Rashba Spin-orbit Coupling Research Articles

Creation of Two-Dimensional Electron Gas at the Heterointerface of CaZrO3/KTaO3 with Tunable Rashba Spin–Orbit Coupling

Creation of Two-Dimensional Electron Gas at the Heterointerface of CaZrO₃/KTaO₃ with Tunable Rashba Spin–Orbit Coupling

Phase diagrams and topological mixed edge states in silicene with intrinsic and extrinsic Rashba effects

We investigate the topological phases and corresponding edge states in silicene with fixed intrinsic and extrinsic Rashba spin-orbit couplings. The results show that silicene can exhibit four distinct topological phases, such as two types of valley-polarized quantum anomalous Hall effect (QAHE) and two types of QAHE that can be effectively controlled by the exchange field and electric field. In addition, these topological phases only correspond to the valley edge states distributed at the boundary of silicene. In particular, when the off-resonant circularly polarized (ORCP) light is further considered, silicene can host richer topological phases that are characterized by the Chern numbers from C=−3 to C=3, where the corresponding mixed edge states—where the valley and nonvalley edge states could coexist—can remarkably appear. More interestingly, the special higher Chern numbers C=±3 determine two valley edge states and one nonvalley edge state, with the right-moving or left-moving direction. These peculiar edge states are attributed to the ORCP light destroying the collapse of the Dirac cones that the valley edge states bridge. Additionally, silicene can experience multiple edge-state transitions for identical Chern numbers C=±2 between the valley edge state, nonvalley edge state, and two types of mixed edge state, under the ORCP light. This work may provide a platform and point of view for investigating other forms of topological edge state and is beneficial for designing multifunctional topological devices. Published by the American Physical Society 2024

Abundant vortex dynamics in spin-1 Bose–Einstein condensates induced by Rashba spin–orbit coupling

The paper studies the dynamics of vortices evolved from the ring dark solitons (RDSs) in spin-1 Bose–Einstein condensates with Rashba spin–orbit coupling (SOC). We find that the SOC induces abundant vortex dynamic phenomena, and the numbers of vortex dipoles and lump-like solitons are all determined by the initial depths’ weighted average of the three components of RDSs. For a shallow RDS, the ring first expands and then contracts into a mini RDS, eventually evolving into late vortex dipoles. For the RDS with a moderate initial depth, during the contraction of the ring, it evolves into early vortex dipoles or lump-like solitons, and they evolve back into the mini RDS, which then evolves into late vortex dipoles during the expansion process. For the deep and black RDSs, the early vortex dipoles continue to transform between vortex dipoles and lump-like solitons without forming mini RDS. The motions of vortices mentioned above have a higher disorder degree than that in the system without SOC, because the SOC breaks the mirror symmetry of the condensate. Finally, we reveal that the SOC strength only influences the number of late vortex dipoles, while it does not affect the number of early vortex dipoles and lump-like solitons. Stronger SOC intensity results in a shallower critical depth, and vortices form only at or beyond this depth.

Dirac-Like Fermions Anomalous Magneto-Transport in a Spin-Polarized Oxide 2D Electron System.

In a 2D electron system (2DES) the breaking of the inversion, time-reversal and bulk crystal-field symmetries is interlaced with the effects of spin-orbit coupling (SOC) triggering exotic quantum phenomena. Here, epitaxial engineering is used to design and realize a 2DES characterized simultaneously by ferromagnetic order, large Rashba SOC and hexagonal band warping at the (111) interfaces between LaAlO3, EuTiO3, and SrTiO3 insulators. The 2DES displays anomalous quantum corrections to the magneto-conductance driven by the time-reversal-symmetry breaking occurring below the magnetic transition temperature. The results are explained by the emergence of a non-trivial Berry phase and competing weak anti-localization/weak localization back-scattering of Dirac-like fermions, mimicking the phenomenology of gapped topological insulators. These findings open perspectives for the engineering of novel spin-polarized functional 2DES holding promises in spin-orbitronics and topologicalelectronics.

