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Ultrafast Condensed Matter Physics at Attoseconds
Shi-Qi Hu and Sheng Meng
Chin. Phys. Lett.    2023, 40 (11): 117801 .   DOI: 10.1088/0256-307X/40/11/117801
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Our understanding of how photons couple to different degrees of freedom in solids forms the bedrock of ultrafast physics and materials sciences. In this review, the emergent ultrafast dynamics in condensed matter at the attosecond timescale have been intensively discussed. In particular, the focus is put on recent developments of attosecond dynamics of charge, exciton, and magnetism. New concepts and indispensable role of interactions among multiple degrees of freedom in solids are highlighted. Applications of attosecond electronic metrology and future prospects toward attosecond dynamics in condensed matter are further discussed. These pioneering studies promise future development of advanced attosecond science and technology such as attosecond lasers, laser medical engineering, and ultrafast electronic devices.
Light-Induced Phonon-Mediated Magnetization in Monolayer MoS$_{2}$
Shengjie Zhang, Yufei Pei, Shiqi Hu, Na Wu, Da-Qiang Chen, Chao Lian, and Sheng Meng
Chin. Phys. Lett.    2023, 40 (7): 077502 .   DOI: 10.1088/0256-307X/40/7/077502
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Light-induced ultrafast spin dynamics in materials is of great importance for developments of spintronics and magnetic storage technology. Recent progresses include ultrafast demagnetization, magnetic switching, and magnetic phase transitions, while the ultrafast generation of magnetism is hardly achieved. Here, a strong light-induced magnetization (up to $0.86\mu_{\scriptscriptstyle{\rm B}}$ per formula unit) is identified in non-magnetic monolayer molybdenum disulfide (MoS$_{2}$). With the state-of-the-art time-dependent density functional theory simulations, we demonstrate that the out-of-plane magnetization can be induced by circularly polarized laser, where chiral phonons play a vital role. The phonons strongly modulate spin-orbital interactions and promote electronic transitions between the two conduction band states, achieving an effective magnetic field $\sim$ $380$ T. Our study provides important insights into the ultrafast magnetization and spin-phonon coupling dynamics, facilitating effective light-controlled valleytronics and magnetism.
Engineering Interlayer Hybridization in Energy Space via Dipolar Overlayers
Bin Shao, Xiao Jiang, Jan Berges, Sheng Meng, and Bing Huang
Chin. Phys. Lett.    2023, 40 (8): 087303 .   DOI: 10.1088/0256-307X/40/8/087303
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The interlayer hybridization (IH) of van der Waals (vdW) materials is thought to be mostly associated with the unignorable interlayer overlaps of wavefunctions ($t$) in real space. Here, we develop a more fundamental understanding of IH by introducing a new physical quantity, the IH admixture ratio $\alpha$. Consequently, an exotic strategy of IH engineering in energy space can be proposed, i.e., instead of changing $t$ as commonly used, $\alpha$ can be effectively tuned in energy space by changing the on-site energy difference (${2\varDelta}$) between neighboring-layer states. In practice, this is feasible via reshaping the electrostatic potential of the surface by deposing a dipolar overlayer, e.g., crystalline ice. Our first-principles calculations unveil that IH engineering via adjusting ${2\varDelta}$ can greatly tune interlayer optical transitions in transition-metal dichalcogenide bilayers, switch different types of Dirac surface states in Bi$_{2}$Se$_{3}$ thin films, and control magnetic phase transition of charge density waves in 1H/1T-TaS$_{2}$ bilayers, opening new opportunities to govern the fundamental optoelectronic, topological, and magnetic properties of vdW systems beyond the traditional interlayer distance or twisting engineering.
Hydrothermally Obtaining Superconductor Single Crystal of FeSe$_{0.2}$Te$_{0.8}$ without Interstitial Fe
Sheng Ma, Shanshan Yan, Jiali Liu, Yizhe Wang, Yuhang Zhang, Zhen Zhao, Zouyouwei Lu, Dong Li, Yue Liu, Jihu Lu, Hua Zhang, Haitao Yang, Fang Zhou, Zian, Li, Xiaoli Dong, and Zhongxian Zhao
Chin. Phys. Lett.    2023, 40 (6): 067402 .   DOI: 10.1088/0256-307X/40/6/067402
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We report a hydrothermal route to remove interstitial excess Fe in non-superconducting iron chalcogenide Fe$_{1+\delta}$Se$_{1-x}$Te$_{x}$ single crystals. The extra-Fe-free ($\delta \sim 0$) FeSe$_{0.2}$Te$_{0.8}$ single crystal thus obtained shows bulk superconductivity at $T_{\rm c} \sim 13.8$ K, which is about 2 K higher than the FeSe$_{0.2}$Te$_{0.8}$ sample obtained by usual post-annealing process. The upper critical field $\mu_{0}H_{\rm c2}$ is estimated to be $\sim$ $42.5$ T, similar to the annealed FeSe$_{0.2}$Te$_{0.8}$. It is surprising to find that the hydrothermal FeSe$_{0.2}$Te$_{0.8}$ exhibits a remarkably small isothermal magnetization hysteresis loop at $T = 3$ K. This yields an extremely low critical current density $J_{\rm c} \sim 1.1\times 10^{2}$ A$\cdot$cm$^{-2}$ (over 100 times smaller than the annealed FeSe$_{0.2}$Te$_{0.8}$) and indicates more free vortices in the hydrothermal FeSe$_{0.2}$Te$_{0.8}$.
