arXiv subjects

Tsung-Wei Chen

Publications and source records attributed to Tsung-Wei Chen.

18 recordsLinked to original sources

arXiv2025

Shift spin photocurrents in two-dimensional systems

The generation of nonlinear spin photocurrents by circularly polarized light in two-dimensional systems is theoretically investigated by calculating the shift spin conductivities. In time-reversal symmetric systems, shift spin photocurrent can be generated under the irradiation of circularly polarized light , while the shift charge photoccurrent is forbidden by symmetry. We show that the $k$-cubic Rashba-Dresselhaus system, the $k$-cubic wurtzite system and Dirac surface states can support the shift spin photocurrent. By symmetry analysis, it is found that in the Rashba type spin-orbit coupled systems, mirror symmetry requires that the spin polarization and the moving direction of the spin photocurrent be parallel, which we name longitudinal shift spin photocurrent. The Dirac surface states with warping term exhibit mirror symmetry, similar to the Rashba type system, and support longitudinal shift spin photocurrent. In contrast, in the Dresselhaus type spin-orbit coupled systems, the parity-mirror symmetry requires that the spin polarization and the moving direction of the spin photocurrent be perpendicular, which we dub transverse shift spin photocurrent. Furthermore, we find that the shift spin photocurrent always vanishes in any $k$-linear spin-orbit coupled system unless the Zeeman coupling is turned on. We find that the splitting of degenerate energy bands due to Zeeman coupling $μ_z$ causes the van Hove singularity. The resulting shift spin conductivity has a significant peak at optical frequency $ω=2μ_z/\hbar$.

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arXiv2022

Two-path interference of single-particle pulses measured by the Unruh-DeWitt-type quantum detector

We study the two-path interference of single-particle pulses measured by the Unruh-DeWitt-type quantum detector, which itself is a quantum state as well as the incoming pulse, and of which the interaction with the pulse is described by unitary quantum evolution instead of a nonunitary collapsing process. Provided that the quantum detector remains coherent in time long enough, the detection probability still manifests the two-path interference pattern even if the length difference between the two paths considerably exceeds the coherence length of the single-particle pulse, contrary to the result measured by an ordinary classical detector. Furthermore, it is formally shown that an ensemble of identical Unruh-DeWitt-type quantum detectors collectively behaves as an ordinary classical detector, if coherence in time of each individual quantum detector becomes sufficiently short. Our study provides a concrete yet manageable theoretical model to investigate the two-path interference measured by a quantum detector and facilitates a quantitative analysis of the difference between classical and quantum detectors. The analysis affirms the main idea of decoherence theory: quantum behavior is lost as a result of quantum decoherence.

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arXiv2020

The semiclassical theory for spin dynamics in a disordered system

We investigate the Drude model of spin dynamics in two-dimensional spin-orbit coupled systems. In the absence of an applied electric field, the spin aligns with the k-dependent effective magnetic field. The influence of disorder (the momentum relaxation time $τ$) on the system is considered. In the presence of an electric field, the change in momentum causes a change in the effective magnetic field. The in-plane spin can precess around the successive change in the orientation of the effective magnetic field. We find that up to the linear order of the electric field, the spin orientation undergoes Larmor-like and non-Larmor like precession. Furthermore, we find that the non-Larmor motion over a very short time ($t\llτ$) exactly equals the result obtained from the Kubo formula. This means that the Kubo formula only captures the system's response over a very short evolution. The spin-Hall conductivity for Larmor and non-Larmor precession in the Rashba system is analytically calculated. We find that the intrinsic spin-Hall current is not a universal constant and correctly drops to zero when the Rashba spin-orbit coupling drops to zero. We also calculate the time-averaged Larmor and non-Larmor spin-Hall conductivities (SHCs) for the k-cubic Rashba system and compare them to experimental values. The time-averaged Larmor SHC vanishes and the non-Larmor SHC is given by $2.1(q/8π)$ which is very close to the experimental value $2.2(q/8π)$ by Wunderlich et al. [Phys. Rev. Lett. {\bf 94}, 047204 (2005)].

