arXiv subjects

Cong Xiao

Publications and source records attributed to Cong Xiao.

90 records · Page 5Linked to original sources

arXiv2019

Scaling parameters in anomalous and nonlinear Hall effects depend on temperature

In the study of the anomalous Hall effect, the scaling relations between the anomalous Hall and longitudinal resistivities play the central role. The scaling parameters by definition are fixed as the scaling variable (longitudinal resistivity) changes. Contrary to this paradigm, we unveil that the electron-phonon scattering can result in apparent temperature-dependence of scaling parameters when the longitudinal resistivity is tuned through temperature. An experimental approach is proposed to observe this hitherto unexpected temperature-dependence. We further show that this phenomenon also exists in the nonlinear Hall effect in nonmagnetic inversion-breaking materials and may help identify experimentally the presence of the side-jump contribution besides the Berry-curvature dipole.

cond-mat.mes-hall↗

arXiv2019

Valley-Contrasting Orbital Magnetic Moment Induced Negative Magnetoresistance

The valley-contrasting orbital magnetic moment of Bloch electrons allows the lifting of valley degeneracy by an out-of-plane magnetic field. We demonstrate that this leads to negative magnetoresistance, utilizing a gapped two-dimensional multi-valley model as an example. An intuitive physical picture in terms of the increased carrier density from a magnetic gating effect is proposed for this negative magnetoresistance. In particular, giant negative magnetoresistance is achieved after one of the two valleys is depleted by the magnetic field. This new mechanism of negative magnetoresistance is argued to be relevant in ionic-liquid gated gapped graphene with small effective mass.

cond-mat.mtrl-sci↗

arXiv2019

Temperature dependence of side-jump spin Hall conductivity

In the conventional paradigm of the spin Hall effect, the side-jump conductivity due to electron-phonon scattering is regarded to be temperature independent. To the contrary, we draw the distinction that, while this side-jump conductivity is temperature independent in the classical equipartition regime where the longitudinal resistivity is linear in temperature, it is temperature dependent below the equipartition regime. The mechanism resulting in this temperature dependence differs from the familiar one of the longitudinal resistivity. In the concrete example of Pt, we show that the change of the spin Hall conductivity with temperature can be as high as 50%. Experimentally accessible high-purity Pt is proposed to be suitable for observing this prominent variation below 80 K.

cond-mat.mes-hall↗

arXiv2019

Theory of phonon side jump contribution in Anomalous Hall transport

The role of electron-phonon scattering in finite-temperature anomalous Hall effect is still poorly understood. In this work, we present a Boltzmann theory for the side-jump contribution from electron-phonon scattering, which is derived from the microscopic quantum mechanical theory. We show that the resulting phonon side-jump conductivity generally approaches different limiting values in the high and low temperature limits, and hence can exhibit strong temperature dependence in the intermediate temperature regime. Our theory is amenable to ab initio treatment, which makes quantitative comparison between theoretical and experimental results possible.

cond-mat.mes-hall↗

arXiv2019

Magnetic field influenced electron-impurity scattering and magnetotransport

We formulate a quasiclassical theory ($ω_cτ\lesssim 1$ with $ω_c$ as the cyclotron frequency and $τ$ as the relaxation time) to study the influence of magnetic field on electron-impurity scattering process in the two-dimensional electron gas. We introduce a general recipe based on an abstraction of the detailed impurity scattering process to define the scattering parameter such as the incoming and outgoing momentum and coordinate jump. In this picture, we can conveniently describe the skew scattering and coordinate jump, which will eventually modify the Boltzmann equation. We find an anomalous Hall resistivity different from the conventional Boltzmann-Drude result and a negative magnetoresistivity parabolic in magnetic field. The origin of these results has been analyzed. The relevance between our theory and recent simulation and experimental works is also discussed. Our theory dominates in dilute impurity system where the correlation effect is negligible.