Superconductivity above 30 K Achieved in Dense Scandium
Xin He, Changling Zhang, Zhiwen Li, Sijia Zhang, Shaomin Feng, Jianfa Zhao, Ke Lu, Baosen Min, Yi Peng, Xiancheng Wang, Jin Song, Luhong Wang, Saori I. Kawaguchi, Cheng Ji, Bing Li, Haozhe Liu, J. S. Tse, and Changqing Jin
Chin. Phys. Lett.    2023, 40 (10): 107403 .   DOI: 10.1088/0256-307X/40/10/107403
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Superconductivity is one of most intriguing quantum phenomena, and the quest for elemental superconductors with high critical temperature ($T_{\rm c}$) is of great scientific significance due to their relatively simple material composition and the underlying mechanism. Here we report the experimental discovery of densely compressed scandium (Sc) becoming the first elemental superconductor with $T_{\rm c}$ breaking into 30 K range, which is comparable to the $T_{\rm c}$ values of the classic La–Ba–Cu–O or LaFeAsO superconductors. Our results show that $T_{\rm c}^{\rm onset}$ of Sc increases from $\sim$ $3$ K at around 43 GPa to $\sim$ $32$ K at about 283 GPa ($T_{\rm c}^{\rm zero} \sim 31$ K), which is well above liquid neon temperature. Interestingly, measured $T_{\rm c}$ shows no sign of saturation up to the maximum pressure achieved in our experiments, indicating that $T_{\rm c}$ may be even higher upon further compression.
High-Temperature Superconductivity in La$_3$Ni$_2$O$_7$
Kun Jiang, Ziqiang Wang, and Fu-Chun Zhang
Chin. Phys. Lett.    2024, 41 (1): 017402 .   DOI: 10.1088/0256-307X/41/1/017402
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Motivated by the recent discovery of high-temperature superconductivity in bilayer La$_3$Ni$_2$O$_7$ under pressure, we study its electronic properties and superconductivity due to strong electron correlation. Using the inversion symmetry, we decouple the low-energy electronic structure into block-diagonal symmetric and antisymmetric sectors. It is found that the antisymmetric sector can be reduced to a one-band system near half filling, while the symmetric bands occupied by about two electrons are heavily overdoped individually. Using the strong coupling mean field theory, we obtain strong superconducting pairing with $B_{\rm 1g}$ symmetry in the antisymmetric sector. We propose that due to the spin-orbital exchange coupling between the two sectors, $B_{\rm 1g}$ pairing is induced in the symmetric bands, which in turn boosts the pairing gap in the antisymmetric band and enhances the high-temperature superconductivity with a congruent d-wave symmetry in pressurized La$_3$Ni$_2$O$_7$.
Realizations, Characterizations, and Manipulations of Two-Dimensional Electron Systems Floating above Superfluid Helium Surfaces
Haoran Wei, Mengmeng Wu, Renfei Wang, Mingcheng He, Hiroki Ikegami, Yang Liu, and Zhi Gang Cheng
Chin. Phys. Lett.    2023, 40 (12): 127301 .   DOI: 10.1088/0256-307X/40/12/127301
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Electron systems in low dimensions are enriched with many superior properties for both fundamental research and technical developments. Wide tunability of electron density, high mobility of motion, and feasible controllability in microscales are the most prominent advantages that researchers strive for. Nevertheless, it is always difficult to fulfill all in one solid-state system. Two-dimensional electron systems (2DESs) floating above the superfluid helium surfaces are thought to meet these three requirements simultaneously, ensured by the atomic smoothness of surfaces and the electric neutrality of helium. Here we report our recent work in preparing, characterizing, and manipulating 2DESs on superfluid helium. We realized a tunability of electron density over one order of magnitude and tuned their transport properties by varying electron distribution and measurement frequency. The work we engage in is crucial for advancing research in many-body physics and for development of single-electron quantum devices rooted in these electron systems.
Electron-Correlation-Induced Charge Density Wave in FeGe
Lin Wu, Yating Hu, Dongze Fan, Di Wang, and Xiangang Wan
Chin. Phys. Lett.    2023, 40 (11): 117103 .   DOI: 10.1088/0256-307X/40/11/117103
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As the first magnetic kagome material to exhibit the charge density wave (CDW) order, FeGe has attracted much attention in recent research. Similar to $A$V$_{3}$Sb$_{5}$ ($A$ = K, Cs, Rb), FeGe exhibits the CDW pattern with an in-plane 2$\times$2 structure and the existence of van Hove singularities near the Fermi level. However, sharply different from $A$V$_{3}$Sb$_{5}$ which has phonon instability at $M$ point, all the theoretically calculated phonon frequencies in FeGe remain positive. Based on first-principles calculations, we surprisingly find that the maximum of nesting function is at $K$ point instead of $M$ point. Two Fermi pockets with Fe-$d_{xz}$ and Fe-$d_{x^{2}-y^{2}}$/$d_{xy}$ orbital characters have large contribution to the Fermi nesting, which evolve significantly with $k_{z}$, indicating the highly three-dimensional (3D) feature of FeGe in contrast to $A$V$_{3}$Sb$_{5}$. Considering the effect of local Coulomb interaction, we reveal that the instability at $K$ point is significantly suppressed due to the sublattice interference mechanism. Meanwhile, the wave functions nested by vector $M$ have many ingredients located at the same Fe site, thus the instability at $M$ point is enhanced. This indicates that the electron correlation, rather than electron-phonon interaction, plays a key role in the CDW transition at $M$ point.
