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 Select Comparative Study of Substitutional N and Substitutional P in Diamond Hong-Yu Yu, Nan Gao, Hong-Dong Li, Xu-Ri Huang, Tian Cui Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/116101 Abstract   HTML   PDF (2560KB) Based on density functional theory calculations, it is found that for substitutional N in diamond the $C_{3v}$ symmetry structure is more stable, while $C_{3v}$ and $D_{2d}$ symmetry patterns for the substitutional P in diamond have comparable energies. Moreover, the substitutional N is a deep donor for diamond, while P is a shallow substitutional n-type dopant. This is attributed to the different doping positions of dopant (the N atom is seriously deviated from the substitutional position, while the P atom nearly locates in the substitutional site), which are determined by the atomic radius.
 Select Molecular Dynamics Study of the Structural Modification of Porous Silica from Low-Energy Recoils Ji-Hua Zhang, Ye Tian, Wei Han, Fang Wang, Fu-Quan Li, Xiao-Dong Yuan, Xia Xiang Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/116102 Abstract   HTML   PDF (637KB) Molecular dynamics simulations are performed to investigate the effects of low-energy recoils on the microscopic structure of porous silica. Exhibiting a logistic growth with the recoil energy, the displacement probability of Si is shown to be smaller than that of O at the same primary knock-on level. Computations of pair distribution functions and bond angle distributions reveal that this material upon irradiation with energies around the displacement thresholds mainly undergoes structural changes in the medium-range order. In the porous network, while the formation of nonbridging oxygen defects tends to induce shorter Si–O bonds than those formed by bridging oxygen atoms, a remarkable increase of inter-tetrahedral bond angles created by multiple recoils can be observed and associated with the rearrangement of ring statistics.
 Select Magnetic Coupling Induced Self-Assembly at Atomic Level Weiyu Xie, Yu Zhu, Jianpeng Wang, Aihua Cheng, Zhigang Wang Chin. Phys. Lett.    2019, 36 (11): 116401.   DOI: 10.1088/0256-307X/36/11/116401 Abstract   HTML   PDF (2734KB) Developing accurate self-assembly is the key for constructing functional materials from a bottom-up approach. At present, it is mainly hindered by building blocks and driving modes. We design a new self-assembly method based on the magnetic coupling between spin-polarized electrons. First-principles calculations show that spin-polarized electrons from different endohedral metallofullerene (EMF) superatoms can pair each other to ensure a one-dimensional extending morphology. Furthermore, without ligand passivation, the EMF superatoms maintain their electronic structures robustly in self-assembly owing to the core-shell structure and the atomic-like electron arrangement rule. Therefore, it should noted that the magnetic coupling of monomeric electron spin polarization can be an important driving mechanism for high-precision self-assembly. These results represent a new paradigm for self-assembly and offer fresh opportunities for functional material construction at the atomic level.
 Select Observation of Simplest Water Chains on Surface Stabilized by a Hydroxyl Group at One End An-Ning Dong, Li-Huan Sun, Xiang-Qian Tang, Yi-Kun Yao, Yang An, Dong Hao, Xin-Yan Shan, Xing-Hua Lu Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/116801 Abstract   HTML   PDF (1547KB) The key to fully understanding water-solid interfaces relies on the microscopic nature of hydrogen bond networks, including their atomic structures, interfacial interactions, and dynamic behaviors. Here, we report the observation of two types of simplest water chains on Au(111) surface which is expected unstable according to the rules of hydrogen network on noble metal surfaces. A common feature at the end of chain structures is revealed in high resolution scanning tunneling microscopy images. To explain the stability in observed hydrogen bond networks, we propose a structure model of the water chains terminated with a hydroxyl group. The model is consistent with detailed image analysis and molecular manipulation. The observation of simplest water chains suggests a new platform for exploring fundamental physics in hydrogen bond networks on surfaces.
 Select High-Pressure Behavior of Nano-Pt in Hydrogen Environment Can Tian, Xiao-li Huang, Yan-ping Huang, Xin Li, Di Zhou, Xin Wang, Tian Cui Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/106101 Abstract   HTML   PDF (901KB) We choose nano-Pt in hydrogen environment to explore the size effect on the formation of metal hydrides. At 30 GPa, a phase transition in the metal lattice from the cubic to hexagonal phase is observed characterized by a drastically increased volume per metal atom, indicating the formation of PtH-$P6_{3}/mmc$. We find that nano-Pt could form PtH at a lower pressure than the bulk Pt due to its high specific surface and structure defects. The present work provides the possible route to new metal hydrides under mild conditions.
