通过多晶与单晶中子衍射及精修分析，研究确定 VBr3 在 26.5 K 以下形成 k=(0,0.5,1) 的锯齿形反铁磁有序，并在 94 K 以下发生面内三重对称破缺。
摘要：
我们报告了对范德华反铁磁体 VBr3 的晶格和磁结构的全面研究，该材料在室温下具有 BiI3 型结构特征。我们对多晶和单晶 VBr3 样品均进行了中子衍射实验，揭示出在 Néel 温度 TN = 26.5 K 以下出现清晰的磁 Bragg 峰。这些磁 Bragg 峰可用六角记号中的 k = (0, 0.5, 1) 进行指标化。我们的精修分析表明，VBr3 中的反铁磁有序表现为锯齿形结构。此外，我们观察到在结构转变温度 Ts = 94 K 以下，HK 平面内的核 Bragg 峰出现峰分裂，表明 ab 平面内的三重对称性发生破缺。
英文摘要：
We report a comprehensive investigation of the lattice and magnetic structure in van der Waals antiferromagnet VBr3, characterized by a BiI3-type structure at room temperature. Neutron diffraction experiments were performed on both polycrystalline and single-crystalline VBr3 samples, revealing clear magnetic Bragg peaks emerging below the Néel temperature of TN = 26.5 K. These magnetic Bragg peaks can be indexed by k = (0, 0.5, 1) in hexagonal notation. Our refinement analysis suggests that the antiferromagnetic order in VBr3 manifests as a zigzag structure. Moreover, we observed peak splitting for nuclear Bragg peaks in the HK-plane below the structure transition temperature of Ts = 94 K, indicating the breaking of 3-fold symmetry within the ab-plane.
Single crystalline samples of the van der Waals antiferromagnet CrPS4 were studied by measurements of specific heat and comprehensive anisotropic temperature- and magnetic field-dependent magnetization. In addition, measurements of the heat capacity and magnetization were performed under pressures of up to ~21 kbar and ~14 kbar respectively. At ambient pressure, two magnetic transitions are observed, second order from a paramagnetic to an antiferromagnetic state at TN ~ 37 K, and a first-order spin reorientation transition at T* ~ 34 K. Anisotropic H - T phase diagrams were constructed using t…
Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal tha…
Kitaev interactions in the van der Waals antiferromagnet VBr3
Fudan University（复旦大学）；J-PARC Center（日本原子力研究开发机构J-PARC中心）；Shanghai Research Center for Quantum Sciences（上海量子科学研究中心）；Institute of Nanoelectronics and Quantum Computing, Fudan University（复旦大学纳米电子与量子计算研究所）
研究通过非弹性中子散射揭示了van der Waals反铁磁体VBr3中的Kitaev相互作用，并表明配体工程可调节Kitaev相互作用。
Antiferromagnetic (AFM) van der Waals (vdW) materials provide a novel platform for synthetic AFM spintronics, in which the spin-related functionalities are derived from manipulating spin configurations between the layers. Metallic vdW antiferromagnets are expected to have several advantages over the widely-studied insulating counterparts in switching and detecting the spin states through electrical currents but have been much less explored due to the lack of suitable materials. Here, utilizing the extreme sensitivity of the vdW interlayer magnetism to material composition, we report the itiner…
Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall (QAH) effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spect…
The recently discovered two-dimensional (2D) van der Waals magnet NiPS$_3$ provides a new route to examine many-body quasiparticles under 2D confinement. Excitons are of particular interest due to their strong coupling to the magnetic ground state in this material. Here, by using a first-principles based approach, we find bright excitons between 1.4 eV to 1.7 eV, similar to the sharp coherent and band edge excitons experimentally observed. Our analysis shows that each exciton in NiPS$_3$ is composed of a combination of $d$-$d$ and charge-transfer character, where the relative ratio of each pai…
We study the thickness dependence of the electronic, magnetic, and optical properties of a NiPS$_3$ thin film, an antiferromagnetic charge-transfer insulator. Utilizing state-of-the-art advanced density functionals, we find the antiferromagnetic Zig-Zag order, the band gap, and the main peaks in the dielectric tensor are all in good agreement with the corresponding experimental values. Upon thinning, the Zig-Zag antiferromagnetic order becomes virtually degenerate with a competing Néel order, consistent with the suppression of long-range order observed by Raman spectroscopy due to strong magne…
