Monolayers of group 6 transition metal dichalcogenides are promising candidates for future spin-, valley-, and charge-based applications. Quantum transport in these materials reflects a complex interplay between real spin and pseudo-spin (valley) relaxation processes, which leads to either positive or negative quantum correction to the classical conductivity. Here we report experimental observation of a crossover from weak localization to weak anti-localization in highly n-doped monolayer MoS2. We show that the crossover can be explained by a single parameter associated with electron spin lifetime of the system. We find that the spin lifetime is inversely proportional to momentum relaxation time, indicating that spin relaxation occurs via Dyakonov-Perel mechanism.
Spin dephasing by the Dyakonov-Perel mechanism in metallic films deposited on insulating substrates is revealed, and quantitatively examined by means of density functional calculations combined with a kinetic equation. The surface-to-substrate asymmetry, probed by the metal wave functions in thin films, is found to produce strong spin-orbit fields and a fast Larmor precession, giving a dominant contribution to spin decay over the Elliott-Yafet spin relaxation up to a thickness of 70 nm. The spin dephasing is oscillatory in time with a rapid (sub-picosecond) initial decay. However, parts of the…
In a heavily doped semiconductor with weak spin-orbital interaction the Dyakonov-Perel spin relaxation rate is known to be proportional to the Drude conductivity. We argue that in the case of weak spin-orbital interaction this proportionality goes beyond the Drude mechanism: it stays valid through the metal-insulator transition and in the range of the exponentially small hopping conductivity.
We report on spin transport features which are unique to high quality bilayer graphene, in absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of propagating wavepacket is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy-dependence of spin lifetime is similar to monolayer graphene with a M-shape profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hol…
Observation of Dyakonov-Perel-type magnon spin relaxation in uniaxial antiferromagnetic insulators
Southern University of Science and Technology（南方科技大学）；Quantum Science Center of Guangdong–Hong Kong–Macao Greater Bay Area (Guangdong)（粤港澳大湾区量子科学中心（广东））；Beijing Normal University（北京师范大学）；Fudan University（复旦大学）
The electron spin dynamics in multilayer GaAs/AlGaAs quantum wells, containing high-mobility dense two-dimensional electron gases, have been studied using time-resolved Kerr rotation and resonant spin amplification techniques. The electron spin dynamics was regulated through the wave function engineering and quantum confinement in multilayer quantum wells. We observed the spin coherence with a remarkably long dephasing time T2* > 13 ns for the structure doped beyond metal-insulator transition. Dyakonov-Perel spin relaxation mechanism, as well as the inhomogeneity of electron g-factor, was sugg…
We investigate how spins relax in intrinsic graphene. The spin-orbit coupling arises from the band structure and is enhanced by ripples. The orbital motion is influenced by scattering centers and ripple-induced gauge fields. Spin relaxation due to Elliot-Yafet and Dyakonov-Perel mechanisms and gauge fields in combination with spin-orbit coupling are discussed. In intrinsic graphene, the Dyakonov-Perel mechanism and spin flip due to gauge fields dominate and the spin-flip relaxation time is inversely proportional to the elastic scattering time. The spin relaxation anisotropy depends on an intri…
We report the anomalous Dyakonov-Perel' spin relaxation in ultracold spin-orbit coupled $^{40}$K gas when the coupling between $|9/2,9/2\ >$ and $|9/2,7/2\ >$ states (atcing as the effective Zeeman magnetic field) is much stronger than the spin-orbit coupled field. Both the transverse and longitudinal spin relaxations are investigated with small and large spin polarizations. It is found that with small spin polarization, the transverse (longitudinal) spin relaxation is divided into four (two) regimes: the normal weak scattering regime, the anomalous Dyakonov-Perel'-like regime, the anomalous E…
In order to describe correctly the interplay of extrinsic and intrinsic spin-orbit mechanisms to the spin Hall effect, it is necessary to consider different sources of spin relaxation. We take into account the spin relaxation time $τ_{DP}$ due to the Dyakonov-Perel mechanism as well as the Elliot-Yafet spin-relaxation time $τ_s$ due to the spin-orbit scattering from impurities. The total spin Hall conductivity depends crucially on the ratio $τ_s /τ_{DP}$.
