Strong Spin-orbit Interaction Research Articles

Link between supercurrent diode and anomalous Josephson effect revealed by gate-controlled interferometry

In Josephson diodes the asymmetry between positive and negative current branch of the current-phase relation leads to a polarity-dependent critical current and Josephson inductance. The supercurrent nonreciprocity can be described as a consequence of the anomalous Josephson effect —a φ0-shift of the current-phase relation— in multichannel ballistic junctions with strong spin-orbit interaction. In this work, we simultaneously investigate φ0-shift and supercurrent diode efficiency on the same Josephson junction by means of a superconducting quantum interferometer. By electrostatic gating, we reveal a direct link between φ0-shift and diode effect. Our findings show that spin-orbit interaction in combination with a Zeeman field plays an important role in determining the magnetochiral anisotropy and the supercurrent diode effect.

Magnetotransport properties of BiSe whiskers

Doped Bi2Se3 superconductors, such as CuxBi2Se3, SrxBi2Se3, NbxBi2Se3 are shown to be nematic superconductors in addition to the observed topological superconductivity. Recently it was found superconductivity in PdxBi2Se3 whiskers. The superconductivity has a residual character and probably exists on the whisker surface. The present paper deals with checking a possible nematicity existing in PdxBi2Se3 whiskers. The suppression of superconductivity was measured in the magnetic fields with induction up to 1.5 T. It was found the dependency of upper critical fields on the angle of magnetic field application. The dependency is explained in the framework of Ginzburg–Landau model and described by rather small anisotropy parameter 1.2. Besides blurring of temperature Curie near 5.3 K was observed of about 0.5 K that additionally confirmed the nematicity in PdxBi2Se3 whiskers. The rather high Curie temperature as well as nematicity existing is probably connected with strong spin–orbit interaction (SOI) of the charge carriers in the whiskers which stabilizing the superconductive state. When the temperature exceeds T c weak antilocalization of charge carriers is observed in the whisker magnetoresistance.

Anisotropic exchange interaction of two hole-spin qubits

AbstractSemiconductor spin qubits offer the potential to employ industrial transistor technology to produce large-scale quantum computers. Silicon hole spin qubits benefit from fast all-electrical qubit control and sweet spots to counteract charge and nuclear spin noise. However, the demonstration of a two-qubit interaction has remained an open challenge. One missing factor is an understanding of the exchange coupling in the presence of a strong spin–orbit interaction. Here we study two hole-spin qubits in a silicon fin field-effect transistor, the workhorse device of today’s semiconductor industry. We demonstrate electrical tunability of the exchange splitting from above 500 MHz to close-to-off and perform a conditional spin-flip in 24 ns. The exchange is anisotropic because of the spin–orbit interaction. Upon tunnelling from one quantum dot to the other, the spin is rotated by almost 180 degrees. The exchange Hamiltonian no longer has the Heisenberg form and can be engineered such that it enables two-qubit controlled rotation gates without a trade-off between speed and fidelity. This ideal behaviour applies over a wide range of magnetic field orientations, rendering the concept robust with respect to variations from qubit to qubit, indicating that it is a suitable approach for realizing a large-scale quantum computer.

Field-Free Spin-Orbit Torque Magnetization Switching in a Perpendicularly Magnetized Semiconductor (Ga,Mn)As Single Layer.

Current-induced spin-orbit torque (SOT) in a perpendicularly magnetized single layer has a strong potential to switch the magnetization using an extremely low current density, which is generally 2-3 orders of magnitude smaller than that required for conventional metal bilayer systems. However, an in-plane external magnetic field has to be applied to break the symmetry and achieve deterministic switching. To further enhance the high-density integration and accelerate the practical application of highly efficient SOT magnetic random-access memory (SOT-MRAM) devices, field-free SOT magnetization switching in a ferromagnetic single layer is strongly needed. In a spin-orbit ferromagnet (a ferromagnet with strong spin-orbit interaction) with crystal inversion asymmetry and a multi-domain structure, the internal Dzyaloshinskii-Moriya effective fields are considered to induce field-free switching. Here, combined with strong spin-orbit coupling and a tilted anisotropy axis induced by a nonuniform Mn distribution and a possible magnetocrystalline anisotropy resulting from a slight substrate tilting, we successfully achieve magnetization switching in a spin-orbit ferromagnet (Ga,Mn)As single layer by utilizing SOT without applying any external magnetic field. Our findings help to deeply elucidate the SOT switching mechanism and can advance the development of a highly efficient MRAM with better scalability.