 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 Reexploration of Structural Changes in Element Bromine through Pressure-Induced Decomposition of Solid HBr Ming-Kun Liu, De-Fang Duan, Yan-Ping Huang, Yong-Fu Liang, Xiao-Li Huang, Tian Cui Chin. Phys. Lett.    2019, 36 (8): 086401.   DOI: 10.1088/0256-307X/36/8/086401 Abstract   HTML   PDF (1213KB) Simple molecular solids have been an important subject in condensed matter physics, particularly for research of pressure-induced molecular dissociation. We re-explore the structural changes of element bromine through pressure-induced decomposition of solid HBr. The phase changes in HBr are investigated by Raman spectroscopy and synchrotron x-ray diffraction up to 125 GPa at room temperature. By applying pressure, HBr decomposes into solid bromine in the pressure range of 18.7–38 GPa. The solid bromine changes from molecular phase to incommensurate phase at 81 GPa, and finally to monatomic phase at 91 GPa. During the process of pressure-induced molecular dissociation, the intermediate incommensurate phase of element bromine is confirmed for the first time from the x-ray diffraction studies. The decomposition of HBr is irreversible since HBr cannot form again upon pressure decompression.
 Select Magnetic and Electronic Properties of $\beta$-Graphyne Doped with Rare-Earth Atoms Juan Ren, Song-Bin Zhang, Ping-Ping Liu Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/076101 Abstract   HTML   PDF (815KB) Structural, electronic and magnetic properties of La-, Ce-, Pr-, Nd-, Pm-, Sm- and Eu-doped $\beta$-graphyne are investigated by comprehensive ab initio calculation based on density functional theory. The adsorption energies indicate that the dopings are suitable. The doped $\beta$-graphyne undergoes transition from semiconductor to metal. Furthermore, the doping of Nd, Pm, Sm and Eu atoms can induce magnetization. The results are useful for spintronics and the design of future electronic devices.
 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 Negative Thermal Expansion of GaFe(CN)$_{6}$ and Effect of Na Insertion by First-Principles Calculations Meng Li, Yuan Li, Chun-Yan Wang, Qiang Sun Chin. Phys. Lett.    2019, 36 (6): .   DOI: 10.1088/0256-307X/36/6/066301 Abstract   HTML   PDF (825KB) We study the negative thermal expansion (NTE) properties and effect of Na insertion on the NTE of the framework material GaFe(CN)$_{6}$ by first-principles calculations based on density functional theory within the quasi-harmonic approximation. The calculated results show that the material exhibits NTE due to the low transverse vibrational modes of the CN groups. The modes demonstrate larger negative values of the mode Grüneisen parameters. Once Na is introduced in the framework of the material, it prefers to locate at the center of the quadrates of the framework material and binds to the four N anions nearby. As a consequence, the transverse vibrational mode of the CN group is clearly hindered and the NTE of the material is weakened. Our theoretical calculations have clarified the mechanisms of NTE and the effect of the guest Na on the NTE of the framework material.
 Select Negative Thermal Expansion of the Dy$_{2}$Fe$_{16}$Cr Compound Li-Yu HAO, Tie YANG, Xiao-Tian WANG, Ming TAN Chin. Phys. Lett.    2019, 36 (6): .   DOI: 10.1088/0256-307X/36/6/066501 Abstract   HTML   PDF (568KB) Structural, thermal expansion, and magnetic properties of the Dy$_{2}$Fe$_{16}$Cr compound are investigated by means of x-ray diffraction and magnetization measurements. The Dy$_{2}$Fe$_{16}$Cr compound has a hexagonal Th$_{2}$Ni$_{17}$-type structure. There exists a negative thermal expansion resulting from a strong spontaneous magnetostriction in the magnetic state of the Dy$_{2}$Fe$_{16}$Cr compound. The average thermal expansion coefficient is $-0.794\times 10^{-5}$/K in the temperature range 292–407 K. The spontaneous magnetostrictive deformation and the Curie temperature are discussed.