Moiré Synaptic Transistor for Homogeneous-Architecture Reservoir Computing
Pengfei Wang, Moyu Chen, Yongqin Xie, Chen Pan, Kenji Watanabe, Takashi Taniguchi, Bin Cheng, Shi-Jun Liang, and Feng Miao
Chin. Phys. Lett.    2023, 40 (11): 117201 .   DOI: 10.1088/0256-307X/40/11/117201
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Reservoir computing has been considered as a promising intelligent computing paradigm for effectively processing complex temporal information. Exploiting tunable and reproducible dynamics in the single electronic device have been desired to implement the “reservoir” and the “readout” layer of reservoir computing system. Two-dimensional moiré materials, with an artificial lattice constant many times larger than the atomic length scale, are one type of most studied artificial quantum materials in community of material science and condensed-matter physics over the past years. These materials are featured with gate-tunable periodic potential and electronic correlation, thus varying the electric field allows the electrons in the moiré potential per unit cell to exhibit distinct and reproducible dynamics, showing great promise in robust reservoir computing. Here, we report that a moiré synaptic transistor can be used to implement the reservoir computing system with a homogeneous reservoir-readout architecture. The synaptic transistor is fabricated based on an h-BN/bilayer graphene/h-BN moiré heterostructure, exhibiting ferroelectricity-like hysteretic gate voltage dependence of resistance. Varying the magnitude of the gate voltage enables the moiré transistor to switch between long-term memory and short-term memory with nonlinear dynamics. By employing the short- and long-term memories as the reservoir nodes and weights of the readout layer, respectively, we construct a full-moiré physical neural network and demonstrate that the classification accuracy of 90.8% can be achieved for the MNIST (Modified National Institute of Standards and Technology) handwritten digits database. Our work would pave the way towards the development of neuromorphic computing based on moiré materials.
Giant 2D Skyrmion Topological Hall Effect with Ultrawide Temperature Window and Low-Current Manipulation in 2D Room-Temperature Ferromagnetic Crystals
Gaojie Zhang, Qingyuan Luo, Xiaokun Wen, Hao Wu, Li Yang, Wen Jin, Luji Li, Jia Zhang, Wenfeng Zhang, Haibo Shu, and Haixin Chang
Chin. Phys. Lett.    2023, 40 (11): 117501 .   DOI: 10.1088/0256-307X/40/11/117501
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The discovery and manipulation of topological Hall effect (THE), an abnormal magnetoelectric response mostly related to the Dzyaloshinskii–Moriya interaction (DMI), are promising for next-generation spintronic devices based on topological spin textures such as magnetic skyrmions. However, most skyrmions and THE are stabilized in a narrow temperature window either below or over room temperature with high critical current manipulation. It is still elusive and challenging to achieve large THE with both wide temperature window till room temperature and low critical current manipulation. Here, using controllable, naturally oxidized sub-20 and sub-10 nm 2D van der Waals room-temperature ferromagnetic Fe$_{3}$GaTe$_{2-x}$ crystals, we report robust 2D skyrmion THE with ultrawide temperature window ranging in three orders of magnitude from 2 to 300 K, in combination with giant THE of $\sim$ 5.4 $µ \Omega\cdot$cm at 10 K and $\sim$ 0.15 $µ \Omega\cdot$cm at 300 K, which is 1–3 orders of magnitude larger than that of all known room-temperature 2D skyrmion systems. Moreover, room-temperature current-controlled THE is also realized with a low critical current density of $\sim$ $6.2\times10^{5}$ A$\cdot$cm$^{-2}$. First-principles calculations unveil natural oxidation-induced highly enhanced 2D interfacial DMI reasonable for robust giant THE. This work paves the way to room-temperature electrically controlled 2D THE-based practical spintronic devices.
Enhanced Magnetic Interaction between Ga and Fe in Two-Dimensional van der Waals Ferromagnetic Crystal Fe$_{3}$GaTe$_{2}$
Heming Zha, Wei Li, Gaojie Zhang, Wenjing Liu, Liwei Deng, Qi Jiang, Mao Ye, Hao Wu, Haixin Chang, and Shan Qiao
Chin. Phys. Lett.    2023, 40 (8): 087501 .   DOI: 10.1088/0256-307X/40/8/087501
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Fe$_{3}$GaTe$_{2}$, a recently discovered van der Waals ferromagnetic crystal with the highest Curie temperature and strong perpendicular magnetic anisotropy among two-dimensional (2D) magnetic materials, has attracted significant attention and makes it a promising candidate for next-generation spintronic applications. Compared with Fe$_{3}$GeTe$_{2}$, which has the similar crystal structure, the mechanism of the enhanced ferromagnetic properties in Fe$_{3}$GaTe$_{2}$ is still unclear and needs to be investigated. Here, by using x-ray magnetic circular dichroism measurements, we find that both Ga and Te atoms contribute to the total magnetic moment of the system with antiferromagnetic coupling to Fe atoms. Our first-principles calculations reveal that Fe$_{3}$GaTe$_{2}$ has van Hove singularities at the Fermi level in nonmagnetic state, resulting in the magnetic instability of the system and susceptibility to magnetic phase transitions. In addition, the calculation results about the density of states in ferromagnetic states of two materials suggest that the exchange interaction between Fe atoms is strengthened by replacing Ge atoms with Ga atoms. These findings indicate the increase of both the itinerate and local moments in Fe$_{3}$GaTe$_{2}$ in view of Stoner and exchange interaction models, which results in the enhancement of the overall magnetism and a higher Curie temperature. Our work provides insight into the underlying mechanism of Fe$_{3}$GaTe$_{2}$'s remarkable magnetic properties and has important implications for searching 2D materials with expected magnetic properties in the future.