Spin Unlocked Vortex Beam Generation on Nonlinear Chiroptical Metasurfaces.

Optical vortices with spin and orbital angular momentum (SAM and OAM) states offer multiple degrees of freedom for manipulating optical fields and thus enable great potentials in optical information processing. Recently, the optical metasurface has become an important platform for vortex beam generation and steering. However, the strong spin-orbit interaction on such metasurfaces usually leads to spin locked OAM generation, which limits the complete control of the angular momentum state of light. Here, we propose to solve this constraint using geometric phase controlled nonlinear chiroptical metasurfaces. The metasurface consists of two types of plasmonic meta-atoms which have opposite handedness and exhibit a strong spin-dependent circular dichroism effect. By encoding specific phase singularities and phase gradients to different channels, we experimentally demonstrate the spin unlocked second harmonic beam steering. The proposed nonlinear chiroptical metasurfaces may have important applications in developing multifunctional nonlinear optical devices.

Strong coupling between a microwave photon and a singlet-triplet qubit

Combining superconducting resonators and quantum dots has triggered tremendous progress in quantum information, however, attempts at coupling a resonator to even charge parity spin qubits have resulted only in weak spin-photon coupling. Here, we integrate a zincblende InAs nanowire double quantum dot with strong spin-orbit interaction in a magnetic-field resilient, high-quality resonator. The quantum confinement in the nanowire is achieved using deterministically grown wurtzite tunnel barriers. Our experiments on even charge parity states and at large magnetic fields, allow us to identify the relevant spin states and to measure the spin decoherence rates and spin-photon coupling strengths. We find an anti-crossing between the resonator mode in the single photon limit and a singlet-triplet qubit with a spin-photon coupling strength of g/2π = 139 ± 4 MHz. This coherent coupling exceeds the resonator decay rate κ/2π = 19.8 ± 0.2 MHz and the qubit dephasing rate γ/2π = 116 ± 7 MHz, putting our system in the strong coupling regime.

Beating-free quantum oscillations in two-dimensional electron gases with strong spin-orbit and Zeeman interactions

Beating-free quantum oscillations in two-dimensional electron gases with strong spin-orbit and Zeeman interactions

Спинтроника немагнитных хиральных сред на примере эффекта Зеебека

Spin transport in chiral materials is considered as the main direction for the spintronics development. Experimental work on the study of the spin Seebeck effect shows that in chiral materials it is possible to achieve effective generation of spin-polarized current at distances up to several millimeters. In the presented work, we develop a theoretical approach to describe the spin Seebeck effect based on the spin Hamiltonian for conduction electrons in a chiral medium. Taking into account this Hamiltonian, the equation of electron spin dynamics averaged over the polycrystalline sample is obtained. In the approximation of ideal Fermi-gas and local-quasi-equilibrium distribution, the spin density operator of conduction electrons is constructed. It is shown that averaging over randomly oriented crystallites does not destroy spin ordering at strong spin-orbit interaction, and the temperature gradient generates spin polarization directed predominantly along the temperature gradient. Spin polarization by the described in the paper mechanism does not require external magnetic fields and remanent magnetization and therefore does not interfere with the operation of micro- and nano-sized structures of spintronics. Since the spin Seebeck effect is reciprocal with the spin Peltier effect, the theoretical approach presented in this work can form the basis of new methods for controlling heat flow in spintronics systems.

