arXiv subjects

X. R. Wang

Publications and source records attributed to X. R. Wang.

At least 37 records · Page 2Linked to original sources

arXiv2023

Unusual anisotropic magnetoresistance due to magnetization-dependent spin-orbit interactions

One of recent surprising discoveries is the unusual anisotropic magnetoresistance (UAMR) that depends on two magnetization components perpendicular to the current differently, in contrast to the conventional anisotropic magnetoresistance (AMR) that predicts no change in resistance when the magnetization varies in the plane perpendicular to the current. Using density functional theory and Boltzmann transport equation calculations for bcc Fe, hcp Co, and bcc FeCo alloys, we show that UAMR can be accounted by the magnetization-dependent spin-orbit interactions (SOI): Magnetization-dependent SOI modifies electron energy bands that, in turn, changes resistance. A phenomenological model reveals the intrinsic connection between SOI and order-parameters. Such a mechanism is confirmed by the strong biaxial stain effect on UAMR. Our findings provide an efficient way of searching and optimizing materials with large UAMR, important in the design of high-performance spintronic devices.

cond-mat.mes-hall↗

arXiv2022

On universal butterfly and antisymmetric magnetoresistances

Butterfly magnetoresistance (BMR) and antisymmetric magnetoresistance (ASMR) are about a butterfly-cross curve and a curve with one peak and one valley when a magnetic field is swept up and down along a fixed direction. Other than the parallelogram-shaped magnetoresistance-curve (MR-curve) often observed in magnetic memory devices, BMR and ASMR are two ubiquitous types of MR-curves observed in diversified magnetic systems, including van der Waals materials, strongly correlated systems, and traditional magnets. Here, we reveal the general principles and the picture behind the BMR and the ASMR that do not depend on the detailed mechanisms of magnetoresistance: 1) The systems exhibit hysteresis loops, common for most magnetic materials with coercivities. 2) The magnetoresistance of the magnetic structures in a large positive magnetic field and in a large negative magnetic field is approximately the same. With the generalized Ohm's law in magnetic materials, these principles explain why most BMR appears in the longitudinal resistance measurements and is very rare in the Hall resistance measurements. Simple toy models, in which the Landau-Lifshitz-Gilbert equation governs magnetization, are used to demonstrate the principles and explain the appearance and disappearance of BMR in various experiments. Our finding provides a simple picture to understand magnetoresistance-related experiments.

cond-mat.mes-hall↗

arXiv2022

Intrinsic anomalous spin Hall effect

Charge-spin interconversion in magnetic materials is investigated by using first-principles calculations. In addition to the conventional spin Hall effect (SHE) that requires mutual orthogonality of the charge current, spin-flow direction, and spin polarization, the recently proposed anomalous SHE (ASHE) is confirmed in Mn2Au and WTe2. The interaction of the order parameter with conduction electrons leads to sizeable nonzero spin Berry curvatures that give rise to nonzero anomalous spin Hall conductivity (ASHC). Our calculations show that the ASHE is intrinsic and originates from the order-parameter-controlled spin-orbit interaction, which generates an extra anomalous effective field. A useful relationship among the order parameter, spin Berry curvature and ASHC is revealed. Our findings open a new avenue for arbitrary-type spin current generation and detection.

cond-mat.mtrl-sci↗

arXiv2022

Chiral Hinge Transport in Disordered Non-Hermitian Second-Order Topological Insulators

The generalized bulk-boundary correspondence predicts the existence of the chiral hinge states in three-dimensional second-order topological insulators (3DSOTIs), resulting in a quantized Hall effect in three dimensions. Chiral hinge states in Hermitian 3DSOTIs are characterized by the quantized transmission coefficients with zero fluctuations, even in the presence of disorders. Here, we show that chiral hinge transport in disordered non-Hermitian systems deviates from the paradigm of the Hermitian case. Our numerical calculations prove the robustness of hinge states of disordered non-Hermitian 3DSOTIs. The mean transmission coefficients may or may not equal the number of chiral hinge channels, depending on the Hermiticity of chiral hinge states, while the fluctuations of transmission coefficients are always non-zero. Such fluctuations are not due to the broken chirality of hinge states but the incoherent scatterings of non-Hermitian potentials. The physics revealed here should also be true for one-dimensional chiral channels in topological materials that support chiral boundary states, such as Chern insulators, three-dimensional anomalous Hall insulators, and Weyl semimetals.