Effective Bi-Layer Model Hamiltonian and Density-Matrix Renormalization Group Study for the High-$T_{\rm c}$ Superconductivity in La$_{3}$Ni$_{2}$O$_{7}$ under High Pressure
Yang Shen, Mingpu Qin, and Guang-Ming Zhang
Chin. Phys. Lett.    2023, 40 (12): 127401 .   DOI: 10.1088/0256-307X/40/12/127401
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High-$T_{\rm c}$ superconductivity with possible $T_{\rm c}\approx 80$ K has been reported in the single crystal of ${\rm La}_{3}{\rm Ni}_{2}{\rm O}_{7}$ under high pressure. Based on the electronic structure given by the density functional theory calculations, we propose an effective bi-layer model Hamiltonian including both $3d_{z^{2}}$ and $3d_{x^{2}-y^{2}}$ orbital electrons of the nickel cations. The main feature of the model is that the $3d_{z^{2}}$ electrons form inter-layer $\sigma$-bonding and anti-bonding bands via the apical oxygen anions between the two layers, while the $3d_{x^{2}-y^{2}}$ electrons hybridize with the $3d_{z^{2}}$ electrons within each NiO$_2$ plane. The chemical potential difference of these two orbital electrons ensures that the $3d_{z^{2}}$ orbitals are close to half-filling and the $3d_{x^{2}-y^{2}}$ orbitals are near quarter-filling. The strong on-site Hubbard repulsion of the $3d_{z^{2}}$ orbital electrons gives rise to an effective inter-layer antiferromagnetic spin super-exchange $J$. Applying pressure can self dope holes on the $3d_{z^{2}}$ orbitals with the same amount of electrons doped on the $3d_{x^{2}-y^{2}}$ orbitals. By performing numerical density-matrix renormalization group calculations on a minimum setup and focusing on the limit of large $J$ and small doping of $3d_{z^{2}}$ orbitals, we find the superconducting instability on both the $3d_{z^{2}}$ and $3d_{x^{2}-y^{2}}$ orbitals by calculating the equal-time spin singlet pair–pair correlation function. Our numerical results may provide useful insights in the high-$T_{\rm c}$ superconductivity in single crystal La$_3$Ni$_2$O$_7$ under high pressure.
Gate-Tunable Negative Differential Conductance in Hybrid Semiconductor–Superconductor Devices
Ming-Li Liu, Dong Pan, Tian Le, Jiang-Bo He, Zhong-Mou Jia, Shang Zhu, Guang Yang, Zhao-Zheng Lyu, Guang-Tong Liu, Jie Shen, Jian-Hua Zhao, Li Lu, and Fan-Ming Qu
Chin. Phys. Lett.    2023, 40 (6): 067301 .   DOI: 10.1088/0256-307X/40/6/067301
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Negative differential conductance (NDC) serves as a crucial characteristic that reveals various underlying physics and transport process in hybrid superconducting devices. We report the observation of gate-tunable NDC outside the superconducting energy gap on two types of hybrid semiconductor–superconductor devices, i.e., normal metal–superconducting nanowire–normal metal and normal metal–superconducting nanowire–superconductor devices. Specifically, we study the dependence of the NDCs on back-gate voltage and magnetic field. When the back-gate voltage decreases, these NDCs weaken and evolve into positive differential conductance dips; and meanwhile they move away from the superconducting gap towards high bias voltage, and disappear eventually. In addition, with the increase of magnetic field, the NDCs/dips follow the evolution of the superconducting gap, and disappear when the gap closes. We interpret these observations and reach a good agreement by combining the Blonder–Tinkham–Klapwijk (BTK) model and the critical supercurrent effect in the nanowire, which we call the BTK-supercurrent model. Our results provide an in-depth understanding of the tunneling transport in hybrid semiconductor–superconductor devices.
Optical Tunable Moiré Excitons in Twisted Hexagonal GaTe Bilayers
Jinsen Han, Kang Lai, Xiaoxiang Yu, Jiahao Chen, Hongli Guo, and Jiayu Dai
Chin. Phys. Lett.    2023, 40 (6): 067801 .   DOI: 10.1088/0256-307X/40/6/067801
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Optical fine-tunable layer-hybridized Moiré excitons are highly in demand for emerging many-body states in two-dimensional semiconductors. We report naturally confined layer-hybridized bright Moiré excitons with long lifetimes in twisted hexagonal GaTe bilayers, using ab initio many-body perturbation theory and the Bethe–Salpeter equation. Due to the hybridization of electrons and holes between layers, which enhances the brightness of excitons, the twisted bilayer system becomes attractive for optical applications. We find that in both R and H-type stacking Moiré superlattices, more than 200 meV lateral quantum confinements occur on exciton energies, which results in two scenarios: (1) The ground state bright excitons $\mathrm{X}_\mathrm{A}$ are found to be trapped at two high-symmetry points, with opposite electric dipoles in the R-stacking Moiré supercell, forming a honeycomb superlattice of nearest-neighbor dipolar attraction. (2) For H-stacking case, the $\mathrm{X}_\mathrm{A}$ is found to be trapped at only one high-symmetry point exhibiting a triangular superlattice. Our results suggest that twisted h-GaTe bilayer is one of the promising systems for optical fine-tunable excitonic devices and provide an ideal platform for realizing strong correlated Bose–Hubbard physics.
Spectrum of the Hole Excitation in Spin-Orbit Mott Insulator Na$_{2}$IrO$_{3}$
Wei Wang, Zhao-Yang Dong, Shun-Li Yu, and Jian-Xin Li
Chin. Phys. Lett.    2023, 40 (8): 087101 .   DOI: 10.1088/0256-307X/40/8/087101
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We study the motion of a hole with internal degrees of freedom, introduced to the zigzag magnetic ground state of Na$_{2}$IrO$_{3}$, by using the self-consistent Born approximation. We find that the low-, intermediate-, and high-energy spectra are primarily attributed to the singlet, triplet, and quintet hole contributions, respectively. The spectral functions exhibit distinct features such as the electron-like dispersion of low-energy states near the $\varGamma$ point, the maximum $M$-point intensity of mid-energy states, and the hole-like dispersion of high-energy states. These features are robust and almost insensitive to the exchange model and Hund's coupling, and are in qualitative agreement with the angular-resolved photoemission spectra observed in Na$_{2}$IrO$_{3}$. Our results reveal that the interference between internal degrees of freedom in different sublattices plays an important role in inducing the complex dispersions.
