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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
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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.
Real-Time Observation of Electron-Hole Coherence Induced by Strong-Field Ionization
Jing Zhao, Jinlei Liu, Xiaowei Wang, Jianmin Yuan, and Zengxiu Zhao
Chin. Phys. Lett.    2022, 39 (12): 123201 .   DOI: 10.1088/0256-307X/39/12/123201
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We introduce and demonstrate a new approach to measure the electron-hole dynamics and coherence induced by strong-field ionization using hole-assisted high-harmonic spectroscopy. The coherent driving of the infrared and XUV pulses correlates the dynamics of the core-hole and the valence-hole by coupling multiple continua, which leads to the otherwise forbidden absorption and emission of high harmonics. An analytical model is developed based on the strong-field approximation by taking into account the essential multielectron configurations. The emission spectra from the core-valence transition and the core-hole recombination are found to modulate strongly as functions of the time delay between the two pulses, suggesting that the coherent electron wave packets in multiple continua can be utilized to temporally resolve the core-valence transition in attoseconds.
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
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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.
Theory of Critical Phenomena with Memory
Shaolong Zeng, Sue Ping Szeto, and Fan Zhong
Chin. Phys. Lett.    2022, 39 (12): 120501 .   DOI: 10.1088/0256-307X/39/12/120501
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Memory is a ubiquitous characteristic of complex systems, and critical phenomena are one of the most intriguing phenomena in nature. Here, we propose an Ising model with memory, develop a corresponding theory of critical phenomena with memory for complex systems, and discover a series of surprising novel results. We show that a naive theory of a usual Hamiltonian with a direct inclusion of a power-law decaying long-range temporal interaction violates radically a hyperscaling law for all spatial dimensions even at and below the upper critical dimension. This entails both indispensable consideration of the Hamiltonian for dynamics, rather than the usual practice of just focusing on the corresponding dynamic Lagrangian alone, and transformations that result in a correct theory in which space and time are inextricably interwoven, leading to an effective spatial dimension that repairs the hyperscaling law. The theory gives rise to a set of novel mean-field critical exponents, which are different from the usual Landau ones, as well as new universality classes. These exponents are verified by numerical simulations of the Ising model with memory in two and three spatial dimensions.
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
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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.
Dynamic Nonreciprocity with a Kerr Nonlinear Resonator
Rui-Kai Pan, Lei Tang, Keyu Xia, and Franco Nori
Chin. Phys. Lett.    2022, 39 (12): 124201 .   DOI: 10.1088/0256-307X/39/12/124201
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On-chip optical nonreciprocal devices are vital components for integrated photonic systems and scalable quantum information processing. Nonlinear optical isolators and circulators have attracted considerable attention because of their fundamental interest and their important advantages in integrated photonic circuits. However, optical nonreciprocal devices based on Kerr or Kerr-like nonlinearity are subject to dynamical reciprocity when the forward and backward signals coexist simultaneously in a nonlinear system. Here, we theoretically propose a method for realizing on-chip nonlinear isolators and circulators with dynamic nonreciprocity. Dynamic nonreciprocity is achieved via the chiral modulation on the resonance frequency due to coexisting self- and cross-Kerr nonlinearities in an optical ring resonator. This work showing dynamic nonreciprocity with a Kerr nonlinear resonator can be an essential step toward integrated optical isolation.
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
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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.
Optically Detected Magnetic Resonance of Diamond Nitrogen-Vacancy Centers under Megabar Pressures
Jian-Hong Dai, Yan-Xing Shang, Yong-Hong Yu, Yue Xu, Hui Yu, Fang Hong, Xiao-Hui Yu, Xin-Yu Pan, and Gang-Qin Liu
Chin. Phys. Lett.    2022, 39 (11): 117601 .   DOI: 10.1088/0256-307X/39/11/117601
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Megabar pressures are of crucial importance for cutting-edge studies of condensed matter physics and geophysics. With the development of diamond anvil cell (DAC), laboratory studies of high pressure have entered the megabar era for decades. However, it is still challenging to implement in situ magnetic sensing under ultrahigh pressures. In this work, we demonstrate optically detected magnetic resonance and coherent quantum control of diamond nitrogen-vacancy (NV) center, a promising quantum sensor inside the DAC, up to 1.4 Mbar. The pressure dependence of optical and spin properties of NV centers in diamond are quantified, and the evolution of an external magnetic field has been successfully tracked at about 80 GPa. These results shed new light on our understanding of diamond NV centers and pave the way for quantum sensing under extreme conditions.
Measuring Quantum Geometric Tensor of Non-Abelian System in Superconducting Circuits
Wen Zheng, Jianwen Xu, Zhuang Ma, Yong Li, Yuqian Dong, Yu Zhang, Xiaohan Wang, Guozhu Sun, Peiheng Wu, Jie Zhao, Shaoxiong Li, Dong Lan, Xinsheng Tan, and Yang Yu
Chin. Phys. Lett.    2022, 39 (10): 100202 .   DOI: 10.1088/0256-307X/39/10/100202
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Topology played an important role in physics research during the last few decades. In particular, the quantum geometric tensor that provides local information about topological properties has attracted much attention. It will reveal interesting topological properties but have not been measured in non-Abelian systems. Here, we use a four-qubit quantum system in superconducting circuits to construct a degenerate Hamiltonian with parametric modulation. By manipulating the Hamiltonian with periodic drivings, we simulate the Bernevig–Hughes–Zhang model and obtain the quantum geometric tensor from interference oscillation. In addition, we reveal its topological feature by extracting the topological invariant, demonstrating an effective protocol for quantum simulation of a non-Abelian system.
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
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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.
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