arXiv subjects

Qian Niu

Publications and source records attributed to Qian Niu.

At least 109 records · Page 6Linked to original sources

arXiv2022

Intrinsic Nonlinear Spin Magnetoelectricity in Centrosymmetric Magnets

We propose an intrinsic nonlinear spin magnetoelectric effect in magnetic materials, offering the potential of all-electric control of spin degree of freedom in centrosymmetric magnets, which reside outside of the current paradigm based on linear spin response. We reveal the band geometric origin of this effect in the momentum and magnetization space Berry connection polarizabilities, and clarify its symmetry characters. As an intrinsic effect, it is determined solely by the material's band structure and represents a material characteristic. Combining our theory with first-principles calculations, we predict sizable nonlinear spin magnetoelectricity in single-layer MnBi$_{2}$Te$_{4}$, which can be detected in experiment. Our theory paves the way for exploring rich nonlinear spintronic effects and novel device concepts based on them.

cond-mat.mes-hall↗

arXiv2021

Semiclassical equations of motion for disordered conductors: extrinsic interband velocity, corrected collision integral and spin-orbit torques

The semiclassical equations of motion are widely used to describe carrier transport in conducting materials. Nevertheless, the substantial challenge of incorporating disorder systematically into the semiclassical model persists, leading to quantitative inaccuracies and occasionally erroneous predictions for the expectation values of physical observables. In the present work we provide a general prescription for reformulating the semiclassical equations of motion for carriers in disordered conductors by taking the quantum mechanical density matrix as the starting point. We focus on external electric fields, without magnetic fields, and spin-independent disorder. The density matrix approach allows averaging over impurity configurations, and the trace of the velocity operator with the disorder-averaged density matrix can be reinterpreted as the semiclassical velocity weighted by the Boltzmann distribution function. Through this rationale the well-known intrinsic group and anomalous velocities are trivially recovered, while we demonstrate the existence of an extrinsic interband velocity, namely a disorder correction to the semiclassical velocity of Bloch electrons, mediated by the interband matrix elements of the Berry connection. A similar correction is present in the non-equilibrium expectation value of the spin operator, contributing to spin-orbit torques. To obtain agreement with diagrammatic approaches the scattering term in the Boltzmann equation is corrected to first order in the electric field, and the Boltzmann equation is solved up to sub-leading order in the disorder potential. Our prescription ensures all vertex corrections present in diagrammatic treatments are taken into account, and to illustrate this we discuss model cases in topological insulators, including the anomalous Hall effect as well as spin-orbit torques.

cond-mat.mes-hall↗

arXiv2021

Conserved current of nonconserved quantities

We provide a unified semiclassical theory for the conserved current of nonconserved quantities, and manifest it in two physical contexts: the spin current of Bloch electrons and the charge current of mean-field Bogoliubov quasiparticles. Several longstanding problems that limit the playground of the conserved spin current of electrons are solved. We reveal that the hitherto overlooked torque quadrupole density and Berry phase correction to the torque dipole density are essential to assure a circulating spin current with vanishing net flow at equilibrium. The band geometric origin of bulk spin transport is ascertained to be the momentum space spin texture and Berry curvature instead of the spin Berry curvature, paving the way for material related studies. In superconductors the attained conserved charge current corresponds to the quasiparticle charge current renormalized by the condensate backflow. Its circulation at equilibrium gives an orbital magnetization, which involves the characteristics of superconductivity, such as the Berry curvature arising from unconventional pairing and an orbital magnetic moment induced by the charge dipole of moving quasiparticles.

cond-mat.mes-hall↗

arXiv2021

Phonon Magnetic Moment from Electronic Topological Magnetization

The traditional theory of magnetic moments for chiral phonons is based on the picture of the circular motion of the Born effective charge, typically yielding a small fractional value of the nuclear magneton. Here we investigate the adiabatic evolution of electronic states induced by lattice vibration of a chiral phonon and obtain an electronic orbital magnetization in the form of a topological second Chern form. We find that the traditional theory needs to be refined by introducing a $\bm{k}$ resolved Born effective charge, and identify another contribution from the phonon-modified electronic energy together with the momentum-space Berry curvature. The second Chern form can diverge when there is a Yang's monopole near the parameter space of interest as illustrated by considering a phonon at the Brillouin zone corner in a gaped graphene model. We also find large magnetic moments for the optical phonon in bulk topological materials where non-topological contribution is also important. The magnetic moment experiences a sign change when the band inversion happens.

