Dyakonov-Perel Spin Relaxation Research Articles

Modulation of Spin Dynamics in 2D Transition-Metal Dichalcogenide via Strain-Driven Symmetry Breaking.

Transition metal dichalcogenides (TMDs) possess intrinsic spin–orbit interaction (SOI) with high potential to be exploited for various quantum phenomena. SOI allows the manipulation of spin degree of freedom by controlling the carrier's orbital motion via mechanical strain. Here, strain modulated spin dynamics in bilayer MoS2 field‐effect transistors (FETs) fabricated on crested substrates are demonstrated. Weak antilocalization (WAL) is observed at moderate carrier concentrations, indicating additional spin relaxation path caused by strain fields arising from substrate crests. The spin lifetime is found to be inversely proportional to the momentum relaxation time, which follows the Dyakonov–Perel spin relaxation mechanism. Moreover, the spin–orbit splitting is obtained as 37.5 ± 1.4 meV, an order of magnitude larger than the theoretical prediction for monolayer MoS2, suggesting the strain enhanced spin‐lattice coupling. The work demonstrates strain engineering as a promising approach to manipulate spin degree of freedom toward new functional quantum devices.

Tunable spin-orbit coupling in two-dimensional InSe

We demonstrate that spin-orbit coupling (SOC) strength for electrons near the conduction band edge in few-layer $\gamma$-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 SOC strengths are compared with the results of weak antilocalization measurements on dual-gated multilayer InSe films, interpreted in terms of Dyakonov-Perel spin relaxation due to SOC, showing a good agreement between theory and experiment.

Contribution of inter-valley scattering in governing the steady state optical spin orientation in Al x Ga1−x As

The impact of inter-valley scattering in modifying the density of optically injected spin polarized electrons in Al x Ga1−x As/GaAs heterostructure is investigated. Polarization resolved photoluminescence excitation measurements are performed for studying the spectral dependence of degree of circular polarization (DCP) of photoluminescence signal emanating from an adjacent quantum well layer. The experimental DCP spectra is found to deviate from the theoretically calculated curve at high excitation energy and the deviation is seen to widen with the Al-content of Al x Ga1−x As barrier layer. In the past, researchers had attributed similar deviations to the Dyakonov–Perel (DP) spin relaxation mechanism which dominates the electrons spin dynamics at high excitation energy. However, we find that the inter-valley scattering along with the DP mechanism decide the shape of optical spin orientation spectra at higher energy. In fact, at higher Al-content, the inter-valley scattering in Al x Ga1–x As layer becomes the most dominant mechanism to govern the steady state electron spin polarization at the band edge. A numerical model is also proposed for estimating the steady state electron spin polarization at zone centre by including the inter-valley scattering, energy relaxation and DP spin relaxation mechanisms.

Charge-spin interconversion in epitaxial Pt probed by spin-orbit torques in a magnetic insulator

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 Dyakonov-Perel spin relaxation in the crystalline Pt film. Furthermore, our results point to a large internal spin-Hall ratio of $\approx$0.8 in epitaxial Pt. Our experimental work takes an essential step towards understanding the mechanisms of charge-spin interconversion and SOTs in Pt-based heterostructures, which are crucial for power-efficient spintronic devices.

Spin diffusion length associated with out-of-plane conductivity of Pt in spin pumping experiments

We present a broadband ferromagnetic resonance study of the Gilbert damping enhancement ($\Delta \alpha$) 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 ($\tau_{\text{s}}$) and momentum relaxation characteristic time ($\tau_{\text{p}}$) is determined by a superposition of DP and EY mechanisms, the qualitative and quantitative agreement with experimental results is excellent. Remarkably, we found that $\tau_{\text{p}}$ must be determined by the out-of-plane electrical resistivity ($\rho$) of the Pt film and hence its spin diffusion length ($\lambda_{\text{Pt}}$) is independent of $t_{\text{Pt}}$. Our work settles the controversy regarding the $t_{\text{Pt}}$ dependence of $\lambda_{\text{Pt}}$ by demonstrating its fundamental connection with $\rho$ considered along the same direction of spin current flow. \end{abstract}

Phonon assisted momentum relaxation, power dissipation and spin relaxation: Drifted Maxwellian approach

