Tunable giant anomalous Hall effect in the Kondo-lattice ferromagnet UBiTe
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Xu, Qiaozhi (ORCID:0009000866640904)↗
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Magnetic critical behavior and anomalous Hall effect in 2H − Co 0.22 TaS 2 single crystals
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Liu, Yu (ORCID:0000000188862876)↗
Proximity-magnetized quantum spin Hall insulator: monolayer 1 T’ WTe2/Cr2Ge2Te6
Abstract Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe 2 , a two-dimensional quantum spin Hall insulator, and few-layer Cr 2 Ge 2 Te 6 , an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe 2 manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.
36 MATERIALS SCIENCE↗
Electrical readout of topological spin order in ultrathin insulators with small magnetization
Probing magnetic order in insulators with small magnetization is particularly challenging in the ultrathin limit, where magnetometry and diffraction lose sensitivity. Here, in this study, we show that topological spin order with vanishingly small magnetization can be electrically detected through an interfacial topological Hall effect in heavy-metal/magnetic-insulator heterostructures. Using Pt coupled to the canted antiferromagnetic insulator hexagonal LuFeO 3 (h-LuFeO 3 ) we observe an unusually large and robust Hall response arising from the transfer of real-space spin topology across the interface via magnetic proximity effect (MPE). This interfacial signal enables detection of magnetic order in h-LuFeO 3 with an extremely small net magnetization (0.025 𝜇 B /Fe), corresponding to Hall-conductivity-magnetization ratio ≈ 2 V −1 , which is one to two orders of magnitude large than other MPE-induced Hall effects and anomalous Hall effects of conducting magnets. This high sensitivity allows detection of magnetic order in h-LuFeO 3 down to a thickness of only 1.5 unit cells. Our results establish an interfacial Hall-based approach for electrically probing topological spin structures in ultrathin insulators and the related interfaces, enabling access to magnetic order in regimes where conventional probes fail.
Topological Hall Effect↗
Intrinsic origin and enhancement of topological responses in ferrimagnetic antiperovskite Mn 4 N
Using first-principles calculations here we investigate the intrinsic origins of the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) in antiperovskite ferrimagnet Mn 4 N. We predict that the AHE is significantly enhanced under both compressive and tensile strain; however, the ANE generally decreases under epitaxial strain, except for 1% compressive strain. We connect this behavior to the evolution of the Berry curvature with strain, suggesting similar strategies for achieving large AHE and ANE changes with modest amounts of strain. Finally, we find that the nonmonotonic characteristics of the AHE and ANE stem from the formation and movement of new Weyl points at the periphery of the Brillouin zone under compressive and tensile strains.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Optical conductivity of multi-Weyl node semimetals due to electromagnetic and axial pseudogauge fields
Here, we compute the optical conductivity of a multi-Weyl semimetal in the presence of electromagnetic and axial fields. In the first part of the paper we use a U(1) gauge field to compute the current of a multi-Weyl semimetal. The current is used in the computation of the optical conductivity within the Kubo-Matsubara formalism. We consider a multi-Weyl semimetal with two nodal points in the z-direction. The multi-Weyl semimetal has a width D in the transverse direction which is smaller than the length L in the longitudinal direction. Due to the width D we replace the model by a sum of transverse modes in 1+1 dimensions. We then regularize the model in 1+1 dimensions. The nodal points are responsible for a constant axial field and a static anomalous Hall effect. In the second part we study the effect of dynamic axial fields. An elastic deformation couples to the two electronic chiralities, and represents the axial pseudogauge field $\vec{a}$ 5 . As a consequence, the system is excited by two kinds of field, the electromagnetic one and the axial srain field. For a Weyl system this is represented by the triangle diagram, which is responsible for the anomalous Hall effect. We can obtain the anomalous Hall effect also by a chiral transformation in 1+1 dimensions using regularization. The chiral transformation modifies the path integration measure and is the source of the induced anomalous Hall effect. The latter is controlled by a combination of strain waves and electromagnetic fields.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Growth, characterization, and Chern insulator state in MnBi 2 Te 4 via the chemical vapor transport method
As the first intrinsic antiferromagnetic topological insulator, MnBi 2 Te 4 has provided a platform to investigate the interplay of band topology and magnetism as well as the emergent phenomena arising from such an interplay. Here we report the chemical-vapor-transport (CVT) growth and characterization of MnBi 2 Te 4 , as well as the observation of the field-induced quantized Hall conductance in 6-layer devices. Through comparative studies between our CVT-grown and flux-grown MnBi 2 Te 4 via magnetic, transport, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy measurements, we find that CVT-grown MnBi 2 Te 4 is marked with higher Mn occupancy on the Mn site, slightly higher Mn Bi antisites, smaller carrier concentration, and a Fermi level closer to the Dirac point. Furthermore, a 6-layer device made from the CVT-grown sample shows by far the highest mobility of 2500 cm 2 V s in MnBi 2 Te 4 devices with the quantized Hall conductance appearing at 1.8 K and 8 T. Furthermore, our study provides a route to obtain high-quality single crystals of MnBi 2 Te 4 that are promising to make superior devices and realize emergent phenomena, such as the layer Hall effect and quantized anomalous Hall effect, etc.