Superexchange Interactions and Magnetic Anisotropy in MnPSe$_3$ Monolayer
Guangyu Wang, Ke Yang, Yaozhenghang Ma, Lu Liu, Di Lu, Yuxuan Zhou, and Hua Wu
Chin. Phys. Lett.    2023, 40 (7): 077301 .   DOI: 10.1088/0256-307X/40/7/077301
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Two-dimensional van der Waals magnetic materials are of great current interest for their promising applications in spintronics. Using density functional theory calculations in combination with the maximally localized Wannier functions method and the magnetic anisotropy analyses, we study the electronic and magnetic properties of MnPSe$_3$ monolayer. Our results show that it is a charge transfer antiferromagnetic (AF) insulator. For this Mn$^{2+}$ $3d^5$ system, although it seems straightforward to explain the AF ground state using the direct exchange, we find that the nearly 90$^\circ$ Mn–Se–Mn charge transfer type superexchange plays a dominant role in stabilizing the AF ground state. Moreover, our results indicate that, although the shape anisotropy favors an out-of-plane spin orientation, the spin-orbit coupling (SOC) leads to the experimentally observed in-plane spin orientation. We prove that the actual dominant contribution to the magnetic anisotropy comes from the second-order perturbation of the SOC, by analyzing its distribution over the reciprocal space. Using the AF exchange and anisotropy parameters obtained from our calculations, our Monte Carlo simulations give the Néel temperature $T_{\rm N}=47$ K for MnPSe$_3$ monolayer, which agrees with the experimental 40 K. Furthermore, our calculations show that under a uniaxial tensile (compressive) strain, Néel vector would be parallel (perpendicular) to the strain direction, which well reproduces the recent experiments. We also predict that $T_{\rm N}$ would be increased by a compressive strain.
Temperature-Dependent Anisotropy and Two-Band Superconductivity Revealed by Lower Critical Field in Organic Superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br
Huijing Mu, Jin Si, Qingui Yang, Ying Xiang, Haipeng Yang, and Hai-Hu Wen
Chin. Phys. Lett.    2023, 40 (6): 067401 .   DOI: 10.1088/0256-307X/40/6/067401
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Resistivity and magnetization have been measured at different temperatures and magnetic fields in organic superconductors $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. The lower critical field and upper critical field are determined, which allow to depict a complete phase diagram. Through the comparison between the upper critical fields with magnetic field perpendicular and parallel to the conducting $ac$-planes, and the scaling of the in-plane resistivity with field along different directions, we find that the anisotropy ${\varGamma}$ is strongly dependent on temperature. It is realized that ${\varGamma}$ is quite large (above 20) near $T_{\rm c}$, which satisfies the 2D model, but approaches a small value in the low-temperature region. The 2D-Tinkham model can also be used to fit the data at high temperatures. This is explained as a crossover from the orbital depairing mechanism in high-temperature and low-field region to the paramagnetic depairing mechanism in the high-field and low-temperature region. The temperature dependence of lower critical field, $H_{\rm c1} (T)$, shows a concave shape in wide temperature region. It is found that neither a single d-wave nor a single s-wave gap can fit the $H_{\rm c1} (T)$, however a two-gap model containing an s-wave and a d-wave can fit the data rather well, suggesting two-band superconductivity and an unconventional pairing mechanism in this organic superconductor.
VASP2KP: $k\!\cdot\! p$ Models and Landé $g$-Factors from ab initio Calculations
Sheng Zhang, Haohao Sheng, Zhi-Da Song, Chenhao Liang, Yi Jiang, Song Sun, Quansheng Wu, Hongming Weng, Zhong Fang, Xi Dai, and Zhijun Wang
Chin. Phys. Lett.    2023, 40 (12): 127101 .   DOI: 10.1088/0256-307X/40/12/127101
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The $k\!\cdot\! p$ method is significant in condensed matter physics for the compact and analytical Hamiltonian. In the presence of magnetic field, it is described by the effective Zeeman's coupling Hamiltonian with Landé $g$-factors. Here, we develop an open-source package VASP2KP (including two parts: vasp2mat and mat2kp) to compute $k\!\cdot\! p$ parameters and Landé $g$-factors directly from the wavefunctions provided by the density functional theory (DFT) as implemented in Vienna ab initio Simulation Package (VASP). First, we develop a VASP patch vasp2mat to compute matrix representations of the generalized momentum operator $\hat{\boldsymbol \pi}=\hat{\boldsymbol p}+\frac{1}{2mc^2}[\hat{{\boldsymbol s}}\times\nabla V({\boldsymbol r})]$, spin operator $\hat{\boldsymbol s}$, time reversal operator $\hat{T}$, and crystalline symmetry operators $\hat{R}$ on the DFT wavefunctions. Second, we develop a python code mat2kp to obtain the unitary transformation $U$ that rotates the degenerate DFT basis towards the standard basis, and then automatically compute the $k\!\cdot\! p$ parameters and $g$-factors. The theory and the methodology behind VASP2KP are described in detail. The matrix elements of the operators are derived comprehensively and computed correctly within the projector augmented wave method. We apply this package to some materials, e.g., Bi$_2$Se$_3$, Na$_3$Bi, Te, InAs and 1H-TMD monolayers. The obtained effective model's dispersions are in good agreement with the DFT data around the specific wave vector, and the $g$-factors are consistent with experimental data. The VASP2KP package is available at https://github.com/zjwang11/VASP2KP.
