Persistent Spin Currents Research Articles

Persistent current in a mesoscopic Holstein-Hubbard ring with Dresselhaus interaction

The effect of electron-phonon coupling, onsite repulsive Coulomb interaction and temperature on the persistent current in a quantum ring is studied in the presence of Dresselhaus spin-orbit interaction. The quantum ring threaded by the Aharonov-Bohm flux is modelled by the one-dimensional Holstein-Hubbard-Dresselhaus Hamiltonian. The electron-phonon interaction and Dresselhaus spin-orbit interaction are decoupled by employing the Lang-Firsov coherent transformation and a unitary transformation respectively. Thereafter, a self-consistent diagonalization technique is performed numerically at the Hartree-Fock level to obtain the effective electronic energy and current. It is shown that the intrinsic Dresselhaus spin-orbit interaction enhances the persistent charge and spin currents significantly. On the other hand, the persistent current is reduced by the onsite and nearest-neighbour electron-phonon interaction and Coulomb interaction. Also, the behaviour of the currents is modified by temperature. The spin-splitting of persistent spin current is enhanced considerably by Dresselhaus spin-orbit interaction and this splitting is tuneable in different regimes of magnetic flux, temperature, chemical potential and the interactions present in the system.

Spin current and internal Zeeman field in spin-orbit coupled rings

We investigate the one-dimensional quantum ring constructed by the spin-orbit coupled material, in which the quantum spin-Hall Bernevig-Zhang (BZ) Hamiltonian and Rashba-Dirac (RD) type spin-orbit coupling are taken into account (called RD-BZ Hamiltonian in this paper). It is known that the curvature of the ring generates an out-of-plane effective magnetic field, acting as an internal Zeeman field. We find that only the BZ coupling can change the strength of the internal Zeeman field, which enables us to detect the effect of the internal Zeeman field. Furthermore, we find that the total angular momentum is conserved in the RD-BZ Hamiltonian, and the energy eigenvalue and wave function must be modified to fit the conserved quantity, which are ignored in the previous studies. The conductance without leads is discussed. Different from the previous results, we find that the conductance behaves like a beat phenomenon resulting from the interplay between the magnetic flux and Aharonov-Casher (AC) phase, and thus, it can oscillate without passing through the insulating state in some regimes of magnetic flux or AC phase. Importantly, we find that the conductance with integer and half-integer magnetic flux provides us a method to measure the AC phase. The cancellation of the internal Zeeman field due to the BZ coupling can be detected by using specific fractional magnetic flux. In the ring with nonvanishing RD and BZ couplings, the conductance can exhibit a quasiplateau near the small RD coupling. The increase in the strength of BZ coupling would result in wider quasiplateau in conductance, which implies that the ring could remain insulating state (or conducting state) regardless of the small change in RD coupling. The thermal average of spin and charge currents are calculated at low temperatures without impurities. Importantly, we find that the persistent spin and charge currents as a function of the magnetic flux exhibit nodelike lines, which amazingly are perpendicular to each other. As a consequence, the persistent spin current could be nonzero even when the charge current vanishes at nonzero magnetic flux. The result suggests a pure spin current in quantum rings, and its direction can be reversed by changing the magnetic flux. The increase in the BZ coupling would exhibit the plateaulike pure spin current.

Charge and spin currents in mesoscopic rings: Role of next nearest-neighbors Rashba spin–orbit coupling using the Green’s functions equation of motion technique

The electronic states of a mesoscopic ring are assessed using the Green functions approach and the equation of motion method. We put forward the Green’s function formalism to evaluate precisely persistent charge and spin currents in a ring exhibiting nearest-neighbors (NN) and next-nearest (NNN) neighbors Rashba spin–orbit interactions. Our present scheme circumvents direct diagonalization of the Hamiltonian, allowing to determine numerically persistent currents with both spin–orbit interactions simultaneously. Our results for the spin currents show that the NN Rashba interaction breaks the spin degeneracy producing a square-like shape; while the NNN Rashba interaction creates an energy-dependent splitting of the levels that generates a pattern of small peaks. The merging of both interactions rises a periodic squared-peaked pattern at half integer values of the quantum flux, which suggests the possibility of controlling the spin current magnitude and polarization by modulating the NNN Rashba parameter.

