046KB) We perform comprehensive density functional theory calculations of strain effect on electronic structure of black phosphorus (BP) and on BP nanoribbons. Both uniaxial and biaxial strain are applied, and the dramatic change of BP's band structure is observed. Under 0–8% uniaxial strain, the band gap can be modulated in the range of 0.55–1.06 eV, and a direct–indirect band gap transition causes strain over 4% in the $y$ direction. Under 0–8% biaxial strain, the band gap can be modulated in the range of 0.35–1.09 eV, and the band gap maintains directly. Applying strain to BP nanoribbon, the band gap value reduces or enlarges markedly either zigzag nanoribbon or armchair nanoribbon. Analyzing the orbital composition and using a tight-binding model we ascribe this band gap behavior to the competition between effects of different bond lengths on band gap. These results would enhance our understanding on strain effects on properties of BP and phosphorene nanoribbon.
 Select Superconducting Single-Layer T-Graphene and Novel Synthesis Routes Qinyan Gu, Dingyu Xing, Jian Sun Chin. Phys. Lett.    2019, 36 (9): 097401 .   DOI: 10.1088/0256-307X/36/9/097401 Abstract   HTML   PDF (15832KB) Single-layer superconductors are ideal materials for fabricating superconducting nano devices. However, up to date, very few single-layer elemental superconductors have been predicted and especially no one has been successfully synthesized yet. Here, using crystal structure search techniques and ab initio calculations, we predict that a single-layer planar carbon sheet with 4- and 8-membered rings called T-graphene is a new intrinsic elemental superconductor with superconducting critical temperature ($T_{\rm c}$) up to around 20.8 K. More importantly, we propose a synthesis route to obtain such a single-layer T-graphene, that is, a T-graphene potassium intercalation compound (C$_4$K with $P4/mmm$ symmetry) is firstly synthesized at high pressure ($>$11.5 GPa) and then quenched to ambient condition; and finally, the single-layer T-graphene can be either exfoliated using the electrochemical method from the bulk C$_4$K, or peeled off from bulk T-graphite C$_4$, where C$_4$ can be obtained from C$_4$K by evaporating the K atoms. Interestingly, we find that the calculated $T_{\rm c}$ of C$_4$K is about 30.4 K at 0 GPa, which sets a new record for layered carbon-based superconductors. The present findings add a new class of carbon-based superconductors. In particular, once the single-layer T-graphene is synthesized, it can pave the way for fabricating superconducting devices together with other 2D materials using the layer-by-layer growth techniques.
 Select Charge Density Wave States and Structural Transition in Layered Chalcogenide TaSe$_{2-x}$Te$_{x}$ Lin-Lin Wei, Shuai-Shuai Sun, Kai Sun, Yu Liu, Ding-Fu Shao, Wen-Jian Lu, Yu-Ping Sun, Huan-Fang Tian, Huai-Xin Yang Chin. Phys. Lett.    2017, 34 (8): 086101 .   DOI: 10.1088/0256-307X/34/8/086101 Abstract   HTML   PDF (2332KB) The structural features and three-dimensional nature of the charge density wave (CDW) state of the layered chalcogenide 1T-TaSe$_{2-x}$Te$_{x}$ ($0\le x\le 2.0$) are characterized by Cs-corrected transmission electron microscopy measurements. Notable changes of both average structure and the CDW state arising from Te substitution for Se are clearly demonstrated in samples with $x>0.3$. The commensurate CDW state characterized by the known star-of-David clustering in the 1T-TaSe$_{2}$ crystal becomes visibly unstable with Te substitution and vanishes when $x=0.3$. The 1T-TaSe$_{2-x}$Te$_{x}$ ($0.3\le x\le 1.3$) samples generally adopt a remarkable incommensurate CDW state with monoclinic distortion, which could be fundamentally in correlation with the strong $q$-dependent electron–phonon coupling-induced period-lattice-distortion as identified in TaTe$_{2}$. Systematic analysis demonstrates that the occurrence of superconductivity is related to the suppression of the commensurate CDW phase and the presence of discommensuration is an evident structural feature observed in the superconducting samples.