cond-mat.mes-hall↗

arXiv2022

Linear level repulsions near exceptional points of non-Hermitian systems

The nearest-neighbor level-spacing distributions are a fundamental quantity of disordered systems and universal. It is well-known that extended and localized states of random Hermitian systems follow the Wigner-Dyson and the Poison distributions, respectively, while the Ginibre distributions describe random non-Hermitian systems with complex eigenvalues. However, the level distribution of systems of neither Hermitian nor non-Hermitian with full complex eigenvalues is still unknown. Here we show a new class of universal level distributions in the vicinity of exceptional points of non-Hermitian Hamiltonians. Two universal distribution functions, $P_{\text{SP}}(s)$ for the symmetry-preserved phase and $P_{\text{SB}}(s)$ for the symmetry-broken phase, are needed to describe the nearest-neighbor level-spacing distributions near exceptional points. Surprisingly, both $P_{\text{SP}}(s)$ and $P_{\text{SB}}(s)$ are proportional to $s$ for small $s$, or a linear level repulsion, in contrast to the cubic level repulsions of the Ginibre ensembles. For non-Hermitian disordered tight-binding Hamiltonians, $P_{\text{SP}}(s)$ and $P_{\text{SB}}(s)$ can be well described by $P_{\text{SP(SB)}}(s)=\tilde{c}_1s\exp[-\tilde{c}_2s^{\tildeα}]$ in the thermodynamic limit (of infinite systems) with a constant $\tildeα$ that depends on the localization nature of states at exceptional points.

cond-mat.dis-nn↗

arXiv2022

Skyrmion pinning by disk-shaped defects

High precision skyrmion pinning by an intentionally created magnetic structure is important in skyrmion manipulation. Here, we consider skyrmion pinning by various types of disks. Among other findings, we clarify that, in terms of pinning position and pinning potential landscape, one needs to distinguish thin-wall skyrmions from thick-wall skyrmions because of fundamental differences. A skyrmion wall prefers areas with a weaker exchange stiffness, a larger DMI constant, or a smaller magnetic anisotropy while a skyrmion core experiences only the magnetic anisotropy, not exchange stiffness and DMI. Depending on disk type, skyrmion type, and the relative size of the disk (to skyrmion), a skyrmion can be pinned at the symmetric (center) or asymmetric (off-center) point of a disk. In case a skyrmion is pinned by a local energy minimum, thermal agitation can depin the skyrmion, and the pinning lifetime follows an Arrhenius law. Interestingly, when the disk size is comparable to the skyrmion size, the skyrmion deforms greatly such that the skyrmion will shrink or expand to fill the whole disk. These findings should be important for skyrmion manipulations.

cond-mat.mes-hall↗

arXiv2022

On field-driven domain wall motion in compensated ferrimagnetic nanowires

The fascinating high-speed field-driven domain wall (DW) motion along ferrimagnetic nanowires near the angular momentum compensation point (AMCP) is solved based on the generic ferrimagnetic dynamics. The physics of the absences of precessional torque and infinite high Walker breakdown field at the AMCP is proved under general conditions. Based on the energy conservation principle, an almost exact DW velocity formula, valid beyond the Walker breakdown field, is obtained. Our results agree with all existing experiments and simulations. This theory provides useful guidances to DW manipulation.

cond-mat.mes-hall↗

arXiv2022

A theory of skyrmion crystal formation

A generic theory about skyrmion crystal (SkX) formation in chiral magnetic thin films and its fascinating thermodynamic behaviours is presented. A chiral magnetic film can have many metastable states with an arbitrary skyrmion density up to a maximal value when a parameter $κ$, which measures the relative Dzyaloshinskii-Moriya interaction (DMI) strength, is large enough. The lowest energy state of an infinite film is a long zig-zagged ramified stripe skyrmion occupied the whole film in the absence of a magnetic field. Under an intermediate field perpendicular to the film, the lowest energy state has a finite skyrmion density. This is why a chiral magnetic film is often in a stripy state at a low field and a SkX only around an optimal field when $κ$ is above a critical value. The lowest energy state is still a stripy helical state no matter with or without a field when $κ$ is below the critical value. The multi-metastable states explains the thermodynamic path dependences of various metastable states of a film. Decrease of $κ$ value with the temperature explains why SkXs become metastable at low temperature in many skyrmion systems. These findings open a new avenue for SkX manipulation and skyrmion-based applications.

cond-mat.mes-hall↗

arXiv2022

Nematic and smectic stripe phases and stripe-SkX transformations

Based on the findings of stripe skyrmions and the metastability of a state of an arbitrary number of skyrmions, precisely controlled manipulation of stripe skyrmions in pre-designed structures and mutual transformation between helical states and skyrmion crystals (SkXs) are demonstrated in chiral magnetic films. As a proof of the concept, we show how to use patterned magnetic fields and spin-transfer torques (STTs) to generate nematic and smectic stripe phases, as well as "UST" mosaic from three curved stripe skyrmions. Cutting one stripe into many pieces and coalescing several skyrmions into one by various external fields are good ways to transform helical states and SkXs from each other.

