Rashba Spin-orbit Interaction Research Articles

Algebraic decay of the nonadiabaticity arising through chiral spin transfer torque in magnetic domain walls with Rashba spin-orbit interaction

Spin transfer torque in a two-dimensional electron gas system without space inversion symmetry was theoretically investigated by solving the Pauli-Schr\"odinger equation for the itinerant electrons inside magnetic domain walls. Due to the presence of the Rashba spin-orbit coupling induced by the broken inversion symmetry, the spin transfer torque is chiral and the nonadiabaticity, which is defined to measure the relative importance of the nonadiabatic fieldlike torque to the adiabatic dampinglike torque, exhibits an inverse power-law decay as the domain wall width is increased. This algebraic decay is much slower than the exponential decay observed for systems without the Rashba spin-orbit coupling, and may find applications in innovative design of spintronic devices utilising magnetic topological textures such as magnetic domain walls and skyrmions.

Lead position and lead-ring coupling effects on the spin-dependent transport properties in a two-dimensional network of quantum nanorings in the presence of Rashba spin–orbit interaction

The effects of lead positions and lead-ring coupling regimes are investigated for spin-related transport through a two-dimensional network of quantum nanorings (2DNQRs) considering Rashba spin–orbit interaction (RSOI) and magnetic flux. A matrix representation of the transmission and reflection coefficients through a single ring connected to the arbitrary number of leads has been introduced. As a specific example of 2DNQRs, the conductance, spin polarization and system efficiency are obtained via a triangular network of quantum rings (TNQRs). TNQRs are completely opaque or transparent versus RSOI strength and wave vector (k) of the incident electron. The periodicity of these functions along the k axis depends on the lead positions and is independent of lead-ring coupling. Also, the symmetric geometry and strong lead-ring coupling regime significantly improve the performance of the system as a multipurpose spintronic device (i.e., a perfect spin filter, spin splitter, spin switching, Stern–Gerlach device and an electronic switching device).

Majorana zero modes in a quantum wire platform without Rashba spin-orbit coupling

We propose a platform for engineering helical fermions in a hybridized double-quantum-wire setup. When our setup is proximity coupled to an $s$-wave superconductor it can become a class $D$ topological superconductor exhibiting Majorana zero modes. The goal of this proposal is to expand the group of available Hamiltonians to those without strong Rashba spin-orbit interactions which are essential to many other approaches. Furthermore, we show that there exist electron-electron interactions that stabilize fractional excitations which obey $\mathbb{Z}^{\,}_{3}$ parafermionic algebra.

Rashba spin-orbit interaction effects on thermal and magnetic properties of parabolic GaAs quantum dot in the presence of donor impurity under external electric and magnetic fields

Based on the effective mass approximation, the magnetic and thermal properties of parabolic GaAs quantum dot have been investigated in the presence of Rashba Spin-Orbit interaction (RSOI), donor impurity and applied magnetic and electric fields. The exact diagonalization method has been used to solve the Hamiltonian of an electron confined in a quantum dot (QD) and obtain the eigenenergies and the binding energy of the donor impurity as a function of various QD physical parameters. We have shown the dependence of the average statistical energy, magnetization, magnetic susceptibility and heat capacity of the donor impurity in the QD on: the Rashba interaction parameter, the magnetic and electric fields, confining frequency, and temperature. The results reveal that these parameters can tune the magnetic properties of the GaAs quantum dot and flip the sign of magnetic susceptibility from negative (diamagnetic) to positive (paramagnetic) type material.