Polar optical phonon assisted momentum relaxation, power dissipation and spin relaxation are studied in GaAs. We adopt the drifted Maxwellian approach in the investigation, where the hot phonons are incorporated via the longitudinal polar optical phonon mechanism in the momentum relaxation. Stating from the rate of change of phonon occupancy in a relaxation time approximation, the hot phonon generation and the electronic power dissipation in the drifted Maxwellian are obtained. The results display the runaway effect, where the enhanced scattering rate reduces the drift velocity so that the field at which runaway occurs is increased in order to achieve the necessary power input. Hot phonons thus tend to stabilize the polar interaction and the reduction of drift velocity. The electron temperature and spin relaxation is also found to increase with the electric field. However, at the moderate fields the spins are found to be almost conserved. The spin relaxation is rapid at higher fields and is almost infinite for field higher than 300 mV μm−1, which might be the result of increase of the electron temperature at higher fields and, consequently, the higher Dyakonov-Perel spin relaxation frequency.

Suppressing effective magnetic field and spin-relaxation rate by tuning barrier compositions in a (111) quantum well

Adjusting compositions of two barriers forming a (111)-oriented InGaAs/AlGaAsSb quantum well (QW) is proposed as a method to eliminate the effective magnetic field due to the Rashba and linear-in-momentum Dresselhaus spin–orbit interactions in the zeroth and first orders in the gate electric field. The resulting suppressed Dyakonov–Perel spin relaxation is used to achieve a high on/off ratio of the spin-relaxation rate at room temperature in a double-QW structure.

Gate-Voltage-Induced Switching of the Spin-Relaxation Rate in a Triple-Quantum-Well Structure

Switching of the Dyakonov-Perel spin relaxation owing to the Rashba spin-orbit interaction (SOI) is theoretically studied in a semiconductor heterostructure with three quantum wells. The action of the present spin-relaxation switching is based on the gate-voltage induced electron transfer from the central well with vanishing Rashba SOI to the left or right well with large Rashba SOI. The spin-relaxation rate is calculated by extending the Dyakonov-Perel theory of the spin relaxation in a single subband to that in more than one subband in order to take into account the contribution from electrons in excited subbands at higher temperatures. It is shown that the on:off ratio of the spin-relaxation rate at room temperature reaches ${10}^{6}$ by choosing widths and compositions of well and barrier layers so as to reduce electron population in excited subbands, which gives an undesirable spin relaxation. The present spin-relaxation switching is expected to improve the on:off ratio of the current in the spin-lifetime field effect transistor.

Spin current polarization and electrical conductivity in metal helimagnets

The peculiarities of spin and charge kinetics in helical magnetic metals that are due to forces acting on the magnetic moment of conduction electrons in inhomogeneous magnetic field have been considered. Analyzing the equations of motion for non-equilibrium spin density shows that an electric field directed along the axis of the helix produces conduction-electron spin polarization along this direction. Under the same conditions, the directions of polarization of the spin current and polarization of the locally equilibrium spin density are collinear. The specific structure of the effective exchange field acting on the conduction electrons in helical magnets causes two additional spin relaxation mechanisms to emerge. In addition to the mechanism of spin-lattice relaxation, “diffusion” and “precession” mechanisms of spin relaxation exist in conductive helical magnets. The diffusion mechanism is analogous to the Dyakonov-Perel spin relaxation mechanism. The key point is that the inhomogeneities of a magnetic field initiate a coupling between the spin and charge systems. It has been shown that, in a helical metal, the electrical conductivity decreases due to the action of the non-uniform exchange magnetic field.

Efficient spin current generation in low-damping Mg(Al, Fe)2O4 thin films

Low-damping magnetic insulators are essential for pure spin current-based electronics as they can generate and transfer spin currents without associated charge currents. Nanometer-thick epitaxial thin films of low-damping magnetic insulators are particularly important in order to control and switch the magnetization via spin transfer torques. We have recently developed films of the ferromagnetic insulator MgAl0.5Fe1.5O4 (MAFO) with a low Gilbert damping parameter (∼0.001). In contrast to Y3Fe5O12 (YIG), MAFO films can be grown on a variety of substrates and have significant in-plane magnetic anisotropy, leading to higher spin-wave frequencies. Here, we demonstrate efficient spin current injection from MAFO into adjacent Pt and β-W layers by ferromagnetic resonance (FMR) broadening and inverse spin Hall effect measurements. Angular dependent magnetoresistance (ADMR) measurements indicate that the proximity effect magnetoresistance is small compared to the spin Hall magnetoresistance associated with spin pumping. FMR and ADMR measurements indicate that MAFO/Pt interfaces have a spin-mixing conductance of ∼2 × 1014 Ω−1 m−2, comparable to that of YIG/Pt. These measurements also show that the spin transport can be described by Dyakonov-Perel spin relaxation combined with an extrinsic spin Hall effect (from skew scattering). These results demonstrate the promise of spinel ferrites for spin current-based spintronics.

