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Magnetic Order and Its Interplay with Structure Phase Transition in van der Waals Ferromagnet VI$_{3}$
Yiqing Hao, Yiqing Gu, Yimeng Gu, Erxi Feng, Huibo Cao, Songxue Chi, Hua Wu, and Jun Zhao
Chin. Phys. Lett.    2021, 38 (9): 096101 .   DOI: 10.1088/0256-307X/38/9/096101
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Van der Waals magnet VI$_{3}$ demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI$_{3}$ that revealed a ferromagnetic order with the moment direction tilted from the $c$-axis by $\sim $$36^{\circ}$ at 4 K. A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed, before the magnetic order completely disappears at $T_{\rm C} = 50$ K. The refined magnetic moment of $\sim $$1.3 \mu_{\scriptscriptstyle {\rm B}}$ at 4 K is much lower than the fully ordered spin moment of $2\mu_{\scriptscriptstyle {\rm B}}$/V$^{3+}$, suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin–orbit coupling in VI$_{3}$. This results in strong magnetoelastic interactions that make the magnetic properties of VI$_{3}$ easily tunable via strain and pressure.
Superconductivity in Shear Strained Semiconductors
Chang Liu, Xianqi Song, Quan Li, Yanming Ma, and Changfeng Chen
Chin. Phys. Lett.    2021, 38 (8): 086301 .   DOI: 10.1088/0256-307X/38/8/086301
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Semiconductivity and superconductivity are remarkable quantum phenomena that have immense impact on science and technology, and materials that can be tuned, usually by pressure or doping, to host both types of quantum states are of great fundamental and practical significance. Here we show by first-principles calculations a distinct route for tuning semiconductors into superconductors by diverse large-range elastic shear strains, as demonstrated in exemplary cases of silicon and silicon carbide. Analysis of strain driven evolution of bonding structure, electronic states, lattice vibration, and electron-phonon coupling unveils robust pervading deformation induced mechanisms auspicious for modulating semiconducting and superconducting states under versatile material conditions. This finding opens vast untapped structural configurations for rational exploration of tunable emergence and transition of these intricate quantum phenomena in a broad range of materials.
High Mixing Entropy Enhanced Energy States in Metallic Glasses
Juntao Huo, Kangyuan Li, Bowen Zang, Meng Gao, Li-Min Wang, Baoan Sun, Maozhi Li, Lijian Song, Jun-Qiang Wang, and Wei-Hua Wang
Chin. Phys. Lett.    2022, 39 (4): 046401 .   DOI: 10.1088/0256-307X/39/4/046401
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Owing to the nonequilibrium nature, the energy state of metallic glasses (MGs) can vary a lot and has a critical influence on the physical properties. Exploring new methods to modulate the energy state of glasses and studying its relationship with properties have attracted great interests. Herein, we systematically investigate the energy state, mixing entropy and physical properties of Zr–Ti–Cu–Ni–Be multicomponent high entropy MGs by experiments and simulations. We find that the energy state increases along with the increase of mixing entropy. The yield strength and thermal stability of MGs are also enhanced by high mixing entropy. These results may open a new door on regulation of energy states and thus physical properties of MGs.
Evidence for a High-Pressure Isostructural Transition in Nitrogen
Chunmei Fan, Shan Liu, Jingyi Liu, Binbin Wu, Qiqi Tang, Yu Tao, Meifang Pu, Feng Zhang, Jianfu Li, Xiaoli Wang, Duanwei He, Chunyin Zhou, and Li Lei
Chin. Phys. Lett.    2022, 39 (2): 026401 .   DOI: 10.1088/0256-307X/39/2/026401
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We observed an isostructural phase transition in the solid nitrogen $\lambda$-N$_{2}$ at approximately 50 GPa accompanied by anomalies in lattice parameters, atomic volume and Raman vibron modes. The anomalies are ascribed to a slight reorientation of the nitrogen molecules, which does not seem to affect the monoclinic symmetry (space group $P2_{1}/c$). Our ab initio calculations further confirm the phenomena, and suggest an optimized structure for the $\lambda$-N$_{2}$ phase. In addition, a new high-pressure amorphous phase of $\eta '$-N$_{2}$ was also discovered by a detailed investigation of the pressure-temperature phase diagram of nitrogen with the aim of probing the phase stability of $\lambda$-N$_{2}$. Our result may provide helpful information about the crystallographic nature of dissociation transitions in diatomic molecular crystals (H$_{2}$, O$_{2}$, N$_{2}$, etc).