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arXiv2020

Topological Anderson insulating phases in the long-range Su-Schrieffer-Heeger model

In the long-range Su-Schriffer-Heeger (SSH) model, in which the next nearest-neighbor hopping is considered, there exhibits a rich topological phase diagram that contains winding numbers $w=0, 1$, and $2$. In the presence of disorder, the change in mean winding number with various disorder strengths is numerically calculated. We find that the disorder drives phase transitions: $w=0 \rightarrow 1, 0 \rightarrow 2 $, $1 \rightarrow 2$ and $2 \rightarrow 1$, in which the non-zero winding numbers driven by disorder are called the topological Anderson insulating (TAI) phases. The transition mechanisms in this long-range SSH model are investigated by means of localization length and self-energy. We find that the localization length has a drastic change at the transition point, and the corresponding critical disorder strength is obtained by further using Born approximation to the self-energy. The origin for the $w = 0$ to $w = 1, 2$ transitions is attributed to the energy gap renormalization (first Born approximation), while for the transitions from $w=1$ to $w=2$ and vice versa are the strong scattering (self-consistent Born approximation). The result of self-consistent Born approximation suggests that in the thermodynamic limit, the gap closing point is not at the Fermi level for the system with localized edge states. This agrees with the topological protection of edge states, such that the pre-existent edge states are protected from being scattered into bulk. Given the experimental feasibility of the long-range SSH model, the predicted TAI phases and transitions could be observed in experiments.

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arXiv2019

Dynamical precession of spin in the two-dimensional spin-orbit coupled systems

We investigate the spin dynamics in the two-dimensional spin-orbit coupled system subject to an in-plane ($x$-$y$ plane) constant electric field, which is assumed to be turned on at the moment $t=0$. The equation of spin precession in linear response to the switch-on of the electric field is derived in terms of Heisenberg's equation by the perturbation method up to the first order of the electric field. The dissipative effect, which is responsible for bringing the dynamical response to an asymptotic result, is phenomenologically implemented à la the Landau-Lifshitz-Gilbert equation by introducing damping terms upon the equation of spin dynamics. Mediated by the dissipative effect, the resulting spin dynamics asymptotes to a stationary state, where the spin and the momentum-dependent effective magnetic field are aligned again and have nonzero components in the out-of-plane ($z$) direction. In the linear response regime, the asymptotic response obtained by the dynamical treatment is in full agreement with the stationary response as calculated in the Kubo formula, which is a time-independent approach treating the applied electric field as completely time-independent. Our method provides a new perspective on the connection between the dynamical and stationary responses.

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arXiv2017

Exact correspondence between classical and Dirac-Pauli spinors in the weak-field limit of static and homogeneous electromagnetic fields

It has long been speculated that the Dirac or, more generally, the Dirac-Pauli spinor in the Foldy-Wouthuysen (FW) representation should behave like a classical relativistic spinor in the low-energy limit when the probability of particle-antiparticle pair creation and annihilation is negligible. In the weak-field limit of static and homogeneous electromagnetic fields, by applying the method of direct perturbation theory inductively on the orders of $1/c$ in the power series, we rigorously prove that it is indeed the case: the FW transformation of the Dirac-Pauli Hamiltonian is in full agreement with the classical counterpart, which is the sum of the orbital Hamiltonian for the Lorentz force equation and the spin Hamiltonian for the Thomas-Bargmann-Michel-Telegdi equation.

quant-ph↗

arXiv2017

Topological phase transitions in an inverted InAs/GaSb quantum well driven by tilted magnetic fields

