arXiv subjects

Junqing Xu

Publications and source records attributed to Junqing Xu.

At least 19 recordsLinked to original sources

arXiv2026

Cross-Layer Anomalous Hall Transport driven by N\'eel-Vector rotating in the Altermagnet candidate V2Te2O

In van der Waals (vdW) materials, weak interlayer coupling generally suppresses vertical dispersion, reinforcing the conventional paradigm that in-plane transport dominates over cross-layer channels. Here, using first-principles calculations and magnetic symmetry analyses, we uncover a giant, symmetry-unlocked cross-layer anomalous Hall conductivity (AHC) in the vdW altermagnet V2Te2O. In the magnetic ground state with Neel vector N//z, horizontal mirror symmetry protects a spin-polarized nodal chain near the Fermi level and strictly enforces zero anomalous Hall response. Tilting the Neel vector explicitly breaks this mirror protection, allowing spin-orbit coupling to gap the nodal chain and activate a sharp cross-layer Hall response. When the Neel vector is rotated into the in-plane configuration (N//x), cross-layer orbital hybridization generates intensive Berry curvature hotspots, boosting the cross-layer component of AHC {\sigma}_{yz} to approximately 255 S/cm, which exceeds in-plane component {\sigma}_{xy} by nearly two orders of magnitude. Furthermore, varying the azimuthal angle systematically redistributes the anomalous Hall response, enabling full directional control of transverse transport. Our findings demonstrate a highly sensitive cross-layer anomalous Hall switch activated by low-barrier spin canting, offering promising avenues for directional tensor selection and low-power multi-axial vdW spintronics.

cond-mat.mtrl-sci↗

arXiv2026

Spin lifetime anisotropy in graphene induced by the SiO2 interface

Understanding how common dielectric substrates influence the spin transport properties of graphene is essential for advancing graphene-based spintronic technologies. Here we use a comprehensive set of numerical simulations to reveal how a SiO$_2$ substrate modifies the spin texture and governs spin relaxation in graphene. Using first-principles density matrix dynamics simulations, as well as tight-binding (TB) transport simulations, we quantify the effects of electron-phonon scattering, impurity scattering, and electrostatic disorder on the spin relaxation process. We find that a 2D SiO$_2$ substrate induces a predominantly Rashba-type helical spin texture in graphene, leading to a spin lifetime anisotropy of 1/2. Meanwhile, bulk SiO$_2$ breaks in-plane symmetry in graphene, leading to anisotropic in-plane and out-of-plane components in the spin texture, which we capture with a newly-developed TB model of graphene. Transport simulations under realistic disorder conditions reveal a spin lifetime anisotropy between 0.5 and 1, similar to what is seen in measurements of graphene spin valves on a SiO$_2$ substrate. Our results reveal a more complex picture of spin relaxation at the ubiquitous graphene/SiO$_2$ interface, beyond the standard Rashba model, providing critical insight for interpreting experiments and guiding substrate engineering for graphene spintronics.

cond-mat.mes-hall↗

arXiv2025

\textit{Ab initio} studies of influence of periodic-direction electric fields on spin lifetime and spin diffusion length and the validation of an \textit{ab initio} matrix-drift-diffusion model

Recently, we developed an \textit{ab initio} approach of spin lifetime (\tau_{s}) and spin diffusion length (l_{s}) in solids [Phys. Rev. Lett. 135, 046705 (2025)], based on a density-matrix master equation with quantum treatment of electron scattering processes. In this work, we extend the method to include the drift term due to an electric field along a periodic direction, implemented using a Wannier-representation-based covariant derivative. We employ this approach to investigate the electric-field effect on \tau_{s} and l_{s} of monolayer WSe_{2}, bulk GaAs, bulk GaN, and graphene-h-BN heterostructure. We find that an electric field reduces \tau_{s} of GaAs, due to the induced D'yakonov-Perel'-type spin relaxation. In GaN and graphene-h-BN, \tau_{s} is significantly affected, partly because the electric field generates an effective magnetic field corresponding to the k-derivative of Rashba spin-orbit (magnetic) field. Our results show that l_{s} can be significantly enhanced or suppressed by a moderate downstream or upstream field respectively. While the standard drift-diffusion model performs well for WSe_{2}, it can introduce large errors of the electric-field-induced changes of l_{s} in GaAs, GaN and graphene-h-BN. Our proposed \textit{ab initio} matrix-drift-diffusion model improves results for GaAs and GaN, but still fails for graphene-h-BN. Thus, to accurately capture the influence of electric fields on l_{s} in realistic materials, it is necessary to go beyond the drift-diffusion model and adopt a microscopic \textit{ab initio} methodology. Moreover, in graphene-h-BN, we find that the field-induced changes of \tau_{s} and l_{s} are not only governed by the drift term in the master equation, but are also significantly affected by the electric-field modification of the equilibrium density matrix away from Fermi-Dirac distribution function.