Thermal Stability of High Power 26650-Type Cylindrical Na-Ion Batteries
Quan Zhou, Yuqi Li, Fei Tang, Kaixuan Li, Xiaohui Rong, Yaxiang Lu, Liquan Chen, and Yong-Sheng Hu
Chin. Phys. Lett.    2021, 38 (7): 076501 .   DOI: 10.1088/0256-307X/38/7/076501
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As a new electrochemical power system, safety (especially thermal safety) of Na-ion batteries (NIBs) is the key towards large-scale industrialization and market application. Thus, research on the thermal stability of NIBs is helpful to evaluate the safety properties and to provide effective strategies to prevent the occurrence of battery safety failure. Thermal stability of the high-power 26650 cylindrical NIBs using Cu-based layered oxide cathode and hard carbon anode is studied. The high power NIBs can achieve fast charge and discharge at 5–10 C rate and maintain 80% capacity after 4729 cycles at 2 C/2 C rate, where the unit C denotes a measure of the rate at which a battery is charge-discharged relative to its maximum capacity. The results of accelerating rate calorimeter and differential scanning calorimetry (ARC-DSC) test results show that NIBs have a higher initial decomposition temperature ($\ge$110 ℃) and a lower maximum thermal runaway temperature ($\le $350 ℃) than those of Li-ion batteries (LIBs), exhibiting a favorable thermal stability. It should be noted that the heat generation of cathode accounts for a large proportion of the total heat generation while the thermal stability of the anode determines the initial thermal runaway temperature, which is similar to LIBs. Finally, the whole temperature characteristics of the NIBs in the range of $-60 $ ℃–1000 ℃ are summarized, which provide guidance for the safety design and applications of NIBs.
High Energy Density Polymeric Nitrogen Nanotubes inside Carbon Nanotubes
Chi Ding, Junjie Wang, Yu Han, Jianan Yuan, Hao Gao, and Jian Sun
Chin. Phys. Lett.    2022, 39 (3): 036101 .   DOI: 10.1088/0256-307X/39/3/036101
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Polymeric nitrogen as a new class of high energy density materials has promising applications. We develop a new scheme of crystal structure searching in a confined space using external confining potentials fitted from first-principles calculations. As a showcase, this method is employed to systematically explore novel polymeric nitrogen structures confined in single-walled carbon nanotubes. Several quasi-one-dimensional (1D) single-bonded polymeric nitrogen structures are realized, two of them are composed of nanotubes instead of chains. These new polymeric nitrogen phases are mechanically stable at ambient pressure and temperature according to phonon calculations and ab initio molecular dynamics simulations. It is revealed that the stabilization of zigzag and armchair chains confined in carbon nanotubes (CNTs) are mostly attributed to the charge transfer from carbon to nitrogen. However, for the novel nitrogen nanotube systems, electrons overlapping in the middle space provide strong Coulomb repulsive forces, which not only induce charge transfer from the middle to the sides but also stabilize the polymeric nitrogen. Our work provides a new strategy for designing novel high-energy-density polymeric nitrogen materials, as well as other new materials with the help of confined space inside porous systems, such as nanotubes, covalent organic frameworks, and zeolites.
Pressure-Driven Ne-Bearing Polynitrides with Ultrahigh Energy Density
Lulu Liu, Shoutao Zhang, and Haijun Zhang
Chin. Phys. Lett.    2022, 39 (5): 056102 .   DOI: 10.1088/0256-307X/39/5/056102
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Neon (Ne) can reveal the evolution of planets, and nitrogen (N) is the most abundant element in the Earth's atmosphere. Considering the inertness of neon, whether nitrogen and neon can react has aroused great interest in condensed matter physics and space science. Here, we identify three new Ne–N compounds (i.e., NeN$_6$, NeN$_{10}$, and NeN$_{22}$) under pressure by first-principles calculations. We find that inserting Ne into N$_2$ substantially decreases the polymeric pressure of the nitrogen and promotes the formation of abundant polynitrogen structures. Especially, NeN$_{22}$ acquires a duplex host-guest structure, in which guest atoms (Ne and N$_2$ dimers) are trapped inside the crystalline host N$_{20}$ cages. Importantly, both NeN$_{10}$ and NeN$_{22}$ not only are dynamically and mechanically stable but also have a high thermal stability up to 500 K under ambient pressure. Moreover, ultra-high energy densities are obtained in NeN$_{10}$ (11.1 kJ/g), NeN$_{22}$ (11.5 kJ/g), tetragonal t-N$_{22}$ (11.6 kJ/g), and t-N$_{20}$ (12.0 kJ/g) produced from NeN$_{22}$, which are more than twice the value of trinitrotoluene (TNT). Meanwhile, their explosive performance is superior to that of TNT. Therefore, NeN$_{10}$, NeN$_{22}$, t-N$_{22}$, and t-N$_{20}$ are promising green high-energy-density materials. This work promotes the study of neon-nitrogen compounds with superior properties and potential applications.