 Select Mechanical Properties of Formamidinium Halide Perovskites FABX$_{3}$ (FA=CH(NH$_{2})_{2}$; B=Pb, Sn; X=Br, I) by First-Principles Calculations Lei Guo, Gang Tang, Jiawang Hong Chin. Phys. Lett.    2019, 36 (5): .   DOI: 10.1088/0256-307X/36/5/056201 Abstract   HTML   PDF (993KB) The mechanical properties of formamidinium halide perovskites FABX$_{3}$ (FA=CH(NH$_{2})_{2}$; B=Pb, Sn; X=Br, I) are systematically investigated using first-principles calculations. Our results reveal that FABX$_{3}$ perovskites possess excellent mechanical flexibility, ductility and strong anisotropy. We shows that the planar organic cation FA$^{+}$ has an important effect on the mechanical properties of FABX$_{3}$ perovskites. In addition, our results indicate that (i) the moduli (bulk modulus $B$, Young's modulus $E$, and shear modulus $G$) of FABBr$_{3}$ are larger than those of FABI$_{3}$ for the same B atom, and (ii) the moduli of FAPbX$_{3}$ are larger than those of FASnX$_{3}$ for the same halide atom. The reason for the two trends is demonstrated by carefully analyzing the bond strength between B and X atoms based on the projected crystal orbital Hamilton population method.
 Select Interplay of Strain and Magnetism in FeSe Monolayers Yun Xie, Junsheng Feng, Hongjun Xiang, Xingao Gong Chin. Phys. Lett.    2019, 36 (5): .   DOI: 10.1088/0256-307X/36/5/056801 Abstract   HTML   PDF (804KB) Superconductivity and its relationship with strain remains elusive in the monolayer FeSe superconductor. Based on first-principles calculations and model studies, we investigate the magnetic properties of FeSe and FeTe monolayers and find that tensile strain induces changes to magnetic phases for both materials. Furthermore, we reveal that electron doping will decrease the difference of effective magnetic interactions between the $a$ and $b$ directions in an FeSe monolayer and hence suppress its nematicity. We suggest that the overall effect of tensile strain combined with electron doping hinders the appearance of both magnetic and nematic orders in an FeSe monolayer, which paves the way for the emergence of superconductivity.
 Select Synthesis and Characteristics of Type Ib Diamond Doped with NiS as an Additive Jian-Kang Wang, Shang-Sheng Li, Ning Wang, Hui-Jie Liu, Tai-Chao Su, Mei-Hua Hu, Fei Han, Kun-Peng Yu, Hong-An Ma Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/046101 Abstract   HTML   PDF (671KB) Large diamond single crystals doped with NiS are synthesized under high pressure and high temperature. It is found that the effects on the surface and shape of the synthesized diamond crystals are gradually enhanced by increasing the NiS additive amount. It is noted that the synthesis temperature is necessarily raised to 1280$^{\circ}\!$C to realize the diamond growth when the additive amount reaches 3.5% in the synthesis system. The results of Fourier transform infrared spectroscopy (FTIR) demonstrate that S is incorporated into the diamond lattice and exists in the form of C–S bond. Based on the FTIR results, it is found that N concentration in diamond is significantly increased, which are ascribed to the NiS additive. The analysis of x-ray photoelectron spectroscopy shows that S is present in states of C–S, S–O and C–S–O bonds. The relative concentration of S compared to C continuously increases in the synthesized diamonds as the amount of additive NiS increases. Additionally, the electrical properties can be used to characterize the obtained diamond crystals and the results show that diamonds doped with NiS crystals behave as n-type semiconductors.
 Select Strain Induced Nanopillars and Variation of Magnetic Properties in La$_{0.825}$Sr$_{0.175}$MnO$_{3}$/LaAlO$_{3}$ Films Xin Li, Jing-Zhi Han, Xiong-Zuo Zhang, Yin-Feng Zhang, Hai-Dong Tian, Ming-Zhu Xue, Kun Li, Xin Wen, Wen-Yun Yang, Shun-Quan Liu, Chang-Sheng Wang, Hong-Lin Du, Xiao-Dong Zhang, Xin-An Wang, Ying-Chang Yang, Jin-Bo Yang Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/046102 Abstract   HTML   PDF (1901KB) To investigate the process of strain relaxation and resultant variation of microstructure and magnetic properties, low-doped La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ epitaxial films with different thicknesses are deposited on LaAlO$_{3}$ substrates and strain induced nanopillars are discovered inside the La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ film. Perpendicular oriented nanopillars mainly exist below 30 nm and tend to disappear above 30 nm. The distribution of nanopillars not only induce the variation of lattice parameters and local structural distortion but also lead to the deviation of easy magnetization axis from the perpendicular direction. Specifically, the out-of-plane lattice parameters of the film decrease quickly with the increase of the thickness but tend to be constant when the thickness is above 30 nm. Meanwhile, the variations of magnetic properties along in-plane and out-of-plane directions would also decline at first and they then remain nearly unchanged. Our work constructs the relationship between nanopillars and magnetic properties inside films. We are able to clearly reveal the effects of inhomogeneous strain relaxation.