Electrically controlled nonlocal transport in antiferromagnet/superconductor junctions with Rashba spin-orbital coupling

Abstract We theoretically investigate nonlocal transport phenomena at an antiferromagnet/normal/superconductor/antiferromagnet junction. Both parallel and antiparallel configurations are considered, with the N\'{e}el vector aligned with the $z$ axis in the left antiferromagnet and with the $z$ and -$z$
 axes in the right antiferromagnet, respectively.
In the parallel configuration, only equal-spin crossed Andreev reflection (CAR) is allowed, as the conventional CAR is forbidden. As the Rashba spin-orbital coupling strength increases in the normal region, the amplitude of equal-spin CAR increases while the amplitude of electron elastic cotunneling (EC) decreases, resulting in equal-spin CAR-dominant nonlocal transport. In the antiparallel configuration, a CAR-dominant nonlocal transport is observed at low Rashba spin-orbital coupling strength, with the conventional CAR process being finite. Furthermore, our results indicate that increasing the staggered sublattice potential enhances the conventional CAR in the parallel configuration, and conversely, in the antiparallel configuration at high Rashba spin-orbital coupling strength, it reduces CAR dominance in favor of EC processes. Therefore, by adjusting the Rashba spin-orbital coupling strength and staggered sublattice potential, CAR-dominant nonlocal transport can be achieved in both the parallel and antiparallel configurations.

Superconducting diode effect in diffusive superconductors and Josephson junctions with Rashba spin-orbit coupling

Superconducting diode effect in diffusive superconductors and Josephson junctions with Rashba spin-orbit coupling

Improving spin manipulation in HgTe/CdTe quantum wells via electrical means

We present a theoretical investigation of electron transport properties through a two-dimensional electron gas (2DEG) HgTe/CdTe quantum wells (QWs) heterostructure that incorporate two Rashba-modulated regions. Our study demonstrates that the transmission can be effectively tuned by varying the Rashba spin-orbit coupling (RSOC) strength, Fermi energy, and electrical voltage. Efficient spin-conserving and spin-flip transmission can be achieved by adjusting the RSOC modulation strength. Furthermore, fully spin-polarized spin or current transmission with PZ=±1, as well as complete spin-flip conversion with a rate of Tsf=1 are attained by fine-tuning the amplitudes of the two Rashba modulations using gate voltage or adjusting the RSOC strength. This electrical mechanism provides a convenient approach to electrically manipulate both spin and charge currents, which holds promise for low-power device applications.

Non-classical correlations and coherence in a two-dimensional electron gas under the influence of Rashba spin–orbit coupling

This investigation delves into the dynamics of quantum correlations and coherence within a two-dimensional electron gas (2DEG) system, taking into account the influence of Rashba spin–orbit coupling (SOC). We utilize metrics such as logarithmic negativity ([Formula: see text]), local quantum uncertainty (LQU ([Formula: see text])), and relative entropy of coherence ([Formula: see text]) to specifically assess these quantum resources among electron spins within non-interacting electron gases. Our study investigates how the separation distance (R) between electrons and the intensity of Rashba SOC ([Formula: see text]), impact the behavior of quantum properties within the 2DEG system. We find that specific strengths of Rashba SOC can effectively regulate quantum correlations and coherence within this system. Particularly significant is our observation that optimal quantum indicators emerge when electron proximity is minimized, underscoring the substantial influence of electron distance on quantum characteristics. Conversely, as electron separation increases, these quantum metrics diminish. Therefore, by adjusting the Rashba parameter, we can enhance the resilience of these quantum measurements against increasing electron separations, providing valuable insights into the potential for controlling and manipulating quantum behavior in similar 2DEG systems.