Ferroelectricity in 2D Elemental Materials
Xuanlin Zhang, Yunhao Lu, and Lan Chen
Chin. Phys. Lett.    2023, 40 (6): 067701 .   DOI: 10.1088/0256-307X/40/6/067701
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Coexistence of Zero-Dimensional Electride State and Superconductivity in AlH$_{2}$ Monolayer
Qiuping Yang, Xue Jiang, and Jijun Zhao
Chin. Phys. Lett.    2023, 40 (10): 107401 .   DOI: 10.1088/0256-307X/40/10/107401
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Electrides, which confine “excess anionic electrons” in subnanometer-sized cavities of a lattice, are exotic ionic crystals. We propose a non-stoichiometric strategy to realize intrinsic two-dimensional (2D) superconducting electride. AlH$_{2}$ monolayer, which is structurally identical to 1H-MoS$_{2}$, possesses zero-dimensionally confined anionic electrons in the interstitial sites of Al triangles, corresponding to a chemical formula of [AlH$_{2}$]$^{+}e^{-}$. The interaction between interstitial anionic electrons (IAEs) and host cation lattice mainly accounts for stabilization of 1H-AlH$_{2}$ electride. Impressively, 1H-AlH$_{2}$ monolayer is an intrinsic Bardeen–Cooper–Schrieffer superconductor with $T_{\rm c}=38$ K, which is the direct consequence of strong coupling of the H-dominated high electronic states with Al acoustic branch vibrations and mid-frequency H-derived phonon softening modes caused by Kohn anomalies. Under tensile strain, IAEs transform into itinerant electrons, favoring the formation of stable Cooper pairs. Therefore, $T_{\rm c}$ reaches up to 53 K at a biaxial fracture strain of 5%. Our findings provide valuable insights into the correlation between non-stoichiometric electrides and superconducting microscopic mechanisms at the 2D limit.
Emergence of High-Temperature Superconducting Phase in Pressurized La$_{3}$Ni$_{2}$O$_7$ Crystals
Jun Hou, Peng-Tao Yang, Zi-Yi Liu, Jing-Yuan Li, Peng-Fei Shan, Liang Ma, Gang Wang, Ning-Ning Wang, Hai-Zhong Guo, Jian-Ping Sun, Yoshiya Uwatoko, Meng Wang, Guang-Ming Zhang, Bo-Sen Wang, and Jin-Guang Cheng
Chin. Phys. Lett.    2023, 40 (11): 117302 .   DOI: 10.1088/0256-307X/40/11/117302
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The recent report of pressure-induced structural transition and signature of superconductivity with $T_{\rm c}\approx80$ K above 14 GPa in La$_{3}$Ni$_{2}$O$_{7}$ crystals has garnered considerable attention. To further elaborate this discovery, we carried out comprehensive resistance measurements on La$_{3}$Ni$_{2}$O$_{7}$ crystals grown in an optical-image floating zone furnace under oxygen pressure (15 bar) using a diamond anvil cell (DAC) and cubic anvil cell (CAC), which employ a solid (KBr) and liquid (glycerol) pressure-transmitting medium, respectively. Sample 1 measured in the DAC exhibits a semiconducting-like behavior with large resistance at low pressures and gradually becomes metallic upon compression. At pressures $P \geqslant13.7$ GPa we observed the appearance of a resistance drop of as much as $\sim$ 50% around 70 K, which evolves into a kink-like anomaly at pressures above 40 GPa and shifts to lower temperatures gradually with increasing magnetic field. These observations are consistent with the recent report mentioned above. On the other hand, sample 2 measured in the CAC retains metallic behavior in the investigated pressure range up to 15 GPa. The hump-like anomaly in resistance around $\sim$ 130 K at ambient pressure disappears at $P\geqslant2$ GPa. In the pressure range of 11–15 GPa we observed the gradual development of a shoulder-like anomaly in resistance at low temperatures, which evolves into a pronounced drop of resistance of 98% below 62 K at 15 GPa, reaching a temperature-independent resistance of 20 $µ \Omega$ below 20 K. Similarly, this resistance anomaly can be progressively shifted to lower temperatures by applying external magnetic fields, resembling a typical superconducting transition. Measurements on sample 3 in the CAC reproduce the resistance drop at pressures above 10 GPa and realize zero resistance below 10 K at 15 GPa even though an unusual semiconducting-like behavior is retained in the normal state. Based on these results, we constructed a dome-shaped superconducting phase diagram and discuss some issues regarding the sample-dependent behaviors on pressure-induced high-temperature superconductivity in the La$_{3}$Ni$_{2}$O$_{7}$ crystals.
Possible Room-Temperature Ferromagnetic Semiconductors
Jing-Yang You, Xue-Juan Dong, Bo Gu, and Gang Su
Chin. Phys. Lett.    2023, 40 (6): 067502 .   DOI: 10.1088/0256-307X/40/6/067502
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Magnetic semiconductors integrate the dual characteristics of magnets and semiconductors. It is difficult to manufacture magnetic semiconductors that function at room temperature. Here, we review a series of our recent theoretical predictions on room-temperature ferromagnetic semiconductors. Since the creation of two-dimensional (2D) magnetic semiconductors in 2017, there have been numerous developments in both experimental and theoretical investigations. By density functional theory calculations and model analysis, we recently predicted several 2D room-temperature magnetic semiconductors, including CrGeSe$_3$ with strain, CrGeTe$_3$/PtSe$_2$ heterostructure, and technetium-based semiconductors (TcSiTe$_3$, TcGeSe$_3$, and TcGeTe$_3$), as well as PdBr$_3$ and PtBr$_3$ with a potential room-temperature quantum anomalous Hall effect. Our findings demonstrated that the Curie temperature of these 2D ferromagnetic semiconductors can be dramatically enhanced by some external fields, such as strain, construction of heterostructure, and electric field. In addition, we proposed appropriate doping conditions for diluted magnetic semiconductors, and predicted the Cr doped GaSb and InSb as possible room-temperature magnetic semiconductors.