 Select Machine Learning to Instruct Single Crystal Growth by Flux Method Tang-Shi Yao, Cen-Yao Tang, Meng Yang, Ke-Jia Zhu, Da-Yu Yan, Chang-Jiang Yi, Zi-Li Feng, He-Chang Lei, Cheng-He Li, Le Wang, Lei Wang, You-Guo Shi, Yu-Jie Sun, Hong Ding Chin. Phys. Lett.    2019, 36 (6): 068101 .   DOI: 10.1088/0256-307X/36/6/068101 Abstract   HTML   PDF (691KB) Growth of high-quality single crystals is of great significance for research of condensed matter physics. The exploration of suitable growing conditions for single crystals is expensive and time-consuming, especially for ternary compounds because of the lack of ternary phase diagram. Here we use machine learning (ML) trained on our experimental data to predict and instruct the growth. Four kinds of ML methods, including support vector machine (SVM), decision tree, random forest and gradient boosting decision tree, are adopted. The SVM method is relatively stable and works well, with an accuracy of 81% in predicting experimental results. By comparison, the accuracy of laboratory reaches 36%. The decision tree model is also used to reveal which features will take critical roles in growing processes.
 Select Distinct Superconducting Gap on Two Bilayer-Split Fermi Surface Sheets in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Superconductor Ping Ai, Qiang Gao, Jing Liu, Yuxiao Zhang, Cong Li, Jianwei Huang, Chunyao Song, Hongtao Yan, Lin Zhao, Guo-Dong Liu, Gen-Da Gu, Feng-Feng Zhang, Feng Yang, Qin-Jun Peng, Zu-Yan Xu, Xing-Jiang Zhou Chin. Phys. Lett.    2019, 36 (6): 067402 .   DOI: 10.1088/0256-307X/36/6/067402 Abstract   HTML   PDF (2816KB) High resolution laser-based angle-resolved photoemission measurements are carried out on an overdoped superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ with a $T_{\rm c}$ of 75 K. Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels: the bonding sheet corresponds to a doping level of 0.14, which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped, giving an overall doping level of 0.20 for the sample. Different superconducting gap sizes on the two Fermi surface sheets are revealed. The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet. The maximum gap difference between the two Fermi surface sheets near the antinodal region is $\sim$2 meV. These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.
 Select Validation of the Ability of Full Configuration Interaction Quantum Monte Carlo for Studying the 2D Hubbard Model Su-Jun Yun, Tie-Kuang Dong, Shi-Ning Zhu Chin. Phys. Lett.    2017, 34 (8): 080201 .   DOI: 10.1088/0256-307X/34/8/080201 Abstract   HTML   PDF (567KB) To validate the ability of full configuration interaction quantum Monte Carlo (FCIQMC) for studying the 2D Hubbard model near half-filling regime, the ground state energies of a $4\times4$ square lattice system with various interaction strengths are calculated. It is found that the calculated results are in good agreement with those obtained by exact diagonalization (i.e., the exact values for a given basis set) when the population of psi particles (psips) is higher than the critical population required to correctly sample the ground state wave function. In addition, the variations of the average computational time per 20 Monte Carlo cycles with the coupling strength and the number of processors are also analyzed. The calculated results show that the computational efficiency of an FCIQMC calculation is mainly affected by the total population of psips and the communication between processors. These results can provide useful references for understanding the FCIQMC algorithm, studying the ground state properties of the 2D Hubbard model for the larger system size by the FCIQMC method and using a computational budget as effectively as possible.
 Select Comparative Study of Micro and Nano Size WO$_{3}$/E44 Epoxy Composite as Gamma Radiation Shielding Using MCNP and Experiment Shahryar Malekie, Nahid Hajiloo Chin. Phys. Lett.    2017, 34 (10): 108102 .   DOI: 10.1088/0256-307X/34/10/108102 Abstract   HTML   PDF (679KB) The radiation shielding characteristics of 50 wt% WO$_{3}$/E44 epoxy composite in various gamma energies from 80 keV to 1.33 MeV are investigated via the MCNP code. Thus two scales are considered for WO$_{3}$ filler particles: micro and nano with sizes of 1 μm and 50 nm, respectively. The simulation results show that WO$_{3}$ nano particles exhibit a larger increase in linear attenuation coefficient in comparison with micro size particles. Finally, validation of simulation results with the published experimental data shows a good agreement.
