Persistent Charge 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 and charge persistent currents in a Kane Mele α-T 3 quantum ring

We conduct a thorough study of different persistent currents in a spin-orbit coupled α-T 3 (pseudospin-1) fermionic quantum ring (QR) that smoothly interpolates between graphene (α = 0, pseudospin-1/2) and a dice lattice (α = 1, pseudospin-1) in presence of an external perpendicular magnetic field. In particular, we have considered effects of intrinsic (ISOC) and Rashba spin-orbit couplings (RSOC) that are both inherent to two dimensional quantum structures and yield interesting consequences. The energy levels of the system comprise of the conduction bands, valence bands, and flat bands which show non-monotonic dependencies on the radius, R of the QR, in the sense that, for small R, the energy levels vary as 1/R , while the variation is linear for large R. The dispersion spectra corresponding to zero magnetic field are benchmarked with those for finite fields to enumerate the role played by the spin–orbit coupling terms therein. Further, it is noted that the flat bands demonstrate dispersive behavior, and hence is able to contribute to the transport properties only for finite ISOC. Moreover, RSOC yields spin-split bands, thereby contributing to the spin-resolved currents. The charge and the spin-polarized persistent currents are hence computed in presence of these spin-orbit couplings. The persistent currents in both the charge and spin sectors oscillate as a function of the magnetic field with a period equal to the flux quantum, as they should be; although they now depend upon the spin–orbit coupling parameters. Interestingly, the ISOC distorts the current profiles, owing to the distribution of the flat band caused by it, whereas RSOC alone preserves the flat band and hence a perfect periodicity of the current characteristic is maintained. Further, we have explored the role played by the parameter α in our entire analysis to enable studies while interpolating from graphene to a dice lattice.

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.

Manipulating the orbital charge-currents of compressed Li and Na atom embedded in quantum plasma

In this study, for the first time, a theoretical investigation is carried out on the persistent orbital charge-currents (cc) and induced magnetic field (imf) of Li and Na atom compressed by spherical confinement, embedded in a quantum plasma. The more general exponential cosine screened Coulomb (MGECSC) potential is considered for interactions in the quantum plasma. There are four different forms of the MGECSC potential in describing Debye and quantum plasmas, but the most general form is used for the most detailed examination. The model potential approach is employed to popularize the interactions of many body electron systems and to overcome the computational difficulty arising from complex electron–electron interactions through a very valid approach. Therefore, the MGECSC potential for the quantum plasma-immersed Li and Na atom is modified within the framework of the model potential approach. The wave equation including the modified potential is solved using the tridiagonal matrix method. The interaction potential between the valence electron and the atomic core alters significantly by the plasma screening effect. This change also creates observable changes in the persistent charge-current (cc) and induced magnetic fields (imf) of Li and Na atom. The effects of plasma screening parameters and spherical confinement strength on the persistent cc and imf are studied in detail. In addition to the effects of plasma and spherical confinement parameters on the spectrum, the cc and imf of Li and Na atom, their critical values, if any, their dominance and alternatives to each other are also discussed in a cause-effect relationship. In the case of stronger spherical confinement, the quantum plasma has a predominant response on the cc and imf compared to a Debye plasma and free-plasma environments.

Электронные состояния и персистентные токи в кольцах с неоднородным спин-орбитальным взаимодействием Рашбы

The paper considers one-dimensional isolated quantum rings in a perpendicular magnetic field, in which Rashba spin-orbit coupling parameter changes along the ring. Electronic spectra and persistent currents of such ring structures are calculated for different values of the structure parameters and the magnitude of the magnetic flux. It is shown that the inhomogeneity of the Rashba parameter leads to anticrossing of energy levels and smoothing of the dependence of the persistent charge current on the flux. The influence of inhomogeneity on the spin persistent current turns out to be more significant: the projection of the spin current on the ring axis ceases to be preserved when moving along the ring; additional extrema or zeros appear on the dependence of the absolute value of the spin current on the magnetic flux for a given number of electrons in the ring.

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.

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

Abstract 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.

Inner and outer ring states of MoS2 quantum rings: Energy spectrum, charge and spin currents

We investigate the energy levels and persistent currents of MoS2 quantum rings having different shapes and edge types in the presence of a perpendicular magnetic field by means of the tight-binding approach. We find states localized at the inner and outer boundaries of the ring. These energy levels exhibit different magnetic field dependences for the inner and outer ring states due to their different localization properties. They both exhibit the usual Aharanov–Bohm oscillations but with different oscillation periods. In the presence of spin–orbit coupling, we show distinct spin and charge persistent currents for inner and outer ring states. We find well-defined spin currents with negligibly small charge currents. This is because the local currents of spin-up and -down states flow in opposite directions.

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.

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.

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.

Equilibrium currents in a Corbino graphene ring

We address the description of a graphene Corbino disk in the context of a tight binding approach that includes both kinetic and Rashba spin-orbit coupling due to an external out-of-plane electric field. Persistent equilibrium currents are induced by an external magnetic field breaking time reversal symmetry. By direct diagonalization, we compute the spectrum and focus on the dispersion near the $K$ points at the Fermi level. The dispersion keenly reproduces that of a continuum model in spite of the complexity of the boundary conditions. We validate the assumptions of the continuum model in terms of predominant zig-zag boundaries conditions and weak sub-band coupling. The wave functions displaying the lowest transverse modes are obtained, showing the predominance of edge states with charge density at the zig-zag edges. The persistent charge currents, nevertheless, do not follow the traditional argument of current cancellation from levels below the Fermi level, and thus they depart in the tight-binding from those found in the continuum model.

Charge- and spin-polarized currents in mesoscopic rings with Rashba spin-orbit interactions coupled to an electron reservoir

The electronic states of a mesoscopic ring are assessed in the presence of Rashba spin-orbit (SO) coupling and a $\text{U}(1)$ gauge field. Spin-symmetric coupling to an ideal lead is implemented following B\"uttiker's voltage probe. The exact density of states is derived using the reservoir uncoupled eigenstates as basis functions mixed by the reservoir coupling. The decay time of uncoupled electron eigenstates is derived by fitting the broadening profiles. Spin and charge persistent currents are computed in the presence of the SO interaction and the reservoir coupling for two distinct scenarios of the electron filling fraction. The degradation of the persistent currents depends uniformly on the reservoir coupling but nonuniformly in temperature, the latter due to the fact that currents emerge from different depths of the Fermi sea, and thus for some regimes of flux, they are provided with a protective gap. Such flux regimes can be tailored by the SO coupling for both charge and spin currents.