Aharonov-Bohm nanoring with periodically modulated Rashba interaction: Energy spectrum and persistent currents

Energy spectra and persistent currents in one-dimensional quantum rings with inhomogeneous spin-orbit Rashba interaction (RSOI) in the presence of Aharonov-Bohm magnetic flux are studied. It is shown that suppression of RSOI in some part of the ring leads to the level anticrossing which results in smoothing the magnetic flux dependence of the charge persistent current. For a periodic ring structure with alternating RSOI and non-RSOI segments the energy levels are grouped into gap-separated subbands. The flux dependence of the charge persistent current of such a ring has a sawtooth shape (as for a ring with uniform RSOI) except for the case of complete filling of several subbands by electrons, where the dependence becomes smooth. We also calculated the persistent spin current. It was shown that the angular dependence of its density satisfies certain symmetry conditions, which are determined by the structure period and the angular width of its non-RSOI part.

Spin torque and persistent currents caused by percolation of topological surface states

The topological insulator/ferromagnetic metal (TI/FMM) bilayer thin films emerged as promising topological surface state-based spintronic devices, most notably in their efficiency of current-induced spin torque. Using a cubic lattice model, we reveal that the surface state Dirac cone of the TI can gradually merge into or be highly intertwined with the FMM bulk bands, and the surface states percolate into the FMM and eventually hybridize with the quantum well states therein. The magnetization can distort the spin-momentum locking of the surface states and yield an asymmetric band structure, which causes a laminar flow of room temperature persistent charge current. Moreover, the proximity to the FMM also promotes a persistent laminar spin current. Through a linear response theory, we elaborate that both the surface state and the FMM bulk bands contribute to the current-induced spin torque, and their real wave functions render the spin torque predominantly field-like, with a magnitude highly influenced by the degree of the percolation of the surface states. On the other hand, impurities can change the spin polarization expected from the Edelstein effect and generate a damping-like torque, and produce a torque even when the magnetization points in-plane and orthogonal to the current direction.

Persistent charge and spin currents in the 1D Holstein-Hubbard ring at half filling and at away from half filling by Bethe-ansatz approach

The persistent charge and spin currents are studied by considering the Holstein-Hubbard model for one dimensional mesoscopic ring. Conventional Lang-Firsov transformation is used to eliminate the phonon coordinates and then the zero phonon state is applied to obtain the effective electronic Hamiltonian which is then treated exactly by the Bethe-Ansatz technique. The coupled transcendental equations that arise in this method are solved numerically and the persistent charge and spin currents are studied with respect to the on-site electron-electron Coulomb correlation (U), on-site electron-phonon interaction strength (g0) and inter-site electron-phonon interaction strength (g1) at half-filling and away from half-filling.

Aharonov–Casher effect and persistent spin currents in a Coulomb-type potential induced by Lorentz symmetry breaking effects

We investigate quantum effects on a nonrelativistic neutral particle with a permanent magnetic dipole moment that interacts with an electric field. This neutral particle is also under the influence of a background that breaks the Lorentz symmetry. We focus on the Lorentz symmetry violation background determined by a space-like vector field. Then, we show that the effects of the violation of Lorentz symmetry can yield an attractive Coulomb-type potential. Furthermore, we obtain the bound state solutions to the Schrödinger–Pauli equation and show that the spectrum of energy is a function of the Aharonov–Casher geometric quantum phase. Finally, we discuss the arising of persistent spin currents.

Persistent currents and spin torque caused by percolated quantum spin Hall state

Motivated by recent experiments, we investigate the quantum spin Hall state in 2D topological insulator/ferromagnetic metal planar junctions by means of a tight-binding model and linear response theory. We demonstrate that whether the edge state Dirac cone is submerged into the ferromagnetic subbands and the direction of the magnetization dramatically affect (i) how the edge state percolates into the ferromagnet, and (ii) the spin-momentum locking of the edge state. Laminar flows of room temperature persistent charge and spin currents near the interface are uncovered. In addition, the current-induced spin polarization at the edge of the 2D topological insulator is found to be dramatically enhanced near the impurities. The current-induced spin polarization in the ferromagnet is mainly polarized in the out-of-plane direction ${\hat{\bf z}}$, rendering a current-induced spin torque that is predominantly field-like $\propto {\bf S}\times{\hat{\bf z}}$.

Persistent spin textures and currents in wurtzite nanowire-based quantum structures

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to bandstructure characteristics which allows to deduce spin-orbit coefficients and other band parameters from measurements.