Генерация и детектирование спинового тока в гетероструктурах иридат/манганит

Kotel'nikov Institute of Radioengineering and Electronics of Russian Academy of Sciences The results of experimental studies on spin current at the interface of iridate/manganite heterostructure SrIrO3/La0.7Sr0.3MnO3 consisted of oxide epitaxial films with nanometer thickness are presented. A pure spin current was induced by microwave irradiation in GHz frequency band under conditions of ferromagnetic resonance. The spin current was detected due to inverse spin-Hall effect measuring the spectral characteristics of charge current arising on electrically conductive SrIrO3 film with strong spin-orbit interaction. The spin-Hall angle, which characterizes the efficiency of spin current conversion to the charge current, was determined by measurements of the angular dependences of spin magnetoresistance of the iridate/manganite interface.

Robust photogalvanic effect, full spin polarization and pure spin current in the BiC photodetector by vacancy and substitution-doping

The photogalvanic effects (PGEs) in low-dimensional devices have attracted great interests recently. Herein, based on non-equilibrium Green’s function combined with density functional theory, we investigated spin-dependent PGE phenomena in the BiC photodetector with the linearly polarized light and zero bias. Due to the presence of strong spin–orbit interaction (SOI) and C 3v symmetry for the BiC monolayer, the armchair and zigzag BiC photodetectors can produce robust spin-dependent PGEs which possess the cos(2θ) and sin(2θ) relations on the photon energies, respectively. Especially, the pristine armchair and armchair Bi-vacancy BiC photodetectors can produce fully spin polarization, and pure spin current was found in the pristine armchair and zigzag BiC photodetector, respectively. Furthermore, after introducing the Bi-vacancy, C-vacancy, Bi-doping and C-doping respectively, the BiC photodetector can produce higher spin-dependent PGEs for their C s symmetry. Additionally, the behaviors of spin-dependent photoresponse are highly anisotropic which can be tuned by the photon energy. This work suggested great potential applications of the BiC monolayer on PGE-driven photodetectors in low energy-consumption optoelectronics and spintronic devices.

Spin orbit induced anisotropic giant effective g factor and reduced effective mass in non-magnetic Sn1−xSrxTe mixed crystal

The binary and ternary telluride systems containing europium as one of the magnetic components exhibit a higher effective g factor and much lower effective mass due to strong spin-orbit and s/p-f hybridized interaction mediated by the dopant impurity. In this communication, we have exploited the same for a ternary mixed crystal Sn1−xSrxTe, with strontium as the non-magnetic counterpart of europium. We have derived a functional equation within the framework of effective mass representation, accounting for both spin-orbit interaction and the presence of a magnetic field. In the context of k→⋅π→ model, at first, two MW (Mitchell-Wallis) band edge states are diagonalized properly, and subsequently, the off-band edge states are treated using perturbation up to the k2 level. We derived the formulas for the effective g factor and effective mass by employing Green's function method, considering the presence of spin-orbit interaction, magnetic field, and Sr impurity and assuming band inversion model as applicable to Sn1−xSrxTe system. Finally, we conduct a comprehensive examination of the effective g factor, effective mass, and their respective anisotropies in Sn1−xSrxTe considering their dependence on carrier concentration and impurity levels at T=300K. We present extremely elevated effective g factor; g=2230, for n−Sn1−xSrxTe and g=2160, for p−Sn1−xSrxTe respectively with large anisotropies and correspondingly low effective mass mc,v=0.003m0 around x=0.014 in the concentration range 0.01×1019 cm−3 to 0.1×1019 cm−3. Irrespective of the unsaturated electronic shell of Sr, the existence of strong spin-orbit interaction refers to the strong coupling of band edge states across the band minima triggered by substitutional impurity. The occurrence of a substantial effective g factor and low effective mass is unique in such non-magnetic Sn1−xSrxTe mixed crystal ever reported.

Phase-Driving Hole Spin Qubits.