36 MATERIALS SCIENCE↗
Concurrence of quantum anomalous Hall and topological Hall effects in magnetic topological insulator sandwich heterostructures
The quantum anomalous Hall (QAH) effect is a consequence of non-zero Berry curvature in momentum space. The QAH insulator harbours dissipation-free chiral edge states in the absence of an external magnetic field. Yet, the topological Hall (TH) effect, a hallmark of chiral spin textures, is a consequence of real-space Berry curvature. Herein, by inserting a topological insulator (TI) layer between two magnetic TI layers, we realized the concurrence of the TH effect and the QAH effect through electric-field gating. The TH effect is probed by bulk carriers, whereas the QAH effect is characterized by chiral edge states. The appearance of the TH effect in the QAH insulating regime is a consequence of chiral magnetic domain walls that result from the gate-induced Dzyaloshinskii–Moriya interaction and occurs during the magnetization reversal process in the magnetic TI sandwich samples. The coexistence of chiral edge states and chiral spin textures provides a platform for proof-of-concept dis- sipationless spin-textured spintronic applications.
36 MATERIALS SCIENCE↗
Large Anomalous and Topological Hall Effect and Nernst Effect in a Dirac Kagome Magnet Fe 3 Ge
Searching for Kagome magnets with novel magnetic and electronic properties has been attracting significant efforts recently. Here, the magnetic, electronic, and thermoelectric properties of Fe 3 Ge single crystals with Fe atoms forming a slightly distorted Kagome lattice are reported. It is shown that Fe 3 Ge exhibits a large anomalous Hall effect and anomalous Nernst effect. The observed anomalous transverse thermoelectric conductivity $|α^A_{xz}|$ reaches ≈4.6 A m −1 K −1 , which is larger than the conventional ferromagnets and most of the topological ferromagnets reported in literature. The first-principles calculations suggest that these exceptional transport properties are dominated by the intrinsic mechanism, which highlights the significant contribution of the Berry curvature of massive Dirac gaps in the momentum space. Additionally, a topological Hall resistivity of 0.9 µΩ cm and a topological Nernst coefficient of 1.2 µV K −1 are also observed, which are presumably ascribed to the Berry phase associated with the field-induced non-zero scalar spin chirality. These features highlight the synergic effects of the Berry phases in both momentum space and real space of Fe 3 Ge, which render it an excellent candidate for room-temperature thermoelectric applications based on transverse transport.