 Select High-Pressure Phase Transitions of Cubic Y$_{2}$O$_{3}$ under High Pressures by In-situ Synchrotron X-Ray Diffraction Sheng Jiang, Jing Liu, Xiao-Dong Li, Yan-Chun Li, Shang-Ming He, Ji-Chao Zhang Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/046103 Abstract   HTML   PDF (954KB) High-pressure phase transitions of cubic Y$_{2}$O$_{3}$ are investigated using in situ synchrotron x-ray diffraction in a diamond anvil cell up to 36.3 GPa. The pressure-induced phase transitions of cubic Y$_{2}$O$_{3}$, which display apparent inconsistencies in previous studies, are verified to be from a cubic phase to a monoclinic phase and further to a hexagonal phase at 11.7 and 21.6 GPa, respectively. The hexagonal Y$_{2}$O$_{3}$ displays noticeable anisotropic compressibility due to its layered structure and it is stable up to the highest pressure in the present study. A third-order Birch–Murnaghan fit based on the observed pressure-volume data yields zero pressure bulk moduli of 180(3), 196(7) and 177(7) GPa for cubic, monoclinic and hexagonal phases, respectively.
 Select Effects of Helium Implantation and Subsequent Electron Irradiation on Microstructures of Fe-11wt.% Cr Model Alloy Bing-Sheng Li, Zhi-Guang Wang, Tie-Long Shen, Kong-Fang Wei, Yan-Bin Sheng, Tamakai Shibayama, Xi-Rui Lu, An-Li Xiong Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/046104 Abstract   HTML   PDF (1410KB) Helium effects on dislocation and cavity formation of Fe-11 wt.% Cr model alloy are investigated. Single-beam (electron) and dual-beam (He$^{+}$/e$^{-}$) irradiations are performed at 350$^\circ\!$C and 400$^\circ\!$C using an ultra-high voltage electron microscope combined with ion accelerators. In-situ observation shows that the growth rate of dislocation loops is reduced in the helium pre-injected specimen. The mean size of cavities decreased in the helium pre-injected specimen. The possible mechanisms are discussed.
 Select Decagonal and Dodecagonal Quasicrystals Obtained by Molecular Dynamics Simulations Jian-Hui Chen, Cheng Cai, Xiu-Jun Fu Chin. Phys. Lett.    2019, 36 (3): .   DOI: 10.1088/0256-307X/36/3/036101 Abstract   HTML   PDF (1778KB) Double-well potentials are used for molecular dynamics simulation in monatomic systems. The potentials change as their parameters are adjusted, resulting in different structures. Of particular interest, we obtain decagonal and dodecagonal quasicrystals by simulations. We also verify the results and explain the formation of quasicrystals from the perspective of potential energy.
 Select An Orthorhombic Phase of Superhard $o$-BC$_{4}$N Nian-Rui Qu, Hong-chao Wang, Qing Li, Zhi-Ping Li, Fa-Ming Gao Chin. Phys. Lett.    2019, 36 (3): .   DOI: 10.1088/0256-307X/36/3/036201 Abstract   HTML   PDF (805KB) A potential superhard $o$-BC$_{4}$N with $Imm2$ space group is identified by ab initio evolutionary methodology using CALYPSO code. The structural, electronic and mechanical properties of $o$-BC$_{4}$N are investigated. The elastic calculations indicate that $o$-BC$_{4}$N is mechanically stable. The phonon dispersions imply that this phase is dynamically stable under ambient conditions. The structure of $o$-BC$_{4}$N is more energetically favorable than $g$-BC$_{4}$N above the pressure of 25.1 GPa. Here $o$-BC$_{4}$N is a semiconductor with an indirect band gap of about 3.95 eV, and the structure is highly incompressible with a bulk modulus of 396.3 GPa and shear modulus of 456.0 GPa. The mechanical failure mode of $o$-BC$_{4}$N is dominated by the shear type. The calculated peak stress of 58.5 GPa in the (100)[001] shear direction sets an upper bound for its ideal strength. The Vickers hardness of $o$-BC$_{4}$N reaches 78.7 GPa, which is greater than that of $t$-BC$_{4}$N and $bc$-BC$_{4}$N proposed recently, confirming that $o$-BC$_{4}$N is a potential superhard material.