cond-mat.mtrl-sci↗

arXiv2025

Predicting and understanding diffusion lengths and lifetimes in solids via a many-body \textit{ab initio} method: The role of coupled dynamics

We present an \textit{ab initio} method of diffusion, relaxation and dephasing processes of arbitrary observables, and corresponding diffusion lengths and lifetimes in solids. The method is based on linearized density-matrix master equation, with quantum treatment of electron scattering processes. It enables clear \textit{ab initio} descriptions of long lifetimes and diffusion lengths using approximate formulas at different levels, such as Dyakonov-Perel and drift-diffusion relations for spin decay and those beyond with coupled dynamics. Our results of graphene-hBN show that the coupling between dynamical processes can significantly affect spin diffusion and relaxation. Our method provides a transparent and powerful tool for predicting and understanding diffusion and relaxation.

physics.comp-ph↗

arXiv2025

Phonon-Assisted Radiative Lifetimes and Exciton Dynamics from First Principles

Exciton-phonon interactions play a fundamental role in phonon-assisted radiative recombination and exciton dynamics in solids. In this work, we present a first-principles framework for computing phonon-assisted radiative lifetimes and exciton dynamics at finite temperatures. Starting from the solution of the Bethe-Salpeter equation, we construct an effective excitonic Hamiltonian that incorporates both exciton-photon and exciton-phonon interactions. Phonon-assisted radiative lifetimes in anisotropic media are evaluated using time-dependent second-order perturbation theory. We further analyze the temperature and phonon-mode dependence of phonon-assisted radiative lifetime and compare our results with available experimental data. We explain the nonmonotonic temperature dependence of the phonon-assisted radiative lifetime by different mechanisms at low and high-temperature regimes. Finally, we perform real-time exciton relaxation at the diagonal approximation of Lindbladian dynamics for time-resolved exciton occupation, providing insights into ultrafast thermalization and scattering pathways. Our ab-initio theory offers a detailed microscopic understanding of phonon-mediated exciton relaxation and recombination processes, and provides in-depth perspectives on phonon-assisted many-body interactions and their influence on optical properties for light-emitting and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic-field dependence of spin-phonon relaxation and dephasing due to g-factor fluctuations from first principles

The electron spin decay lifetime in materials can be characterized by relaxation (T1) and irreversible (T2) and reversible (T2*) decoherence processes. Their interplay leads to a complex dependence of spin relaxation times on the direction and magnitude of magnetic fields, relevant for spintronics and quantum information applications. Here, we use real-time first-principles density matrix dynamics simulations to directly simulate Hahn echo measurements, disentangle dephasing from decoherence, and predict T1, T2 and T2* spin lifetimes. We show that g-factor fluctuations lead to non-trivial magnetic field dependence of each of these lifetimes in inversion-symmetric crystals of CsPbBr3 and silicon, even when only intrinsic spin-phonon scattering is present. Most importantly, fluctuations in the off-diagonal components of the g-tensor lead to a strong magnetic field dependence of even the T1 lifetime in silicon. Our calculations elucidate the detailed role of anisotropic g-factors in determining the spin dynamics even in simple, low spin-orbit coupling materials such as silicon.

cond-mat.mtrl-sci↗

arXiv2024

Spin Dynamics in Hybrid Halide Perovskites -- Effect of Dynamical and Permanent Symmetry Breaking