Unexpected Selective Absorption of Lithium in Thermally Reduced Graphene Oxide Membranes
Jie Jiang, Liuhua Mu, Yu Qiang, Yizhou Yang, Zhikun Wang, Ruobing Yi, Yinwei Qiu, Liang Chen, Long Yan, and Haiping Fang
Chin. Phys. Lett.    2021, 38 (11): 116802 .   DOI: 10.1088/0256-307X/38/11/116802
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Lithium plays an increasingly important role in scientific and industrial processes, and it is extremely important to extract lithium from a high Mg$^{2+}$/Li$^{+}$ mass ratio brine or to recover lithium from the leachate of spent lithium-ion batteries. Conventional wisdom shows that Li$^{+}$ with low valence states has a much weaker adsorption (and absorption energy) with graphene than multivalent ions such as Mg$^{2+}$. Here, we show the selective adsorption of Li$^{+}$ in thermally reduced graphene oxide (rGO) membranes over other metal ions such as Mg$^{2+}$, Co$^{2+}$, Mn$^{2+}$, Ni$^{2+}$, or Fe$^{2+}$. Interestingly, the adsorption strength of Li$^{+}$ reaches up to 5 times the adsorption strength of Mg$^{2+}$, and the mass ratio of a mixed Mg$^{2+}$/Li$^{+}$ solution at a very high value of $ 500\!:\!1$ can be effectively reduced to $ 0.7\!:\!1$ within only six experimental treatment cycles, demonstrating the excellent applicability of the rGO membranes in the Mg$^{2+}$/Li$^{+}$ separation. A theoretical analysis indicates that this unexpected selectivity is attributed to the competition between cation–$\pi$ interaction and steric exclusion when hydrated cations enter the confined space of the rGO membranes.
Defects in Statically Unstable Solids: The Case for Cubic Perovskite $\alpha$-CsPbI$_3$
Xiaowei Wu, Chen Ming, Jing Shi, Han Wang, Damien West, Shengbai Zhang, and Yi-Yang Sun
Chin. Phys. Lett.    2022, 39 (4): 046101 .   DOI: 10.1088/0256-307X/39/4/046101
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High-temperature phases of solids are often dynamically stable only. First-principles study of point defects in such solids at 0 K is prohibited by their static instability, which results in random structures of the defect-containing supercell so that the total energy of the supercell is randomly affected by structural distortions far away from the defect. Taking cubic perovskite $\alpha$-CsPbI$_3$ as an example, we first present the problem incurred by the static instability and then propose an approach based on molecular dynamics to carry out ensemble average for tackling the problem. Within affordable simulation time, we obtain converged defect ionization energies, which are unattainable by a standard approach and allow us to evaluate its defect tolerance property. Our work paves the way for studying defects in statically unstable solids.
Lithium Ion Batteries Operated at $-100\,^{\circ}\!$C
Jianli Gai, Jirong Yang, Wei Yang, Quan Li, Xiaodong Wu, and Hong Li
Chin. Phys. Lett.    2023, 40 (8): 086101 .   DOI: 10.1088/0256-307X/40/8/086101
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Enabling lithium-ion batteries (LIBs) to operate in a wider temperature range, e.g., as low or high as possible or capable of both, is an urgent need and shared goal. Here we report, for the first time, a low-temperature electrolyte consisting of traditional ethylene carbonate, methyl acetate, butyronitrile solvents, and 1 M LiPF$_{6}$ salt, attributed to its very low freezing point ($T_{\rm f} = -126.3\,^{\circ}\!$C) and high ion conductivity at extremely low temperatures (0.21 mS/cm at $-100\,^{\circ}\!$C), successfully extends the service temperature of a practical 9.6 Ah LIB down to $-100\,^{\circ}\!$C (49.6% capacity retention compared to that at room temperature), which is the lowest temperature reported for practical cells so far as we know, and is lower than the lowest natural temperature ($-89.2\,^{\circ}\!$C) recorded on earth. Meanwhile, the high-temperature performance of lithium-ion batteries is not affected. The capacity retention is 88.2% and 83.4% after 800 cycles at 25$\,^{\circ}\!$C and 45$\,^{\circ}\!$C, respectively. The progress also makes LIB a proper power supplier for space vehicles in astronautic explorations.