Signature of Superconductivity in Pressurized La$_{4}$Ni$_{3}$O$_{10}$
Qing Li, Ying-Jie Zhang, Zhe-Ning Xiang, Yuhang Zhang, Xiyu Zhu, and Hai-Hu Wen
Chin. Phys. Lett.    2024, 41 (1): 017401 .   DOI: 10.1088/0256-307X/41/1/017401
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The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$ under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La$_{4}$Ni$_{3}$O$_{10}$ under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La$_{4}$Ni$_{3}$O$_{10}$ samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature ($T_{\rm c}$) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La$_{4}$Ni$_{3}$O$_{10}$. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-$T_{\rm c}$ superconductivity in nickelates.
Highly Tunable Perpendicular Magnetic Anisotropy and Anisotropic Magnetoresistance in Ru-Doped La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ Epitaxial Films
Enda Hua, Kunjie Dai, Qing Wang, Huan Ye, Kuan Liu, Jinfeng Zhang, Jingdi Lu, Kai Liu, Feng Jin, Lingfei Wang, and Wenbin Wu
Chin. Phys. Lett.    2023, 40 (7): 077501 .   DOI: 10.1088/0256-307X/40/7/077501
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As a prototypical half-metallic ferromagnet, La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) has been extensively studied due to its versatile physical properties and great potential in spintronic applications. However, the weak perpendicular magnetic anisotropy (PMA) limits the controllability and detection of magnetism in LSMO, thus hindering the realization of oxide-based spintronic devices with low energy consumption and high integration level. Motivated by this challenge, we develop an experimental approach to enhance the PMA of LSMO epitaxial films. By cooperatively introducing 4$d$ Ru doping and a moderate compressive strain, the maximum uniaxial magnetic anisotropy in Ru-doped LSMO can reach $3.0 \times 10^{5}$ J/m$^{3}$ at 10 K. Furthermore, we find a significant anisotropic magnetoresistance effect in these Ru-doped LSMO films, which is dominated by the strong PMA. Our findings offer an effective pathway to harness and detect the orientations of magnetic moments in LSMO films, thus promoting the feasibility of oxide-based spintronic devices, such as spin valves and magnetic tunnel junctions.
Gate-Dependent Nonlinear Hall Effect at Room Temperature in Topological Semimetal GeTe
N. N. Orlova, A. V. Timonina, N. N. Kolesnikov, and E. V. Deviatov
Chin. Phys. Lett.    2023, 40 (7): 077302 .   DOI: 10.1088/0256-307X/40/7/077302
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We experimentally investigate nonlinear Hall effect as zero-frequency and second-harmonic transverse voltage responses to ac electric current for topological semimetal GeTe. A thick single-crystal GeTe flake is placed on the Si/SiO$_2$ substrate, where the p-doped Si layer serves as a gate electrode. We confirm that electron concentration is not gate-sensitive in thick GeTe flakes due to the gate field screening by bulk carriers. In contrast, by transverse voltage measurements, we demonstrate that the nonlinear Hall effect shows pronounced dependence on the gate electric field at room temperature. Since the nonlinear Hall effect is a direct consequence of a Berry curvature dipole in topological media, our observations indicate that Berry curvature can be controlled by the gate electric field. This experimental observation can be understood as a result of the known dependence of giant Rashba splitting on the external electric field in GeTe. For possible applications, the zero-frequency gate-controlled nonlinear Hall effect can be used for the efficient broad-band rectification.
Signatures of Temperature-Driven Lifshitz Transition in Semimetal Hafnium Ditelluride
Qixuan Li, Bin Wang, Nannan Tang, Chushan Li, Enkui Yi, Bing Shen, Donghui Guo, Dingyong Zhong, and Huichao Wang
Chin. Phys. Lett.    2023, 40 (6): 067101 .   DOI: 10.1088/0256-307X/40/6/067101
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Temperature-driven change of Fermi surface has been attracting attention recently as it is fundamental and essential to understand a metallic system. We report the magnetotransport anomalies in the semimetal HfTe$_{2}$ single crystals. The magnetoresistance behavior at high temperatures obeys Kohler's rule which can lead to the field-induced resistivity upturn behavior as observed. When the temperature is decreased to around 30 K, Kohler's rule becomes inapplicable, indicating the change of the Fermi surface in HfTe$_{2}$. The Hall analyses and extended Kohler's plot reveal abrupt change of carrier densities and mobilities near 30 K. These results suggest that the chemical potential may shift as the temperature increases and the shift causes an electron pocket to vanish. Our work of the temperature-driven Lifshitz transition in HfTe$_{2}$ is relevant to understanding of the transport anomalies and exotic physical properties in transition-metal dichalcogenides.
