Persistent Spin Research Articles

Spin torque and persistent currents caused by percolation of topological surface states

The topological insulator/ferromagnetic metal (TI/FMM) bilayer thin films emerged as promising topological surface state-based spintronic devices, most notably in their efficiency of current-induced spin torque. Using a cubic lattice model, we reveal that the surface state Dirac cone of the TI can gradually merge into or be highly intertwined with the FMM bulk bands, and the surface states percolate into the FMM and eventually hybridize with the quantum well states therein. The magnetization can distort the spin-momentum locking of the surface states and yield an asymmetric band structure, which causes a laminar flow of room temperature persistent charge current. Moreover, the proximity to the FMM also promotes a persistent laminar spin current. Through a linear response theory, we elaborate that both the surface state and the FMM bulk bands contribute to the current-induced spin torque, and their real wave functions render the spin torque predominantly field-like, with a magnitude highly influenced by the degree of the percolation of the surface states. On the other hand, impurities can change the spin polarization expected from the Edelstein effect and generate a damping-like torque, and produce a torque even when the magnetization points in-plane and orthogonal to the current direction.

Persistent charge and spin currents in the 1D Holstein-Hubbard ring at half filling and at away from half filling by Bethe-ansatz approach

The persistent charge and spin currents are studied by considering the Holstein-Hubbard model for one dimensional mesoscopic ring. Conventional Lang-Firsov transformation is used to eliminate the phonon coordinates and then the zero phonon state is applied to obtain the effective electronic Hamiltonian which is then treated exactly by the Bethe-Ansatz technique. The coupled transcendental equations that arise in this method are solved numerically and the persistent charge and spin currents are studied with respect to the on-site electron-electron Coulomb correlation (U), on-site electron-phonon interaction strength (g0) and inter-site electron-phonon interaction strength (g1) at half-filling and away from half-filling.

Aharonov–Casher effect and persistent spin currents in a Coulomb-type potential induced by Lorentz symmetry breaking effects

We investigate quantum effects on a nonrelativistic neutral particle with a permanent magnetic dipole moment that interacts with an electric field. This neutral particle is also under the influence of a background that breaks the Lorentz symmetry. We focus on the Lorentz symmetry violation background determined by a space-like vector field. Then, we show that the effects of the violation of Lorentz symmetry can yield an attractive Coulomb-type potential. Furthermore, we obtain the bound state solutions to the Schrödinger–Pauli equation and show that the spectrum of energy is a function of the Aharonov–Casher geometric quantum phase. Finally, we discuss the arising of persistent spin currents.

Discovery Principles and Materials for Symmetry-Protected Persistent Spin Textures with Long Spin Lifetimes

Summary Persistent spin textures (PSTs) in solid-state materials arise from a unidirectional spin-orbit field in momentum space and offer a route to deliver the necessary long carrier spin lifetimes utilized in future quantum microelectronic devices. Nonetheless, few bulk materials host PSTs owing to crystal symmetry and chemical requirements, with even fewer experimentally demonstrated examples. This scarcity makes both PST materials discovery and performance assessment challenging. Here, we demonstrate that bulk PSTs exist in the family of layered A3B2O7 oxides and that a persistent spin helix (PSH) occurs over a large region of the Brillouin zone—a feature essential to experimental realization. By solving the spin-diffusion equations in the strong coupling limit and using the PST performance criteria we formulate, we find that the spin lifetime of the PSH is ∼63 ns in Sr3Hf2O7—substantially larger than that predicted (∼10 ns) and demonstrated experimentally (∼600 ps) in GaAs/AlGaAs quantum wells.

Kondo effect under the influence of spin–orbit coupling in a quantum wire

The analysis of the impact of spin–orbit coupling (SOC) on the Kondo state has generated considerable controversy, mainly regarding the dependence of the Kondo temperature TK on SOC strength. Here, we study the one-dimensional (1D) single impurity Anderson model (SIAM) subjected to Rashba (α) and Dresselhaus (β) SOC. It is shown that, due to time-reversal symmetry, the hybridization function between impurity and quantum wire is diagonal and spin independent (as it is the case for the zero-SOC SIAM), thus the finite-SOC SIAM has a Kondo ground state similar to that for the zero-SOC SIAM. This similarity allows the use of the Haldane expression for TK, with parameters renormalized by SOC, which are calculated through a physically motivated change of basis. Analytic results for the parameters of the SOC-renormalized Haldane expression are obtained, facilitating the analysis of the SOC effect over TK. It is found that SOC acting in the quantum wire exponentially decreases TK while SOC at the impurity exponentially increases it. These analytical results are fully supported by calculations using the numerical renormalization group (NRG), applied to the wide-band regime, and the projector operator approach, applied to the infinite-U regime. Literature results, using quantum Monte Carlo, for a system with Fermi energy near the bottom of the band, are qualitatively reproduced, using NRG. In addition, it is shown that the 1D SOC SIAM for arbitrary α and β displays a persistent spin helix SU(2) symmetry similar to the one for a 2D Fermi sea with the restriction α = β.