The helical edge states in a quantum spin Hall insulator are presumably protected by time- reversal symmetry. However, even in the presence of magnetic field which breaks time-reversal symmetry, the helical edge conduction can still exist, dubbed as pseudo quantum spin Hall effect. In this work, the effects of the magnetic fields on the pseudo quantum spin Hall effect and the phase transitions are studied. We show that an in-plane magnetic field drives a pseudo quantum spin Hall state to metallic state at a high field. Moreover, at a fixed in-plane magnetic field, an increasing out-of-plane magnetic field leads to a reentrance of pseudo quantum spin Hall state in an inverted InAs/GaSb quantum well. The edge state probability distribution and Chern numbers are calculated to verify that the reentrant states are topologically nontrivial. The origin of the reentrant behavior is attributed to the nonmonotonic bending of Landau levels and the Landau level mixing caused by the orbital effect induced by the in-plane magnetic field. The robustness to disorder is demonstrated by the numerically calculated quantized conductance for disordered nanowires within Landauer-Büttiker formalism.

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arXiv2016

Exact Foldy-Wouthuysen transformation of the Dirac-Pauli Hamiltonian in the weak-field limit by the method of direct perturbation theory

We apply the method of direct perturbation theory for the Foldy-Wouthuysen (FW) transformation upon the Dirac-Pauli Hamiltonian subject to external electromagnetic fields. The exact FW transformations exist and agree with those obtained by Eriksen's method for two special cases. In the weak-field limit of static and homogeneous electromagnetic fields, by mathematical induction on the orders of $1/c$ in the power series, we rigorously prove the long-held speculation: the FW transformed Dirac-Pauli Hamiltonian is in full agreement with the classical counterpart, which is the sum of the orbital Hamiltonian for the Lorentz force equation and the spin Hamiltonian for the Thomas-Bargmann-Michel-Telegdi equation.

quant-ph↗

arXiv2016

Out-of-plane spin polarization of edge currents in Chern insulator with Rashba spin-orbit interaction

We investigate the change in the non-zero Chern number and out-of-plane spin polarization of the edge currents in a honeycomb lattice with the Haldane-Rashba interaction. This interaction breaks the time-reversal symmetry due to the Haldane phase caused by a current loop at site-I and -II atoms and also accounts for the Rashba-type spin-orbit interaction. The Rashba spin-orbit interaction increases the number of Dirac points and the band-touching phenomenon can be generated by tuning the on-site potential in the non-zero Haldane phase. By using the Pontryagin winding number and numerical Berry curvature methods, we find that the Chern number pattern is $\{+2, -1, 0\}$ and $\{-2, +1, 0\}$ for the positive and negative Haldane phase, respectively. A non-zero Chern number is called a Chern-insulating phase. We discovered that changes in both the Haldane phase and on-site potential leads to a change in the orientation of the bulk spin polarization of site-I and site-II atoms. Interestingly, in a ribbon with a zigzag edge, which naturally has site-I atoms at one outer edge and site-II atoms at the opposite outer edge, the spin polarization of the edge states approximately obeys the properties of bulk spin polarization regardless of the change in the Chern number. In addition, even when the Chern number changes from $+2$ to $-1$ (or $-2$ to $+1$), by tuning the strength of the on-site potential, the sign of the spin polarization of the edge states persists. This approximate bulk-edge correspondence of the spin polarization in the Haldane-Rashba system would play an important role in spintronics, because it enables us to control the orientation of the spin polarization in a single Chern-insulating phase.

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arXiv2014

Topological-Berry-phase-induced spin torque current in a two-dimensional system with generic $k$-linear spin-orbit interaction