Strong interest has arisen recently on low-dimensional systems with strong spin-orbit interaction due to their peculiar properties of interest for some spintronic applications. Here, the time evolution of the electron spin polarization of a disordered two-dimensional electron gas is calculated exactly within the Boltzmann formalism for arbitrary couplings to a Rashba spin-orbit field. The classical Dyakonov-Perel mechanism of spin relaxation is shown to fail for sufficiently strong Rashba fields, in which case new regimes of spin decay are identified. These results suggest that spin manipulati…
We investigate spin transport in quasi 2DEG formed by III-V semiconductor heterojunctions using the Monte Carlo method. The results obtained with and without electron-electron scattering are compared and appreciable difference between the two is found. The electron-electron scattering leads to suppression of Dyakonov-Perel mechanism (DP) and enhancement of Elliott-Yafet mechanism (EY). Finally, spin transport in InSb and GaAs heterostructures is investigated considering both DP and EY mechanisms. While DP mechanism dominates spin decoherence in GaAs, EY mechanism is found to dominate in high m…
We study the spin-orbit interaction (SOI) in InAs/ GaSb and InAs quantum wells. We show through temperature- and gate-dependent magnetotransport measurements of weak antilocalization that the dominant spin-orbit relaxation mechanism in our low-mobility heterostructures is Elliott-Yafet and not Dyakonov-Perel in the form of the Rashba or Dresselhaus SOI as previously suggested. We compare our findings with recent work on this material system and show that the SOI length lies within the same range. The SOI length may be controlled using an electrostatic gate, opening up prospects for developing …
Spatiotemporal spin transport from first principles
Rensselaer Polytechnic Institute（伦斯勒理工学院）；University of Wisconsin–Madison（威斯康星大学麦迪逊分校）；nOhm Devices, Inc.（nOhm 设备有限公司）；Catalan Institute of Nanoscience and Nanotechnology (ICN2)（加泰罗尼亚纳米科学与技术研究所）
In the absence of an external field, the Rashba spin-orbit interaction (SOI) in a two-dimensional electron gas in a semiconductor quantum well arises entirely from the screened electrostatic potential of ionized donors. We adjust the wave functions of a quantum well so that electrons occupying the first (lowest) subband conserve their spin projection along the growth axis (Sz), while the electrons occupying the second subband precess due to Rashba SOI. Such a specially designed quantum well may be used as a spin relaxation trigger: electrons conserve Sz when the applied voltage (or current) is…
Recent experiments demonstrate that a ballistic version of spin resonance, mediated by spin-orbit interaction, can be induced in narrow channels of a high-mobility GaAs two-dimensional electron gas by matching the spin precession frequency with the frequency of bouncing trajectories in the channel. Contrary to the typical suppression of Dyakonov-Perel' spin relaxation in confined geometries, the spin relaxation rate increases by orders of magnitude on resonance. Here, we present Monte Carlo simulations of this effect to explore the roles of varying degrees of disorder and strength of spin-orbi…
A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As quantum wells grown on (110) oriented substrate. The spin lifetime of spins perpendicular to the growth direction is about one order of magnitude shorter compared to spins along (110). The spin lifetimes of both spin orientations decrease monotonically above a temperature of 80 and 120 K, respectively. The decrease is very surprising for spins along (110) direction and cannot be explained by the usual Dyakonov Perel dephasing mechanism. A novel spin dephasing mechanism is put forward that is based on scattering of el…
The spin-dependent transport properties of armchair graphene nanoribbons in the presence of extrinsic spin-orbit coupling induced by a random distribution of Nickel adatoms is studied. By combining a recursive Green's function formalism with density functional theory, we explore the influence of ribbon length and metal adatom concentration on the conductance. At a given length, we observed a significant enhancement of the spin-flip channel around resonances and at energies right above the Fermi level. We also estimate the spin-relaxation length, finding values on the order of tens of micromete…