 Select Dynamic Spallation in Uranium under Laser Shock Loading Da-Wu Xiao, Hua Shu, Dong-Li Zou, Chao Lu, Li-Feng He Chin. Phys. Lett.    2019, 36 (2): .   DOI: 10.1088/0256-307X/36/2/026201 Abstract   HTML   PDF (1801KB) The spall behavior of uranium is investigated using direct laser ablation loading experiments. The uranium targets are cut and ground to 0.05 mm, 0.1 mm, and 0.15 mm in thickness. Laser energies are varied to yield a constant peak pressure. This results in different strain rates and varying degrees of damage to the uranium targets. The spall strength is calculated and analyzed from the free surface velocity histories recorded using a line velocity interferometer for any reflections system. The spall strength increases from 4.3 GPa to 9.4 GPa with strain rates ranging from $4.0\times10^{6}$ s$^{-1}$ to $1.7\times10^{7}$ s$^{-1}$. Post-mortem analysis is performed on the recovered samples, revealing the twin-matrix interfaces together with the inclusions to be the primary factor governing the spall fracture of uranium.
 Select CH$_{3}$NH$_{3}$ Formed by Electron Injection at Heterojunction Inducing Peculiar Properties of CH$_{3}$NH$_{3}$PbI$_{3}$ Material Ao Zhang, Yun-Lin Chen, Chun-Xiu Zhang, Jun Yan Chin. Phys. Lett.    2019, 36 (2): .   DOI: 10.1088/0256-307X/36/2/026701 Abstract   HTML   PDF (2680KB) The effect of formed CH$_{3}$NH$_{3}$ at the heterojunction on properties of CH$_{3}$NH$_{3}$PbI$_{3}$ material is investigated based on experiment and theoretical calculation. Our calculation results show that the giant dielectric constant, anomalous hysteresis and long-lasting polarization for CH$_{3}$NH$_{3}$PbI$_{3}$ originate from the formed CH$_{3}$NH$_{3}$ at the heterojunction. It is found that the induced weak EPS by the reorientation of CH$_{3}$NH$_{3}$ sub-group along the built-in electric field enables us to effectively increase the ordering of entire lead-halide framework. In addition, the heterojunction has an advantage of channel separation between carrier transport and electron diffusion. These properties of the heterojunction are the main origin of the high efficiency of CH$_{3}$NH$_{3}$PbI$_{3}$ solar cells.
 Select Probing Lattice Vibrations at SiO$_{2}$/Si Surface and Interface with Nanometer Resolution Yue-Hui Li, Mei Wu, Rui-Shi Qi, Ning Li, Yuan-Wei Sun, Cheng-Long Shi, Xue-Tao Zhu, Jian-Dong Guo, Da-Peng Yu, Peng Gao Chin. Phys. Lett.    2019, 36 (2): .   DOI: 10.1088/0256-307X/36/2/026801 Abstract   HTML   PDF (2461KB) Recent advances in monochromatic aberration corrected electron microscopy make it possible to detect the lattice vibrations with both high-energy resolution and high spatial resolution. Here, we use sub-10 meV electron energy loss spectroscopy to investigate the local vibrational properties of the SiO$_{2}$/Si surface and interface. The energy of the surface mode is thickness dependent, showing a blue shift as $z$-thickness (parallel to the fast electron beam) of SiO$_{2}$ film increases, while the energy of the bulk mode and the interface mode keeps constant. The intensity of the surface mode is well-described by a Bessel function of the second kind. The mechanism of the observed spatially dependent vibrational behavior is discussed and compared with dielectric response theory analysis. Our nanometer scale measurements provide useful information on the bonding conditions at the surface and interface.