Honeycomb Lattice in Metal-Rich Chalcogenide Fe$_{2}$Te
Jia-Qi Guan, Li Wang, Pengdong Wang, Wei Ren, Shuai Lu, Rong Huang, Fangsen Li, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue
Chin. Phys. Lett.    2021, 38 (11): 116801 .   DOI: 10.1088/0256-307X/38/11/116801
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Two-dimensional honeycomb crystals have inspired intense research interest for their novel properties and great potential in electronics and optoelectronics. Here, through molecular beam epitaxy on SrTiO$_{3}$(001), we report successful epitaxial growth of metal-rich chalcogenide Fe$_{2}$Te, a honeycomb-structured film that has no direct bulk analogue, under Te-limited growth conditions. The structural morphology and electronic properties of Fe$_{2}$Te are explored with scanning tunneling microscopy and angle resolved photoemission spectroscopy, which reveal electronic bands cross the Fermi level and nearly flat bands. Moreover, we find a weak interfacial interaction between Fe$_{2}$Te and the underlying substrates, paving a newly developed alternative avenue for honeycomb-based electronic devices.
Pressure-Induced Color Change in the Lutetium Dihydride LuH$_{2}$
Pengfei Shan, Ningning Wang, Xiquan Zheng, Qingzheng Qiu, Yingying Peng, and Jinguang Cheng
Chin. Phys. Lett.    2023, 40 (4): 046101 .   DOI: 10.1088/0256-307X/40/4/046101
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The lutetium dihydride LuH$_{2}$ is stable at ambient conditions. Here we show that its color undergoes sequential changes from dark blue at ambient pressure to pink at $\sim$ $2.2$ GPa and then to bright red at $\sim$ $4$ GPa upon compression in a diamond anvil cell. Such a pressure-induced color change in LuH$_{2}$ is reversible and it is very similar to that recently reported in the N-doped lutetium hydride [Nature 615, 244 (2023)]. However, our preliminary resistance measurements on LuH$_{2}$ under pressures up to $\sim$ $7$ GPa evidenced no superconductivity down to 1.5 K.
First-Principles Calculations about Elastic and Li$^{+}$ Transport Properties of Lithium Superoxides under High Pressure and High Temperature
Yufeng Li, Shichuan Sun, Yu He, and Heping Li
Chin. Phys. Lett.    2022, 39 (2): 026101 .   DOI: 10.1088/0256-307X/39/2/026101
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Lithium superoxides, Li$_{2}$O$_{3}$, LiO$_{2}$, and LiO$_{4}$, have been synthesized under high pressure. These materials have potential applications in energy storage devices. Here, we use first-principles calculations to investigate the elastic and Li$^{+}$ transport properties of these oxides at high pressure and high temperature. The elastic constants are calculated at 20–80 GPa, and they satisfy the Born stability criteria, indicating the good mechanical stability of these oxides. Their sound velocities calculated with elastic constants are close to each other, but difference in velocity anisotropy is obvious. LiO$_{2}$ presents significant shear sound wave anisotropy over 80%. The Li$^{+}$ transport properties are investigated using first principles molecular dynamics (FPMD) and climbing-image nudged elastic band methods. The lowest Li$^{+}$ migration barrier energies increase from 0.93, 0.86 and 1.22 eV at 20 GPa to 1.43, 1.12 and 1.77 eV at 50 GPa for Li$_{2}$O$_{3}$, LiO$_{2}$, and LiO$_{4}$, respectively. The most favorable path for LiO$_{2}$ and LiO$_{4}$ is along the [001] direction. The FPMD results suggest that these oxides become unstable with increasing temperature up to 2000 K due to O–O dimer clusters in these superoxides. Consequently, a superionic transition is not observed in the simulations.
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
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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.
Chiral Dirac Fermion in a Collinear Antiferromagnet
Ao Zhang, Ke Deng, Jieming Sheng, Pengfei Liu, Shiv Kumar, Kenya Shimada, Zhicheng Jiang, Zhengtai Liu, Dawei Shen, Jiayu Li, Jun Ren, Le Wang, Liang Zhou, Yoshihisa Ishikawa, Takashi Ohhara, Qiang Zhang, Garry McIntyre, Dehong Yu, Enke Liu, Liusuo Wu, Chaoyu Chen, and Qihang Liu
Chin. Phys. Lett.    2023, 40 (12): 126101 .   DOI: 10.1088/0256-307X/40/12/126101
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In a Dirac semimetal, the massless Dirac fermion has zero chirality, leading to surface states connected adiabatically to a topologically trivial surface state as well as vanishing anomalous Hall effect. Recently, it is predicted that in the nonrelativistic limit of certain collinear antiferromagnets, there exists a type of chiral “Dirac-like” fermion, whose dispersion manifests four-fold degenerate crossing points formed by spin-degenerate linear bands, with topologically protected Fermi arcs. Such an unconventional chiral fermion, protected by a hidden $SU(2)$ symmetry in the hierarchy of an enhanced crystallographic group, namely spin space group, is not experimentally verified yet. Here, by angle-resolved photoemission spectroscopy measurements, we reveal the surface origin of the electron pocket at the Fermi surface in collinear antiferromagnet CoNb$_{3}$S$_{6}$. Combining with neutron diffraction and first-principles calculations, we suggest a multidomain collinear antiferromagnetic configuration, rendering the existence of the Fermi-arc surface states induced by chiral Dirac-like fermions. Our work provides spectral evidence of the chiral Dirac-like fermion caused by particular spin symmetry in CoNb$_{3}$S$_{6}$, paving an avenue for exploring new emergent phenomena in antiferromagnets with unconventional quasiparticle excitations.