Kagome magnet↗
1/4 is the new 1/2 when topology is intertwined with Mottness
In non-interacting systems, bands from non-trivial topology emerge strictly at half-filling and exhibit either the quantum anomalous Hall or spin Hall effects. Here we show using determinantal quantum Monte Carlo and an exactly solvable strongly interacting model that these topological states now shift to quarter filling. A topological Mott insulator is the underlying cause. The peak in the spin susceptibility is consistent with a possible ferromagnetic state at T = 0. The onset of such magnetism would convert the quantum spin Hall to a quantum anomalous Hall effect. While such a symmetry-broken phase typically is accompanied by a gap, we find that the interaction strength must exceed a critical value for this to occur. Hence, we predict that topology can obtain in a gapless phase but only in the presence of interactions in dispersive bands. These results explain the recent quarter-filled quantum anomalous Hall effects seen in moiré systems.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Gate-Tunable Multiband Transport in ZrTe 5 Thin Devices
Interest in ZrTe 5 has been reinvigorated in recent years owing to its potential for hosting versatile topological electronic states and intriguing experimental discoveries. However, the mechanism of many of its unusual transport behaviors remains controversial: for example, the characteristic peak in the temperature-dependent resistivity and the anomalous Hall effect. Here, through employing a clean dry-transfer fabrication method in an inert environment, we successfully obtain high-quality ZrTe 5 thin devices that exhibit clear dual-gate tunability and ambipolar field effects. Such devices allow us to systematically study the resistance peak as well as the Hall effect at various doping densities and temperatures, revealing the contribution from electron–hole asymmetry and multiple-carrier transport. By comparing with theoretical calculations, we suggest a simplified semiclassical two-band model to explain the experimental observations. Finally, our work helps to resolve the longstanding puzzles on ZrTe 5 and could potentially pave the way for realizing novel topological states in the two-dimensional limit.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Recent progress in MnBi2 n Te3 n +1 intrinsic magnetic topological insulators: crystal growth, magnetism and chemical disorder
ABSTRACT The search for magnetic topological materials has been at the forefront of condensed matter research for their potential to host exotic states such as axion insulators, magnetic Weyl semimetals, Chern insulators, etc. To date, the MnBi2nTe3n+1 family is the only group of materials showcasing van der Waals-layered structures, intrinsic magnetism and non-trivial band topology without trivial bands at the Fermi level. The interplay between magnetism and band topology in this family has led to the proposal of various topological phenomena, including the quantum anomalous Hall effect, quantum spin Hall effect and quantum magnetoelectric effect. Among these, the quantum anomalous Hall effect has been experimentally observed at record-high temperatures, highlighting the unprecedented potential of this family of materials in fundamental science and technological innovation. In this paper, we provide a comprehensive review of the research progress in this intrinsic magnetic topological insulator family, with a focus on single-crystal growth, characterization of chemical disorder, manipulation of magnetism through chemical substitution and external pressure, and important questions that remain to be conclusively answered.
36 MATERIALS SCIENCE↗
Large asymmetric anomalous Nernst effect in the antiferromagnet SrIr 0.8 Sn 0.2 O 3
A large anomalous Nernst effect is essential for thermoelectric energy-harvesting in the transverse geometry without external magnetic field. It's often connected with anomalous Hall effect, especially when electronic Berry curvature is believed to be the driving force. This approach implicitly assumes the same symmetry for the Nernst and Hall coefficients, which is however not necessarily true. Here we report a large anomalous Nernst effect in antiferromagnetic SrIr 0.8 Sn 0.2 O 3 that defies the antisymmetric constraint on the anomalous Hall effect imposed by the Onsager reciprocal relation. The observed spontaneous Nernst thermopower quickly reaches the sub-μV/K level below the Néel transition around 250 K, which is comparable with many topological antiferromagnetic semimetals and far excels other magnetic oxides. Our analysis indicates that the coexistence of significant symmetric and antisymmetric contributions plays a key role, pointing to the importance of extracting both contributions and a new pathway to enhanced anomalous Nernst effect for transverse thermoelectrics.
Hall effect↗
Topological magnons and topological electronic properties in metallic Kagome magnets
The primary goal of this project is to study metallic Kagome magnets that potentially host topological electronic properties and topological magnetic excitations, aiming to understand the effect of lattice geometry on topological character of magnons and electronic bands and to study the interplay between magnetism and topological electronic properties. Through this DOE-funded research program, we have unraveled novel electronic and thermoelectric properties of RMn 6 Sn 6 (R = Y, Tb, and Er) and Fe 3 Ge Kagome magnets, including anomalous Hall effect and anomalous Nernst effect, which are associated with the Berry curvature contribution from the massive Dirac gaps in the 3D momentum space. In addition, we have also revealed topological Hall effect and topological Nernst effect, and we ascribed these phenomena to the Berry curvature associated with the magnetic field-induced non-zero scalar spin chirality. These features highlight the synergic effects of the Berry phases in both momentum space and real space of Kagome magnets, which render them excellent candidates for room-temperature electronic and thermoelectric applications based on transverse transport. In addition, through this program we have also study novel thermal transport properties of quantum materials by probing the thermal Hall effect, which can provide crucial insights into nontrivial topological character and chirality of quasiparticles like magnons and magnon-polarons. This report provides an overview of some project activities related to these accomplishments.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Observation of anomalous thermal Hall effect in a Kagome superconductor
Broken time-reversal symmetry (TRS) in superconductors can induce not only spontaneous magnetization by the finite angular momentum of Cooper pairs but also the anomalous thermal Hall effects (ATHEs), whose detection has been extremely challenging. Here, we report the successful observation of an ATHE developing below the superconducting transition temperature at zero magnetic field in the kagome-lattice superconductor CsV 3 Sb 5 . This finding is verified by the absence of a signal in a conventional type-II superconductor using the same setup and by ruling out the trapped-vortex effects through micro-Hall array measurements. Both the temperature dependence and the magnitude of the observed anomalous thermal Hall conductivity are quite different from those expected for the quantized thermal edge current of an intrinsic ATHE but consistent with extrinsic impurity-induced ATHEs in chiral superconductivity. Our study of ATHE offers an alternative approach to probe TRS breaking in the superconducting states.