Possible Occurrence of Superconductivity by the π-flux Dirac String Formation Due to Spin-Twisting Itinerant Motion of Electrons

We show that the Rashba spin-orbit interaction causes spin-twisting itinerant motion of electrons in metals and realizes the quantized cyclotron orbits of conduction electrons without an external magnetic field. From the view point of the Berry connection, the cause of this quantization is the appearance of a non-trivial Berry connection A fic = − ℏ 2 e ∇ χ ( χ is an angular variable with period 2 π ) that generates π flux (in the units of ℏ = 1 , e = 1 , c = 1 ) inside the nodal singularities of the wave function (a “Dirac string”) along the centers of spin-twisting. Since it has been shown in our previous work that the collective mode of ∇ χ is stabilized by the electron-pairing and generates supercurrent, the π -flux Dirac string created by the spin-twisting itinerant motion will be stabilized by the electron-pairing and produce supercurrent.

Unconventional superconducting phases in multilayer films with layer-dependent Rashba spin-orbit interactions

We present resistivity measurements of bilayer and trilayer films with layer-dependent Rashba spin-orbit interactions. A sharp upturn is observed in the temperature dependence of the parallel upper critical field. We show that it corresponds to a transition from a complex-stripe phase to a helical phase. Moreover, we propose the phase diagram of a multilayer system, which is obtained from experimental observations with the support of numerical calculations. These results pave the way for the clarification of non-trivial superconducting states in multilayer systems composed of two-dimensional Rashba superconductors.

Electronic transport in one-dimensional Floquet topological insulators via topological and nontopological edge states

Based on probing electronic transport properties we propose an experimental test for the recently discovered rich topological phase diagram of one-dimensional Floquet topological insulators with Rashba spin-orbit interaction [Kennes \emph{et al.}, Phys. Rev. B {\bf 100}, 041103(R) (2019)]. Using the Keldysh-Floquet formalism we compute electronic transport properties of these nanowires, where we propose to couple the leads in such a way, as to primarily address electronic states with a large weight at one edge of the system. By tuning the Fermi energy of the leads to the center of the topological gap we are able to directly address the topological edge states, granting experimental access to the topological phase diagram. Surprisingly, when tuning the lead Fermi energy to special values in the bulk which coincide with extremal points of the dispersion relation, we find additional peaks of similar magnitude to those caused by the topological edge states. These peaks reveal the presence of continua of states centered around aforementioned extremal points whose wavefunctions are linear combinations of delocalized bulk states and exponentially localized edge states, where the ratio of edge- to bulk-state amplitude is maximal at the extremal point of the dispersion. We discuss the transport properties of these \emph{non-topological edge states}, explain their emergence in terms of an intuitive yet quantitative physical picture and discuss their relationship with Van Hove singularities in the bulk of the system. The mechanism giving rise to these states is not specific to the model we consider here, suggesting that they may be present in a wide class of one-dimensional systems.

Spin filtering in a HgTe topological insulator PN junction via Rashba spin–orbit interaction

This work presents theoretically investigate the electron transport through a PN junction in two-dimensional HgTe/CdTe topological insulator. We find that the transmission can be tuned by changing the incidence angle, gate voltage, Fermi energy and the Rashba spin–orbit interaction (RSOI). By tuning the strength of RSOI modulation, the efficient spin-flip conversion for the spin-up and spin-down channels are found. Furthermore, a perfect one-channel transmission mode with T=1 is formed due to the spin-split induced by the RSOI. This RSOI-induced spin-split mechanism provides us a way of pure electrical modulation to manipulate the spin and charge currents.

Role of spin-orbit interactions on the entropy and heat capacity of a quantum dot helium placed in an external magnetic field