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
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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.
Magnetic-Field-Induced Sign Changes of Thermal Expansion in DyCrO$_{4}$
Jin-Cheng He, Zhao Pan, Dan Su, Xu-Dong Shen, Jie Zhang, Da-Biao Lu, Hao-Ting Zhao, Jun-Zhuang Cong, En-Ke Liu, You-Wen Long, and Young Sun
Chin. Phys. Lett.    2023, 40 (6): 066501 .   DOI: 10.1088/0256-307X/40/6/066501
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The anharmonicity of lattice vibration is mainly responsible for the coefficient of thermal expansion (CTE) of materials. External stimuli, such as magnetic and electric fields, thus cannot effectively change the CTE, much less the sign variation from positive to negative or vice versa. In this study, we report significant magnetic field effects on the CTE of zircon- and scheelite-type DyCrO$_{4}$ prepared at ambient and high pressures, respectively. At zero field, the zircon-type DyCrO$_{4}$ exhibits a negative CTE below the ferromagnetic-order temperature of 23 K. With increasing field up to $\ge $1.0 T, however, the sign of the CTE changes from negative to positive. In the scheelite phase, magnetic field can change the initially positive CTE to be negative with a field up to 2.0 T, and then a reentrant positive CTE is induced by enhanced fields $\ge $3.5 T. Both zircon and scheelite phases exhibit considerable magnetostrictive effects with the absolute values as high as $\sim$ $800$ ppm at 2 K and 10 T. The strong spin–lattice coupling is discussed to understand the unprecedented sign changes of the CTE caused by applying magnetic fields. The current DyCrO$_{4}$ provides the first example of field-induced sign change of thermal expansion, opening up a way to readily control the thermal expansion beyond the conventional chemical substitution.
Abnormal Elastic Changes for Cubic-Tetragonal Transition of Single-Crystal SrTiO$_{3}$
Caizi Zhang, Fangfei Li, Xinmiao Wei, Mengqi Guo, Yingzhan Wei, Liang Li, Xinyang Li, and Qiang Zhou
Chin. Phys. Lett.    2022, 39 (9): 096201 .   DOI: 10.1088/0256-307X/39/9/096201
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Strontium titanate (SrTiO$_{3}$) is a typical perovskite-type ceramic oxide and studying its high-pressure phases are critical to understand the ferroelastic phase transition. SrTiO$_{3}$ also can be used as an important analog of davemaoite (CaSiO$_{3}$) to understand the compositional and velocity structure of the Earth's interior. However, the high-pressure studies on the cubic-to-tetragonal phase transition pressure and elastic properties remain unclear for SrTiO$_{3}$. Here, we investigate the phase transition and elasticity of single-crystal SrTiO$_{3}$ by Raman and Brillouin scattering combined with diamond anvil cell. The acoustic velocities of single-crystal SrTiO$_{3}$ and the independent elastic constants of cubic and tetragonal SrTiO$_{3}$ are determined up to 27.5 GPa at room temperature. This study indicates that $C_{11}$, $C_{12}$, and $C_{44}$ exhibit abnormal changes at 10.3 GPa, which is related to the cubic-to-tetragonal phase transition. Interestingly, a significant softening on shear modulus and a large anisotropy of shear wave splitting ($A_{\mathrm{S}}^{\mathrm{PO}}$) jump are observed at 10.3 GPa. Using obtained elastic constants, the coefficients of the Landau potential are calculated to understand the phase transition between cubic and tetragonal. The calculated coefficients of the Landau potential are, $\lambda_{2} = 3.12\times 10^{-2}$ GPa, $\lambda_{4} = -2.02 \times 10^{-2}$ GPa, $B^* = 1.34 \times 10^{-4}$ GPa and $B = 1.66\times 10^{-4}$ GPa. The elastic results have profound implications in understanding the structure of the Earth's interior and indicate that the presence of tetragonal Ti-bearing CaSiO$_{3}$ helps to explain the large $A_{\mathrm{S}}^{\mathrm{PO}}$ of the Earth's mid-mantle.