The quasi-two-dimensional antiferromagnet Ni$_2$P$_2$S$_6$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of Ni$_2$P$_2$S$_6$. The results enable one to reliably determine the positive, "easy"-plane type of the single ion anisotropy of the Ni$^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7$ meV. The resonance response reveals strongly anisotropic …
Transition metal halides are quasi-two-dimensional van der Waals magnets that can potentially host antiferromagnetic (AFM) skyrmions induced by competing exchange interactions. We study theoretically magnetic states of Fe-doped NiBr$_2$ using spin interactions obtained by fitting experimental data. We find that AFM interlayer interactions suppress skyrmion lattices but allow for skyrmions with topological charges $1,2$ and $3$ confined to a single magnetic layer. These single-layer skyrmions exist in three different magnetic phases and their dynamics changes drastically at the transition from …
The magnetic excitations in CoPS$_3$, a two-dimensional van der Waals (vdW) antiferromagnet with spin $S=3/2$ on a honeycomb lattice, has been measured using powder inelastic neutron scattering. Clear dispersive spin waves are observed with a large spin gap of ~13 meV. The magnon spectra were fitted using an $XXZ$-type $J_1-J_2-J_3$ Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange $J_1=-2.08$ meV and $J_2=-0.26$ meV for the nearest and second-nearest neighbors and a sizeable antif…
The interplay between a multitude of electronic, spin, and lattice degrees of freedom underlies the complex phase diagrams of quantum materials. Layer stacking in van der Waals (vdW) heterostructures is responsible for exotic electronic and magnetic properties, which inspires stacking control of two-dimensional magnetism. Beyond the interplay between stacking order and interlayer magnetism, we discover a spin-shear coupling mechanism in which a subtle shear of the atomic layers can have a profound effect on the intralayer magnetic order in a family of vdW antiferromagnets. Using time-resolved …
CeTe3 is a unique platform to investigate the itinerant magnetism in a van der Waals (vdW) coupled metal. Despite chemical pressure being a promising route to boost quantum fluctuation in this system, a systematic study on the chemical pressure effect on Ce3+(4f1) states is absent. Here, we report on the successful growth of a series of Se doped single crystals of CeTe3. We found a fluctuation driven exotic magnetic rotation from the usual easy-axis ordering to an unusual hard-axis ordering. Unlike in localized magnetic systems, near-critical magnetism can increase itinerancy hand-in-hand with…
Revealing the Electronic Structure of van der Waals Antiferromagnetic NiPS$_3$ through Synchrotron-Based $μ$-ARPES and Alkali Metal Dosing
Boston University（波士顿大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Rice University（莱斯大学）；Universidade Federal de Ouro Preto（欧罗普雷托联邦大学）；Universidade Federal de Minas Gerais（米纳斯吉拉斯联邦大学）；University of Bristol（布里斯托大学）
Atomically Resolved Acoustic Dynamics Coupled with Magnetic Order in a van der Waals Antiferromagnet
Materials Science Division, Argonne National Laboratory（阿贡国家实验室材料科学部）；Department of Physics, University of Washington（华盛顿大学物理系）；Center for Ultrafast Phase Transformation, Department of Physics, Sogang University（西江大学物理系超快相变中心）；Department of Materials Science and Engineering, University of Washington（华盛顿大学材料科学与工程系）；Pohang Accelerator Laboratory, POSTECH（韩国科学技术院浦项加速器实验室）；Institute for Condensed Matter and Complex Systems, School of Physics and Astronomy, The University of Edinburgh（爱丁堡大学物理与天文学学院凝聚态与复杂系统研究所）；Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；Department of Physics, Cambridge, Massachusetts Institute of Technology（麻省理工学院物理系）；Departments of Physics and Applied Physics, Stanford University（斯坦福大学物理与应用物理系）
