In-Plane Anomalous Hall Effect in PT-Symmetric Antiferromagnetic Materials

Intrinsic Nonlinear Spin Hall Effect and Manipulation of Perpendicular Magnetization

We propose an intrinsic nonlinear spin Hall effect, which enables the generation of collinearly polarized spin current in a large class of nonmagnetic materials with the corresponding linear response being symmetry forbidden. This opens a new avenue for field-free switching of perpendicular magnetization, which is required for the next-generation information storage technology. We develop the microscopic theory of this effect and clarify its quantum origin in band geometric quantities which can be enhanced by topological nodal features. Combined with first-principles calculations, we predict pronounced effects at room temperature in topological metals PbTaSe_{2} and PdGa. Our work establishes a fundamental nonlinear response in spin transport and opens the door to exploring spintronic applications based on nonlinear spin Hall effect.

Planar Hall plateau in magnetic Weyl semimetals

Despite the rapid progress in the study of planar Hall effect (PHE) in recent years, all the previous works only showed that the PHE is connected to local geometric quantities, such as Berry curvature. Here, for the first time, we point out that the PHE in magnetic Weyl semimetals is directly related to a global quantity, namely, the Chern number of the Weyl point. This leads to a remarkable consequence that the PHE observation predicted here is robust against many system details, including the Fermi energy. The main difference between non-magnetic and magnetic Weyl points is that the latter breaks time-reversal symmetry T, thus generally possessing an energy tilt. Via semiclassical Boltzmann theory, we investigate the PHE in generic magnetic Weyl models with energy tilt and arbitrary Chern number. We find that by aligning the magnetic and electric fields in the same direction, the trace of the PHE conductivity contributed by Berry curvature and orbital moment is proportional to the Chern number and the energy tilt of the Weyl points, resulting in a previously undiscovered quantized PHE plateau by varying the Fermi energy. We further confirm the existence of PHE plateaus in a more realistic lattice model without T symmetry. By proposing a new quantized physical quantity, our work not only provides a new tool for extracting the topological character of the Weyl points but also suggests that the interplay between topology and magnetism can give rise to intriguing physics.

In-Plane Anomalous Hall Effect Associated with Orbital Magnetization: Measurements of Low-Carrier Density Films of a Magnetic Weyl Semimetal

For over a century, the Hall effect, a transverse effect under an out-of-plane magnetic field or magnetization, has been a cornerstone for magnetotransport studies and applications. Modern theoretical formulation based on the Berry curvature has revealed the potential that even an in-plane magnetic field can induce an anomalous Hall effect, but its experimental demonstration has remained difficult due to its potentially small magnitude and strict symmetry requirements. Here, we report observation of the in-plane anomalous Hall effect by measuring low-carrier density films of magnetic Weyl semimetal EuCd_{2}Sb_{2}. Anomalous Hall resistance exhibits distinct threefold rotational symmetry for changes in the in-plane field component, and this can be understood in terms of out-of-plane Weyl points splitting or orbital magnetization induced by the in-plane field, as also confirmed by model calculation. Our findings demonstrate the importance of the in-plane field to control the Hall effect, accelerating materials development and further exploration of various in-plane field-induced phenomena.

Antisymmetric planar Hall effect in rutile oxide films induced by the Lorentz force

The conventional Hall effect is linearly proportional to the field component or magnetization component perpendicular to a film. Despite the increasing theoretical proposals on the Hall effect to the in-plane field or magnetization in various special systems induced by the Berry curvature, such an unconventional Hall effect has only been experimentally reported in Weyl semimetals and in a heterodimensional superlattice. Here, we report an unambiguous experimental observation of the antisymmetric planar Hall effect (APHE) with respect to the in-plane magnetic field in centrosymmetric rutile RuO2 and IrO2 single-crystal films. The measured Hall resistivity is found to be linearly proportional to the component of the applied in-plane magnetic field along a particular crystal axis and to be independent of the current direction or temperature. Both the experimental observations and theoretical calculations confirm that the APHE in rutile oxide films is induced by the Lorentz force. Our findings can be generalized to ferromagnetic materials for the discovery of anomalous Hall effects and quantum anomalous Hall effects induced by in-plane magnetization. In addition to significantly expanding knowledge of the Hall effect, this work opens the door to explore new members in the Hall effect family.

Heterodimensional Kondo superlattices with strong anisotropy

Localized magnetic moments in non-magnetic materials, by interacting with the itinerary electrons, can profoundly change the metallic properties, developing various correlated phenomena such as the Kondo effect, heavy fermion, and unconventional superconductivity. In most Kondo systems, the localized moments are introduced through magnetic impurities. However, the intrinsic magnetic properties of materials can also be modulated by the dimensionality. Here, we report the observation of Kondo effect in a heterodimensional superlattice VS2-VS, in which arrays of the one-dimensional (1D) VS chains are encapsulated by two-dimensional VS2 layers. In such a heterodimensional Kondo superlattice, we observe the typical Kondo effect but with intriguing anisotropic field dependence. This unique anisotropy is determined to originate from the magnetic anisotropy which has the root in the unique 1D chains in the structure, as corroborated by the first-principles calculation. Our results open up a novel avenue of studying exotic correlated physics in heterodimensional materials.

An ideal candidate for observing anomalous Hall effect induced by the in-plane magnetic field

The anomalous Hall effect induced by the in-plane magnetic field (anomalous planar Hall effect) has recently attracted a lot of interests due to its numerous advantages. Although several schemes have been put forward in theory, experimental observations in many materials so far are often accompanied by planar Hall effects due to other mechanisms, rather than the pure anomalous planar Hall effect (APHE). We propose the surface state of the strained topological insulator as an ideal candidate to observe this effect. The surface state exhibits a pure APHE, characterized by a linear dependence on the magnetic field and a 2πperiodicity, which remains robust against the scattering of non-magnetic and various magnetic impurities, as long as the uniaxial strain preserves mirror symmetry. Although a general strain that breaks the mirror symmetry can induce the conventional Drude Hall effect, the anomalous contribution remains dominant. Furthermore, we present a feasible scheme to distinguish between the two contributions based on their distinct magnetic field dependencies. Our work is of great significance for promoting experimental observation of the APHE and provides reference value in the search for other realistic materials.

Crystal Symmetry-Dependent In-Plane Hall Effect

The Hall effect has played a vital role in unraveling the intricate properties of electron transport in solid materials. Here, we report on a crystal symmetry-dependent in-plane Hall effect (CIHE) observed in a CuPt/CoPt ferromagnetic heterostructure. Unlike the planar Hall effect (PHE), the CIHE in CuPt/CoPt strongly depends on the current flowing direction (ϕI) with respect to the crystal structure. It reaches its maximum when the current is applied along the low crystal-symmetry axes and vanishes when applied along the high crystal-symmetry axes, exhibiting an unconventional angular dependence of cos(3ϕI). Utilizing a symmetry analysis based on the Invariant Theory, we demonstrate that the CIHE can exist in magnetic crystals possessing C3v symmetry. Using a tight-binding model and realistic first-principles calculations on the metallic heterostructure, we find that the CIHE originates from the trigonal warping of the Fermi surface. Our observations highlight the critical role of crystal symmetry in generating new types of Hall effects.