Tunable spin textures in polar antiferromagnetic hybrid organic\u2013inorganic perovskites by electric and magnetic fields

The hybrid organic–inorganic perovskites (HOIPs) have attracted much attention for their potential applications as novel optoelectronic devices. Remarkably, the Rashba band splitting, together with specific spin orientations in k-space (i.e., spin texture), has been found to be relevant for the optoelectronic performances. In this work, by using first-principles calculations and symmetry analysis, we study the electric polarization, magnetism, and spin texture properties of the antiferromagnetic (AFM) ferroelectric HOIP TMCM-MnCl3 (TMCM = (CH3)3NCH2Cl+, trimethylchloromethyl ammonium). This recently synthesized compound is a prototype of order–disorder and displacement-type ferroelectric with a large piezoelectric response, high ferroelectric transition temperature, and excellent photoluminescence properties as reported by You (Science 357:306, 2017). The most interesting result is that the inversion symmetry breaking coupled to the spin–orbit coupling gives rise to a Rashba-like band splitting and a related robust persistent spin texture (PST) and/or typical spiral spin texture, which can be manipulated by tuning the ferroelectric or, surprisingly, also by the AFM order parameter. The tunability of spin texture upon switching of AFM order parameter is largely unexplored and our findings not only provide a platform to understand the physics of AFM spin texture but also support the AFM HOIP ferroelectrics as a promising class of optoelectronic materials.

Strain-controlled spin-splitting in the persistent spin helix state of two-dimensional SnSe monolayer

Abstract Recently, ferroelectric materials have attracted much attention in the field of spintronics since they enable to manipulation of spin splitting and spin textures via electric control of electric polarization. Here, two-dimensional ferroelectric SnSe monolayer (ML) comprises a promising candidate for spintronics since it has large and switchable in-plane electric polarization. However, the application of strain strongly modifies the electronic properties of the SnSe ML, which is expected to significantly affect the properties of the spin-splitting bands. By using fully-relativistic density-functional theory calculations, we predict that a strongly anisotropic spin splitting is observed in the energy bands near both the conduction band minimum (CBM) and valence band maximum (VBM), which can be effectively tuned by applying a small uniaxial in-plane strain. We demonstrate that these spin-splitting bands exhibit a unidirectional out-of-plane spin configuration in the momentum space, giving rise to a long-lived persistent spin helix (PSH) state. Importantly, a giant spin splitting (up to 245 meV) is achieved under the uniaxial in-plane tensile strain without losing the nature of the PSH state, thus offering a possibility for realizing efficient spintronic devices enabling operation at room temperature.

Persistent spin dynamics and absence of spin freezing in the H−T phase diagram of the two-dimensional triangular antiferromagnet YbMgGaO4

We report results of muon spin relaxation and rotation ($\mu$SR) experiments on the spin-liquid candidate~YbMgGaO$_{4}$. No static magnetism $\gtrsim 0.003\mu_B$ per Yb ion, ordered or disordered, is observed down to 22~mK, a factor of two lower in temperature than previous measurements. Persistent (temperature-independent) spin dynamics are observed up to 0.20~K and at least 1~kOe, thus extending previous zero-field $\mu$SR results over a substantial region of the $H$-$T$ phase diagram. Knight shift measurements in a 10-kOe transverse field reveal two lines with nearly equal amplitudes. Inhomogeneous muon depolarization in a longitudinal field, previously characterized by stretched-exponential relaxation due to spatial inhomogeneity, is fit equally well with two exponentials, also of equal amplitudes. We attribute these results to two interstitial muon sites in the unit cell, rather than disorder or other spatial distribution. Further evidence for this attribution is found from agreement between the ratio of the two measured relaxation rates and calculated mean-square local Yb$^{3+}$ dipolar fields at candidate muon sites. Zero-field data can be understood as a combination of two-exponential dynamic relaxation and quasistatic nuclear dipolar fields.