Thermal transport is a pivotal probe for studying low-energy, charge-neutral quasi-particles in insulating magnets. In this Letter, we report an observation of large magneto-thermal conductivity and thermal Hall effect (THE) in a van der Waals antiferromagnet FeCl2. The magneto-thermal conductivity reaches over ~700%, indicating strong magnon-phonon coupling. Furthermore, we find an appreciable thermal Hall signal which changes sign concurrently with the spin-flip transition from the antiferromagnetic state to the polarized ferromagnetic state. Our theoretical calculations suggest that, in add…
Stripe antiferromagnetism in van der Waals metal HoTe3 decoupled from charge density wave order
The University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；The Institute for Solid State Physics, The University of Tokyo（东京大学固体物理研究所）；J-PARC Center, Japan Atomic Energy Agency（日本原子能机构J-帕克中心）
The crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe3 have been investigated on single crystal and polycrystalline materials. The crystal structure contains layers made up of lozenge shaped Cr_4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3eV. The magnetic susceptibility exhibits a broad maximum near 300K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55K, and…
Although the magnetic order is suppressed by a strong magnetic frustration, it is maintained but appears in complex order forms such as a cycloid or spin density wave in weakly frustrated systems. Herein, we report a weakly magnetic-frustrated two-dimensional van der Waals material CrPSe$_3$. Polycrystalline CrPSe$_3$ was synthesized at an optimized temperature of 700$^\circ$C to avoid the formation of any secondary phases (e.g., Cr$_2$Se$_3$). The antiferromagnetic transition appeared at $T_N\sim 126$ K with a large Curie-Weiss temperature $T_{\rm CW} \sim -371$ via magnetic susceptibility me…
Layered van-der-Waals 2D magnetic materials are of great interest in fundamental condensed-matter physics research, as well as for potential applications in spintronics and device physics. We present neutron powder diffraction data using new ultra-high-pressure techniques to measure the magnetic structure of Mott-insulating 2D honeycomb antiferromagnet FePS$_3$ at pressures up to 183 kbar and temperatures down to 80 K. These data are complemented by high-pressure magnetometry and reverse Monte Carlo modeling of the spin configurations. As pressure is applied, the previously-measured ambient-pr…
Matter-light interaction is at the center of diverse research fields from quantum optics to condensed matter physics, opening new fields like laser physics. A magnetic exciton is one such rare example found in magnetic insulators. However, it is relatively rare to observe that external variables control matter-light interaction. Here, we report that the broken inversion symmetry of multiferroicity can act as an external knob enabling the magnetic exciton in van der Waals antiferromagnet NiI2. We further discover that this magnetic exciton arises from a transition between Zhang-Rice-triplet and…
Collective excitations of bound electron-hole pairs -- known as excitons -- are ubiquitous in condensed matter, emerging in systems as diverse as band semiconductors, molecular crystals, and proteins. Recently, their existence in strongly correlated electron materials has attracted increasing interest due to the excitons' unique coupling to spin and orbital degrees of freedom. The non-equilibrium driving of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phases beyond thermodynamic equilibrium. Here, we achieve this in the van der Wa…
Two-dimensional van der Waals compounds with magnetic ions on a honeycomb lattice are hosts to a variety of exotic behavior. The magnetic interactions in one such compound, MnPSe$_3$, are investigated with elastic and inelastic neutron scattering. Magnetic excitations are observed in the magnetically ordered regime and persist to temperatures well above the ordering temperature, $\rm T_N$ = 74 K, consistent with low dimensional magnetic interactions. The inelastic neutron scattering results allow a model spin Hamiltonian to be presented that includes dominant intralayer interactions of $J_{1ab…