The hybrid organic-inorganic halide perovskite (HOIP), for example MAPbBr3, exhibits extended spin lifetime and apparent spin lifetime anisotropy in experiments. The underlying mechanisms of these phenomena remain illusive. By utilizing our first-principles densitymatrix dynamics approach with quantum scatterings including electron-phonon and electronelectron interactions and self-consistent spinorbit coupling, we present temperature- and magnetic field-dependent spin lifetimes in hybrid perovskites, in agreement with experimental observations. For centrosymmetric hybrid perovskite MAPbBr3, the experimentally observed spin lifetime anisotropy is mainly attributed to the dynamical Rashba effect arising from the interaction between organic and inorganic components and the rotation of the organic cation. For noncentrosymmetric perovskite, such as MPSnBr3, we found persistent spin helix texture at the conduction band minimum, which significantly enhances the spin lifetime anisotropy. Our study provides theoretical insight to spin dynamics in HOIP and strategies for controlling and optimizing spin transport.

cond-mat.mtrl-sci↗

arXiv2024

Spin relaxation in graphite due to spin-orbital-phonon interaction from first-principles density-matrix approach

We predict "intrinsic" spin relaxation times ($T_{1}$) of graphite due to spin-orbit-phonon interaction, i.e., the combination of spin-orbit coupling and electron-phonon interaction, using our developed first-principles density-matrix approach. We obtain ultralong $T_{1}$, e.g., $\sim$600 ns at 300 K, which leads to ultralong in-plane spin diffusion length $\sim$110 $μ$m within the drift-diffusion model. Our prediction sets the upper bound of $T_{1}$ of graphite at each given temperature and Fermi level. The anisotropy ratios of $T_{1}$ or values of $T_{1z}/T_{1x}$ are found small and around 0.6. We examine the applicability of the well-known Elliot-Yafet (EY) relation, which declares that spin relaxation rate $T_{1α}^{-1}$ ($α=x,y,z$) is proportional to the product of the ensemble average of spin mixing parameter $\left\langle b_α^{2}\right\rangle $ and carrier relaxation rate $τ_{p}^{-1}$. Our numerical tests suggest that the EY relation works qualitatively if the degeneracy threshold $t^{\mathrm{deg}}$ for evaluating $b_α^{2}$ is elatively large (not much smaller than or comparable to $k_{B}T$), e.g., $10^{-3}$ eV or larger, but fails if $t^{\mathrm{deg}}$ is too tiny (much smaller than $k_{B}T$), e.g., $10^{-6}$ eV or smaller.

physics.comp-ph↗

arXiv2024

Spin Relaxation and Diffusion in Monolayer 1T'-WTe$_2$ from First-Principles

Understanding spin relaxation in topological systems such as quantum spin-hall (QSH) insulator is critical for realizing coherent transport at high temperature. WTe$_{2}$, known as a QSH insulator with a high transition temperature of 100K, is an important test-bed of unveiling spin relaxation mechanism in topological materials. In this work, we employ our recently-developed \emph{ab initio} density-matrix dynamics approach to investigate spin relaxation mechanism, and calculate spin lifetime and diffusion length of monolayer 1T'-WTe$_{2}$, at finite temperature under an external electric field. We found the spin lifetime of electrons have the largest anisotropy when measuring along the canted-spin-texture direction. Moreover, we found an opposite trend between spin and carrier relaxation against applied electric field. Most importantly, the relaxation mechanism under intermediate electric field around 1V/nm can not be explained by either Eillot-Yafet or Dyakonov-Perel models, which highlights the generality of our \emph{ab initio} density-matrix framework. We then proposed analytical models to explain its mechanism and compare well with \emph{ab initio} results at small and large electric field. We predict that spin lifetime and spin diffusion length of bulk-state electrons are $\sim$1 ps and $\sim$30 nm at room temperature respectively, suggesting its promise for spintronic applications.

cond-mat.mes-hall↗

arXiv2024

\textit{Ab initio} Wannier-representation-based calculations of photocurrent in semiconductors and metals