The Berry phase on the Fermi surface and its influence on the conserved spin current in a two-dimensional system with generic $k$-linear spin-orbit interaction are investigated. We calculate the response of the effective conserved spin current to the applied electric field, which is composed of conventional and spin torque currents, by using the Kubo formula. We find that the conventional spin current is not determined by the Berry phase effect. Remarkably, the spin torque Hall current is found to be proportional to the Berry phase, and the longitudinal spin torque current vanishes because of the Berry phase effect. When the $k$-linear spin-orbit interaction dominates the system, the Berry phase on the Fermi surface maintains two invariant properties. One is that the magnitude of the spin torque current protected by the Berry phase is unchanged by a small fluctuation of energy dispersion. The other one is that the change in the direction of the applied electric field does not change the magnitude of the spin torque current even if the energy dispersion is not spherically symmetric; i.e., the Berry phase effect has no dependence on the two-dimensional material orientation. The spin torque current is a universal value for all $k$-linear systems, such as Rashba, Dresselhaus, and Rashba-Dresselhaus systems. The topological number attributed to the Berry phase on the Fermi surface represents the phase of the orbital chirality of spin in the $k$-linear system. The change in the topological number results in a phase transition in which the orbital chirality of spin $s_z$ and $-s_z$ is exchanged. We found that the spin torque current can be experimentally measured.

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arXiv2014

High-order Foldy-Wouthuysen transformations of the Dirac and Dirac-Pauli Hamiltonians in the weak-field limit

The low-energy and weak-field limit of Dirac equation can be obtained by an order-by-order block diagonalization approach to any desired order in the parameter $π/mc$ ($π$ is the kinetic momentum and $m$ is the mass of the particle). In the previous work, it has been shown that, up to the order of $(π/mc)^8$, the Dirac-Pauli Hamiltonian in the Foldy-Wouthuysen (FW) representation may be expressed as a closed form and consistent with the classical Hamiltonian, which is the sum of the classical relativistic Hamiltonian for orbital motion and the Thomas-Bargmann-Michel-Telegdi (T-BMT) Hamiltonian for spin precession. In order to investigate the exact validity of the correspondence between classical and Dirac-Pauli spinors, it is necessary to proceed to higher orders. In this paper, we investigate the FW representation of the Dirac and Dirac-Pauli Hamiltonians by using Kutzelnigg's diagonalization method. We show that the Kutzelnigg diagonalization method can be further simplified if nonlinear effects of static and homogeneous electromagnetic fields are neglected (in the weak-field limit). Up to the order of $(π/mc)^{14}$, we find that the FW transformation for both Dirac and Dirac-Pauli Hamiltonians is in agreement with the classical Hamiltonian with the gyromagnetic ratio given by $g=2$ and $g\neq2$ respectively. Furthermore, with higher-order terms at hand, it is demonstrated that the unitary FW transformation admits a closed form in the low-energy and weak-field limit.

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arXiv2014

Correspondence between classical and Dirac-Pauli spinors in view of the Foldy-Wouthuysen transformation

The classical dynamics for a charged spin particle is governed by the Lorentz force equation for orbital motion and by the Thomas-Bargmann-Michel-Telegdi (T-BMT) equation for spin precession. In static and homogeneous electromagnetic fields, it has been shown that the Foldy-Wouthuysen (FW) transform of the Dirac-Pauli Hamiltonian, which describes the relativistic quantum theory for a spin-1/2 particle, is consistent with the classical Hamiltonian (with both the orbital and spin parts) up to the order of $1/m^{14}$ ($m$ is the particle's mass) in the low-energy/weak-field limit. In this paper, we extend this correspondence to the case of inhomogeneous fields. Regardless of the field gradient (e.g., Stern-Gerlach) force, the T-BMT equation is unaltered and thus the classical Hamiltonian remains the same, but subtleties arise and need to be clarified. For the relativistic quantum theory, we apply Eriksen's method to obtain the exact FW transformations for the two special cases, which in conjunction strongly suggest that, in the weak-field limit, the FW transformed Dirac-Pauli Hamiltonian (except for the Darwin term) is in agreement with the classical Hamiltonian in a manner that classical variables correspond to quantum operators via a specific Weyl ordering. Meanwhile, the Darwin term is shown to have no classical correspondence.

