Rashba Spin-orbit Coupling Research Articles

Quantum correlations of a two-dimensional electron gas with Rashba spin-orbit coupling

Quantum correlations of a two-dimensional electron gas with Rashba spin-orbit coupling

The Superconducting Dome in Artificial High-Tc Superlattices Tuned at the Fano–Feshbach Resonance by Quantum Design

While the search for new high-temperature superconductors had been driven by the empirical “trials and errors” method for decades, we now report the synthesis of Artificial High-Tc Superlattices (AHTS) designed by quantum mechanics theory at the nanoscale. This discovery paves the way for engineering a new class of high-temperature superconductors, following the predictions of the Bianconi Perali Valletta (BPV) theory recently implemented in 2022 by Mazziotti et al. including Rashba spin-orbit coupling to create nanoscale AHTS composed of quantum wells. The high-Tc superconducting properties within these superlattices are controlled by a conformational parameter of the superlattice geometry, specifically, the ratio L/d which represents the thickness of La2CuO4 layers (L) relative to the superlattice period (d). Using molecular beam epitaxy, we have successfully grown numerous AHTS samples. These samples consist of initial layers of stoichiometric La2CuO4 units with a thickness L, doped by interface space charge, and intercalated with second layers of non-superconducting metallic material, La1.55Sr0.45CuO4 with thickness denoted as W = d − L. This configuration forms a quantum superlattice with periodicity d. The agreement observed between the experimental dependence Tc (the superconducting transition temperature) versus L/d ratio and the predictions of the BPV theory for AHTS in the form of the superconducting dome validates the hypothesis that the superconducting dome arises from the Fano–Feshbach or shape resonance in multigap superconductivity driven by quantum nanoscale confinement.

Topological superconductivity from forward phonon scatterings

Searching for topological superconductors with non-Abelian states has been attracting broad interest. The most commonly used recipe for building topological superconductors utilizes the proximity effect, which significantly limits the working temperature. Here, we propose a mechanism to attain topological superconductivity via forward phonon scatterings. Our crucial observation is that electron-phonon interactions with small momentum transfers favor spin-triplet Cooper pairing under an applied magnetic field. This process facilitates the formation of chiral topological superconductivity even without Rashba spin-orbit coupling. As a proof of concept, we propose an experimentally feasible heterostructure to systematically study the entangled relationship among forward-phonon scatterings, Rashba spin-orbit coupling, pairing symmetries, and the topological property of the superconducting state. This theory not only deepens our understanding of the superconductivity induced by the electron-phonon interaction but also sheds light on the critical role of the electron-phonon coupling in pursuing non-Abelian Majorana quasiparticles.

Impact of Bychkov-Rashba Spin Splitting on Dual Emissions for Lead Halide Perovskite Nanowires.

Bychkov-Rashba spin-orbit coupling (SOC) is decisive for photoinduced photoluminescence (PL) in terms of double emissions. It turns out to be remarkable for one-dimensional lead halide perovskite nanowires (PeNWs). This is primarily due to large surface to volume ratios and structural symmetry breaking fields in the reduced dimension. Systematic studies of the effect of Rashba SOC on PL and its discrimination with the self-trapped exciton in wide temperature and illumination intensity ranges are considerably important and, heretofore, have not been performed. Here, highly crystalline methylammonium lead triiodine (MAPbI3) PeNWs are demonstrated to be able to produce remarkable dual emissions at low temperatures. With extensive analyses by a photoelectrical device-based spin-photogalvanic effect and magnetophotoluminescence, the Rashba effect is proven to be the only factor that governs the dual emissions. We believe a complete understanding of the PL character of PeNWs is beneficial for the development of novel perovskite nanophotonic devices.