The effects of Rashba, Dresselhaus and Coulomb interactions on the entropy and heat capacity of a two-electron quantum dot placed in an external magnetic field are studied at finite temperature. The effect of electron-electron interaction is studied by using the Johnson-Payne harmonic model which is exactly soluble, while the Rashba and Dresselhus spin-orbit coupling effects are dealt with using unitary transformations. It is observed that as a function of the magnetic field, the heat capacity has a double peak structure with the minima between the peaks occurring at the singlet-triplet transition point and the e-e interaction shifts the peaks to lower magnetic fields. It is also observed that the coulomb correlation reduces the heat capacity in the low temperature regime while in the high temperature regime it enhances it. Interestingly, however, in the high field regime, the heat capacity does not seem to be influenced by the electron-electron interaction. The entropy is found to show a single peak as a function of magnetic field at the singlet-triplet transition point. The Rashba spin-orbit interaction reduces the heat capacity and the Dresselhaus interaction enhances it whereas the opposite effect is exhibited by the entropy. The temperature behaviour of both the entropy and heat capacity clearly demonstrates the effect of zero field spin-splitting. • A quantum dot with two electrons is considered in a magnetic field at finite temperature. • Role of Rashba and Dresselhauss interactions (RI & DI) are studied on heat capacity ( C v ) and entropy (S). • Coulomb interaction between electrons is treated by the exactly soluble Johnson-Payne model. • RI reduces C v and DI enhances it while S shows just an opposite behaviour. • Temperature behaviour of C v and S exhibits zero-field splitting.

Reentrant Kondo effect for a quantum impurity coupled to a metal-semiconductor hybrid contact

Using NRG, we show that a system containing a quantum impurity (QI), strongly coupled to a semiconductor (gap $2 \Delta$) and weakly coupled to a metal, displays a 'reentrant' Kondo stage at low temperatures. The NRG analysis of the corresponding Single Impurity Anderson Model (SIAM) shows that the reentrant stage is characterized by a second sequence of SIAM fixed points: free orbital (FO) > local moment (LM) > strong coupling (SC). In the first stage, the SC fixed point (Kondo temperature $T_{K1}$) is unstable, while the second stage exhibits a much lower Kondo temperature $T_{K2}$, associated to a stable SC fixed point. The results indicate that the reentrant Kondo screening is associated to an effective SIAM, with an effective repulsion $U_{eff}$. This low temperature effective SIAM, which we dub as 'reentrant' SIAM, behaves as a 'replica' of the high temperature (bare) SIAM. The intuitive picture that emerges is that the first Kondo state develops through impurity screening by semiconducting electrons, while the second Kondo state involves screening by metallic electrons, once the semiconducting electrons are out of reach to thermal excitations ($T < \Delta$) and only the metallic spectral weight inside the gap is available for impurity screening. In addition, we analyze a hybrid system formed by a QI `sandwiched' between an armchair graphene nanoribbon (AGNR) and a scanning tunneling microscope (STM) tip, with respective couplings set to reproduce the generic model described above. The energy gap in the AGNR can be externally tuned by an electric-field-induced Rashba spin-orbit interaction. We analyzed this system for realistic parameter values, using NRG, and concluded that the reentrant SIAM, with its associated second stage Kondo, is worthy of experimental investigation.

Role of Rashba spin-orbit interaction on polaron Zeeman effect in a two-dimensional quantum dot with parabolic confinement

We calculate the energies of the ground and the first excited states of a free polaron and that of a polaron bound to a Coulomb impurity in a quantum dot with harmonic confinement in the presence of Rashba spin-orbit interaction by employing the variation theory of Lee, Low and Pines as modified by Huybrechts for an all-coupling range of the electron-phonon interaction and arbitrary confinement length. We show that in both cases, the Rashba interaction removes the two-fold spin-degeneracy of the first-excited states even in the absence of any applied magnetic field, though the ground state does not show any such spin splitting. The self-energy corrections due to the polaronic effect are however not affected by the Rashba interaction. We also investigate the combined effect of Rashba and polaronic interactions in the presence of an external magnetic field using the Rayleigh-Schrödinger perturbation theory. Application of our results to GaAs and CdS quantum dots shows that the suppression of the phonon-induced size-dependent Zeeman splitting in a quantum dot is reduced by the Rashba coupling.