Partially Diffusive Helium-Silica Compound under High Pressure
Cong Liu, Junjie Wang, Xin Deng, Xiaomeng Wang, Chris J. Pickard, Ravit Helled, Zhongqing Wu, Hui-Tian Wang, Dingyu Xing, and Jian Sun
Chin. Phys. Lett.    2022, 39 (7): 076101 .   DOI: 10.1088/0256-307X/39/7/076101
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Helium is the second most abundant element in the universe, and together with silica, they are important components of giant planets. Exploring the reactivity and state of helium and silica under high pressure is crucial for understanding of the evolution and internal structure of giant planets. Here, using first-principles calculations and crystal structure predictions, we identify four stable phases of a helium-silica compound with seven/eight-coordinated silicon atoms at pressure of 600–4000 GPa, corresponding to the interior condition of the outer planets in the solar system. The density of HeSiO$_{2}$ agrees with current structure models of the planets. This helium-silica compound exhibits a superionic-like helium diffusive state under the high-pressure and high-temperature conditions along the isentropes of Saturn, a metallic fluid state in Jupiter, and a solid state in the deep interiors of Uranus and Neptune. These results show that helium may affect the erosion of the rocky core in giant planets and may help to form a diluted core region, which not only highlight the reactivity of helium under high pressure but also provide evidence helpful for building more sophisticated interior models of giant planets.
Hydrodynamics of a Multi-Component Bosonic Superfluid
Fan Zhang and Lan Yin
Chin. Phys. Lett.    2023, 40 (6): 066701 .   DOI: 10.1088/0256-307X/40/6/066701
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We obtain the superfluid hydrodynamic equations of a multi-component Bose gas with short-ranged interactions at zero temperature under the local equilibrium assumption and show that the quantum pressure is generally present in the nonuniform case. Our approach can be extended to systems with long-range interactions such as dipole-dipole interactions by treating the Hartree energy properly. For a highly symmetric superfluid, we obtain the excitation spectrum and show that except for the density phonon, all other excitations are all degenerate. The implication of our results is discussed.
High-Temperature Superconductivity in Doped Boron Clathrates
Liang Ma, Lingrui Wang, Yifang Yuan, Haizhong Guo, and Hongbo Wang
Chin. Phys. Lett.    2023, 40 (8): 086201 .   DOI: 10.1088/0256-307X/40/8/086201
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The recent discoveries of near-room-temperature superconductivity in clathrate hydrides present compelling evidence for the reliability of theory-orientated conventional superconductivity. Nevertheless, the harsh pressure conditions required to maintain such high $T_{\rm c}$ limit their practical applications. To address this challenge, we conducted extensive first-principles calculations to investigate the doping effect of the recently synthesized LaB$_{8}$ clathrate, intending to design high-temperature superconductors at ambient pressure. Our results demonstrate that these clathrates are highly promising for high-temperature superconductivity owing to the coexistence of rigid boron covalent networks and the tunable density of states at the Fermi level. Remarkably, the predicted $T_{\rm c}$ of BaB$_{8}$ could reach 62 K at ambient pressure, suggesting a significant improvement over the calculated $T_{\rm c}$ of 14 K in LaB$_{8}$. Moreover, further calculations of the formation enthalpies suggest that BaB$_{8}$ could be potentially synthesized under high-temperature and high-pressure conditions. These findings highlight the potential of doped boron clathrates as promising superconductors and provide valuable insights into the design of light-element clathrate superconductors.
Novel Boron Nitride Polymorphs with Graphite-Diamond Hybrid Structure
Kun Luo, Baozhong Li, Lei Sun, Yingju Wu, Yanfeng Ge, Bing Liu, Julong He, Bo Xu, Zhisheng Zhao, and Yongjun Tian
Chin. Phys. Lett.    2022, 39 (3): 036301 .   DOI: 10.1088/0256-307X/39/3/036301
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Both boron nitride (BN) and carbon (C) have $sp$, $sp^{2}$ and $sp^{3}$ hybridization modes, thus resulting in a variety of BN and C polymorphs with similar structures, such as hexagonal BN (hBN) and graphite, cubic BN (cBN) and diamond. Here, five types of BN polymorph structures are proposed theoretically, inspired by the graphite-diamond hybrid structures discovered in a recent experiment. These BN polymorphs with graphite-diamond hybrid structures possess excellent mechanical properties with combined high hardness and high ductility, and also exhibit various electronic properties such as semi-conductivity, semi-metallicity, and even one- and two-dimensional conductivity, differing from known insulators hBN and cBN. The simulated diffraction patterns of these BN hybrid structures could account for the unsolved diffraction patterns of intermediate products composed of so-called “compressed hBN” and diamond-like BN, caused by phase transitions in previous experiments. Thus, this work provides a theoretical basis for the presence of these types of hybrid materials during phase transitions between graphite-like and diamond-like BN polymorphs.