cond-mat.mes-hall↗

arXiv2022

General Exceptional Points

Exceptional points are interesting physical phenomena in non-Hermitian physics at which the eigenvalues are degenerate and the eigenvectors coalesce. In this paper, we find that the universal feature of arbitrary non-Hermitian two level systems with singularities is basis defectiveness rather than energy degeneracy or state coalescence. This leads to the discovery of general exceptional points (GEPs). For GEPs, more subtle structures (e.g., Bloch peach), additional classification, and' 'hidden" quantum phase transitions are explored. By using the topologically protected subspace from two edge states in the non-Hermitian SSH model as an example, we illustrate the physical properties of different types of GEPs.

quant-ph↗

arXiv2022

A theory for anisotropic magnetoresistance in materials with two vector order parameters

Anisotropic magnetoresistance (AMR) and related planar Hall resistance (PHR) are ubiquitous phenomena of magnetic materials. Although the universal angular dependences of AMR and PHR in magnetic polycrystalline materials with one order parameter are well known, no similar universal relation for other class of magnetic materials are known to date. Here I present a general theory of galvanomagnetic effects in magnetic materials with two vector order parameters, such as magnetic single crystals with a dominated crystalline axis or polycrystalline non-collinear ferrimagnetic materials. It is shown that AMR and PHR have a universal angular dependence. In general, both longitudinal and transverse resistivity are non-reciprocal in the absence of inversion symmetry: Resistivity takes different value when the current is reversed. Different from simple magnetic polycrystalline materials where AMR and PHR have the same magnitude, and $π/4$ out of phase, the magnitude of AMR and PHR of materials with two vector order parameters are not the same in general, and the phase difference is not $π/4$. Instead of $π$ periodicity of the usual AMR and PHR, the periodicities of materials with two order parameters are $2π$.

cond-mat.str-el↗

arXiv2021

Hermitian chiral boundary states in non-Hermitian topological insulators

Eigenenergies of a non-Hermitian system without parity-time symmetry are complex in general. Here, we show that the chiral boundary states of higher-dimensional (two-dimensional and three-dimensional) non-Hermitian topological insulators without parity-time symmetry can be Hermitian with real eigenenergies under certain conditions. Our approach allows one to construct Hermitian chiral edge and hinge states from non-Hermitian two-dimensional Chern insulators and three-dimensional second-order topological insulators, respectively. Such Hermitian chiral boundary channels have perfect transmission coefficients (quantized values) and are robust against disorders. Furthermore, a non-Hermitian topological insulator can undergo the topological Anderson insulator transition from a topological trivial non-Hermitian metal or insulator to a topological Anderson insulator with quantized transmission coefficients at finite disorders.

cond-mat.mes-hall↗

arXiv2021

Dancing Synchronization in Coupled Spin-Torque Nano-Oscillators

We are reporting a new type of synchronization, termed dancing synchronization, between two spin-torque nano-oscillators (STNOs) coupled through spin waves. Different from the known synchronizations in which two STNOs are locked with various fixed relative phases, in this new synchronized state two STNOs have the same frequency, but their relative phase varies periodically within the common period, resulting in a dynamic waving pattern. The amplitude of the oscillating relative phase depends on the coupling strength of two STNOs, as well as the driven currents. The dancing synchronization turns out to be universal, and can exist in two nonlinear Van der Pol oscillators coupled both reactively and dissipativly. Our findings open doors for new functional STNO-based devices.

cond-mat.mes-hall↗

arXiv2021

Skyrmion size in skyrmion crystals

A magnetic skyrmion is a topological object that can exist as a solitary embedded in the vast ferromagnetic phase, or coexists with a group of its "siblings" in various stripy phases as well as skyrmion crystals (SkXs). Isolated skyrmions and skyrmions in an SkX are circular while a skyrmion in other phases is a stripe of various forms. Unexpectedly, the sizes of the three different types of skyrmions depend on material parameters differently. For chiral magnetic films with exchange stiffness constant $A$, the Dzyaloshinskii-Moriya interaction (DMI) strength $D$, and perpendicular magnetic anisotropy $K$, $κ\equivπ^2D^2/(16AK)=1$ separates isolated skyrmions from condensed skyrmion states. In contrast to isolated skyrmions whose size increases with $D/K$ and is insensitive to $κ\ll1$ and stripe skyrmions whose width increases with $A/D$ and is insensitive to $κ\gg1$, the size of skyrmions in SkXs is inversely proportional to the square root of skyrmion number density and decreases with $A/D$. This finding has important implications in our search for stable smaller skyrmions at the room temperature in applications.