cond-mat.mes-hall↗

arXiv2021

Switching magnon chirality in artificial antiferromagnet

Magnons in antiferromagnets can support both right-handed and left-handed chiralities, which shed a light on the chirality-based spintronics. Here we demonstrate the switching and reading of magnon chirality in an artificial antiferromagnet. The coexisting antiferromagnetic and ferromagnetic characteristic resonance modes are discovered, which permits a high tunability in the modulation of magnon chirality. The reading of the chirality is accomplished via the chirality-dependent spin pumping as well as spin rectification effect. Our result illustrates an ideal antiferromagnetic platform for handling magnon chirality and paves the way for chirality-based spintronics.

physics.app-ph↗

arXiv2021

Topological electric driving of magnetization dynamics in insulators

Established forms of electromagnetic coupling are usually conservative (in insulators) or dissipative (in metals and semiconductors). Here we point out the possibility of nondissipative electric driving of magnetization dynamics, if the valence electronic states have nontrivial topology in the combined space of crystal momentum and magnetization configuration. We provide a hybrid insulator system to demonstrate that the topology-based nonconservative electrical generalized force is capable of supporting sustained magnetization motion in the presence of Gilbert damping, with quantized and steady energy pumping into magnetization motion from the electric field. We also generalize our results to magnetic textures, and discuss electric field induced Dzyaloshinskii-Moriya interaction which can be nonconservative.

cond-mat.mes-hall↗

arXiv2021

Giant $c$-axis nonlinear anomalous Hall effect in T$_d$-MoTe$_2$ and WTe$_2$

While the anomalous Hall effect can manifest even without an external magnetic field, time reversal symmetry is nonetheless still broken by the internal magnetization of the sample. Recently, it has been shown that certain materials without an inversion center allow for a nonlinear type of anomalous Hall effect whilst retaining time reversal symmetry. The effect may arise from either Berry curvature or through various asymmetric scattering mechanisms. Here, we report the observation of an extremely large $c$-axis nonlinear anomalous Hall effect in the non-centrosymmetric T$_d$ phase of MoTe$_2$ and WTe$_2$ without intrinsic magnetic order. We find that the effect is dominated by skew-scattering at higher temperatures combined with another scattering process active at low temperatures. Application of higher bias yields an extremely large Hall ratio of $E_\perp /E_\parallel$=2.47 and corresponding anomalous Hall conductivity of order 8x10$^7$S/m.

cond-mat.mes-hall↗

arXiv2020

Van der Waals Heterostructure Pt$_{2}$HgSe$_{3}$/CrI$_3$ for Topological Valleytronics

We identify a valley-polarized Chern insulator in the van der Waals heterostructure, Pt$_{2}$HgSe$_{3}$/CrI$_3$, for potential applications with interplay between electric, magnetic, optical, and mechanical effects. The interlayer proximity magnetic coupling nearly closes the band gap of Pt$_{2}$HgSe$_{3}$ and the strong intra-layer spin-orbit coupling further lifts the valley degeneracy by over 100 meV leading to positive and negative band gaps at opposite valleys. In the valley with negative gap, the interfacial Rashba spin-orbit coupling opens a topological band gap of 17.8 meV, which is enlarged to 30.8 meV by adding an $h$-BN layer. We find large orbital magnetization in Pt$_{2}$HgSe$_{3}$ layer that is much larger than spin, which can induce measurable optical Kerr effect. The valley polarization and Chern number are coupled to the magnetic order of the nearest neighboring CrI$_3$ layer, which is switchable by electric, magnetic, and mechanical means in experiments. The presence of $h$-BN protects the topological phase allowing the construction of superlattices with valley, spin, and layer degrees of freedoms.

cond-mat.mtrl-sci↗

arXiv2020

Floquet-Bloch Oscillations and Intraband Zener Tunneling in an Oblique Spacetime Crystal