Toggling Valley-Spin Locking and Nonlinear Optical Properties of Single-Element Multiferroic Monolayers via Light

Ferroicities in structural ferroic materials can effectively tune many different fascinating physical properties, leading to potential applications in the future. Usually, in order to break inversion symmetry in ferroelectrics, the material needs to be composed of multiple types of element. Here, we study a single element multiferroic material with time-reversal symmetry, monolayer $\mathrm{Bi}(110)$, which possesses multiple types of ferroic (ferroelectric and ferroelastic) order. By performing first-principles calculations combined with a group-theory analysis, we reveal that this multiferroic material exhibits various interesting properties, such as an out-of-plane persistent spin helix, valleytronics, and promising nonlinear optical properties, which depend on its orientation variant. We show further that under illumination with linearly polarized light, different orientation variants in the $\mathrm{Bi}(110)$ monolayer possess different free energies. Thus, one can use this noncontacting approach to control the thermodynamic stability of different types of order. The switching of the orientation variant is ultrafast and can occur in times on the order of picoseconds. During this process, no electron-hole pairs are generated. Therefore, one can eliminate waste heat during operation, ensuring good reversibility.

Persistent currents and spin torque caused by percolated quantum spin Hall state

Motivated by recent experiments, we investigate the quantum spin Hall state in 2D topological insulator/ferromagnetic metal planar junctions by means of a tight-binding model and linear response theory. We demonstrate that whether the edge state Dirac cone is submerged into the ferromagnetic subbands and the direction of the magnetization dramatically affect (i) how the edge state percolates into the ferromagnet, and (ii) the spin-momentum locking of the edge state. Laminar flows of room temperature persistent charge and spin currents near the interface are uncovered. In addition, the current-induced spin polarization at the edge of the 2D topological insulator is found to be dramatically enhanced near the impurities. The current-induced spin polarization in the ferromagnet is mainly polarized in the out-of-plane direction ${\hat{\bf z}}$, rendering a current-induced spin torque that is predominantly field-like $\propto {\bf S}\times{\hat{\bf z}}$.

Enhanced longevity of the spin helix in low-symmetry quantum wells

In a semiconductor, collective excitations of spin textures usually decay rather fast due to D'yakonov-Perel' spin relaxation. The latter arises from spin-orbit coupling, which induces wave-vector-dependent spin rotations that, in conjunction with random disorder scattering, generate spin decoherence. However, symmetries occurring under certain conditions can prevent the relaxation of particular homogeneous and inhomogeneous spin textures. The inhomogeneous spin texture, termed as persistent spin helix, is especially appealing as it enables us to manipulate the spin orientation while retaining a long spin lifetime. Recently, it was predicted that such symmetries can be realized in zinc-blende two-dimensional electron gases if at least two growth-direction Miller indices agree in modulus and the coefficients of the Rashba and linear Dresselhaus spin-orbit couplings are suitably matched [PRL 117, 236801 (2016)]. In the present paper, we systematically analyze the impact of the symmetry-breaking cubic Dresselhaus spin-orbit coupling, which generically coexists in these systems, on the stability of the emerging spin helices with respect to the growth direction. We find that, as an interplay between orientation and strength of the effective magnetic field induced by the cubic Dresselhaus terms, the spin relaxation is weakest for a low-symmetry growth direction that can be well approximated by a [225] lattice vector. These quantum wells yield a 30\% spin-helix lifetime enhancement compared to [001]-oriented electron gases and, remarkably, require a negligible Rashba coefficient. The rotation axis of the corresponding spin helix is only slightly tilted out of the quantum-well plane. This makes the experimental study of the spin-helix dynamics readily accessible for conventional optical spin orientation measurements where spins are excited and detected along the quantum-well growth direction.