We present a general ab initio method based on Wannier functions using the covariant derivative for simulating the photocurrent in solids. The method is widely applicable to charge/spin DC and AC photocurrent at any perturbation levels in both semiconductors and metals for both linearly and circularly polarized light. This is because the method is theoretically complete (within the relaxation time approximation), that is to say, it includes all intraband, interband and their cross terms. It is also free from the degeneracy issue, i.e., applicable to arbitrary band structures with arbitrary numbers of degenerate bands. We apply the method to various semiconductors and metals, including GaAs, graphene-hBN heterostructure, monolayer WS2, a 2D ferroelectric material - monolayer GeS, bilayer anti-ferromagnetic MnBi2Te4 and topological Weyl semimetal RhSi, to simulate their charge and/or spin, DC and/or AC photocurrent. Our theoretical results are in agreement with previous theoretical works. Our numerical tests of GaAs, WS2 and GeS suggest setting the degeneracy threshold in the conventional method as \hbarΓ^{2}, with Γ^{2} the relaxation rate of the off-diagonal elements of the density matrix between two states with close energies. We find that compared with the conventional Wannier-function-based method using non-dgenerate perturbation theory, the numerical errors of optical susceptibilities of bilayer anti-ferromagnetic MnBi2Te4 with the PT symmetry can be reduced by 1-2 orders of magnitude by our method for circularly polarized light. Our method provides a universal computational tool for reliable and accurate predictions of abundant weak-field photocurrent phenomena in disparate materials.

physics.comp-ph↗

arXiv2023

Circular Dichroism of Crystals from First Principles

Chiral crystals show promise for spintronic technologies on account of their high spin selectivity, which has led to significant recent interest in quantitative characterization and first-principles prediction of their spin-optoelectronics properties. Here, we outline a computational framework for efficient ab-initio calculations of circular dichroism (CD) in crystalline materials. We leverage direct calculations of orbital angular momentum and quadrupole matrix element calculations in density-functional theory (DFT) and Wannier interpolation to calculate CD in complex materials, removing the need for band convergence and accelerating Brillouin-zone convergence compared to prior approaches. We find strong agreement with measured CD signals in molecules and crystals ranging in complexity from small bulk unit cells to 2D hybrid perovskites, and show the importance of the quadrupole contribution to the anisotropic CD in crystals. Spin-orbit coupling affects the CD of crystals with heavier atoms, as expected, but this is primarily due to changes in the electronic energies, rather than due to direct contributions from the spin matrix elements. We showcase the capability to predict CD for complex structures on a 2D hybrid perovskite, finding strong orientation dependence and identifying the eigen-directions of the unit cell with the strongest CD. We additionally decompose CD into separate contributions from inorganic, organic, and mixed organic-inorganic transitions, finding the chiral molecules to dominate the CD, with the inorganic lattice contributing at higher frequencies in specific directions. This unprecedented level of detail in CD predictions in crystals will facilitate experimental development of complex chiral crystals for spin selectivity.

cond-mat.mtrl-sci↗

arXiv2023

How Spin Relaxes and Dephases in Bulk Halide Perovskites

Spintronics in halide perovskites has drawn significant attention in recent years, due to highly tunable spin-orbit fields and intriguing interplay with lattice symmetry. Spin lifetime -- a key parameter that determines the applicability of materials for spintronics and spin-based quantum information applications -- has been extensively measured in halide perovskites, but not yet assessed from first-principles calculations. Here, we leverage our recently-developed \emph{ab initio} density-matrix dynamics framework to compute the spin relaxation time ($T_{1}$) and ensemble spin dephasing time ($T_{2}^{*}$) in a prototype halide perovskite, namely CsPbBr$_{3}$ with self-consistent spin-orbit coupling (SOC) and quantum descriptions of the electron scattering processes. We also implement the Landé $g$-factor for solids from first principles and take it into account in our dynamics, which is required to accurately capture spin dephasing at external magnetic fields. We thereby predict intrinsic spin lifetimes as an upper bound for experiments, identify the dominant spin relaxation pathways, and evaluate the dependence on temperature, external fields, carrier density,and impurities. Importantly, we find that the Fr{ö}hlich interaction that dominates carrier relaxation contributes negligibly to spin relaxation, consistent with the spin-conserving nature of this interaction. We investigated the effect of spin-orbit field with inversion asymmetry on spin lifetime, and we demonstrated from our calculation, persistent spin helix can enhance spin lifetime when the spin-split is large, but it can not be realized by Rashba SOC. Our theoretical approach may lead to new strategies to optimize spin and carrier transport properties in spintronics and quantum information applications.