Interaction-driven Chern insulating phases in the [formula omitted] lattice with Rashba spin-orbit coupling

The magnetic interaction is a necessary ingredient to break the time-reversal symmetry in realizing quantum anomalous Hall, or Chern insulating phases. Here, we study topological phases in the model, a minimal theoretical model supporting the flat band, taking account of Rashba spin-orbit coupling and flat-band-induced spontaneous ferromagnetism. By analyzing the interaction-driven phase diagrams, band structures, topological edge states, and topological invariants, we demonstrate that this system offers a platform for realizing a wide range of phases, including normal insulators, semimetals, and Chern insulators. Uniquely, there exist both high-Chern-number insulators and valley-polarized Chern insulators. In the latter phase, edge channels exist in the single valley, leading to nearly valley polarization. These findings demonstrate the potential of interaction-driven systems in realizing exotic phases and their promising role in future applications in topology electronics and valleytronics.

Equal-spin Andreev reflection in antiferromagnet/normal/superconductor junctions with Rashba spin–orbit coupling

We have theoretically investigated the Andreev reflection (AR)-induced conductance spectra through antiferromagnet/normal layer/superconductor junctions with hexagonal lattices. When the PT symmetry is broken by the staggered sublattice potential, antiferromagnet may exhibit spin polarization. A gap-edge conductance peak is usually shown, reflecting the characteristic of conventional AR. Equal-spin AR can be generated by the spin-flip scattering caused by Rashba spin–orbit coupling in the normal layer. Surprisingly, when the equal-spin AR process dominates, the conductance peak divides into two peaks near the singlet-gap energy, indicating the existence of spin-triplet pairings in the antiferromagnet. Furthermore, as the amplitudes of the conventional and equal-spin ARs can be modulated by the staggered sublattice potential and electrostatic potential, a conversion from the conductance peak to the conductance peak splitting can be realized, which can help us to distinguish between the spin-singlet and spin-triplet pairings. These findings make the antiferromagnet/superconductor junctions as promising platforms for future superconducting spintronics applications.

Patterning by dynamically unstable spin–orbit-coupled Bose–Einstein condensates

In a two-dimensional atomic Bose–Einstein condensate, we demonstrate Rashba spin–orbit coupling can always introduce dynamical instability into specific zero-quasimomentum states in all parameter regimes. During the evolution of the zero-quasimomentum states, such spin–orbit-coupling-induced instability can fragment the states and lead to a dynamically patterning process. The features of formed patterns are identified from the symmetries of the Bogoliubov–de Gennes Hamiltonian. We show that spin–orbit-coupled Bose–Einstein condensates provide an interesting platform for the investigation of pattern formations.

Topological phase transition on the ruby lattice with Rashba spin–orbit coupling and an exchange field

We investigate the topological phases in a ruby lattice when considering the effects of the intrinsic spin–orbit coupling, the Rashba spin–orbit coupling and an exchange field. The combination of the Rashba coupling and the exchange field leads to the quantum anomalous Hall effect characterized by a nonzero Chern number. We show that the variation of the exchange field drives interesting topological phase transitions between different topological insulating phases, including the quantum spin Hall insulator with spin Chern number Cs=±1, the quantum anomalous Hall insulator with Chern number C=±1 and C=±2. Particularly, the quantum anomalous Hall phases with high Chern numbers C=−4 and C=6 can be realized by tuning the strength of the exchange field. Each phase transition is accompanied by closing of the bulk gap. Furthermore, we find a new topological phase which exhibits the electronic characteristics of both the quantum spin Hall effect and the quantum anomalous Hall effect. The band structures of the edge states confirm the classification of various topological gapped phases.

Dynamical excitation of spin-orbit coupled spin-1 Bose-Einstein condensate in a narrow box trap

We demonstrate the dynamics of a spin-1 ferromagnetic Bose-Einstein condensate (BEC) with Rashba spin-orbit (SO) coupling in a narrow square box trap when an obstacle potential is moving in it. The ground-state energy would be enhanced with the box trap becomes narrower. From the Bogoliubov analytical excitation spectrum lines, we obtain the dependence of critical excitation velocity and momentum on the trap geometry under different SO coupling. Numerically, when the obstacle moves along the direction of plane wave ground state, in addition to spin-density waves being excited periodically, a long density belt accompanied by density island chain is found ahead of the obstacle. Such density distribution is mainly determined by the narrow box trap. A series of density islands arranged more closely are generated as the obstacle moves along opposite direction. There exists another velocity threshold for different density excitation under a larger SO coupling.