We investigate an oblique spacetime crystal realized by a monoatomic crystal in which a mode of sound propagates. We provide a systematic analysis of the crystal and obtain the corresponding band structure, based on which the electron dynamics under an external electric field is studied. Several unique band topologies are revealed, which lead to novel Floquet-Bloch oscillations in the electron's motion. We also discover intraband Zener tunneling in the oblique spacetime crystal beyond the adiabatic limit, which effectively converts between the different band topologies. Our results indicate the possibility of a prototypical quantum acoustoelectric generator that converts energy between the sound wave and a DC electric field in quantized units.

cond-mat.other↗

arXiv2020

Thermoelectric Generation of Orbital Magnetization in Metals

We propose an orbital magnetothermal effect wherein a temperature gradient generates an orbital magnetization (OM) for Bloch electrons, and we present a unified theory for electrically and thermally induced OM, valid for both metals and insulators. We reveal that there exists an intrinsic response of OM, for which the susceptibilities are completely determined by the band geometric quantities such as interband Berry connections, interband orbital moments, and the quantum metric. The theory can be readily combined with first-principles calculations to study real materials. As an example, we calculate the OM response in CrI$_{3}$ bilayers, where the intrinsic contribution dominates. The temperature scaling of intrinsic and extrinsic responses, the effect of phonon drag, and the phonon angular momentum contribution to OM are discussed.

cond-mat.mes-hall↗

arXiv2020

Berry curvature effects on quasiparticle dynamics in superconductors

We construct a theory for the semiclassical dynamics of superconducting quasiparticles by following their wave-packet motion and reveal rich contents of Berry curvature effects in the phase-space spanned by position and momentum. These Berry curvatures are traced back to the characteristics of superconductivity, including the nontrivial momentum-space geometry of superconducting pairing, the real-space supercurrent, and the charge dipole of quasiparticles. The Berry-curvature effects strongly influence the spectroscopic and transport properties of superconductors, such as the local density of states and the thermal Hall conductivity. As a model illustration, we apply the theory to study the twisted bilayer graphene with a $d_{x^{2}+y^{2}}+id_{xy}$ superconducting gap function, and demonstrate Berry-curvature induced effects.

cond-mat.supr-con↗

arXiv2020

Josephson radiation from nonlinear dynamics of Majorana zero modes

Josephson radiation is a powerful method to probe Majorana zero modes in topological superconductors. Recently, Josephson radiation with half the Josephson frequency has been experimentally observed in a HgTe-based junction, possibly from Majorana zero modes. However, this radiation vanishes above a critical voltage, sharply contradicting previous theoretical results. In this work, we theoretically obtain a radiation spectrum quantitatively in agreement with the experiment after including the nonlinear dynamics of the Majorana states into the standard resistively shunted junction model. We further predict two new structures of the radiation spectrum for future experimental verification: an interrupted emission line and a chaotic regime. We develop a fixed-point analysis to understand all these features. Our results resolve an apparent discrepancy between theory and experiments, and will inspire reexamination of structures in radiation spectra of various topological Josephson junctions.

cond-mat.supr-con↗

arXiv2020

WKB estimate of bilayer graphene's magic twist angles

Graphene bilayers exhibit zero-energy flat bands at a discrete series of magic twist angles. In the absence of intra-sublattice inter-layer hopping, zero-energy states satisfy a Dirac equation with a non-abelian SU(2) gauge potential that cannot be diagonalized globally. We develop a semiclassical WKB approximation scheme for this Dirac equation by introducing a dimensionless Planck's constant proportional to the twist angle, solving the linearized Dirac equation around AB and BA turning points, and connecting Airy function solutions via bulk WKB wavefunctions. We find zero energy solutions at a discrete set of values of the dimensionless Planck's constant, which we obtain analytically. Our analytic flat band twist angles correspond closely to those determined numerically in previous work.

cond-mat.mes-hall↗

arXiv2020

Unified bulk semiclassical theory for intrinsic thermal transport and magnetization currents