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arXiv2013

Maximum intrinsic spin-Hall conductivity in two-dimensional systems with k-linear spin-orbit interaction

We analytically calculate the intrinsic spin-Hall conductivity (ISHC) ($σ^z_{xy}$ and $σ^z_{yx}$) in a clean, two-dimensional system with generic k-linear spin-orbit interaction. The coefficients of the product of the momentum and spin components form a spin-orbit matrix $\widetildeβ$. We find that the determinant of the spin-orbit matrix $\detbeta$ describes the effective coupling of the spin $s_z$ and orbital motion $L_z$. The decoupling of spin and orbital motion results in a sign change of the ISHC and the band-overlapping phenomenon. Furthermore, we show that the ISHC is in general unsymmetrical ($σ^z_{xy}\neq-σ^z_{yx}$), and it is governed by the asymmetric response function $\Deltabeta$, which is the difference in band-splitting along two directions: those of the applied electric field and the spin-Hall current. The obtained non-vanishing asymmetric response function also implies that the ISHC can be larger than $e/8π$, but has an upper bound value of $e/4π$. We will that the unsymmetrical properties of the ISHC can also be deduced from the manifestation of the Berry curvature at the nearly degenerate area. On the other hand, by investigating the equilibrium spin current, we find that $\detbeta$ determines the field strength of the SU(2) non-Abelian gauge field.

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arXiv2011

High Chern number quantum anomalous Hall phases in graphene ribbons with Haldane orbital coupling

We investigate possible phase transitions among the different quantum anomalous Hall (QAH) phases in a zigzag graphene ribbon under the influence of the exchange field. The effective tight-binding Hamiltonian for graphene is made up of the hopping term, the Kane-Mele and Rashba spin-orbit couplings as well as the Haldane orbital term. We find that the variation of the exchange field results in bulk gap-closing phenomena and phase transitions occur in the graphene system. If the Haldane orbital coupling is absent, the phase transition between the chiral (anti-chiral) edge state $ν=+2$ ($ν=-2$) and the pseudo-quantum spin Hall state ($ν=0$) takes place. Surprisingly, when the Haldane orbital coupling is taken into account, an intermediate QSH phase with two additional edge modes appears in between phases $ν=+2$ and $ν=-2$. This intermediate phase is therefore either the hyper-chiral edge state of high Chern number $ν=+4$ or anti-hyper-chiral edge state of $ν=-4$ when the direction of exchange field is reversed. We present the band structures, edge state wave functions and current distributions of the different QAH phases in the system. We also report the critical exchange field values for the QAH phase transitions.

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arXiv2010

Foldy-Wouthuysen transformation for a Dirac-Pauli dyon and the Thomas-Bargmann-Michel-Telegdi equation

The classical dynamics for a charged point particle with intrinsic spin is governed by a relativistic Hamiltonian for the orbital motion and by the Thomas-Bargmann-Michel-Telegdi equation for the precession of the spin. It is natural to ask whether the classical Hamiltonian (with both the orbital and spin parts) is consistent with that in the relativistic quantum theory for a spin-1/2 charged particle, which is described by the Dirac equation. In the low-energy limit, up to terms of the 7th order in $1/E_g$ ($E_g=2mc^2$ and $m$ is the particle mass), we investigate the Foldy-Wouthuysen (FW) transformation of the Dirac Hamiltonian in the presence of homogeneous and static electromagnetic fields and show that it is indeed in agreement with the classical Hamiltonian with the gyromagnetic ratio being equal to 2. Through electromagnetic duality, this result can be generalized for a spin-1/2 dyon, which has both electric and magnetic charges and thus possesses both intrinsic electric and magnetic dipole moments. Furthermore, the relativistic quantum theory for a spin-1/2 dyon with arbitrary values of the gyromagnetic and gyroelectric ratios can be described by the Dirac-Pauli equation, which is the Dirac equation with augmentation for the anomalous electric and anomalous magnetic dipole moments. The FW transformation of the Dirac-Pauli Hamiltonian is shown, up to the 7th order again, to be also in accord with the classical Hamiltonian.