The electronic structures, lattice dynamics, and magnetic properties of crystal β-FeSe under hydrostatic pressure have been studied by using the first-principles electronic structure calculations with van der Waals (vdW) corrections. With applied pressures, the energy bands around the Fermi energy level consisting mainly of Fe-3d orbitals show obvious energy shifts and occupation variations, and meanwhile the frequencies of all optical phonon modes increase. Among these phonon modes, the A1g mode, which relates to the Se height from the Fe-Fe plane, shows a clear frequency jump in the range be…
Field-rigid Ising antiferromagnetism with giant spin-flip fields in Van der Waals UOTe
Washington University in St. Louis（圣路易斯华盛顿大学）；National High Magnetic Field Laboratory, Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室国家高磁场实验室）；Neutron Scattering Division, Oak Ridge National Laboratory（橡树岭国家实验室中子散射部）；Harvard University（哈佛大学）；National High Magnetic Field Laboratory, Florida State University（佛罗里达州立大学国家高磁场实验室）
Microscopic NMR evidence for successive antiferroelectric and antiferromagnetic order in the van der Waals magnet CuCrP$_2$S$_6$
Leibniz Institute for Solid State and Materials Research（德累斯顿固体与材料研究莱布尼茨研究所）；TU Dresden（德累斯顿工业大学）；Institute for Solid State and Materials Physics, TU Dresden（德累斯顿工业大学固体与材料物理研究所）；International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所MagTop国际研究中心）；Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden（维尔茨堡-德累斯顿卓越集群ct.qmat，德累斯顿工业大学）
The MnBi$_{2}$Te$_{4}$ family has attracted significant attention due to its rich topological states such as the quantum anomalous Hall (QAH) insulator state, the axion insulator state, and the magnetic Weyl semimetal state. Nevertheless, the intrinsic antiferromagnetic (AFM) interlayer coupling in MnBi$_{2}$Te$_{4}$ partly hinders the realization of "high-temperature" QAH effect. Here, by using first-principles electronic structure calculations, we design a new class of materials Mn\emph{X}$_{2}$\emph{B}$_{2}$\emph{T}$_{6}$ (\emph{X}=Ge, Sn, or Pb; \emph{B}=Sb or Bi; \emph{T}=Se or Te) based …
We grew high-quality single crystals of the 5f electron van der Waals compound UI$_3$ and investigated them by measurements of specific heat and magnetization as functions of temperature and magnetic field. UI$_3$ behaves as an antiferromagnet with a first-order magnetic phase transition at the Neél temperature $T_N$ = 2.65 K. It is characterized by a sharp symmetric specific-heat peak which is gradually shifted to lower temperatures by increasing magnetic field applied along either the a- or b-axis. The behavior in magnetic fields reveals orthorhombic magnetocrystalline anisotropy with the ha…
The current family of experimentally realized two-dimensional magnetic materials consist of 3$d$ transition metals with very weak spin-orbit coupling. In contrast, we report a new platform in a chemically bonded and layered 4$d$ oxide, with strong electron correlations and competing spin-orbit coupling. We synthesize ultra-thin sheets of SrRu$_2$O$_6$ using scalable liquid exfoliation. These exfoliated sheets are characterized by complementary experimental and theoretical techniques. The thickness of the nano-sheets varies between three to five monolayers, and within the first-principles calcu…
Spontaneous magnetic order is a routine instance in three-dimensional (3D) materials but for a long time, it remained elusive in the 2D world. Recently, the first examples of (stand-alone) 2D van der Waals (vdW) crystals with magnetic order, either antiferromagnetic or ferromagnetic, have been reported. In this review, we describe the state of the art of the nascent field of magnetic 2D materials focusing on synthesis, engineering, and theory aspects. We also discuss challenges and some of the many different promising directions for future work.
Magnetic Field-Tunable Repulsive Exciton-Exciton Interaction in the van der Waals Antiferromagnet NiPS$_3$
The Institute for Solid State Physics, The University of Tokyo（东京大学固体研究所）；Department of Physics and Astronomy, Seoul National University (SNU)（首尔大学物理天文学系）