Yoshida, Hiroki [Univ. of Tokyo, Chiba (Japan)] (O↗
Discovery and Synthesis of a Family of Boride Altermagnets
Borides are a rich material family. To push the boundaries of borides’ properties and applications into broader fields, we have conducted systematic theoretical and experimental searches for synthesizable phases in ternary borides TM 2 B 2 (T = 3d, M = 4d/5d transition metals). We find that TM 2 B 2 in the FeMo 2 B 2 -type and CoW 2 B 2 -type structures form a large family of stable/metastable materials of 120 members. Among them, we identify 40 materials with stable magnetic solutions. Further, we discover 11 altermagnets in the FeMo 2 B 2 -type structure. So far, boride altermagnets are rare. In these altermagnets, T = Fe or Mn atoms are arranged in parallel T-chains with strong ferromagnetic intrachain couplings and antiferromagnetic interchain couplings. They simultaneously exhibit electronic band spin splitting, typical of ferromagnetism, and zero net magnetization, typical of antiferromagnetism. They also exhibit magnonic band chiral splitting. Both effects originate from the unique altermagnetic symmetries crucially constrained by the nonmagnetic atoms in the structure. Transport properties of relevance to spintronic applications, including the strain-induced spin-splitter effect and anomalous Hall effect, are predicted. An iodine-assisted synthesis method for TM 2 B 2 is developed, using which 7 of the predicted low-energy phases are experimentally synthesized and characterized, including 4 altermagnets. This work expands the realm of borides by offering new opportunities for studying altermagnetism and altermagnons in borides. It also provides valuable insights into the discovery and design of altermagnets. Here, by demonstrating that altermagnets can exist as families sharing a common motif, this work paves a feasible route for discovering altermagnets by elemental substitutions and high-throughput computations.
Chemical structure↗
Optical control of orbital magnetism in magic-angle twisted bilayer graphene
Flat bands in twisted graphene structures host various strongly correlated and topological phenomena. Optically probing and controlling them can reveal important information such as symmetry and dynamics, but this has been challenging due to the small energy gap compared with optical wavelengths. Here, in this study, we report on the near-infrared optical control of orbital magnetism and associated anomalous Hall effects in a magic-angle twisted bilayer graphene on a monolayer WSe 2 device. We demonstrate control over the hysteresis and amplitude of the anomalous Hall effect near integer moiré fillings using circularly polarized light. By modulating the light helicity, we observe periodic modulation of the transverse resistance in a wide range of fillings, indicating light-induced orbital magnetization through a large inverse Faraday effect. At the transition between metallic and anomalous Hall effect regimes, we also reveal large and random switching of the Hall resistivity, which we attribute to the light-tuned percolating cluster of magnetic domains. Our results demonstrate the potential of the optical manipulation of correlation and topology in moiré structures.
Persky, Eylon [Stanford Univ., CA (United States);↗
Topological Hall effect in magnetic topological insulator films
Geometric Berry phase can be induced either by spin–orbit coupling, giving rise to the anomalous Hall effect in ferromagnetic materials, or by chiral spin texture, such as skyrmions, leading to the topological Hall effect. Recent experiments have revealed that both phenomena can occur in topological insulator films with magnetic doping, thus providing us with an intriguing platform to study the interplay between these two phenomena. In this work, we report on a numerical simulation of the anomalous Hall and topological Hall effects in a four-band model that can properly describe the quantum well states in the magnetic topological insulator films by combining Landauer–Büttiker formula and the iterative Green’s function method. Our numerical results suggest that spin–orbit coupling in this model plays a different role in the quantum transport in the clean and disordered limits. In the clean limit, spin–orbit coupling mainly influences the longitudinal transport but does not have much effect on topological Hall conductance. In the disordered limit, the longitudinal transport is determined by disorder scattering and spin–orbit coupling is found to affect strongly the topological Hall conductance. Here, this sharp contrast unveils a dramatic interplay between spin–orbit coupling and disorder effect in topological Hall effect in magnetic topological insulator systems.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