Valley-dependent odd quantum Hall states induced by side potential in graphene

The valley degree of freedom endows graphene with many novel physical properties. In this paper, we propose a new method to construct valley-related quantum Hall states using side potentials. We found that due to the effect of side potential, zigzag graphene nanoribbons have rich and practical edge states related to valleys. If the side potential is combined with the out-of-plane exchange field and the Rashba spin–orbit coupling, the zigzag graphene nanoribbons have the characteristics of valley selective anomalous quantum Hall effect. If we further consider the staggered sublattice potential, as long as the strength of the staggered sublattice potential is weaker than the strength of the out-of-plane exchange field, this valley selection feature is robust to the staggered sublattice potential. However, once the strength of the staggered sublattice potential is stronger than that of the out-of-plane exchange field, the valley selection will disappear. Still if a specific side potential is applied, the system will have the characteristics of a typical valley Hall effect. Here we want to emphasize that in the absence of side potential, although the edge state of the system will have the characteristics of the valley hall effect, the energy band of such edge state is mixed with the bulk band; if a specific side potential is applied, these edge states can be brought into the bulk band gap. In order to verify the effectiveness of our proposed method, we further use this method to construct the quantum valley-spin Hall effect in the toy Kane–Mele model.

Nonlinear Photocurrent Responses in Janus WSSe Monolayer

Janus WSSe monolayer is a novel two-dimensional (2D) material that breaks the out-of-plane mirror symmetry and has a large built-in electric field. These features lead to sizable Rashba spin-orbit coupling and enhanced nonlinear optical properties, making it a promising material platform for various spintronic and optoelectronic device applications. In recent years, nonlinear photocurrent responses such as shift and injection currents were found to be closely related to the quantum geometry and Berry curvature of materials, indicating that these responses can serve as powerful tools for probing the novel quantum properties of materials. In this work, we investigate the second-order nonlinear photocurrent responses in a Janus WSSe monolayer theoretically based on first-principles calculations and the Wannier interpolation method. It is demonstrated that the Janus WSSe monolayer exhibits significant out-of-plane nonlinear photocurrent coefficients, which is distinct from the non-Janus structures. Our results also suggest that the second-order nonlinear photocurrent response in the Janus WSSe monolayer can be effectively tuned by biaxial strain or an external electric field. Thus, the Janus WSSe monolayer offers a unique opportunity for both exploring nonlinear optical phenomena and realizing flexible 2D optoelectronic nanodevices.

Shot noise in strained WSe2 multiple barrier structure under spin–orbit interaction

The Rashba spin–orbit coupling (RSOC) effect on the polarization and shot noise properties in a strained WSe2 multiple barrier structure is studied by transfer-matrix technique. The results indicate that the spin-dependent Fano factor depends on the strain and RSOC strength with oscillatory evolution. Thus, the value of Fano factor with spin rotation can be tuned mechanically and by an external electric field. Also, Fano factor shows high sensitivity to the number of electrostatic barriers in strained WSe2 multiple barrier structure. Meanwhile, in a strained WSe2 multiple barrier structure both sign and value of the valley and spin polarizations could be adjusted by tuning both RSOC and strain strength. These findings show that a WSe2 multiple barrier structure under strain and RSOC can be useful for future spintronics, straintronic and valleytronic devices.

Decoupling multiphase superconductivity from normal state ordering in CeRh2As2

${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ is a multiphase superconductor with ${T}_{\text{c}}=0.26$ K. The two superconducting (SC) phases, SC1 and SC2, observed for a magnetic field $H$ parallel to the $c$ axis of the tetragonal unit cell, have been interpreted as even- and odd-parity SC states, separated by a phase boundary at ${\ensuremath{\mu}}_{0}{H}^{*}=4$ T. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally noncentrosymmetric environments of the globally centrosymmetric lattice. The existence of another ordered state (phase I) below a temperature ${T}_{0}\ensuremath{\approx}0.4$ K suggests an alternative interpretation of the ${H}^{*}$ transition: It separates a mixed SC$+$I (SC1) and a pure SC (SC2) state. Here, we present a detailed study of higher-quality single crystals of ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$, showing much sharper signatures at ${T}_{\text{c}}=0.31$ K and ${T}_{0}=0.48$ K. We refine the $T\ensuremath{-}H$ phase diagram of ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ and demonstrate that ${T}_{0}(H)$ and ${T}_{\text{c}}(H)$ lines meet at ${\ensuremath{\mu}}_{0}H\ensuremath{\approx}6$ T, well above ${H}^{*}$, implying no influence of phase I on the SC phase switching. A basic analysis with the Ginzburg-Landau theory indicates a weak competition between the two orders.