Infrared Nano-Imaging of Electronic Phase across the Metal–Insulator Transition of NdNiO$_3$ Films
Fanwei Liu, Sisi Huang, Sidan Chen, Xinzhong Chen, Mengkun Liu, Kuijuan Jin, and Xi Chen
Chin. Phys. Lett.    2022, 39 (7): 076801 .   DOI: 10.1088/0256-307X/39/7/076801
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NdNiO$_3$ is a typical correlated material with temperature-driven metal–insulator transition. Resolving the local electronic phase is crucial in understanding the driving mechanism behind the phase transition. Here we present a nano-infrared study of the metal–insulator transition in NdNiO$_3$ films by a cryogenic scanning near-field optical microscope. The NdNiO$_3$ films undergo a continuous transition without phase coexistence. The nano-infrared signal shows significant temperature dependence and a hysteresis loop. Stripe-like modulation of the optical conductivity is formed in the films and can be attributed to the epitaxial strain. These results provide valuable evidence to understand the coupled electronic and structural transformations in NdNiO$_3$ films at the nano-scale.
Ideal Spin Hydrodynamics from the Wigner Function Approach
Hao-Hao Peng, Jun-Jie Zhang, Xin-Li Sheng, and Qun Wang
Chin. Phys. Lett.    2021, 38 (11): 116701 .   DOI: 10.1088/0256-307X/38/11/116701
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Based on the Wigner function in local equilibrium, we derive hydrodynamical quantities for a system of polarized spin-1/2 particles: the particle number current density, the energy-momentum tensor, the spin tensor, and the dipole moment tensor. Compared with ideal hydrodynamics without spin, additional terms at the first and second orders in the Knudsen number ${Kn}$ and the average spin polarization $\chi_{s}$ have been derived. The Wigner function can be expressed in terms of matrix-valued distributions, whose equilibrium forms are characterized by thermodynamical parameters in quantum statistics. The equations of motion for these parameters are derived by conservation laws at the leading and next-to-leading order ${Kn}$ and $\chi_{s}$.
Effect of Oxide Content of Graphene Oxide Membrane on Remarkable Adsorption for Calcium Ions
Jie Jiang, Long Yan, and Haiping Fang
Chin. Phys. Lett.    2021, 38 (10): 106801 .   DOI: 10.1088/0256-307X/38/10/106801
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Graphene oxide membranes (GOMs), as one of the most promising novel materials, have gained great interest in the field of adsorption. However, the oxygen content of graphene oxide is directly related to its adsorption properties, such as suspension stability, adsorption capacity, and reusability of GOMs. Here, a series of reduced GOMs with oxygen content from 28% to 12% were conveniently prepared by the thermally reduced and the corresponding interlayer spacing of these membranes changed from 8.0 Å to 3.7 Å. These prepared GOMs have remarkable Ca$^{2+}$ adsorption capacity, which increases with the oxygen content or interlayer spacing of GOMs. Importantly, the max adsorption capacity of the mass ratio between adsorbed Ca$^{2+}$ and pristine GOMs can reach up to 0.481 g/g, which is about one order of magnitude higher than the adsorption capacity of activated sludge, magnetic Fe$_{3}$O$_{4}$, functionalized silica, zeolite molecular sieve, and other reported previously. Moreover, GOMs show excellent stability and the Ca$^{2+}$ can be easily desorbed by water, so that the GOMs can be reused. Our previous theoretical analysis suggests that this remarkable adsorption is attributable to the strong interactions between Ca$^{2+}$ and GO sheets, including the ion-$\pi$ interactions between Ca$^{2+}$ and aromatic graphitic rings as well as the electrostatic interaction between Ca$^{2+}$ and oxygen-containing groups.
Computational Prediction of a Novel Superhard $sp^{3}$ Trigonal Carbon Allotrope with Bandgap Larger than Diamond
Ruoyun Lv, Xigui Yang, Dongwen Yang, Chunyao Niu, Chunxiang Zhao, Jinxu Qin, Jinhao Zang, Fuying Dong, Lin Dong, and Chongxin Shan
Chin. Phys. Lett.    2021, 38 (7): 076101 .   DOI: 10.1088/0256-307X/38/7/076101
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Searching for new carbon allotropes with superior properties has been a longstanding interest in material sciences and condensed matter physics. Here we identify a novel superhard carbon phase with an 18-atom trigonal unit cell in a full-$sp^{3}$ bonding network, termed tri-C$_{18}$ carbon, by first-principles calculations. Its structural stability has been verified by total energy, phonon spectra, elastic constants, and molecular dynamics simulations. Furthermore, tri-C$_{18}$ carbon has a high bulk modulus of 400 GPa and Vickers hardness of 79.0 GPa, comparable to those of diamond. Meanwhile, the simulated x-ray diffraction pattern of tri-C$_{18}$ carbon matches well with the previously unexplained diffraction peaks found in chimney soot, indicating the possible presence of tri-C$_{18}$ carbon. Remarkably, electronic band structure calculations reveal that tri-C$_{18}$ carbon has a wide indirect bandgap of 6.32 eV, larger than that of cubic diamond, indicating its great potential in electronic or optoelectronic devices working in the deep ultraviolet region.