cond-mat.mes-hall↗

arXiv2019

$τ$-slice algebras of $n$-translation algebras and quasi $n$-Fano algebras

In this paper, we show that the $n$-APR tilts of dual $τ$-slice algebras of acyclic stable $n$-translation algebras are realized as $τ$-mutations. Such dual $τ$-slice algebras are quasi $(n-1)$-Fano when the $n$-translation algebra is Koszul, and a recursive construction of higher quasi Fano algebras for quasi $n$-Fano algebra obtained in this way is given. The $τ_n$-closure and $ν_n$-closure of such algebras are studied and we show that for an acyclic dual $n$-translation algebras with bound quiver $Q^{\perp}$, the Auslander-Reiten quivers of its $τ_n$-closures are truncation of the quiver $\mathbb{Z}|_n Q^{\perp}$, and the Auslander-Reiten quiver of its $ν_n$-closure is $\mathbb{Z}|_n Q^{\perp}$ when it is $n$-representation infinite.

math.RT↗

arXiv2019

Subdomain POD-TPWL with Local Parameterization for Large-Scale Reservoir History Matching Problems

We have recently proposed an efficient subdomain POD-TPWL (Xiao et al, 2018) for history matching without the need of model intrusion. From a computational point of view, a local parameterization where the parameters are separately defined in each subdomain is very attractive. In this study we present one such local parameterization through integrating principle component analysis with domain decomposition to independently represent the spatial parameter field within low-order parameter subspaces in each subdomain. This local parameterization allows us to perturb parameters in each subdomain simultaneously and the effects of all these perturbations can be computed with only a few full-order model simulations. To avoid non-smoothness at the boundaries of neighboring subdomains, the optimal local parameters are projected onto a global parameterization. The performance of subdomain POD-TPWL with local parameterization has been assessed through two cases tested on a modified version of SAIGUP model. This local parameterization has high scalability, since the number of training models depends primarily on the number of the local parameters in each subdomain and not on the dimension of the underlying full-order model. Activating more subdomains results in less local parameter patterns and enables us to run less model simulations. For a large-scale case-study in this work, to optimize 282 global parameters, LP-SD POD-TPWL only needs 90 fidelity simulations. In comparison to the finite-difference based history matching, and subdomain POD-TPWL with global parameterization where the parameters are defined over the entire domain, the CPU cost is reduced by a factor of several orders of the magnitude while reasonable accuracy remains.

physics.comp-ph↗

arXiv2018

Boltzmann transport from density matrix theory: interband and intraband coherences

To account for the anomalous/spin Hall conductivities and spin-orbit torque in the zeroth order of electron scattering time in strongly spin-orbit coupled systems, the Boltzmann transport theory in the case of weak disorder-potentials has been augmented by adding some interband coherence effects by hand. In this work these interband coherence terms are derived systematically from analyzing the equation of motion of the single-particle density matrix in the Bloch representation. Interband elements of the out-of-equilibrium density matrix are related to only one part of interband-coherence responses. Disorder-induced off-diagonal elements of the equilibrium density matrix are shown to be vital in producing the coordinate-shift anomalous driving term in the modified Boltzmann equation. Moreover, intraband coherence is inherent in the Boltzmann equation, whose contribution to anomalous/spin Hall conductivities is parametrically the same as the interband coherence.

cond-mat.mes-hall↗

arXiv2018

Conserved spin current for the Mott relation

The conserved bulk spin current [Shi et al., Phys. Rev. Lett. 96, 076604 (2006)], defined as the time derivative of the spin displacement operator, ensures automatically the Onsager relation between the spin Hall effect (SHE) and the inverse SHE. Here, we reveal another desirable property of this conserved spin current: the Mott relation linking the SHE and its thermal counterpart, the spin Nernst effect (SNE). According to the Mott relation, the SNE is known once the SHE is understood. In a two-dimensional Dirac-Rashba system with a smooth scalar disorder potential, we find a sign change of the spin Nernst conductivity when tuning the chemical potential.