We investigate spin transport in InSb/InAlSb heterostructure using the Monte Carlo approach, generalized by including density matrix description of spin for taking spin dynamics into account. In addition to the dominant Dyakonov-Perel (DP) mechanism for spin control and relaxation, we consider magnetic interaction between electrons which assumes importance due to high electronic Lande g-factor in the material. It is found that while the effect of magnetic interaction is not important at low densities, it reduces the spin relaxation length by as much as 50% at higher densities. We also present …
In this contribution, we investigated the spin coherence of high-mobility dense two-dimensional electron gases confined in multilayer systems. The dynamics of optically-induced spin polarization was experimentally studied employing the time-resolved Kerr rotation and resonant spin amplification techniques. For both the double and triple quantum wells doped beyond the metal-insulator transition, where the spin coherence is greatly suppressed, we found remarkably long spin lifetimes limited by the Dyakonov-Perel mechanism and spin hopping process between the donor sites as well as the spread of …
We demonstrate that spin-orbit coupling (SOC) strength for electrons near the conduction band edge in few-layer $γ$-InSe films can be tuned over a wide range. This tunability is the result of a competition between film-thickness-dependent intrinsic and electric-field-induced SOC, potentially, allowing for electrically switchable spintronic devices. Using a hybrid $\mathbf{k\cdot p}$ tight-binding model, fully parameterized with the help of density functional theory computations, we quantify SOC strength for various geometries of InSe-based field-effect transistors. The theoretically computed S…
We investigate spin relaxation in graphene spin valves and observe strongly contrasting behavior for single layer graphene (SLG) and bilayer graphene (BLG). In SLG, the spin lifetime (τs) varies linearly with the momentum scattering time (τp) as carrier concentration is varied, indicating the dominance of Elliot-Yafet (EY) spin relaxation at low temperatures. In BLG, τs and τp exhibit an inverse dependence, which indicates the dominance of Dyakonov-Perel spin relaxation at low temperatures. The different behavior is due to enhanced screening and/or reduced surface sensitivity of BLG, which gre…
The understanding of spin dynamics and relaxation mechanisms in clean graphene and the upper time and length scales on which spin devices can operate are prerequisites to realizing graphene spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with spin-orbit Rashba fields as low as a few tens of micron eV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from …
We suggest an approach to the problem of free electron spin evolution in a semiconductor with arbitrary anisotropy or quantum structure in a magnetic field. The developed approach utilizes quantum kinetic equations for average spin components. These equations represent the relaxation in terms of correlation functions for fluctuating effective fields responsible for spin relaxation. In a particular case when autocorrelation functions are dominant, the kinetic equations reduce to the Bloch equations. The developed formalism is applied to the problem of electron spin relaxation due to exchange sc…
We develop a theory of effects of electron-electron collisions on the Dyakonov-Perel' spin relaxation in multi-valley quantum wells. It is shown that the electron-electron scattering rate which governs the spin relaxation is different from that in a single-valley system. The theory is applied to Si/SiGe (001)-grown quantum wells where two valleys are simultaneously populated by free carriers. The dependences of the spin relaxation rate on temperature, electron concentration and valley-orbit splitting are calculated and discussed. We demonstrate that in a wide range of temperatures the electron…
The optical spin orientation effect in a GaAs/(Ga,Al)As quantum well containing a high-mobility 2D electron gas was found to be due to spin-polarized minority carriers, the holes. The observed oscillations of both the intensity and polarization of the photoluminescence in a magnetic field are well described in a model whose main elements are resonant absorption of the exciting light by the Landau levels and mixing of the heavy- and light-hole subbands. After subtraction of these effects, the observed influence of magnetic fields on the spin polarization can be well interpreted by a standard ap…
We report on the measurements of spin relaxation in GaAs quasi-one-dimensional mesowires, relying on spin noise spectroscopy, thus adding to the existing body of spin relaxation studies in bulk, two-dimensional and zero dimensional systems. In addition to temperature and magnetic field dependence, we modify the spin relaxation time via applied electric field and aspect ratio of the mesowires, suggesting that scalable spintronics devices with controllable spin relaxation are achievable. Overall, we observed higher spin-relaxation time in mesowires compared to bulk with a spin noise exhibiting D…