Improvement of Cyclic Stability of Na$_{0.67}$Mn$_{0.8}$Ni$_{0.1}$Co$_{0.1}$O$_{2}$ via Suppressing Lattice Variation
Zhongmin Ren, Muqin Wang, Shuaishuai Chen, Lei Ding, Hua Li, Jian Liu, Jieyun Zheng, Zhihong Liu, Deyu Wang, and Mingkui Wang
Chin. Phys. Lett.    2021, 38 (7): 076102 .   DOI: 10.1088/0256-307X/38/7/076102
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Strategies to prolong operational life are highly pursued to strengthen the advantage of cost-effectiveness on sodium-ion batteries (SIBs). We demonstrate the crucial influence of particles' internal mechanical strains on durability of cathode, which does not attract enough attentions from the community. Among the investigated samples, 2% Ti-modified-Na$_{0.67}$Ni$_{0.1}$Co$_{0.1}$Mn$_{0.8}$O$_{2}$ suppresses the $c$-axis lattice variation by 38%, attains the reversible capacity 86% higher after 200 cycles, and still keeps intact morphology. This approach indicates that the mechanical properties could tailor cyclic stability of cathode, which is particular important to further improve competitiveness for SIBs.
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
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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.
A Free-Volume Model for Thermal Expansion of Metallic Glass
Tong Lu, Song Ling Liu, Yong Hao Sun, Wei-Hua Wang, and Ming-Xiang Pan
Chin. Phys. Lett.    2022, 39 (3): 036401 .   DOI: 10.1088/0256-307X/39/3/036401
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Many mechanical, thermal and transport behaviors of polymers and metallic glasses are interpreted by the free-volume model, whereas their applications on thermal expansion behaviors of glasses is rarely seen. Metallic glass has a range of glassy states depending on cooling rate, making their coefficients of thermal expansion vary with the glassy states. Anharmonicity in the interatomic potential is often used to explain different coefficients of thermal expansion in crystalline metals or in different metallic-glass compositions. However, it is unclear how to quantify the change of anharmonicity in the various states of metallic glass of the same composition and to connect it with coefficient of thermal expansion. In the present work, isothermal annealing is applied, and the dimensional changes are measured for La$_{62}$Al$_{14}$Cu$_{11.7}$Ag$_{2.3}$Ni$_{5}$Co$_{5}$ and Zr$_{52.5}$Cu$_{17.9}$Ni$_{14.6}$Al$_{10}$Ti$_{5}$ metallic glasses, from which changes in density and the coefficients of thermal expansion of the specimens are both recorded. The coefficients of thermal expansion linearly decrease with densification reflecting the role of free volume in thermal expansion. Free volume is found to have not only volume but also entity with an effective coefficient of thermal expansion similar to that of gases. Therefore, the local regions containing free volume inside the metallic glass are gas-like instead of liquid-like in terms of thermal expansion behaviors.
Realization of $^{87}$Rb Bose–Einstein Condensates in Higher Bands of a Hexagonal Boron-Nitride Optical Lattice
Jin-Yu Liu, Xiao-Qiong Wang, and Zhi-Fang Xu
Chin. Phys. Lett.    2023, 40 (8): 086701 .   DOI: 10.1088/0256-307X/40/8/086701
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Ultracold neutral atoms in higher bands of an optical lattice provide a natural avenue to emulate orbital physics in solid state materials. Here, we report the realization of $^{87}$Rb Bose–Einstein condensates in the fourth and seventh Bloch bands of a hexagonal boron-nitride optical lattice, exhibiting remarkably long coherence time through active cooling. Using band mapping spectroscopy, we observe that atoms condensed at the energy minimum of $\varGamma$ point ($K_{1}$ and $K_{2}$ points) in the fourth (seventh) band as sharp Bragg peaks. The lifetime for the condensate in the fourth (seventh) band is about 57.6 (4.8) ms, and the phase coherence of atoms in the fourth band persists for a long time larger than 110 ms. Our work thus offers great promise for studying unconventional bosonic superfluidity of neutral atoms in higher bands of optical lattices.
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