Spin-dependent polarization and quantum Hall conductivity in decorated graphene: influence of locally induced spin–orbit-couplings and impurities

We study spin transport through graphene-like substrates in the presence of one or several, locally induced spin–orbit coupling (SOC) terms resulting from periodically placed strips, on their top and decorated with a random distribution of impurities. Intrinsic SOC, Rashba SOC and/or pseudo-spin-inversion-asymmetry coupling are considered. A systematic investigation of the spin conductance identifies the main SOC terms which lead to its energy dependence as well as the extent to which the impurity concentration and each SOC term can affect or tune it, In addition, the spin current flow is considered in the presence of different SOC impurities and their related group symmetry such C 6v , C 3v , D 6h and D 3h . Further, we show that the quantum spin-Hall effect (QSHE) related to the spin edge states depends only on the spin character when the PIA and ISO terms are not sublattice resolved, and on both the spin and sublattice character when they are. In addition, we show that the RSO term plays a major role in obtaining edge states that are either protected on both edges or only on one edge against backscattering. This Rashba term creates an anticrosing gap that affects the symmetry in the edge localizations and leads to half-topological states. The results can facilitate the experimental choice of appropriately decorated strips to (i) develop spin-transistor devices by tuning the Fermi energy, (ii) control the robustness of the QSHE against backscattering even in the presence of on-site sublattice asymmetry induced by a transverse electric field or functionalizations, and (iii) provide a strong theoretical support for spintronic quantum devices.

Spin-singlet topological superconductivity in the attractive Rashba-Hubbard model

Fully gapped, spin-singlet superconductors with antisymmetric spin-orbit coupling in a Zeeman magnetic field provide a promising route to realize superconducting states with non-Abelian topological order and therefore fault-tolerant quantum computation. Here we use a quantum Monte Carlo dynamical cluster approximation to study the superconducting properties of a doped two-dimensional attractive Hubbard model with Rashba spin-orbit coupling in a Zeeman magnetic field. We generally find that the Rashba coupling has a beneficial effect towards $s$-wave superconductivity. In the presence of a finite Zeeman field, when superconductivity is suppressed by Pauli pair breaking, the Rashba coupling counteracts the spin imbalance created by the Zeeman field by mixing the spins, and thus restores superconductivity at finite temperatures. We show that this favorable effect of the spin-orbit coupling is traced to a spin-flip driven enhancement of the amplitude for the propagation of a pair of electrons in time-reversed states. Moreover, by inspecting the Fermi surface of the interacting model, we show that for sufficiently large Rashba coupling and Zeeman field, the superconducting state is expected to be topologically nontrivial.

A controllable spin flip and filter in zigzag graphene nanoribbons with triangular defect

Based on the recursive Green-function method together with Landauer-Büttiker formalism, we theoretically study the controllable spin-dependent electron transport properties by using zigzag graphene nanoribbons (ZGNRs) with triangular nanoconstrictions in the presence of the ferromagnetic strips and spatial modulation of Rashba spin-orbit coupling (RSOC). In this structure, the peaks of spin-flip and spin-conserved conductance can be modulated by the strength of RSOC and exchange magnetic field. The distribution of the local density of states (LDOS) around triangular nanoconstrictions is predicted. The spin filtering effect of ZGNRs with triangular and rectangular nanoconstrictions are compared by spin conductance from left (L) to right (R) lead and R to L lead, respectively. The results might be useful in designing a bidirectional spin converter and spin filter with the modulation of the multi-parameter.