The spin-orbit interaction in spin qubits enables spin-flip transitions, resulting in Rabi oscillations when an external microwave field is resonant with the qubit frequency. Here, we introduce an alternative driving mechanism mediated by the strong spin-orbit interactions in hole spin qubits, where a far-detuned oscillating field couples to the qubit phase. Phase-driving at radio frequencies, orders of magnitude slower than the microwave qubit frequency, induces highly nontrivial spin dynamics, violating the Rabi resonance condition. By using a qubit integrated in a silicon fin field-effect transistor, we demonstrate a controllable suppression of resonant Rabi oscillations and their revivals at tunable sidebands. These sidebands enable alternative qubit control schemes using global fields and local far-detuned pulses, facilitating the design of dense large-scale qubit architectures with local qubit addressability. Phase-driving also decouples Rabi oscillations from noise, an effect due to a gapped Floquet spectrum and can enable Floquet engineering high-fidelity gates in future quantum processors.

Spin-filtered measurements of Andreev bound states in semiconductor-superconductor nanowire devices

Semiconductor nanowires coupled to superconductors can host Andreev bound states with distinct spin and parity, including a spin-zero state with an even number of electrons and a spin-1/2 state with odd-parity. Considering the difference in spin of the even and odd states, spin-filtered measurements can reveal the underlying ground state. To directly measure the spin of single-electron excitations, we probe an Andreev bound state using a spin-polarized quantum dot that acts as a bipolar spin filter, in combination with a non-polarized tunnel junction in a three-terminal circuit. We observe a spin-polarized excitation spectrum of the Andreev bound state, which can be fully spin-polarized, despite strong spin-orbit interaction in the InSb nanowires. Decoupling the hybrid from the normal lead causes a current blockade, by trapping the Andreev bound state in an excited state. Spin-polarized spectroscopy of hybrid nanowire devices, as demonstrated here, is proposed as an experimental tool to support the observation of topological superconductivity.

Zeeman splitting in doped antiferromagnetic semiconductors

Zeeman splitting of conduction electrons in doped antiferromagnetic materials in the presence of a strong spin–orbit interaction is considered. Using symmetry arguments the Hamiltonian and the spectrum of conduction electrons near some time-reversal invariant momenta were constructed. It was shown, that in the vicinity of some time-reversal invariant momenta the external magnetic field perpendicular to the Neel axis does not lead to the Zeeman splitting. The Kramers degeneracy is preserved in that case. The absence of the Zeeman splitting in antiferromagnets leads to the absence of spin zeros in quantum oscillation experiments. Recent experimental confirmation of the predicted effects in organic antiferromagnet and High-Tc Superconductors is discussed.

Spin magnetoresistance of thin film structures of manganite and material with strong spin-orbit interaction.

Spin magnetoresistance of thin film structures of manganite and material with strong spin-orbit interaction.

Oscillator Strengths and Cross Sections of the Valence-Shell Excitations of HBr Studied by Fast Electron Impact.

The oscillator strengths and cross sections of the valence-shell excitations of HBr were determined by fast electron scattering with an incident electron energy of 1500 eV and an energy resolution of 80 meV. The momentum transfer dependence behaviors of the generalized oscillator strengths have been used to elucidate the transition characteristics. The present results show that the strong spin-orbital interaction results in the observation of some triplet states in the (Λ, S) coupling and the constant generalized oscillator strength ratios for the pair states with the same electronic configuration and quantum number Ω, and the quantitative spin-orbit coupling coefficients of b3Π1(v = 0) and C1Π(v = 0) are determined. The optical oscillator strengths of the valence-shell excitations were obtained by extrapolating the generalized oscillator strengths to the limit of zero squared momentum transfer. The present optical oscillator strengths give an independent cross-check of the previous experimental and theoretical results, and the comparison shows that the line-saturation effect is more severe for the high Rydberg states with large intensities and narrow natural widths. The integral cross sections of the valence-shell excitations of HBr were obtained from the excitation threshold to 5000 eV by the BE-scaling method. The present oscillator strengths and cross sections supplement the fundamental molecular database of HBr and can be used for modeling in the semiconductor industry, astrophysics, and atmospheric chemistry.