Anomalous hybridized excitons induced by combined effects of Van der Waals coupling and Rashba spin–orbit coupling

As a typical transition-metal dichalcogenide, MoS2has drawn wide attention due to its good stability and excellent physicochemical properties, making it suitable for visible-region optoelectronic devices. To expand its application, bandgap engineering via heterostructure, thus far, was conventionally employed to tune the band gap. However, this strategy has the disadvantage that energy levels of bands do not show obvious changes compared to the isolated components, limiting the range of applications. Here, we achieve hybridized excitons induced by combined effects of Van der Waals (vdW) coupling and Rashba spin-orbit coupling (SOC), with a small exciton energy of 0.65 eV. For this purpose, we design a MoS2/MoWC heterostructure, where a built-in field (due to the absence of mirror symmetry) induces the Rashba SOC and contributes to the anomalous hybridized states, combined with the vdW coupling. An effective model is proposed to demonstrate the anomalous hybridized states for the heterostructure. Our approach reveals a novel mechanics model for hybridized excitons states, providing new physical ways to achieve infrared-region devices.

Spin-dependent Goos-Hänchen shift for electron in single-layered semiconductor microstructure modulated by Rashba spin–orbit coupling

We theoretically investigate Goos-Hänchen effect for electron in single-layered semiconductor microstructure (SLSM) modulated by Rashba spin–orbit coupling (SOC). Due to the SOC effect, GH displacement is obviously dependent on spins, which allows electron spins to be separated in space dimension and results in spin polarization of electrons in semiconductors. Spin polarization ratio is associated with incident energy, incident direction and in-plane wave vector, e.g., it reaches maximum at resonance, but no spin polarization effect appears at normal incidence. In particular, both magnitude and sign of spin polarization ratio are controlled by external electric field or semiconductor-layer thickness, therefore, a manipulable spatial electron-spin splitter is obtained for semiconductor spintronics device applications.

Spin-orbit coupling effects in single-layer phosphorene

The electronic band structure of monolayer phosphorene is thoroughly studied by considering the presence of spin-orbit interaction. We employ a multiorbital Slater-Koster tight-binding approach to derive effective k·p-type Hamiltonians that describes the dominant spin-orbit coupling (SOC) effects of the Rashba and intrinsic origin at the high Γ and S high symmetry points in phosphorene. In the absence of SOC effects a minimal admixture of pz and py atomic orbitals suffices to reproduce the well-known anisotropy of highest valence and the lowest conduction bands at the Γ point, consistent with density functional theory (DFT) and k·p methods. In contrast, the inclusion of the px and s atomic orbitals are rather crucial for an adequate description of the SOC effects in phosphorene at low energies, particularly at the S point. We introduce useful analytical expressions for the Rashba and intrinsic SOC parameters in terms of the relevant Slater-Koster integrals. In addition, we report simple formulas for the interband dipole strengths, revealing the nature of the strong anisotropic behavior of its lower bands. Our findings can be useful for further studies of electronic and spin transport properties in monolayer phosphorene and its nanoribbons. Published by the American Physical Society 2024

Influence of the Hall-bar geometry on texture-induced topological spin transport in two-dimensional Rashba spin–orbit ferromagnets

While the recent prediction and observation of magnetic skyrmions bears inspiring promise for next-generation spintronic devices, how to detect and track their position becomes an important issue. In this work, we investigate the spin transport in a two-dimensional magnetic nanoribbon with the Hall-bar geometry in the presence of Rashba spin–orbit coupling and magnetic skyrmions. We employ the Kwant tight-binding code to compute the Hall conductance and local spin-polarized current density. We consider two versions of the model: One with single skyrmion and one with two separate skyrmions. It is found that the size and position of the skyrmions strongly modulate the Hall conductance near the Hall-bar position. The geometry of the Hall bar also has a strong influence on the Hall conductance of the system. With the decreasing of the width of Hall leads, the peak of Hall conductance becomes sharper. We also show the spatial distribution of the spin-polarized current density around a skyrmion located at different positions. We extend this study toward two separate skyrmions, where the Hall conductance also reveals a sizable dependence on the position of the skyrmions and their distance. Our numerical analysis offers the possibility of electrically detecting the skyrmion position, which could have potential applications in ultrahigh-density storage design.