cond-mat.mtrl-sci↗

arXiv2023

Ab-initio predictions of spin relaxation, dephasing and diffusion in solids

Spin relaxation, dephasing and diffusion are at the heart of spin-based information technology. Accurate theoretical approaches to simulate spin lifetimes ($τ_s$), determining how fast the spin polarization and phase information will be lost, are important to the understandings of underlying mechanism of these spin processes, and invaluable to search for promising candidates of spintronic materials. Recently, we develop a first-principles real-time density-matrix (FPDM) approach to simulate spin dynamics for general solid-state systems. Through the complete first-principles' descriptions of light-matter interaction and scattering processes including electron-phonon, electronimpurity and electron-electron scatterings with self-consistent spin-orbit coupling, as well as ab initio Land'e g-factor, our method can predict $τ_s$ at various conditions as a function of carrier density and temperature, under electric and magnetic fields. By employing this method, we successfully reproduce experimental results of disparate materials and identify the key factors affecting spin relaxation, dephasing, and diffusion in different materials. Specifically, we predict that germanene has long $τ_s$ (~100 ns at 50 K), a giant spin lifetime anisotropy and spin-valley locking effect under electric fields, making it advantageous for spin-valleytronic applications. Based on our theoretical derivations and ab initio simulations, we propose a new useful electronic quantity, named spin-flip angle $θ^{\uparrow\downarrow}$, for the understanding of spin relaxation through intervalley spin-flip scattering processes. Our method can be further applied to other emerging materials and extended to simulate exciton spin dynamics and steady-state photocurrents due to photogalvanic effect.

cond-mat.mtrl-sci↗

arXiv2022

Substrate Effects on Spin Relaxation in Two-Dimensional Dirac Materials with Strong Spin-Orbit Coupling

Understanding substrate effects on spin dynamics and relaxation in two-dimensional (2D) materials is of key importance for spintronics and quantum information applications. However, the key factors that determine the substrate effect on spin relaxation, in particular for materials with strong spin-orbit coupling, have not been well understood. Here we performed first-principles real-time density-matrix dynamics simulations with spin-orbit coupling (SOC) and quantum descriptions of electron-phonon and electron-impurity scattering for the spin lifetimes of supported/free-standing germanene, a prototypical strong SOC 2D Dirac material. We show that the effects of different substrates on spin lifetime can surprisingly differ by two orders of magnitude. We find that substrate effects on $τ_s$ are closely related to substrate-induced modifications of the SOC-field anisotropy, which changes the spin-flip scattering matrix elements. We propose a new electronic quantity, named spin-flip angle $θ^{\uparrow\downarrow}$, to characterize spin relaxation caused by intervalley spin-flip scattering. We find that the spin relaxation rate is approximately proportional to the averaged value of $\mathrm{sin}^{2}\left(θ^{\uparrow\downarrow}/2\right)$, which can be used as a guiding parameter of controlling spin relaxation.

cond-mat.mes-hall↗

arXiv2022

Electric fields and substrates dramatically accelerate spin relaxation in graphene

Electrons in graphene are theoretically expected to retain spin states much longer than most materials, making graphene a promising platform for spintronics and quantum information technologies. Here, we use first-principles density-matrix (FPDM) dynamics simulations to show that interaction with electric fields and substrates strongly enhance spin relaxation through scattering with phonons. Consequently, the relaxation time at room temperature reduces from microseconds in free-standing graphene to nanoseconds in graphene on hexagonal boron nitride (hBN) substrate, the order of magnitude typically measured in experiments. Further, inversion symmetry breaking by hBN introduces a stronger asymmetry in electron and hole spin lifetimes, than predicted by the conventional D'yakonov-Perel' (DP) model for spin relaxation. Deviations from the conventional DP model are stronger for in-plane spin relaxation, resulting in out-of-plane to in-plane lifetime ratios much greater than 1/2 with a maximum close to the Dirac point. These FPDM results, independent of symmetry-specific assumptions or material-dependent parameters, also validate recent modifications of the DP model to explain such deviations. Overall, our results indicate that spin-phonon relaxation in the presence of substrates may be more important in graphene than typically assumed, requiring consideration for graphene-based spin technologies at room temperature.