Persistent currents in the two-chain correlated electron model

For the recently proposed exactly solvable two-chain correlated electron model with the anisotropic spin-spin interaction between electrons and the spin-orbit interaction the ground state persistent currents are calculated. The model describes the quasi-one-dimensional type II superconductor. It is shown, that the spin-orbit coupling determines the initial phase of oscillations of charge and spin persistent currents, related to unbound electron states. On the other hand, Cooper-like singlet pairs define oscillations of only charge persistent currents with the period, characteristic to pairs. Depending on the value of the external magnetic field and the band filling, the system can reveal the complicated picture of the interference of several kinds of oscillations.

Hybridization effects on persistent currents in rings of topological insulators

We investigate spin dynamics and persistent charge and spin currents of electrons in rings of Topological Insulator (TI) thick and ultrathin films. Interesting effects are observed, in particular, precession of the tangential and longitudinal components of the spin transport is very small, while the radial component does not precess and thus behaves as a conserved quantity in the system dynamics. Further, we find that persistent charge current vanishes due to the equal and opposite contributions of the clockwise and counterclockwise propagating modes, whereas longitudinal spin polarized current vanishes due to the inversion symmetry along the cylindrical axis in both cases. Strikingly, the x- and y-components of spin polarized current oscillate with finite phase shift in the ring of thick film, whereas they vanish in the ring of ultrathin film due to the cancellation of antiparallel spin polarizations of the counterclockwise and clockwise propagating modes originated from the hybridization between the topologically protected conducting states of the outer and inner surfaces of the ring.

Spin and charge transport of electron on a mesoscopic ring of topological insulator thin film in uniform magnetic field

Spin dynamics and persistent spin and charge currents of an electron on a mesoscopic ring of topological insulator (TI) thin film in a uniform magnetic field are investigated. We find that the circular symmetry of TI in the magnetic field leads to a shift of the valence band maxima and conduction band minima in the energy spectrum from the charge neutrality point depending on the strength of the magnetic field, in addition to the bandgap induced by the hybridization and Zeeman energies. The numerical analysis of the dynamical equations obtained from the Heisenberg equation of motion shows that the tangential, radial, and longitudinal components of the electron’s spin exhibit periodic oscillations. Interestingly, the longitudinal component of spin polarized current vanishes due to the inversion symmetry along the cylindrical axis, whereas its x- and y-components oscillate with a finite phase shift. The persistent charge current on the ring of the TI thin film changes sign from positive to negative approaching maximal saturated values at large magnetic fluxes. Moreover, we investigate the effect of dephasing on persistent currents when the ring is coupled to an electron reservoir. Strikingly, both charge and spin persistent currents dissipate significantly with increasing the coupling parameter.

Spin dynamics and persistent currents on a cylindrical surface of topological insulator

Spin dynamics and persistent charge and spin currents of an electron on a cylindrical surface of a topological insulator are investigated. We find that the cylindrical symmetry of topological insulator leads to opening a band gap in the energy spectrum of the topological surface states of the system depending on the radius of the cylinder. In addition, the spectrum acquires a shift in the aforementioned symmetry. The numerical analysis of the dynamical equations obtained from the Heisenberg equation of motion shows that the precession of the tangential, radial, and longitudinal components of the spin transport is very small, reflecting the robustness of the conducting metallic surface states of topological insulators. Moreover, the persistent charge current on the cylindrical surface of TI vanishes, as expected in the absence of any time-reversal symmetry breaking perturbation. On the other hand, the longitudinal component of the persistent spin-polarized current remains constant in the course of electron dynamics on the cylindrical surface, whereas its x- and y-components oscillate with finite phase shift.

Hidden spin current in doped Mott antiferromagnets

We investigate the nature of doped Mott insulators using exact diagonalization and density matrix renormalization group methods. Persistent spin currents are revealed in the ground state, which are concomitant with a nonzero total momentum or angular momentum associated with the doped hole. The latter determines a nontrivial ground state degeneracy. By further making superpositions of the degenerate ground states with zero or unidirectional spin currents, we show that different patterns of spatial charge and spin modulations will emerge. Such anomaly persists for the odd numbers of holes, but the spin current, ground state degeneracy, and charge/spin modulations completely disappear for even numbers of holes, with the two-hole ground state exhibiting a d-wave symmetry. An understanding of the spin current due to a many-body Berry-like phase and its impact on the momentum distribution of the doped holes will be discussed.