We present a unified theoretical framework for the study of spin dynamics and relativistic transport phenomena in disordered two-dimensional Dirac systems with pseudospin-spin coupling. The formalism is applied to the paradigmatic case of graphene with uniform Bychkov-Rashba interaction and shown to capture spin relaxation processes and associated charge-to-spin interconversion phenomena in response to generic external perturbations, including spin density fluctuations and electric fields. A controlled diagrammatic evaluation of the generalized spin susceptibility in the diffusive regime of we…
We study spin relaxation in dilute magnetic semiconductors near a ferromagnetic transition, where spin fluctuations become strong. An enhancement in the scattering rate of itinerant carriers from the spin fluctuations of localized impurities leads to a change in the dominant spin relaxation mechanism from Dyakonov-Perel to spin flips in scattering. On the ferromagnetic side of the transition, we show that due to the presence of two magnetic components -- the itinerant carriers and the magnetic impurities -- with different gyromagnetic ratios, the relaxation rate of the total magnetization can …
The classical Bloch equations of spin magnetization transport is extended to fully time-dependent and highly-nonlinear nonequilibrium spin magnetization quantum distribution function transport (SMQDFT) equations. The relevant variables are the spinor correlation functions which separate into charge and spin magnetization distributions that becomes highly coupled in SMQDFT equa- tions. The leading terms consist of the Boltzmann kinetic equation with spin-orbit coupling in a magnetic eld together with spin-dependent scattering terms which contribute to the torque. These do not have analogue with…
Measurements of spin dynamics of electrons in a degenerate two dimensional electron gas, where the Dyakonov-Perel mechanism is dominant, have been used to investigate the electron scattering time (tp*) as a function of energy near the Fermi energy. Close to the Fermi energy the spin evolution is oscillatory, indicating a quasi-collision-free regime of spin dynamics. As the energy is increased a transition to exponential, collision-dominated, spin decay occurs. The frequency and the value of tp* are extracted using a Monte Carlo simulation method. At the Fermi energy tp* is very close to the en…
Through the theoretical study of electron spin lifetime in the 2DEG of doped Si, we highlight a dominant spin relaxation mechanism induced by the impurity central-cell potential near an interface via intervalley electron scattering. At low temperatures and with modest doping, this Yafet spin flip mechanism can become much more important than the Dyakonov-Perel spin relaxation indcued by the structural Rashba spin-orbit coupling field. As the leading-order impurity-induced spin flip happens only between two non-opposite valleys in Si, two-dimensional electron gas (2DEG) systems in Si MOSFETs or…
Intrinsic electron spin relaxation due to the D'yakonov-Perel' mechanism is studied in monolayer Molybdenum Disulphide. An intervalley in-plane spin relaxation channel is revealed due to the opposite effective magnetic fields perpendicular to the monolayer Molybdenum Disulphide plane in the two valleys together with the intervalley electron-phonon scattering. The intervalley electron-phonon scattering is always in the weak scattering limit, which leads to a rapid decrease of the in-plane spin relaxation time with increasing temperature. A decrease of the in-plane spin relaxation time with the …
Dynamic response of two-dimensional electron systems with spin-orbit interaction is studied theoretically on the basis of quantum kinetic equation, taking into account elastic scattering of electrons. The spin polarization and spin current induced by the applied electric field are calculated for the whole class of electron systems described by p-linear spin-orbit Hamiltonians. The absence of nonequilibrium intrinsic static spin currents is confirmed for these systems with arbitrary (nonparabolic) electron energy spectrum. Relations between the spin polarization, spin current, and electric curr…
The intrinsic contribution to the spin Hall effect in a 2DEG with non-magnetic impurities is studied in a quantum Boltzmann approach. It is shown that if the steady state response is perturbative in the spin-orbit coupling parameter $λ$, then the precession term--vital for Dyakonov-Perel relaxation and the key to the spin Hall effect in previous similar Boltzmann studies--must be left out to first order in spin-orbit coupling. In such a case one would have that to lowest order in the parameters electric field, spin-orbit coupling, impurity strength and impurity concentration there is no intrin…