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arXiv2009

Transverse force generated by an electric field and transverse charge imbalance in spin-orbit coupled systems

We use linear response theory to study the transverse force generated by an external electric field and hence possible charge Hall effect in spin-orbit coupled systems. In addition to the Lorentz force that is parallel to the electric field, we find that the transverse force perpendicular to the applied electric field may not vanish in a system with an anisotropic energy dispersion. Surprisingly, in contrast to the previous results, the transverse force generated by the electric field does not depend on the spin current, but in general, it is related to the second derivative of energy dispersion only. Furthermore, we find that the transverse force does not vanish in the Rashba-Dresselhaus system. Therefore, the non-vanishing transverse force acts as a driving force and results in charge imbalance at the edges of the sample. The estimated ratio of the Hall voltage to the longitudinal voltage is $\sim 10^{-3}$. The disorder effect is also considered in the study of the Rashba-Dresselhaus system. We find that the transverse force vanishes in the presence of impurities in this system because the vertex correction and the anomalous velocity of the electron accidently cancel each other.

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arXiv2009

Torque and conventional spin-Hall currents in two-dimensional spin-orbit coupled systems: Universal relation and hyper-selection rule

We investigate torque and also conventionally defined spin-Hall currents in two-dimensional (2D) spin-orbit coupled systems of spin-1/2 particles within the linear response Kubo formalism. We obtain some interesting relations between the conventional and torque spin-Hall conductivities for the generic effective Hamiltonian $H_0=ε_k^0+A(k)σ_x-B(k)σ_y$, where $A(k)=η^A_ik_i+η^A_{ij}k_ik_j+η^A_{ijl}k_ik_jk_l+...$, $B(k)=η^B_ik_i+η^B_{ij}k_ik_j+η^B_{ijl}k_ik_jk_l+...$, and $η$'s are the specific system-dependent coefficients. Specifically, we find that in the intrinsic case the magnitude of torque spin-Hall conductivity $σ^{τ_z}_{xy}(0)$ is always twice larger than the conventional spin-Hall conductivity $σ^{s_z}_{xy}(0)$, and the two conductivities have the opposite signs, i.e., $σ^{τ_z}_{xy}(0)=-2σ^{s_z}_{xy}(0)$. This universal relation also holds in the presence of an uniform in-plane magnetic field. We also find that if the energy dispersion is rotationally invariant, there exists a hyper-angular momentum $I_z = (k\times \partialθ/\partial k)_z s_z + L_z$ which is conserved. Furthermore, the hyper-angular momentum current $<{1/2}\{I_z,v_x\}>$ vanishes, and this leads to a hyper selection rule for the conventional spin-Hall current.

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arXiv2006

Conserved Spin and Orbital Angular Momentum Hall Current in a Two-Dimensional Electron System with Rashba and Dresselhaus Spin-orbit Coupling

We study theoretically the spin and orbital angular momentum (OAM) Hall effect in a high mobility two-dimensional electron system with Rashba and Dresselhuas spin-orbit coupling by introducing both the spin and OAM torque corrections, respectively, to the spin and OAM currents. We find that when both bands are occupied, the spin Hall conductivity is still a constant (i.e., independent of the carrier density) which, however, has an opposite sign to the previous value. The spin Hall conductivity in general would not be cancelled by the OAM Hall conductivity. The OAM Hall conductivity is also independent of the carrier density but depends on the strength ratio of the Rashba to Dresselhaus spin-orbit coupling, suggesting that one can manipulate the total Hall current through tuning the Rashba coupling by a gate voltage. We note that in a pure Rashba system, though the spin Hall conductivity is exactly cancelled by the OAM Hall conductivity due to the angular momentum conservation, the spin Hall effect could still manifest itself as nonzero magnetization Hall current and finite magnetization at the sample edges because the magnetic dipole moment associated with the spin of an electron is twice as large as that of the OAM. We also evaluate the electric field-induced OAM and discuss the origin of the OAM Hall current. Finally, we find that the spin and OAM Hall conductivities are closely related to the Berry vector (or gauge) potential.

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