Josephson diode effect in a ballistic single-channel nanowire

When time-reversal and inversion symmetry are broken, superconducting circuits may exhibit a so-called diode effect, where the critical currents for opposite directions of the current flow differ. In recent years, this effect has been observed in a multitude of systems, and the different physical ingredients that may yield such an effect are well understood. On a microscopic level, the interplay between spin–orbit coupling and a Zeeman field may give rise to a diode effect in a single Josephson junction. However, so far, there is no analytical description of the effect within a simple model. Here, we study a single-channel nanowire with Rashba spin–orbit coupling and in the presence of a Zeeman field. We show that the different Fermi velocities and spin projections of the two pseudo-spin bands lead to a diode effect. Simple analytical expressions for the diode efficiency can be obtained in limiting cases.

Spin orbit coupling enhanced thermionic cooling in two-dimensional semiconductor structures

We theoretically investigate the effect of Rashba spin–orbit coupling (RSO) on thermionics properties of two-dimensional semiconductor. Due to the hybridization of parabolic and linear energy dispersion, the thermal energy equipartition is modified. The thermionic emission from the lower spin branch is enhanced and that from the upper spin branch is reduced. For a double quantum well cooling structure under lower doping, the net heat transported through the structure is enhanced by RSO. The coefficient of performance (COP) is improved by about 20% in the low bias regime and around 3% in the high bias regime. Since the RSO can be further tuned by an applied field, the COP can be further optimized.

Phase jumps in Josephson junctions with time-dependent spin–orbit coupling

Planar Josephson junctions (JJs), based on common superconductors and III–V semiconductors, are sought for Majorana states and fault-tolerant quantum computing. However, with gate-tunable spin–orbit coupling (SOC), we show that the range of potential applications of such JJs becomes much broader. The time-dependent SOC offers unexplored mechanisms for switching JJs, accompanied by the 2π-phase jumps and the voltage pulses corresponding to the single-flux-quantum transitions, key to high-speed and low-power superconducting electronics. In a constant applied magnetic field, with Rashba and Dresselhaus SOC, anharmonic current-phase relations, calculated microscopically in these JJs, yield a nonreciprocal transport and superconducting diode effect. Together with the time-dependent SOC, this allows us to identify a switching mechanism at no applied current bias, which supports fractional-flux-quantum superconducting circuits and neuromorphic computing.

Giant asymmetric proximity-induced spin–orbit coupling in twisted graphene/SnTe heterostructure

We analyze the spin–orbit coupling effects in a 3∘-degree twisted bilayer heterostructure made of graphene and an in-plane ferroelectric SnTe, with the goal of transferring the spin–orbit coupling from SnTe to graphene, via the proximity effect. Our results indicate that the point-symmetry breaking due to the incompatible mutual symmetry of the twisted monolayers and a strong hybridization has a massive impact on the spin splitting in graphene close to the Dirac point, with the spin splitting values greater than 20 meV. The band structure and spin expectation values of graphene close to the Dirac point can be described using a symmetry-free model, triggering different types of interaction with respect to the threefold symmetric graphene/transition-metal dichalcogenide heterostructure. We show that the strong hybridization of the Dirac cone’s right movers with the SnTe band gives rise to a large asymmetric spin splitting in the momentum space. Furthermore, we discover that the ferroelectricity-induced Rashba spin–orbit coupling in graphene is the dominant contribution to the overall Rashba field, with the effective in-plane electric field that is almost aligned with the (in-plane) ferroelectricity direction of the SnTe monolayer. We also predict an anisotropy of the in-plane spin relaxation rates. Our results demonstrate that the group-IV monochalcogenides MX (M = Sn, Ge; X = S, Se, Te) are a viable alternative to transition-metal dichalcogenides for inducing strong spin–orbit coupling in graphene.