 Select Instability of Epitaxially Strained Thin Films Based on Nonlocal Elasticity Wang-Min Zhou, Wang-Jun Li Chin. Phys. Lett.    2019, 36 (1): .   DOI: 10.1088/0256-307X/36/1/016801 Abstract   HTML   PDF (458KB) We perform a linear analysis of the elastic fields and stability of epitaxially strained thin films based on nonlocal elasticity. We derive expressions of perturbed stresses to the first order of perturbation amplitude, which show that the stresses are directly proportional to the lattice mismatch and the perturbation amplitude, and decrease with an increase in the perturbation wavelength. The critical perturbation wavelength distinguishes whether the flat film for the perturbation is stable, which is inversely proportional to the square of the mismatch and decreases with the thickness of the film.
 Select Blistering and Helium Retention of Tungsten and 5% Chromium Doped Tungsten Exposed to 60keV Helium Ions Irradiation Shu-qin Lv, Wen-jia Han, Jian-gang Yu, Hang Zhou, Mi Liu, Chang-an Chen, Kai-gui Zhu Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/126101 Abstract   HTML   PDF (899KB) Pure tungsten (W) and chromium doped W (W-5%Cr) are prepared by powder metallurgy. The microstructure, blistering and helium retention are investigated by x-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermal desorption spectroscopy (TDS). These results show that the average size and density of helium blisters on the surface of pure W are much larger than those on the W-5%Cr alloy. Vacancy-impurity pairs can reduce the migration coefficients of vacancy and vacancy-helium complexes, and Cr may play a role of such an impurity. Moreover, the TDS result shows that the highest desorption peak moves to higher temperature, which is attributed to the He$_{m}$Cr$_{k}$V$_{n}$ complexes in the W-Cr alloy. In addition, the helium retention is found to be higher in W than in W-5%Cr.
 Select A Calorimetric Study Assisted with First Principle Calculations of Specific Heat for Si-Ge Alloys within a Broad Temperature Range Qing Wang, Hai-Peng Wang, De-Lu Geng, Ming-Xing Li, Bing-Bo Wei Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/126501 Abstract   HTML   PDF (785KB) Calorimetric measurements are performed to determine the specific heat of Si-$x$ at.% Ge (where $x=0$, 10, 30, 50, 70, 90 and 100) alloys within a broad temperature range from 123 to 823 K. The measured specific heat increases dramatically at low temperatures, and the composition dependence of specific heat is evaluated from the experimental results. Meanwhile, the specific heat at constant volume, the thermal expansion, and the bulk modulus of Si and Ge are investigated by the first principle calculations combined with the quasiharmonic approximation. The negative thermal expansion is observed for both Si and Ge. Furthermore, the isobaric specific heat of Si and Ge is calculated correspondingly from 0 K to their melting points, which is verified by the measured results and accounts for the temperature dependence in a still boarder range.
 Select Structural Evolution and Phase Change Properties of C-Doped Ge$_{2}$Sb$_{2}$Te$_{5}$ Films During Heating in Air Long Zheng, Xing-Ming Yang, Yi-Feng Hu, Liang-Jun Zhai, Jian-Zhong Xue, Xiao-Qin Zhu, Zhi-Tang Song Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/126801 Abstract   HTML   PDF (763KB) We elucidate the importance of a capping layer on the structural evolution and phase change properties of carbon-doped Ge$_{2}$Sb$_{2}$Te$_{5}$ (C-GST) films during heating in air. Both the C-GST films without and with a thin SiO$_{2}$ capping layer (C-GST and C-GST/SiO$_{2}$) are deposited for comparison. Large differences are observed between C-GST and C-GST/SiO$_{2}$ films in resistance-temperature, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectra, data retention capability and optical band gap measurements. In the C-GST film, resistance-temperature measurement reveals an unusual smooth decrease in resistance above 110$^{\circ}\!$C during heating. X-ray diffraction result has excluded the possibility of phase change in the C-GST film below 170$^{\circ}\!$C. The x-ray photoelectron spectroscopy experimental result reveals the evolution of Te chemical valence because of the carbon oxidation during heating. Raman spectra further demonstrate that phase changes from an amorphous state to the hexagonal state occur directly during heating in the C-GST film. The quite smooth decrease in resistance is believed to be related with the formation of Te-rich GeTe$_{4-n}$Ge$_{n}$ ($n=0$, 1) units above 110$^{\circ}\!$C in the C-GST film. The oxidation of carbon is harmful to the C-GST phase change properties.