cond-mat.mes-hall↗

arXiv2018

Non-intrusive Subdomain POD-TPWL Algorithm for Reservoir History Matching

This paper presents a non-intrusive subdomain POD-TPWL (SD POD-TPWL) algorithm for reservoir data assimilation through integrating domain decomposition (DD), radial basis function (RBF) interpolation and the trajectory piecewise linearization (TPWL). It is an efficient approach for model reduction and linearization of general non-linear time-dependent dynamical systems without intruding the legacy source code. In the subdomain POD-TPWL algorithm, firstly, a sequence of snapshots over the entire computational domain are saved and then partitioned into subdomains. From the local sequence of snapshots over each subdomain, a number of local basis vectors is formed using POD, and then the RBF interpolation is used to estimate the derivative matrices for each subdomain. Finally, those derivative matrices are substituted into a POD-TPWL algorithm to form a reduced-order linear model in each subdomain. This reduced-order linear model makes the implementation of the adjoint easy and resulting in an efficient adjoint-based parameter estimation procedure. The performance of the new adjoint-based parameter estimation algorithm has been assessed through several synthetic cases. Comparisons with the classic finite-difference based history matching show that our proposed subdomain POD-TPWL approach is obtaining comparable results. The number of full-order model simulations required is roughly 2-3 times the number of uncertain parameters. Using different background parameter realizations, our approach efficiently generates an ensemble of calibrated models without additional full-order model simulations.

math.NA↗

arXiv2018

Boltzmann scaling of spontaneous Hall current and nonequilibrium spin-polarization

We extend the semiclassical Boltzmann formalism for the anomalous Hall effect (AHE) in nondegenerate multiband electron systems to the spin Hall effect (SHE) and unconventional Edelstein effect (UEE, cannot be accounted for by the conventional Boltzmann equation, unlike the conventional Edelstein effect). This extension is confirmed by extending the Kohn-Luttinger density-matrix transport theory in the weak disorder-potential regime. By performing Kubo linear response calculations in a prototypical multiband model, the Boltzmann scaling for the AHE/SHE and UEE is found to be valid only if the disorder-broadening of bands is quite smaller than the minimal intrinsic energy-scale around the Fermi level. Discussions on this criterion in various multiband systems are also presented. A qualitative phase diagram is proposed to show the influences of changing independently the impurity density and strength of disorder potential on the AHE/SHE and UEE.

cond-mat.mes-hall↗

arXiv2018

Boltzmann approach to spin-orbit-induced transport in effective quantum theories

In model studies of the spin/anomalous Hall effect, effective Hamiltonians often serve as the starting point. However, a complete effective quantum theory contains not only the effective Hamiltonian but also the relation linking the physical observables to the canonical ones. We construct the semiclassical Boltzmann (SB) transport framework in the weak disorder-potential regime directly in the level of the effective quantum theory, and confirm this construction by formulating a generalized Kohn-Luttinger density matrix transport theory also in this level. The link and difference between the present SB theory and previous phenomenological Boltzmann, quantum kinetic and usual Kubo-Streda theories are clarified. We also present the slightly generalized Kubo-Streda formula in the level of the effective quantum theory. In this level, it is the generalized Kubo-Streda formula rather than the usual one that leads to the same physical interpretations as the present SB theory. In the application to a Rashba 2D effective model, a nonzero spin Hall effect important in the case of strong Rashba coupling but neglected in previous theories is found.

cond-mat.mes-hall↗

arXiv2017

Semiclassical Boltzmann theory of spin Hall effects in giant Rashba systems

For the spin Hall effect arising from strong band-structure spin-orbit coupling, a semiclassical Boltzmann theory reasonably addressing the intriguing disorder effect called side-jump is still absent. In this paper we describe such a theory of which the key ingredient is the spin-current-counterpart of the semiclassical side-jump velocity (introduced in the context of the anomalous Hall effect). Applying this theory to spin Hall effects in a two-dimensional electron gas with giant Rashba spin-orbit coupling, we find largely enhanced spin Hall angle in the presence of magnetic impurities when only the lower Rashba band is partially occupied.

cond-mat.mes-hall↗

arXiv2017

Semiclassical theory of spin-orbit torques in disordered multiband electron systems

We study spin-orbit torques (SOT) in non-degenerate multiband electron systems in the weak disorder limit. In order to have better physical transparency a semiclassical Boltzmann approach equivalent to the Kubo diagrammatic approach in the non-crossing approximation is formulated. This semiclassical framework accounts for the interband- coherence effects induced by both the electric field and static impurity scattering. Using the two-dimensional Rashba ferromagnet as a model system, we show that the antidamping-like SOT arising from disorder-induced interband-coherence effects is very sensitive to the spin structure of disorder and may have the same sign as the intrinsic SOT in the presence of spin-dependent disorder. While the cancellation of this SOT and the intrinsic one occurs only in the case of spin-independent short-range disorder.

