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 Select A Time-Dependent Random State Approach for Large-Scale Density Functional Calculations Weiqing Zhou and Shengjun Yuan Chin. Phys. Lett.    2023, 40 (2): 027101 .   DOI: 10.1088/0256-307X/40/2/027101 Abstract   HTML   PDF (7108KB) We develop a self-consistent first-principle method based on the density functional theory. Physical quantities such as the density of states, Fermi energy and electron density are obtained using a time-dependent random state method without diagonalization. The numerical error for calculating either global or local variables always scales as $1/\sqrt{SN_{\rm e}}$, where $N_{\rm e}$ is the number of electrons and $S$ is the number of random states, leading to a sublinear computational cost with the system size. In the limit of large systems, one random state could be enough to achieve reasonable accuracy. The accuracy and scaling properties of using the method are derived analytically and verified numerically in different condensed matter systems. Our time-dependent random state approach provides a powerful strategy for large-scale density functional calculations.
 Select Quasi-Two-Dimensional Nature of High-$T_{\rm c}$ Superconductivity in Iron-Based (Li,Fe)OHFeSe Dong Li, Yue Liu, Zouyouwei Lu, Peiling Li, Yuhang Zhang, Sheng Ma, Jiali Liu, Jihu Lu, Hua Zhang, Guangtong Liu, Fang Zhou, Xiaoli Dong, and Zhongxian Zhao Chin. Phys. Lett.    2022, 39 (12): 127402 .   DOI: 10.1088/0256-307X/39/12/127402 Abstract   HTML   PDF (5277KB) The intercalated iron selenide (Li,Fe)OHFeSe has a strongly layered structure analogous to the quasi-two-dimensional (2D) bismuth cuprate superconductors, and exhibits both high-temperature ($T_{\rm c}$) and topological superconductivity. However, the issue of its superconductivity dimensionality has not yet been fully investigated so far. Here we report that the quasi-2D superconductivity features, including the high anisotropy $\gamma = 151$ and the associated quasi-2D vortices, are also revealed for (Li,Fe)OHFeSe, based on systematic experiments of the electrical transport and magnetization and model fittings. Thus, we establish a new vortex phase diagram for (Li,Fe)OHFeSe, which delineates an emergent quasi-2D vortex-liquid state, and a subsequent vortex-solid dimensional crossover from a pancake-like to a three-dimensional state with decreasing temperature and magnetic field. Furthermore, we find that all the quasi-2D characteristics revealed here for the high-$T_{\rm c}$ iron selenide superconductor are very similar to those reported for high-$T_{\rm c}$ bismuth cuprate superconductors.
 Select Large Room-Temperature Magnetoresistance in van der Waals Ferromagnet/Semiconductor Junctions Wenkai Zhu, Shihong Xie, Hailong Lin, Gaojie Zhang, Hao Wu, Tiangui Hu, Ziao Wang, Xiaomin Zhang, Jiahan Xu, Yujing Wang, Yuanhui Zheng, Faguang Yan, Jing Zhang, Lixia Zhao, Amalia Patanè, Jia Zhang, Haixin Chang, and Kaiyou Wang Chin. Phys. Lett.    2022, 39 (12): 128501 .   DOI: 10.1088/0256-307X/39/12/128501 Abstract   HTML   PDF (4671KB) A magnetic tunnel junction (MTJ) is the core component in memory technologies, such as the magnetic random-access memory, magnetic sensors and programmable logic devices. In particular, MTJs based on two-dimensional van der Waals (vdW) heterostructures offer unprecedented opportunities for low power consumption and miniaturization of spintronic devices. However, their operation at room temperature remains a challenge. Here, we report a large tunnel magnetoresistance (TMR) of up to 85% at room temperature ($T = 300$ K) in vdW MTJs based on a thin ($< 10$ nm) semiconductor spacer WSe$_{2}$ layer embedded between two Fe$_{3}$GaTe$_{2}$ electrodes with intrinsic above-room-temperature ferromagnetism. The TMR in the MTJ increases with decreasing temperature up to 164% at $T = 10$ K. The demonstration of TMR in ultra-thin MTJs at room temperature opens a realistic and promising route for next-generation spintronic applications beyond the current state of the art.
 Select NMR Evidence for Universal Pseudogap Behavior in Quasi-Two-Dimensional FeSe-Based Superconductors B. L. Kang, M. Z. Shi, D. Zhao, S. J. Li, J. Li, L. X. Zheng, D. W. Song, L. P. Nie, T. Wu, and X. H. Chen Chin. Phys. Lett.    2022, 39 (12): 127401 .   DOI: 10.1088/0256-307X/39/12/127401 Abstract   HTML   PDF (5078KB) Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors [(TBA)$_{x}$FeSe and (CTA)$_{x}$FeSe] with superconducting transition temperatures ($T_{\rm c}$) above 40 K have been discovered. Due to the large interlayer distance ($\sim$15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above $T_{\rm c}$ is revealed by nuclear magnetic resonance (NMR) measurements on $^{77}$Se nuclei, suggesting a preformed pairing scenario similar to that of cuprates. Here, we report another new kind of organic-ion-intercalated FeSe superconductor, (PY)$_{x}$FeSe, with a reduced interlayer distance ($\sim$10 Å) compared to (TBA)$_{x}$FeSe and (CTA)$_{x}$FeSe. By performing $^{77}$Se NMR and transport measurements, we observe a similar pseudogap behavior well above $T_{\rm c}$ of $\sim$40 K and a large resistivity anisotropy of $\sim$$10^{\boldsymbol{4}}$ in (PY)$_{x}$FeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.
 Select Cobalt-Dimer Nitrides: A Potential Novel Family of High-Temperature Superconductors Yuhao Gu, Kun Jiang, Xianxin Wu, and Jiangping Hu Chin. Phys. Lett.    2022, 39 (9): 097401 .   DOI: 10.1088/0256-307X/39/9/097401 Abstract   HTML   PDF (11283KB) We predict that the square lattice layer formed by [Co$_2$N$_2$]$^{2-}$ diamond-like units can host high-temperature superconductivity. The layer appears in the stable ternary cobalt nitride, BaCo$_2$N$_2$. The electronic physics of the material stems from Co$_2$N$_2$ layers where the dimerized Co pairs form a square lattice. The low energy physics near Fermi energy can be described by an effective two-orbital model. Without considering interlayer couplings, the two orbitals are effectively decoupled. This electronic structure satisfies the “gene” character proposed for unconventional high-temperature superconductors. We predict that the leading superconducting pairing instability is driven from an extended $s$-wave ($s^\pm$) to a $d$-wave by hole doping, e.g., in Ba$_{1-x}$K$_x$Co$_2$N$_2$. This study provides a new platform to establish the superconducting mechanism of unconventional high-temperature superconductivity.