Reconfigurable logic gates in nanowires with Rashba spin-orbit interaction

Abstract Two configurations are proposed for the implementation of several logic gates in ballistic nanowires with Rashba spin-orbit interaction. The logic states are encoded in Rashba coefficients/applied gate voltages on up to three regions surrounded by areas with no spin-orbit coupling. In this way, only one spin-polarized current is needed in order to implement multiple-bits gates, minimizing the technological difficulties currently associated to spintronic devices.

Artificial oxide heterostructures with non-trivial topology

In the quest for topological insulators with large band gaps, heterostructures with Rashba spin–orbit interactions come into play. Transition metal oxides with heavy ions are especially interesting in this respect. We discuss the design principles for stacking oxide Rashba layers. Assuming a single layer with a two-dimensional electron gas (2DEG) on both interfaces as a building block, a two-dimensional topological insulating phase is present when negative coupling between the 2DEGs exists. When stacking multiple building blocks, a two-dimensional or three-dimensional topological insulator is artificially created, depending on the intra- and interlayer coupling strengths and the number of building blocks. We show that the three-dimensional topological insulator is protected by reflection symmetry, and can therefore be classified as a topological crystalline insulator. In order to isolate the topological states from bulk states, the intralayer coupling term needs to be quadratic in momentum. It is described how such a quadratic coupling could potentially be realized by taking buckling within the layers into account. The buckling, thereby, brings the idea of stacked Rashba system very close to the alternative approach of realizing the buckled honeycomb lattice in [111]-oriented perovskite oxides.

Brightening odd-parity excitons in transition-metal dichalcogenides: Rashba spin-orbit interaction, skyrmions, and cavity polaritons

Odd-angular momentum exciton states are dark to light in monolayers of transition-metal dichalcogenides and can be addressed only by two-photon probes. Besides, $2p$ excitons states are expected to show a fine splitting that arises from the peculiar electronic band structure of the material, characterized by a finite Berry curvature around each valley. In this work we study in detail the coupling of photons to $p$ exciton states and we find that the Rashba spin-orbit interaction or a Skyrmion in the transition-metal dichalcogenide substrate can be exploited to engineer finite optical selection rules. The basis mechanism relies on a matching of the exciton angular momentum with the winding of the Rashba spin-orbit interaction or the Skyrmion topological charge. In a photonic cavity, the coupling is enhanced due to photon confinement and the resulting polaritonic branches acquire a mixing with $2p^\pm$ excitons, thus providing a useful tool to detect exciton fine-splitting and and enable novel uses of odd-parity dark excitons.

Bound Electron Pairs Formed by the Spin–Orbit Interaction in 2D Gated Structures

The bound electron pairs (BEPs) arising due to the pair spin–orbit interaction (PSOI) in 2D structures are explored with a gate that allows the BEPs to be manipulated. The gate breaks the in‐plane reflection symmetry of the pair Coulomb field and creates a one‐particle Rashba spin–orbit interaction. It is found that the normal component of the electric field substantially affects the BEPs but the key role in forming the BEPs belongs to the in‐plane component. The ground state of a BEP with zero total momentum, which is doubly degenerate in the absence of the gate, splits into two states. One of them is tunable by varying the gate voltage, whereas the other is on the contrary robust. The tunable BEP has a higher binding energy which grows as the gate voltage increases, with its orbital and spin structure changing continuously. At large negative voltage, the tunable BEP decays. The orbital and spin structure of the robust BEP does not depend on the gate voltage. Its energy level crosses the conduction band bottom at high gate voltage of any polarity, but the robust BEP remains bound and localized even when in continuum.

Hinge states in a system of coupled Rashba layers

We consider a system of stacked tunnel-coupled two-dimensional electron- and hole-gas layers with Rashba spin-orbit interactions subjected to a staggered Zeeman field. The interplay of different intra-layer tunnel couplings results in a phase transition to a topological insulator phase in three dimensions hosting gapless surface states. The staggered Zeeman field further enriches the topological phase diagram by generating a second-order topological insulator phase hosting gapless hinge states. The emergence of the topological phases is proven analytically in the regime of small Zeeman field and confirmed by numerical simulations in the non-perturbative region of the phase diagram. The topological phases are stable against external perturbations and disorder.