We reveal the unexpected role of the material inhomogeneity in unifying the formulation of intrinsic thermal and thermoelectric transport as well as magnetization currents. The smooth inhomogeneity leads to the position dependent local band dispersion and phase-space Berry curvature, enabling a general and rapid access to transport and magnetization currents displaying the momentum-space Berry curvature physics. Our theory does not invoke the boundary current, the thermodynamic approach to magnetization or any mechanical counterpart of statistical forces. By introducing a fictitious inhomogeneity, it applies to homogeneous samples as well, promoting the inhomogeneity to be a basic trick in semiclassical transport theories. Such a trick works regardless of the driving force of transport, e.g., temperature gradient, in contrast to the trick of fictitious gravitational field in quantum transport theories. We thus include more general mechanical driving forces and establish the Mott relation between the resulting transport thermal and electric currents, whereas this relation for these two currents was previously only known when an electric field is the driving force.

cond-mat.mes-hall↗

arXiv2020

Engineering Corner States from Two-Dimensional Topological Insulators

We theoretically demonstrate that the second-order topological insulator with robust corner states can be realized in two-dimensional $\mathbb{Z}_2$ topological insulators by applying an in-plane Zeeman field. Zeeman field breaks the time-reversal symmetry and thus destroys the $\mathbb{Z}_2$ topological phase. Nevertheless, it respects some crystalline symmetries and thus can protect the higher-order topological phase. By taking the Kane-Mele model as a concrete example, we find that spin-helical edge states along zigzag boundaries are gapped out by Zeeman field whereas in-gap corner state at the intersection between two zigzag edges arises, which is independent on the field orientation. We further show that the corner states are robust against the out-of-plane Zeeman field, staggered sublattice potentials, Rashba spin-orbit coupling, and the buckling of honeycomb lattices, making them experimentally feasible. Similar behaviors can also be found in the well-known Bernevig-Hughes-Zhang model.

cond-mat.mes-hall↗

arXiv2020

Linear magnetoresistance induced by intra-scattering semiclassics of Bloch electrons

The weak field magnetoresistance has seen a revived interest due to the distinct role played by the momentum-space Berry curvature of Bloch electrons. While most previous studies in this regard focus on the inter-scattering motion of semiclassical Bloch electrons in electromagnetic fields, the intra-scattering effects of the semiclassical dynamics augmented by the Berry curvature, magnetic moment and shift vector on the magnetoresistance have been largely overlooked. Here we uncover that these intra-scattering effects, which are neglected in the field-independent relaxation time approximation to the Boltzmann collision integral, can be as important as the inter-scattering ones. Concrete calculations on the two dimensional gapped Dirac model show that the sign of the negative linear magnetoresistance given by the Berry curvature alone is reversed when one considers the magnetic moment and shift vector.

cond-mat.mtrl-sci↗

arXiv2020

Berry-phase effects in dipole density and Mott relation

We provide a unified semiclassical theory for thermoelectric responses of any observable represented by an operator $\hat{\boldsymbolθ}$ that is well-defined in periodic crystals. The Einstein and Mott relations are established generally, in the presence of Berry-phase effects, for various physical realizations of $\hat{\boldsymbolθ}$ in electronic systems, including the familiar case of the electric current as well as the currently controversial cases of the spin polarization and spin current. The magnetization current, which has been proven indispensable in the thermoelectric response of electric current, is generalized to the cases of various $\hat{\boldsymbolθ}$. In our theory the dipole density of a physical quantity emerges and plays a vital role, which contains not only the statistical sum of the dipole moment of $\hat{\boldsymbolθ}$ but also a Berry-phase correction.

cond-mat.mes-hall↗

arXiv2019

Theory of nonlinear Hall effects: renewed semiclassics from quantum kinetics

We propose a modified Boltzmann nonlinear electric-transport framework which differs from the nonlinear generalization of the linear Boltzmann formalism by a contribution that has no counterpart in linear response. This contribution follows from the interband-coherence effect of dc electric-fields during scattering and is related to the interband Berry connection. As an application, we demonstrate it in the second-order nonlinear Hall effect of the tilted massive Dirac model. The intuitive Boltzmann constructions are confirmed by a quantum kinetic theory, which shows that arbitrary $n$th-order nonlinear dc response up to the first three leading contributions in the weak disorder potential is handled by the same few gauge-invariant semiclassical ingredients.

cond-mat.mes-hall↗