Unconventional spin excitations in the S=32 triangular antiferromagnet RbAg2Cr[VO4]2

We present muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements of the $S=3/2$ undistorted triangular lattice established in ${\mathrm{RbAg}}_{2}\mathrm{Cr}{[{\mathrm{VO}}_{4}]}_{2}$. The zero- (ZF) and longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ spectra evidence the absence of spin freezing and long-range magnetic ordering down to $T=25$ mK, supporting the formation of a dynamic ground state. Noticeably, we observe an anomalous temperature dependence of the ZF muon spin relaxation rate ${\ensuremath{\lambda}}_{Z}F(T)$, featuring a decrease below $T=20$ K. This suggests the alteration of the dominant relaxation mechanism by the development of short-range magnetic correlations. A subsequent leveling off of ${\ensuremath{\lambda}}_{Z}F(T)$ below $T=2$ K indicates persistent spin dynamics and reveals the presence of exotic magnetic excitations. The field dependence of the muon spin relaxation rate at $T=25$ mK is well described by a diffusive spin transport model with algebraic spin-spin correlations. The suppressed long-range order and the peculiar temperature-dependent behavior of ${\ensuremath{\lambda}}_{Z}F(T)$ will be discussed in terms of the exchange interaction between ${\mathrm{Cr}}^{3+}$ moments via nonmagnetic ${[{\mathrm{VO}}_{4}]}^{3\ensuremath{-}}$ entities. In the title compound, the degeneracy of the ${t}_{2g}$-orbital set is not lifted by a space group symmetry reduction or subject to significant anisotropy resulting from spin-orbit coupling.

Persistent spin dynamics in the pressurized spin-liquid candidate YbMgGaO4

Single-crystal x-ray diffraction, density-functional band-structure calculations, and muon spin relaxation ($\mu$SR) are used to probe pressure evolution of the triangular spin-liquid candidate YbMgGaO$_4$. The rhombohedral crystal structure is retained up to at least 10 GPa and shows a nearly uniform compression along both in-plane and out-of-plane directions, whereas local distortions caused by the random distribution of Mg$^{2+}$ and Ga$^{3+}$ remain mostly unchanged. The $\mu$SR data confirm persistent spin dynamics up to 2.6 GPa and down to 250 mK with no change in the muon relaxation rate. Longitudinal-field $\mu$SR reveals power-law behavior of the spin-spin autocorrelation function, both at ambient pressure and upon compression.

Persistent spin textures and currents in wurtzite nanowire-based quantum structures

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to bandstructure characteristics which allows to deduce spin-orbit coefficients and other band parameters from measurements.

Critical supercurrent and φ0 state for probing a persistent spin helix

We theoretically study the profile of a supercurrent in two-dimensional Josephson junctions with Rashba-Dresselhaus spin-orbit interaction (RDSOI) in the presence of a Zeeman field. Through investigating self-biased supercurrent (so called $\varphi_0$-Josephson state), we obtain explicit expressions for the functionality of the $\varphi_0$ state with respect to RDSOI parameters ($\alpha,\beta$) and in-plane Zeeman field components ($h_x,h_y$). Our findings reveal that, when the chemical potential ($\mu$) is high enough compared to the energy gap ($\Delta$) in superconducting electrodes, i.e., $\mu \gg \Delta$, RSOI and DSOI with equal strengths ($|\alpha|=|\beta|$) cause vanishing $\varphi_0$ state independent of magnetization and the type of RDSOI. A Zeeman field with unequal components, i.e., $|h_x|\neq |h_y|$, however, can counteract and nullify the destructive impact of equal-strength RDSOIs (for one type only), where $\mu\sim\Delta$, although $|h_x|= |h_y|$ can still eliminate the $\varphi_0$ state. Remarkably, in the $\mu\sim\Delta$ limit, the $\varphi_0$ state is proportional to the multiplication of both components of an in-plane Zeeman field, i.e., $h_xh_y$, which is absent in the $\mu \gg \Delta$ limit. Furthermore, our results of critical supercurrents demonstrate that the persistent spin helices can be revealed in a high enough chemical potential regime $\mu\gg \Delta$, while an opposite regime, i.e., $\mu\sim\Delta$, introduces an adverse effect. In the ballistic regime, the "maximum" of the critical supercurrent occurs at $|\alpha|=|\beta|$ and the Zeeman field can boost this feature. The presence of disorder and nonmagnetic impurities change this picture drastically so the "minimum" of the critical supercurrent occurs at and around the symmetry lines $|\alpha|=|\beta|$.