We perform extensive first-principles calculations for heterostructures composed of monolayer graphene and hexagonal boron nitride (hBN). Employing a symmetry-derived minimal tight-binding model, we extract orbital and spin-orbit coupling (SOC) parameters for graphene on hBN, as well as for hBN encapsulated graphene. Our calculations show that the parameters depend on the specific stacking configuration of graphene on hBN. We also perform an interlayer distance study for the different graphene/hBN stacks to find the corresponding lowest energy distances. For very large interlayer distances, on…
We investigate the in-plane spin relaxation of electrons due to the D'yakonov-Perel' and Elliot-Yafet mechanisms including the intra- and inter-valley processes in monolayer MoS$_2$. We construct the effective Hamiltonian for the conduction band using the Löwdin partition method from the anisotropic two-band Hamiltonian with the intrinsic spin-orbit coupling of the conduction band included. The spin-orbit coupling of the conduction band induces the intra- and inter-valley D'yakonov-Perel' spin relaxation. In addition, the Elliot-Yafet spin relaxation also takes place due to the interband spin …
An effective Hamiltonian is derived to describe the conduction band of monolayer black phosphorus (phosphorene) in the presence of spin-orbit coupling and external electric field. Envelope function approximation along with symmetry arguments and Lowdin partitioning are utilized to derive extrinsic spin-orbit coupling. The resulting spin splitting appears in fourth order perturbation terms and is shown to be linear in both the magnitude of the external electric field and the strength of the atomic spin-orbit coupling, similar to the Bychkov-Rashba expression but with an in-plane anisotropy. The…
We measure spin-orbit torques (SOTs) in a unique model system of all-epitaxial ferrite/Pt bilayers to gain insights into charge-spin interconversion in Pt. With negligible electronic conduction in the insulating ferrite, the crystalline Pt film acts as the sole source of charge-to-spin conversion. A small field-like SOT independent of Pt thickness suggests a weak Rashba-Edelstein effect at the ferrite/Pt interface. By contrast, we observe a sizable damping-like SOT that depends on the Pt thickness, from which we deduce the dominance of an extrinsic spin-Hall effect (skew scattering) and Dyakon…
Spin-charge interconversion is currently the focus of intensive experimental and theoretical research both for its intrinsic interest and for its potential exploitation in the realization of new spintronic functionalities. Spin-orbit coupling is one of the key microscopic mechanisms to couple charge currents and spin polarizations. The Rashba spin-orbit coupling in a two-dimensional electron gas has been shown to give rise to the inverse spin galvanic effect, i.e. the generation of a non-equilibrium spin polarization by a charge current. Whereas the Rashba model may be applied to the interpret…
Tellurium (Te) has attracted great research interest due to its unique crystal structure since 1970s. However, the conduction band of Te is rarely studied experimentally because of the intrinsic p-type nature of Te crystal. By atomic layer deposited dielectric doping technique, we are able to access the conduction band transport properties of Te in a controlled fashion. In this paper, we report on a systematic study of weak-antilocalization (WAL) effect in n-type two-dimensional (2D) Te films. We find that the WAL agrees well with Iordanskii, Lyanda-Geller, and Pikus (ILP) theory. The gate and…
We use microscopic linear response theory to derive a set of equations that provide a complete description of coupled spin and charge diffusive transport in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO) interaction. These equations capture a number of interrelated effects including spin accumulation and diffusion, Dyakonov-Perel spin relaxation, magnetoelectric, and spin-galvanic effects. They can be used under very general circumstances to model transport experiments in 2DEG systems that involve either electrical or optical spin injection. We comment on the relationshi…
We present a broadband ferromagnetic resonance study of the Gilbert damping enhancement ($Δα$) due to spin pumping in NiFe/Pt bilayers. The bilayers, which have negligible interfacial spin memory loss, are studied as a function of the Pt layer thickness ($t_{\text{Pt}}$) and temperature (100-293 K). Within the framework of diffusive spin pumping theory, we demonstrate that Dyakonov-Perel (DP) or Elliot-Yaffet (EY) spin relaxation mechanisms acting alone are incompatible with our observations. In contrast, if we consider that the relation between spin relaxation characteristic time ($τ_{\text{s…