Promises and technological prospects of two-dimensional Rashba materials

The Rashba spin–orbit coupling effect, primarily arising from structural-inversion asymmetry in periodic crystals, has garnered considerable attention due to its tunability and potential applications in spintronics. Its capability to manipulate electron spin without an external magnetic field opens new avenues for spintronic device design, particularly in semiconductor technology. Within this framework, 2D Rashba materials hold special interest due to their inherent characteristics, which facilitate miniaturization and engineering capabilities. In this Perspective article, we provide an overview of recent advancements in the research of 2D Rashba materials, aiming to offer a comprehensive understanding of the diverse manifestations and multifaceted implications of the Rashba effect in material science. Rather than merely presenting a list of materials, our approach involves synthesizing various viewpoints, assessing current trends, and addressing challenges within the field. Our objective is to bridge the gap between fundamental research and practical applications by correlating each material with the necessary advancements required to translate theoretical concepts into tangible technologies. Furthermore, we highlight promising avenues for future research and development, drawing from insights gleaned from the current state of the field.

Second-order charge and spin transport in LaO/STO system in the presence of cubic Rashba spin orbit couplings

Under an applied electric field, certain non-centrosymmetric materials with broken time-reversal symmetry may exhibit non-reciprocal transport behavior in which the charge and spin currents contain components that are second order in the electric field. In this study, we investigate the second-order spin accumulation and charge and spin responses in the LaAlO3/SrTiO3 (LaO/STO) system with magnetic dopants under the influence of linear and cubic Rashba spin–orbit coupling (RSOC) terms. We explain the physical origin of the second-order response and perform a symmetry analysis of the first- and second-order responses for different dopant magnetization directions relative to the applied electric field. We then numerically solve the Boltzmann transport equation by extending the approach of Schliemann and Loss (2003 Phys. Rev. B 68 165311) to higher orders in the electric field. We show that the sign of the second-order responses can be switched by varying the magnetization direction of the magnetic dopants or relative strengths of the two cubic RSOC terms and explain these trends by considering the Fermi surfaces of the respective systems. These findings provide insights into the interplay of multiple SOC effects in the LaO/STO system and how the resulting first- and second-order charge and spin responses can be engineered by exploiting the symmetries of the system.

Zero energy bound states on nano atomic line defect in iron-based high temperature superconductors

Motivated by recent scanning tunneling microscopy experiments on Fe atomic line defect in iron-based high temperature superconductors, we explore the origin of the zero energy bound states near the endpoints of the line defect by employing the two-orbit four-band tight binding model. With increasing the strength of the Rashba spin–orbit coupling along the line defect, the zero energy resonance peaks move simultaneously forward to negative energy for s+− pairing symmetry, but split for s++ pairing symmetry. The superconducting order parameter correction due to As(Te, Se) atoms missing does not shift the zero energy resonance peaks. Such the zero energy bound states are induced by the weak magnetic order rather than the strong Rashba spin–orbit coupling on Fe atomic line defect.

Emergent Quantum Phenomena of a Noncentrosymmetric Charge Density Wave in 1T-Transition Metal Dichalcogenides.

1T-transition metal dichalcogenides (TMDs) have been an exciting platform for exploring the intertwinement of charge density waves and strong correlation phenomena. While the David star structure has been conventionally considered as the underlying charge order in the literature, recent scanning tunneling probe experiments on several monolayer 1T-TMD materials have motivated a new, alternative structure, namely, the anion-centered David star structure. In this Letter, we show that this novel anion-centered David star structure manifestly breaks inversion symmetry, resulting in flat bands with pronounced Rashba spin-orbit couplings. These distinctive features unlock novel possibilities and functionalities for 1T-TMDs, including the giant spin Hall effect, the emergence of Chern bands, and spin liquid that spontaneously breaks crystalline rotational symmetry. Our findings establish promising avenues for exploring emerging quantum phenomena of monolayer 1T-TMDs with this novel noncentrosymmetric structure.