Theoretical study on slit experiments in Rashba electron systems

We develop a theory of designing slit experiments in two-dimensional electron systems with the Rashba spin-orbit interaction. By simulating the spatiotemporal dynamics of electrons passing through a single slit or a double slit, we find that the interference fringes of the electron probability density attain specific spin orientations via the precession of spins around effective magnetic fields mediated by the Rashba spin-orbit interaction whose directions are determined by the propagation path. The spin orientations of the fringes can be controlled by tuning the Rashba spin-orbit parameter, which can be achieved by applying an electric gate voltage. This phenomenon can be exploited to implement electrically tunable transmission of spin information and to generate spin-polarized currents.

Spin Hall Conductance In a Two-Dimensional Tight-Binding Model In The Presence Of Rashba Spin-Orbit Interaction And Random Impurities

The spin-Hall conductance in a two-dimensional tight-binding model is studied in the presence of Rashba spin-orbit interaction and random impurities. The conventional definition of spin current falls through for a system with Rashba spin-orbit interaction because of the non-conservation of the spin magnetic moment in the presence of Rashba coupling. Using a modified definition of spin-current operator, the relaxation time for electron-impurity scattering is calculated with the help of Matsubara Green function and the spin-Hall conductivity is determined by employing the Kubo-Greenwood formalism. Our results show the explicit behavior of the spin-Hall conductance as a function of chemical potential, Rashba spin-orbit coupling constant and the impurity strength

Coherent quantum transport in a transitionmetal dichalcogenides superconducting double junction with Rashba spin-orbit interaction

In this study, the coherent quantum transport problem in a transitionmetal dichalcogenides superconducting double junctions with the Rashba spin-orbit interaction has been investigated. It is found that the coherent tunneling conductance is characterized by the nontrivial Rashba metallic states which can be achieved by tuning the Rashba spin-orbit interaction and Fermi energy. Remarkably, the hump structure and the reentrant behavior of the coherent tunneling conductances can be served as a promising option to detect the Rashba ring metal state, the normal Rashba metal state and the anomalous Rashba metal state. Since the retro-normal reflection and the specular Andreev reflection occur in the Rashba ring metal state, we thus expect that it can also offer a potential route to detect the retro-normal reflection and the specular Andreev reflection based on the transitionmetal dichalcogenides based superconducting double junctions.

Long-ranged triplet supercurrent in a single in-plane ferromagnet with spin-orbit coupled contacts to superconductors

By converting conventional spin-singlet Cooper pairs to polarized spin-triplet pairs, it is possible to sustain long-ranged spin-polarized supercurrents flowing through strong ferromagnets. Obtaining such a conversion via spin-orbit interactions, rather than magnetic inhomogeneities, has recently been explored in the literature. A challenging aspect with regard to experimental detection has been that in order for Rashba spin-orbit interactions, present, e.g., at interfaces due to inversion symmetry breaking, to generate such long-ranged supercurrents, an out-of-plane component of the magnetization is required. This limits the choice of materials and can induce vortices in the superconducting region complicating the interpretation of measurements. Therefore, it would be desirable to identify a way in which Rashba spin-orbit interactions can induce long-ranged supercurrents for purely in-plane rotations of the magnetization. Here, we show that this is possible in a lateral Josephson junction where two superconducting electrodes are placed in contact with a ferromagnetic film via two thin, heavy normal metals. The magnitude of the supercurrent in such a setup becomes tunable by the in-plane magnetization angle when using only a single magnetic layer. These results could provide a new and simpler way to generate controllable spin-polarized supercurrents than previous experiments which utilized complicated magnetically textured Josephson junctions.