Negative-to-Positive Tunnel Magnetoresistance in van der Waals Fe$_{3}$GeTe$_{2}$/Cr$_{2}$Ge$_{2}$Te$_{6}$/Fe$_{3}$GeTe$_{2}$ Junctions
Zi-Ao Wang, Xiaomin Zhang, Wenkai Zhu, Faguang Yan, Pengfei Liu, Zhe Yuan, and Kaiyou Wang
Chin. Phys. Lett.    2023, 40 (7): 077201 .   DOI: 10.1088/0256-307X/40/7/077201
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The emergent van der Waals magnetic material is a promising component for spintronic devices with novel functionalities. Here, we report a transition of negative-to-positive magnetoresistance in Fe$_{3}$GeTe$_{2}$/Cr$_{2}$Ge$_{2}$Te$_{6}$/ Fe$_{3}$GeTe$_{2}$ van der Waals all-magnetic tunnel junctions with increasing the applied bias voltage. A negative magnetoresistance is observed first in Fe$_{3}$GeTe$_{2}$/Cr$_{2}$Ge$_{2}$Te$_{6}$/Fe$_{3}$GeTe$_{2}$ tunnel junctions, where the resistance with antiparallel aligned magnetization of two Fe$_{3}$GeTe$_{2}$ electrodes is lower than that with parallel alignment, which is due to the opposite spin polarizations of two Fe$_{3}$GeTe$_{2}$ electrodes. With the bias voltage increasing, the spin polarization of the biased Fe$_{3}$GeTe$_{2}$ electrode is changed so that the spin orientations of two Fe$_{3}$GeTe$_{2}$ electrodes are the same. Our experimental observations are supported by the calculated spin-dependent density of states for Fe$_{3}$GeTe$_{2}$ electrodes under a finite bias. The significantly bias voltage-dependent spin transport properties in van der Waals magnetic tunnel junctions open a promising route for designing electrical controllable spintronic devices based on van der Waals magnets.
Near-Field Radiative Heat Transfer between Disordered Multilayer Systems
Peng Tian, Wenxuan Ge, Songsong Li, Lei Gao, Jianhua Jiang, and Yadong Xu
Chin. Phys. Lett.    2023, 40 (6): 067802 .   DOI: 10.1088/0256-307X/40/6/067802
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Near-field radiative heat transfer (NFRHT) research is an important research project after a major breakthrough in nanotechnology. Based on the multilayer structure, we find that due to the existence of inherent losses, the decoupling of hyperbolic modes (HMs) after changing the filling ratio leads to suppression of heat flow near the surface mode resonance frequency. It complements the physical landscape of enhancement of near-field radiative heat transfer by HMs and more surface states supported by multiple surfaces. More importantly, considering the difficulty of accurate preparation at the nanoscale, we introduce the disorder factor to describe the magnitude of the random variation of the layer thickness of the multilayer structure and then explore the effect on heat transfer when the layer thickness is slightly different from the exact value expected. We find that the near-field radiative heat flux decreases gradually as the disorder increases because of interlayer energy localization. However, the reduction in heat transfer does not exceed an order of magnitude, although the disorder is already very large. At the same time, the regulation effect of the disorder on NFRHT is close to that of the same degree of filling ratio, which highlights the importance of disordered systems. This work qualitatively describes the effect of disorder on heat transfer and provides instructive data for the fabrication of NFRHT devices.
Multiple Magnetic Phase Transitions and Critical Behavior in Single-Crystal SmMn$_{2}$Ge$_{2}$
Xiao-Yan Wang, Jun-Fa Lin, Xiang-Yu Zeng, Huan Wang, Xiao-Ping Ma, Yi-Ting Wang, Kun Han, and Tian-Long Xia
Chin. Phys. Lett.    2023, 40 (6): 067503 .   DOI: 10.1088/0256-307X/40/6/067503
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Magnetic materials with noncollinear spin configurations have engendered significant interest in condensed matter physics due to their intriguing physical properties. We direct our attention towards the magnetic properties and critical behavior of single-crystal SmMn$_{2}$Ge$_{2}$, an itinerant magnet with numerous temperature-dependent magnetic phase transitions. Notably, SmMn$_{2}$Ge$_{2}$ displays significant magnetic anisotropy with easy magnetization direction switching from the $c$ axis to the $ab$ plane as temperature decreases. The critical behavior of the ferromagnetic transition occurring above room temperature is thoroughly examined. Reliable and self-consistent critical exponents, including $\beta = 0.292(2)$, $\gamma=0.924(8)$, and $\delta = 4.164(6)$, along with the Curie temperature $T_{\rm c}=347$ K, are extracted through various methods, which provide evidence for the coexistence of multiple magnetic interactions in SmMn$_{2}$Ge$_{2}$. Further analysis reveals that the magnetic interaction of SmMn$_{2}$Ge$_{2}$ is a long-range type with the interaction distance decaying as $J(r)\sim r^{-4.35}$.
Ultra-Broadband Thermal Emitter for Daytime Radiative Cooling with Metal-Insulator-Metal Metamaterials
Huaiyuan Yin and Chunzhen Fan
Chin. Phys. Lett.    2023, 40 (7): 077801 .   DOI: 10.1088/0256-307X/40/7/077801
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A novel thermal emitter with metal-insulator-metal design is proposed to realize efficient daytime radiative cooling. It can achieve ultrahigh absorption of 99.67% in the first atmospheric window and strong reflection of 94.86% in solar band. Analysis on the cooling performance with different real and imaginary parts of refractive index is carried out to provide a guide line in the material choice. As a case study, three inorganic materials are substituted to get enhanced absorption and it is verified that the refractive index matching is desirable to obtain high absorption. In addition, such high emissivity persists under different incident angles in both TE and TM modes. A net cooling power of 96.39 W/m$^{2}$ is achieved in the daytime with the incorporation of convection coefficients. Finally, this thermal emitter achieves an average temperature drop of 5.1 ℃ based on the solution of conduction equation at 300 K. Therefore, our design with an excellent cooling ability can further bolster development in managements of radiative cooling or thermal radiation.
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