cond-mat.mtrl-sci↗

arXiv2021

Giant Spin Lifetime Anisotropy and Spin-Valley Locking in Silicene and Germanene from First-Principles Density-Matrix Dynamics

Through First-Principles real-time Density-Matrix (FPDM) dynamics simulations, we investigate spin relaxation due to electron-phonon and electron-impurity scatterings with spin-orbit coupling in two-dimensional Dirac materials - silicene and germanene, at finite temperatures and under external fields. We discussed the applicability of conventional descriptions of spin relaxation mechanisms by Elliott-Yafet (EY) and D'yakonov-Perel' (DP) compared to our FPDM method, which is determined by a complex interplay of intrinsic spin-orbit coupling, external fields, and electron-phonon coupling strength, beyond crystal symmetry. For example, the electric field dependence of spin relaxation time is close to DP mechanism for silicene at room temperature, but rather similar to EY mechanism for germanene. Due to its stronger spin-orbit coupling strength and buckled structure in sharp contrast to graphene, germanene has a giant spin lifetime anisotropy and spin valley locking effect under nonzero Ez and relatively low temperature. More importantly, germanene has extremely long spin lifetime (~100 ns at 50 K) and ultrahigh carrier mobility, which makes it advantageous for spin-valleytronic applications.

cond-mat.mtrl-sci↗

arXiv2021

Ab initio Ultrafast Spin Dynamics in Solids

Spin relaxation and decoherence is at the heart of spintronics and spin-based quantum information science. Currently, theoretical approaches that can accurately predict spin relaxation of general solids including necessary scattering pathways and capable for ns to ms simulation time are urgently needed. We present a first-principles real-time density-matrix approach based on Lindblad dynamics to simulate ultrafast spin dynamics for general solid-state systems. Through the complete first-principles descriptions of pump, probe and scattering processes including electron-phonon, electron-impurity and electron-electron scatterings with self-consistent electronic spin-orbit couplings, our method can directly simulate the ultrafast pump-probe measurements for coupled spin and electron dynamics over ns at any temperatures and doping levels. We first apply this method to a prototypical system GaAs and obtain excellent agreement with experiments. We find that the relative contributions of different scattering mechanisms and phonon modes differ considerably between spin and carrier relaxation processes. In sharp contrast to previous work based on model Hamiltonians, we point out that the electron-electron scattering is negligible at room temperature but becomes dominant at low temperatures for spin relaxation in n-type GaAs. We further examine ultrafast dynamics in novel spin-valleytronic materials - monolayer and bilayer WSe2 with realistic defects. We find that spin relaxation is highly sensitive to local symmetry and chemical bonds around defects. Our work provides a predictive computational platform for spin dynamics in solids, which has unprecedented potentials for designing new materials ideal for spintronics and quantum information technology.

cond-mat.mtrl-sci↗

arXiv2021

Intersystem Crossing and Exciton-Defect Coupling of Spin Defects in Hexagonal Boron Nitride

Despite the recognition of two-dimensional (2D) systems as emerging and scalable host materials of single photon emitters or spin qubits, uncontrolled and undetermined chemical nature of these quantum defects has been a roadblock to further development. Leveraging the design of extrinsic defects can circumvent these persistent issues and provide an ultimate solution. Here we established a complete theoretical framework to accurately and systematically design quantum defects in wide-bandgap 2D systems. With this approach, essential static and dynamical properties are equally considered for spin qubit discovery. In particular, many-body interactions such as defect-exciton couplings are vital for describing excited state properties of defects in ultrathin 2D systems. Meanwhile, nonradiative processes such as phonon-assisted decay and intersystem crossing rates require careful evaluation, which compete together with radiative processes. From a thorough screening of defects based on first-principles calculations, we identify promising single photon emitters such as SiVV and spin qubits such as TiVV and MoVV in hexagonal boron nitride. This work provided a complete first-principles theoretical framework for defect design in 2D materials.

cond-mat.mtrl-sci↗