 Select Origin of Negative Imaginary Part of Effective Permittivity of Passive Materials Kai-Lun Zhang, Zhi-Ling Hou, Ling-Bao Kong, Hui-Min Fang, Ke-Tao Zhan Chin. Phys. Lett.    2017, 34 (9): 097701 .   DOI: 10.1088/0256-307X/34/9/097701 Abstract   HTML   PDF (650KB) The anti-resonant phenomenon of effective electromagnetic parameters of metamaterials has aroused controversy due to negative imaginary permittivity or permeability. It is experimentally found that the negative imaginary permittivity can occur for the natural passive materials near the Fabry–Perot resonances. We reveal the nature of negative imaginary permittivity, which is correlated with the magnetoelectric coupling. The anti-resonance of permittivity is a non-inherent feature for passive materials, while it can be inherent for devices or metamaterials. Our finding validates that the negative imaginary part of effective permittivity does not contradict the second law of thermodynamics for metamaterials owing to the magnetoelectric coupling.
 Select Low-Frequency Noise in Amorphous Indium Zinc Oxide Thin Film Transistors with Aluminum Oxide Gate Insulator Ya-Yi Chen, Yuan Liu, Zhao-Hui Wu, Li Wang, Bin Li, Yun-Fei En, Yi-Qiang Chen Chin. Phys. Lett.    2018, 35 (4): 048502 .   DOI: 10.1088/0256-307X/35/4/048502 Abstract   HTML   PDF (781KB) Low-frequency noise (LFN) in all operation regions of amorphous indium zinc oxide (a-IZO) thin film transistors (TFTs) with an aluminum oxide gate insulator is investigated. Based on the LFN measured results, we extract the distribution of localized states in the band gap and the spatial distribution of border traps in the gate dielectric, and study the dependence of measured noise on the characteristic temperature of localized states for a-IZO TFTs with Al$_2$O$_3$ gate dielectric. Further study on the LFN measured results shows that the gate voltage dependent noise data closely obey the mobility fluctuation model, and the average Hooge's parameter is about $1.18\times10^{-3}$. Considering the relationship between the free carrier number and the field effect mobility, we simulate the LFN using the $\Delta N$–$\Delta\mu$ model, and the total trap density near the IZO/oxide interface is about $1.23\times 10^{18}$ cm$^{-3}$eV$^{-1}$.
 Select Superconductivity and Fermi Surface Anisotropy in Transition Metal Dichalcogenide NbTe$_{2}$ Xi Zhang, Tianchuang Luo, Xiyao Hu, Jing Guo, Gongchang Lin, Yuehui Li, Yanzhao Liu, Xiaokang Li, Jun Ge, Ying Xing, Zengwei Zhu, Peng Gao, Liling Sun, Jian Wang Chin. Phys. Lett.    DOI: 10.1088/0256-307X/36/5/057402 Abstract   HTML   PDF (1541KB) Transition metal dichalcogenides, featuring layered structures, have aroused enormous interest as a platform for novel physical phenomena and a wide range of potential applications. Among them, special interest has been placed upon WTe$_{2}$ and MoTe$_{2}$, which exhibit non-trivial topology both in single layer and bulk as well as pressure induced or enhanced superconductivity. We study another distorted 1T material NbTe$_{2}$ through systematic electrical transport measurements. Intrinsic superconductivity with onset transition temperature ($T_{\rm c}^{\rm onset}$) up to 0.72 K is detected where the upper critical field ($H_{\rm c}$) shows unconventional quasi-linear behavior, indicating spin-orbit coupling induced p-wave paring. Furthermore, a general model is proposed to fit the angle-dependent magnetoresistance, which reveals the Fermi surface anisotropy of NbTe$_{2}$. Finally, non-saturating linear magnetoresistance up to 50 T is observed and attributed to the quantum limit transport.
 Select A Unified Approach to the Thermodynamics and Quantum Scaling Functions of One-Dimensional Strongly Attractive $SU(w)$ Fermi Gases Yi-Cong Yu, Xi-Wen Guan Chin. Phys. Lett.    2017, 34 (7): 070501 .   DOI: 10.1088/0256-307X/34/7/070501 Abstract   HTML   PDF (776KB) We present a unified derivation of the pressure equation of states, thermodynamics and scaling functions for the one-dimensional (1D) strongly attractive Fermi gases with $SU(w)$ symmetry. These physical quantities provide a rigorous understanding on a universality class of quantum criticality characterized by the critical exponents $z=2$ and correlation length exponent $\nu=1/2$. Such a universality class of quantum criticality can occur when the Fermi sea of one branch of charge bound states starts to fill or becomes gapped at zero temperature. The quantum critical cone can be determined by the double peaks in specific heat, which serve to mark two crossover temperatures fanning out from the critical point. Our method opens to further study on quantum phases and phase transitions in strongly interacting fermions with large $SU(w)$ and non-$SU(w)$ symmetries in one dimension.