 Select Interaction between Dislocation and Twinning Boundary under Incremental Loading in $\alpha$-Titanium Xiang-Yue Liu, Hong Zhang, Xin-Lu Cheng Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/116201 Abstract   HTML   PDF (10428KB) The lattice dislocation interacting with grain boundary in the polycrystal exerts an evident influence on the materials' strength and toughness. A comprehensive study regarding the dislocation–twinning boundary (TB) interaction in $\alpha$-titanium and TB migration is performed by employing molecular dynamic simulation. We analyze the interactions between dislocation and TB, under the conditions of plastic deformation and thermal stress, including the interaction between pure edge $\langle a\rangle$ dislocation and $({11\bar{2}2})$ TB and the interaction between mixed type $\langle a\rangle$ dislocations and $({10\bar{1}1})$ TB at 10 K/300 K. The $\langle {c+a} \rangle$ pyramidal transmitting slip mode is motivated in the case of edge dislocation–$({11\bar{2}2})$ interaction at 300 K and then transforms into basal-dissociated dislocation after experiencing the complex dissociation and combination. The basal-dissociated pyramidal partial dislocation located in the second grain can be driven to penetrate through the second grain leaving the multiple stacking faults behind. Dissociation of incident basal dislocation on $({10\bar{1}1})$ TB results in a nucleation of a $({10\bar{1}1})$ twin embryo in twin crystals at room temperature. We determine the nature of the generated defects by means of the Burgers circuit analysis.
 Select Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd$_{3}$As$_{2}$ Fei Sun, M. Yang, M. W. Yang, Q. Wu, H. Zhao, X. Ye, Youguo Shi, Jimin Zhao Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/116301 Abstract   HTML   PDF (1553KB) Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd$_{3}$As$_{2}$ is carried out. An acoustic phonon (AP) mode with central frequency $f=0.037$ THz (i.e., 1.23 cm$^{-1}$ or 0.153 meV) is unambiguously generated and detected, which we attribute to laser-induced thermal strain. An AP chirping (i.e., variation of the phonon frequency) is clearly detected, which is ascribed to heat capacity variation with time. By comparing our experimental results and the theoretical model, we obtain a chirping time constant, which is 31.2 ps at 6 K and 19.8 ps at 300 K, respectively. Significantly, we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping, instead of a Fano resonance. Moreover, we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence, as well as temperature, which endows Cd$_{3}$As$_{2}$ application potentials in thermoelectric devices.
 Select Nonlinear Dicke Quantum Phase Transition and Its Quantum Witness in a Cavity-Bose–Einstein-Condensate System Wang-Jun Lu, Zhen Li, Le-Man Kuang Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/116401 Abstract   HTML   PDF (1308KB) We investigate nonlinear Dicke quantum phase transition (QPT) induced by inter-atomic nonlinear interaction and its quantum witness in a cavity-Bose–Einstein-condensate (BEC) system. It is shown that inter-atomic nonlinear interaction in a cavity BEC system can induce first-order Dicke QPT. It is found that this nonlinear Dicke QPT can happen in an arbitrary coupling regime of the cavity and atoms. It is demonstrated that the quantum speed limit time can witness the Dicke QPT through its sudden change at the critical point of the QPT.
 Select Surface Tension and Electrostriction in a Suspended Bridge of Dielectric Liquid Yan Cen, Chuanshan Tian Chin. Phys. Lett.    2018, 35 (10): .   DOI: 10.1088/0256-307X/35/10/106801 Abstract   HTML   PDF (701KB) The mechanism of the formation of a surprisingly long suspended liquid bridge subjected to a dc electric field has been intensively studied in the past few decades. However, the role of electrostriction and quantitative evaluation of surface tension in the bridge have not been evaluated. We present combined theoretical and experimental studies on this issue. Electrostriction is pointed out to be the driving force that pushes liquid upward against gravity and into the gap between two containers and forms the suspended bridge, which is within the framework of the Maxwell pressure tensor. Through a comparison between experiment and theory, the surface tension is found to play an important role in holding the long suspended bridge. Ignorance of the surface tension leads to much smaller bridge length than the experimental values. The dynamic stability of the bridge with respect to its diameter, length and conductance is also discussed.
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