Light‐Induced Giant Rashba Spin–Orbit Coupling at Superconducting KTaO3(110) Heterointerfaces (Adv. Mater. 25/2023)

Superconductivity In article number 2300582, Yulin Gan, Zhicheng Zhong, Yunzhong Chen, Hong Ding, and co-workers report a giant Rashba spin–orbit coupling and light-induced large enhancement at the novel superconducting amorphous-Hf0.5Zr0.5O2/KTaO3(110) heterointerface. These intriguing properties make such a system more competitive than various mainstream materials in both basic science research and applications such as topological superconductivity and highly energy-efficient spintronic devices.

Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor.

In superconductors possessing both time and inversion symmetries, the Zeeman effect of an external magnetic field can break the time-reversal symmetry, forming a conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state characterized by Cooper pairings with finite momentum1,2. In superconductors lacking (local)inversion symmetry, the Zeeman effect may still act as the underlying mechanism of FFLO states by interacting with spin-orbit coupling (SOC). Specifically, the interplay between the Zeeman effect and Rashba SOC can lead to the formation of more accessible Rashba FFLO states that cover broader regions in the phase diagram3-5. However, when the Zeeman effect is suppressed because of spin locking in the presence of Ising-type SOC, the conventional FFLO scenarios are no longer effective. Instead, an unconventional FFLO state is formed by coupling the orbital effect of magnetic fields with SOC, providing an alternative mechanism in superconductors with broken inversion symmetries6-8. Here we report the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite-momentum Cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of a normal metal, a uniform Ising superconducting phase and a six-fold orbital FFLO state. This study highlights an alternative route to achieving finite-momentum superconductivity and provides a universal mechanism to preparing orbital FFLO states in similar materials with broken inversion symmetries.

Twist-angle dependent proximity induced spin-orbit coupling in graphene/topological insulator heterostructures

The proximity-induced spin-orbit coupling (SOC) in heterostructures of twisted graphene and topological insulators (TIs) Bi$_2$Se$_3$ and Bi$_2$Te$_3$ is investigated from first principles. To build commensurate supercells, we strain graphene and correct thus resulting band offsets by applying a transverse electric field. We then fit the low-energy electronic spectrum to an effective Hamiltonian that comprises orbital and spin-orbit terms. For twist angles 0$^\circ\leq\Theta \lessapprox 20^\circ$, we find the dominant spin-orbit couplings to be of the valley-Zeeman and Rashba types, both a few meV strong. We also observe a sign change in the induced valley-Zeeman SOC at $\Theta\approx 10^\circ$. Additionally, the in-plane spin structure resulting from the Rashba SOC acquires a non-zero radial component, except at $0^\circ$ or $30^\circ$. At $30^\circ$ the graphene Dirac cone interacts directly with the TI surface state. We therefore explore this twist angle in more detail, studying the effects of gating, TI thicknesses, and lateral shifts on the SOC parameters. We find, in agreement with previous results, the emergence of the proximitized Kane-Mele SOC, with a change in sign possible by electrically tuning the Dirac cone within the TI bulk band gap.

Large Rashba spin-orbit coupling and high-temperature quantum anomalous Hall effect in Re-intercalated graphene/CrI3 heterostructure

In 2010, the quantum anomalous Hall effect (QAHE) in graphene was proposed in the presence of Rashba spin-orbit coupling and a ferromagnetic exchange field. After a decade of experimental exploration, the anomalous Hall conductance can only reach about 0.25 in units of $2{e}^{2}/h$, which was attributed to the tiny Rashba spin-orbit coupling. Here, we show theoretically that Re-intercalation in a $\text{graphene}/{\mathrm{CrI}}_{3}$ heterostructure can not only induce sizable Rashba spin-orbit coupling ($>40$ meV), but also open up large band gaps at valleys $K$ (22.2 meV) and ${K}^{\ensuremath{'}}$ (30.3 meV), and a global band gap over 5.5 meV (19.5 meV with random Re distribution) hosting QAHE. A low-energy continuum model is constructed to explain the underlying physical mechanism. We find that Rashba spin-orbit coupling is robust against external stress whereas a tensile strain can increase the global bulk gap. Furthermore, we comprehensively explore the electronic properties of $3d, 4d, 5d$ transition-metal intercalation in graphene/${\mathrm{CrI}}_{3}$ systems.