Despite its extremely weak intrinsic spin-orbit coupling (SOC), graphene has been shown to acquire considerable SOC by proximity coupling with exfoliated transition metal dichalcogenides (TMDs). Here we demonstrate strong induced Rashba SOC in graphene that is proximity coupled to a monolayer TMD film, MoS2 or WSe2, grown by chemical vapor deposition with drastically different Fermi level positions. Graphene/TMD heterostructures are fabricated with a pickup-transfer technique utilizing hexagonal boron nitride, which serves as a flat template to promote intimate contact and therefore a strong i…
We report on efficient spin injection in p-doped InGaAs/GaAs quantum-dot (QD) spin light emitting diode (spin-LED) under zero applied magnetic field. A high degree of electroluminescence circular polarization (Pc) ~19% is measured in remanence up to 100K. This result is obtained thanks to the combination of a perpendicularly magnetized CoFeB/MgO spin injector allowing efficient spin injection and an appropriate p-doped InGaAs/GaAs QD layer in the active region. By analyzing the bias and temperature dependence of the electroluminescence circular polarization, we have evidenced a two-step spin r…
We study the spin dynamics in the presence of impurity and electron-electron (e-e) scattering in a III-V semiconductor quantum well with arbitrary spin-orbit coupling (SOC) strength and symmetry at finite temperature. We derive the coupled spin-charge dynamic equations in the presence of inelastic scattering and provide a new formalism that describes the spin relaxation and dynamics in both the weak and the strong SOC regime in a unified way. In the weak SOC regime, as expected, our theory reproduces all previous zero-temperature results, most of which have focused on impurity-scattering induc…
Bilayer MoS2 is a centrosymmetric semiconductor with degenerate spin states in the six valleys at the corners of the Brillouin zone. It has been proposed that breaking of this inversion symmetry by an out-of-plane electric field breaks this degeneracy, allowing for spin and valley lifetimes to be manipulated electrically in bilayer MoS2 with an electric field. In this work, we report phase-coherent transport properties of double-gated mono-, bi-, and tri-layer MoS2. We observe a similar crossover from weak localization to weak anti-localization, from which we extract the spin relaxation time a…
We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ~1000 and the spin injection/detection efficiency was greatly enhanced from ~1% (transparent contacts) to ~30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. F…
We have analysed spin relaxation behaviour of various II-VI semiconductors for nanowire structure and 2-D channel by simulating spin polarized transport through a semi-classical approach. Monte Carlo simulation method has been applied to simulate our model. D'yakanov-Perel mechanism and Elliot-Yafet mechanism are dominant for spin relaxation in II-VI semiconductors. Variation in spin relaxation length with external field has been analysed and comparison is drawn between nanowire and 2-D channels. Spin relaxation lengths of various II-VI semiconductors are compared at an external field of 1kV/c…
Spintronic devices usually rely on long spin relaxation times and/or lengths for optimum performance. Therefore, the ability to modulate these quantities with an external agent offers unique possibilities. The dominant spin relaxation mechanism in most technologically important semiconductors is the D'yakonov-Perel' (DP) mechanism which vanishes if the spin carriers (electrons) are confined to a single conduction subband in a quantum wire grown in certain crystallographic directions, or polycrystalline quantum wires. Here, we report modulating the DP spin relaxation rate (and hence the spin re…
Graphene grown by chemical vapor deposition (CVD) is the most promising material for industrial-scale applications based on graphene monolayers. It also holds promise for spintronics; despite being polycrystalline, spin transport in CVD graphene has been measured over lengths up to 30 $μ$m, which is on par with the best measurements made in single-crystal graphene. These results suggest that grain boundaries (GBs) in CVD graphene, while impeding charge transport, may have little effect on spin transport. However, to date very little is known about the true impact of disordered networks of GBs …