Spin-locked transport in a two-dimensional electron gas

Spin-orbit interactions in solids have inspired innovative physics for spin-based technologies. One such example is the persistent spin helix, where spin-orbit interactions from the semiconductor lattice are balanced with those in asymmetric quantum wells, to create long-lived spin textures. Spin transport in the presence of the momentum-dependent spin-orbit interactions lead to Larmor precession and subsequent dephasing that challenges the design of current spin-based information processing devices. We demonstrate that external magnetic fields can be applied to overcome this issue for spin-polarized charge carriers transported by in-plane electric fields. A frame of reference picture is introduced to describe the emergence and dynamics of the polarization-locked spin-wave packet after optical excitation. Applying well-matched magnetic fields maintains the persistent spin-helix profile regardless of whether the frame of reference is in motion or not. Monte Carlo simulations allow this traveling persistent spin-helix concept to be extended to a proposed spin Hall-effect transistor to ease design requirements.

Spin splitting with persistent spin textures induced by the line defect in the 1T phase of monolayer transition metal dichalcogenides

The spin splitting driven by spin-orbit coupling in monolayer (ML) transition metal dichalcogenides (TMDCs) family has been widely studied only for the 1H-phase structure, while it is not profound for the 1T-phase structure due to the centrosymmetric of the crystal. Based on first-principles calculations, we show that significant spin splitting can be induced in the ML 1T-TMDCs by introducing the line defect. Taking the ML PtSe2 as a representative example, we considered the most stable form of the line defects, namely Se-vacancy line defect (Se-VLD). We find that large spin splitting is observed in the defect states of the Se-VLD, exhibiting a highly unidirectional spin configuration in the momentum space. This peculiar spin configuration may yield the so-called persistent spin textures (PST), a specific spin structure resulting in protection against spin-decoherence and supporting an extraordinarily long spin lifetime. Moreover, by using k.p perturbation theory supplemented with symmetry analysis, we clarified that the emerging of the spin splitting maintaining the PST in the defect states is originated from the inversion symmetry breaking together with one-dimensional nature of the Se-VLD engineered ML PtSe2. Our findings pave a possible way to induce the significant spin splitting in the ML 1T-TMDCs, which could be highly important for designing spintronic devices.

Zero-field spin precession dynamics of high-mobility two-dimensional electron gas in persistent spin helix regime

We investigated the spin-orbit (SO) effective magnetic-field-induced spin precession of a high-mobility two-dimensional electron gas (2DEG) in a modulation-doped (001) GaAs/AlGaAs quantum well close to the persistent spin helix (PSH) state. The oscillating spin signal induced by the SO fields in zero external magnetic fields was clearly observed. When the photoexcited carrier density is sufficiently small compared with the 2DEG density, the spin precession length in the space domain was obtained by analyzing the precession frequency in the PSH regime. The excitation density dependence of the spin dynamics was reproduced well using the scattering time-dependent spin dynamics simulation. We revealed that the increase in the photoexcited carriers results in a transition from ballistic motion to diffusive motion, leading to a decrease in the spin-diffusion coefficient.

Nodal plane and persistent spin texture in a Weyl semimetal without mirror symmetry

By utilizing symmetry analysis and electronic structure calculations, we investigated the low-temperature orthorhombic phase of Ag$_{2}$Se in ${\cal SG}$~17. In addition to the discovery of a nodal plane at $k_{z}=\pi$ protected by the joint operation of time-reversal (${\cal T}$) and the 2-fold screw rotation $S_{2z}$, we found 24 Weyl points mainly residing at the $k_{y}=0$ plane with notable Fermi arc and large quasiparticle interference pattern (QPI). Due to the absence of mirror symmetry, a pair of Weyl points with opposite chirality reside at different binding energies, which makes this system an excellent material candidate for realizing the novel chiral anomaly related phenomenon, such as the quantized circular photogalvanic and the chiral magnetic effects. Furthermore, we also reveal the striking spin textures at $k_{z}=\pi$ plane which demonstrates, in a large region of the surface Brillouin Zone, a direction-selective spin polarization, which has a strong implication to spintronic applications.

μSR study of the triangular Ising antiferromagnet ErMgGaO4

We present a detailed magnetic study of the triangular antiferromagnet ErMgGaO$_4$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the Er$^{3+}$ moment along the c axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.5~K and no evidence of residual entropy as temperature approaching zero. Zero field (ZF) $\mu$SR measurements shows no sign of static uniform or random field and longitudinal field (LF) $\mu$SR measurements exhibit persistent spin fluctuations down to our lowest temperature of 25~mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ErMgGaO$_4$.