cond-mat.mes-hall↗

arXiv2021

Stripe skyrmions and skyrmion crystals

Skyrmions are important in topological quantum field theory for being soliton solutions of a nonlinear sigma model and in information technology for their attractive applications. Skyrmions are believed to be circular and stripy spin textures appeared in the vicinity of skyrmion crystals are termed spiral, helical, and cycloid spin orders, but not skyrmions. Here we present convincing evidences showing that those stripy spin textures are skyrmions, "siblings" of circular skyrmions in skyrmion crystals and "cousins" of isolated circular skyrmions. Specifically, isolated skyrmions are excitations when skyrmion formation energy is positive. The skyrmion morphologies are various stripy structures when the ground states of chiral magnetic films are skyrmions. The density of skyrmion number determines the morphology of condensed skyrmion states. At the extreme of one skyrmion in the whole sample, the skyrmion is a ramified stripe. As the skyrmion number density increases, individual skyrmion shapes gradually change from ramified stripes to rectangular stripes, and eventually to disk-like objects. At a low skyrmion number density, the natural width of stripes is proportional to the ratio between the exchange stiffness constant and Dzyaloshinskii-Moriya interaction coefficient. At a high skyrmion number density, skyrmion crystals are the preferred states. Our findings reveal the nature and properties of stripy spin texture, and open a new avenue for manipulating skyrmions, especially condensed skyrmions such as skyrmion crystals.

cond-mat.mes-hall↗

arXiv2021

Experimental observation of edge-dependent quantum pseudospin Hall effect

It is a conventional wisdom that the helical edge states of quantum spin Hall (QSH) insulator are particularly stable due to the topological protection of time-reversal symmetry. Here, we report the first experimental observation of an edge-dependent quantum (pseudo-)spin Hall effect by employing two Kekule electric circuits with molecule-zigzag and partially-bearded edges, where the chirality of the circulating current in the unit cell mimics the electron spin. We observe a helicity flipping of the topological in-gap modes emerging in opposite parameter regions for the two edge geometries. Experimental findings are interpreted in terms of the mirror winding number defined in the unit cell, the choice of which exclusively depends on the edge shape. Our work offers a deeper understanding of the boundary effect on the QSH phase, and pave the way for studying the spin-dependent topological physics in electric circuits.

cond-mat.mes-hall↗

arXiv2020

Recent progress in antiferromagnetic dynamics

Spintronics, since its inception, has mainly focused on ferromagnetic materials for manipulating the spin degree of freedom in addition to the charge degree of freedom, whereas much less attention has been paid to antiferromagnetic materials. Thanks to the advances of micro-nano-fabrication techniques and the electrical control of the Néel order parameter, antiferromagnetic spintronics is booming as a result of abundant room temperature materials, robustness against external fields and dipolar coupling, and rapid dynamics in the terahertz regime. For the purpose of applications of antiferromagnets, it is essential to have a comprehensive understanding of the antiferromagnetic dynamics at the microscopic level. Here, we first review the general form of equations that govern both antiferromagnetic and ferrimagnetic dynamics. This general form unifies the previous theories in the literature. We also provide a survey for the recent progress related to antiferromagnetic dynamics, including the motion of antiferromagnetic domain walls and skyrmions, the spin pumping and quantum antiferromagnetic spintronics. In particular, open problems in several topics are outlined. Furthermore, we discuss the development of antiferromagnetic quantum magnonics and its potential integration with modern information science and technology.

cond-mat.mes-hall↗

arXiv2020

Robustness of Helical Hinge States of Weak Second-Order Topological Insulators

Robustness of helical hinge states of three-dimensional weak second-order topological i sulators (WSOTIs) against disorders is studied. The pure WSOTI is obtained from a weak $\mathbb{Z}_2$ first-order topological insulator through a surface band inversion. Both bulk states and surface states in the WSOTI are gapped, and in-gap valley-momentum locked helical hinge states are topologically protected by the surface valley-Chern number. In the presence of weak disorders, helical hinge states are robust against disorders while the quantized conductance of the states is fragile due to the inter-valley scattering. As disorder increases, the system undergoes a series of quantum phase transitions: from the WSOTI to the weak first-order topological insulator, then to a diffusive metal and finally to an Anderson insulator. Our results thus fully establish the WSOTI phase as a genuine state of matters and open a door for the second-order valleytronics that allows one to control the valley degree of freedom through helical hinge states.

cond-mat.mes-hall↗