cond-mat.mes-hall↗

arXiv2017

Rashba torque beyond the Boltzmann regime

We study spin torques induced by Rashba spin-orbit coupling in two-dimensional ferromagnets under the good-metal condition $ε_{F}τ/\hbar\gg1$ ($ε_{F}$ the Fermi energy, $τ$ the electron lifetime) by employing the Kubo formula. We find that, in the presence of spin-dependent disorder the Rashba torque changes greatly as the system evolves out of the weak disorder limit where $\hbar/τ$ is much smaller than any intrinsic energy scale characterizing the multiband structure. The antidamping-like component of Rashba torque can be comparable to and larger than the field-like one out of the weak disorder limit. The semiclassical Boltzmann theory produces the same results as microscopic linear response calculations only in the weak disorder limit. Our analysis indicates that rich behaviors of various nonequilibrium phenomena beyond the Boltzmann theory may also be present even when $ε_{F}τ/\hbar\gg1$ in multiband systems where $ε_{F}$ is not the unique intrinsic energy scale.

cond-mat.mes-hall↗

arXiv2016

The role of band-index-dependent transport relaxation times in anomalous Hall effect

We revisit model calculations of the anomalous Hall effect (AHE) and show that, in isotropic Rashba-coupled two-dimensional electron gas (2DEG) with pointlike potential impurities, the full solution of the semiclassical Boltzmann equation (SBE) may differ from the widely-used $1/τ^{||}$ & $1/τ^{\perp}$ solution [Phys. Rev. B 68, 165311 (2003)]. Our approach to AHE is analogous to the SBE-based analysis of the anisotropic magnetoresistance leading to an integral equation for the distribution function [Phys. Rev. B 79, 045427 (2009)] but in the present case, we reduce the description to band-index-dependent transport relaxation times. When both Rashba bands are partially occupied, these are determined by solving a system of linear equations. Detailed calculations show that, for intrinsic and hybrid skew scatterings the difference between $1/τ^{||}$ & $1/τ^{\perp}$ and the full solution of SBE is notable for large Fermi energies. For coordinate-shift effects, the side-jump velocity acquired in the inter-band elastic scattering process is shown to be more important for larger Rashba coupling and may even exceed the intra-band one for the outer Rashba band. The coordinate-shift contribution to AHE in the considered case notably differs from that in the limit of smooth disorder potential analyzed before.

cond-mat.str-el↗

arXiv2016

Thermoelectric response of spin polarization in Rashba spintronic systems

Motivated by recent discovery of strongly spin-orbit coupled two-dimensional (2D) electron gas near the surface of Rashba semiconductors BiTeX (X=Cl, Br, I), we calculate thermoelectric responses of spin polarization in 2D Rashba model using an exact solution of the linearized Boltzmann equation for elastic scattering. When the Fermi energy $E_{F}$ lies below the band crossing point we find a non-Edelstein electric-field induced spin polarization which is linear in $E_{F}$. We show that the Mott-like relation between spin polarizations induced by the temperature gradient and electric field breaks down significantly when $E_{F}$ lies in the vicinity of the band crossing point. As the temperature tends to zero, the temperature-gradient induced spin polarization vanishes. These results differ from previous ones obtained by relaxation time approximations.

cond-mat.mes-hall↗

arXiv2015

Unconventional thermoelectric behaviors and enhancement of figure of merit in Rashba spintronic systems

Thermoelectric transport in strongly spin-orbit coupled two-dimensional Rashba system is studied using the exact solution of the linearized Boltzmann equation. Some unusual transport behaviors are revealed. We show that the electrical conductivity takes a Drude form when the Fermi energy $E_{F}$ is above the band crossing point, but a non-Drude form which is a quadratic function of $E_{F}$ when $E_{F}$ lies below the band crossing point. The Mott relation breaks down when $E_{F}$ lies in the vicinity of the band crossing point. It is shown that the thermopower and thermoelectric figure of merit are strongly enhanced when $E_{F}$ downs below the band crossing point. This enhancement is attributed to not only the one-dimensional-like density of state but also the unconventional intraband elastic scattering below the band crossing point. The differences between these results and those obtained by the relaxation time approximation are discussed in detail.

cond-mat.mes-hall↗