Evidence of pseudogravitational distortions of the Fermi surface geometry in the antiferromagnetic metal FeRh

The confluence between high-energy physics and condensed matter has produced groundbreaking results via unexpected connections between the two traditionally disparate areas. In this work, we elucidate additional connectivity between high-energy and condensed matter physics by examining the interplay between spin-orbit interactions and local symmetry-breaking magnetic order in the magnetotransport of thin-film magnetic semimetal FeRh. We show that the change in sign of the normalized longitudinal magnetoresistance observed as a function of increasing in-plane magnetic field results from changes in the Fermi surface morphology. We demonstrate that the geometric distortions in the Fermi surface morphology are more clearly understood via the presence of pseudogravitational fields in the low-energy theory. The pseudogravitational connection provides additional insights into the origins of a ubiquitous phenomenon observed in many common magnetic materials and points to an alternative methodology for understanding phenomena in locally-ordered materials with strong spin-orbit interactions.

Hole-Spin Driving by Strain-Induced Spin-Orbit Interactions.

Hole spins in semiconductor quantum dots can be efficiently manipulated with radio-frequency electric fields owing to the strong spin-orbit interactions in the valence bands. Here we show that the motion of the dot in inhomogeneous strain fields gives rise to linear Rashba spin-orbit interactions (with spatially dependent spin-orbit lengths) and g-factor modulations that allow for fast Rabi oscillations. Such inhomogeneous strains build up spontaneously in the devices due to process and cool down stress. We discuss spin qubits in Ge/GeSi heterostructures as an illustration. We highlight that Rabi frequencies can be enhanced by 1 order of magnitude by shear strain gradients as small as 3×10^{-6} nm^{-1} within the dots. This underlines that spins in solids can be very sensitive to strains and opens the way for strain engineering in hole spin devices for quantum information and spintronics.

Spectroscopic study of cooperative states of Nd3+ pair centers in fluorite-type crystals: A competition of dipole-dipole and quadrupole-quadrupole energy exchange

Pair centers of Nd3+ ions in the CdF2, CaF2, and SrF2 doped crystals at T=6.5K were studied under tunable narrow-linewidth laser excitation. It has been established that the exchange of electronic excitation leads to the splitting of the one-exciton cooperative 4G5/2(1) x 4I9/2(1) state by 2–3 cm−1 with a non-monotonic dependence on the distance between the ions in the pairs in these crystals. This dependence is explained by the competition of forced dipole-dipole and quadrupole-quadrupole interactions with spin flip. The high rate of such exchange mechanisms is due to strong spin–orbit interaction and small energy difference between the 4f levels of Nd3+ ions. It is shown that the pair centers in all one-exciton cooperative states, in contrast to the ground and two-exciton excited states, have nonzero static dipole moments. This makes it possible to use the latter states as ultrafast qubits, and one-exciton states as control qubit to implement a CNOT quantum gate.

Electric Control of Helicity-Dependent Photocurrent and Surface Polarity Detection on Two-Dimensional Bi2O2Se Nanosheets.

Bismuth oxyselenide (Bi2O2Se) is a two-dimensional (2D) layered semiconductor material with high electron Hall mobility and excellent environmental stability as well as strong spin-orbit interaction (SOI), which has attracted intense attention for application in spintronic and spin optoelectronic devices. However, a comprehensive study of spin photocurrent and its microscopic origin in Bi2O2Se is still missing. Here, the helicity-dependent photocurrent (HDPC) was investigated in Bi2O2Se nanosheets. By analyzing the dependence of HDPC on the angle of incidence, we find that the HDPC originates from surface states with Cs symmetry in Bi2O2Se, which can be attributed to the circular photogalvanic effect (CPGE) and circular photon drag effect (CPDE). It is revealed that the HDPC current almost changes linearly with the source-drain voltage. Furthermore, we demonstrate effective tuning of HDPC in Bi2O2Se by ionic liquid gating, indicating that the spin splitting of the surface electronic structure is effectively tuned. By analyzing the gate voltage dependence of HDPC, we can unambiguously identify the surface polarity and the surface electronic structure of Bi2O2Se. The large HDPC in Bi2O2Se nanosheets and its efficient electrical tuning demonstrate that 2D Bi2O2Se nanosheets may provide a good platform for opto-spintronics devices.