Magneto-optical quantum interferences in a system of spinor excitons

In this work we investigate magneto-optical properties of two-dimensional semiconductor quantum-ring excitons with Rashba and Dresselhaus spin-orbit interactions threaded by a magnetic flux perpendicular to the plane of the ring. By calculating the excitonic Aharonov-Bohm spectrum, we study the Coulomb and spin-orbit effects on the Aharonov-Bohm features. From the light-matter interactions of the excitons, we find that for scalar excitons, there are open channels for spontaneous recombination resulting in a bright photoluminescence spectrum, whereas the forbidden recombination of dipolar excitons results in a dark photoluminescence spectrum. We investigate the generation of persistent charge and spin currents. The exploration of spin orientations manifests that by adjusting the strength of the spin-orbit interactions, the exciton can be constructed as a squeezed complex with specific spin polarization. Moreover, a coherently moving dipolar exciton acquires a nontrivial dual Aharonov-Casher phase, creating the possibility to generate persistent dipole currents and spin dipole currents. Our study reveals that in the presence of certain spin-orbit generated fields, the manipulation of the magnetic field provides a potential application for quantum-ring spinor excitons to be utilized in nano-scaled magneto-optical switches.

Controllable persistent spin-polarized charge current in a Rashba ring

We theoretically predict the appearance of a persistent charge current in a Rashba ring with a normal and a ferromagnetic lead under no external bias. This charge current is the result of the breaking of the time inversion symmetry in the original persistent pure spin current induced by the Rashba spin-orbit coupling (RSOC) in the ring due to the existence of the ferromagnetic lead. With the Keldysh Green’s function technique, we find that not only the magnitude and sign but also the spin polarization of the generated charge current is determined by the system parameters such as the magnetization direction of the ferromagnetic lead, the tunneling coefficient, the strength of the RSOC and the exchange energy of the ferromagnetic lead, which are all tunable in experiments, that is, a controllable persistent spin-polarized charge current can be obtained in such a device.

Nonequilibrium spin transport in integrable spin chains: Persistent currents and emergence of magnetic domains

We construct exact steady states of unitary non-equilibrium time evolution in the gapless XXZ spin-1/2 chain where integrability preserves ballistic spin transport at long-times. We characterize the quasi-local conserved quantities responsible for this feature and introduce a computationally effective way to evaluate their expectation values on generic matrix product initial states. We employ this approach to reproduce the long-time limit of local observables in all quantum quenches which explicitly break particle-hole or time-reversal symmetry. We focus on a class of initial states supporting persistent spin currents and our predictions remarkably agree with numerical simulations at long times. Furthermore, we propose a protocol for this model where interactions, even when antiferromagnetic, are responsible for the unbounded growth of a macroscopic magnetic domain.

Characteristics of persistent spin current components in a quasi-periodic Fibonacci ring with spin–orbit interactions: Prediction of spin–orbit coupling and on-site energy

In the present work we investigate the behavior of all three components of persistent spin current in a quasi-periodic Fibonacci ring subjected to Rashba and Dresselhaus spin–orbit interactions. Analogous to persistent charge current in a conducting ring where electrons gain a Berry phase in presence of magnetic flux, spin Berry phase is associated during the motion of electrons in presence of a spin–orbit field which is responsible for the generation of spin current. The interplay between two spin–orbit fields along with quasi-periodic Fibonacci sequence on persistent spin current is described elaborately, and from our analysis, we can estimate the strength of any one of two spin–orbit couplings together with on-site energy, provided the other is known.

Quantum nano ring composed of quantum dots as a source of pure persistent spin or charge current

Spin-dependent persistent current in a quantum ring constituted by two normal and one magnetic quantum dots, in the presence of Rashba spin–orbit interaction is studied by using Green function technique. It is shown that the presence of the magnetic quantum dot breaks the degeneracy of the density of states of electrons with different spin states. Besides, the Rashba spin–orbit interaction along with the magnetic quantum dot develops tunable persistent spin and charge currents. Moreover, the persistent charge current induces a fully adjustable magnetic flux whose direction and magnitude can be tuned by altering the strength of the Rashba spin–orbit interaction.

On a relation of the angular frequency to the Aharonov–Casher geometric phase in a quantum dot

By analysing the behaviour of a neutral particle with permanent magnetic dipole moment confined to a quantum dot in the presence of a radial electric field, Coulomb-type and linear confining potentials, then, an Aharonov–Bohm-type effect for bound states and a dependence of the angular frequency of the system on the Aharonov–Casher geometric phase and the quantum numbers associated with the radial modes, the angular momentum and the spin are obtained. In particular, the possible values of the angular frequency and the persistent spin currents associated with the ground state are investigated in two different cases.