Unidirectional Spin-Orbit Interaction Induced by the Line Defect in Monolayer Transition Metal Dichalcogenides for High-Performance Devices.

Spin-orbit (SO) interaction is an indispensable element in the field of spintronics for effectively manipulating the spin of carriers. However, in crystalline solids, the momentum-dependent SO effective magnetic field generally results in spin randomization by a process known as the Dyakonov-Perel spin relaxation, leading to the loss of spin information. To overcome this obstacle, the persistent spin helix (PSH) state with a unidirectional SO field was proposed but difficult to achieve in real materials. Here, on the basis of first-principles calculations and tight-binding model analysis, we report for the first time a unidirectional SO field in monolayer transition metal dichalcogenides (TMDs, MX2, M = Mo, W; and X = S, Se) induced by two parallel chalcogen vacancy lines. By changing the relative positions of the two vacancy lines, the direction of the SO field can be tuned from x to y. Moreover, using k·p perturbation theory and group theory analysis, we demonstrate that the emerging unidirectional SO field is subject to both the structural symmetry and 1D nature of such defects engineered in 2D TMDs. In particular, through transport calculations, we confirm that the predicted SO states carry highly coherent spin current. Our findings shed new light on creating PSH states for high-performance spintronic devices.

Electron spin dynamics in mesoscopic GaN nanowires

The electron spin dynamics in spontaneously formed GaN nanowires (NWs) on Si(111) is investigated by time-resolved magneto-optical Kerr-rotation spectroscopy for temperatures from 15 to 260 K. A strong increase in the electron spin relaxation time by more than an order of magnitude is found as compared to bulk GaN. The temperature dependence of spin relaxation is characterized by two regimes, which are explained by a model taking into account the coexistence of two different mechanisms. As a result, the spin lifetime is limited by hyperfine interaction of localized electron spins with nuclear spins at low temperatures. The mesoscopic electron confinement in the NWs leads to a dominance of Dyakonov-Perel spin relaxation driven by interface-induced contributions at high temperatures, resulting in a slow-down, but not complete suppression of spin relaxation as compared to bulk GaN. These findings underline the important role of the high surface-to-volume ratio in NWs.

Tailoring multilayer quantum wells for spin devices

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 suggested as the major limiting factors for the spin coherence time. In the metallic regime, we found that the electron-electron collisions become dominant over microscopic scattering on the electron spin relaxation with the Dyakonov-Perel mechanism. Furthermore, the data analysis indicated that in our structure, due to the spin relaxation anisotropy, Dyakonov-Perel spin relaxation mechanism is efficient for the spins oriented in-plane and suppressed along the quantum well growth direction resulting in the enhancement of T2*. Our findings, namely, long-lived spin coherence persisting up to about room temperature, spin polarization decay time with and without a magnetic field, the spin-orbit field, single electron relaxation time, transport scattering time, and the electron-electron Coulomb scattering time highlight the attractiveness of n-doped multilayer systems for spin devices.

Transition from spin-orbit to hyperfine interaction dominated spin relaxation in a cold fluid of dipolar excitons

We measure the spin-resolved transport of dipolar excitons in a biased GaAs double quantum well structure. From these measurements we extract both spin lifetime and mobility of the excitons. We find that below a temperature of $4.8$K, there is a sharp increase in the spin lifetime of the excitons, together with a sharp reduction in their mobility. Below a critical power the spin lifetime increases with increasing mobility and density, while above the critical power the opposite trend is observed. We interpret this transition as an evidence of the interplay between two different spin dephasing mechanisms: at low mobility the dephasing is dominated by the hyperfine interaction with the lattice nuclei spins, while at higher mobility the spin-orbit interaction dominates, and a Dyakonov-Perel spin relaxation takes over. The excitation power and temperature regime where the hyperfine interaction induced spin dephasing is observed correlates with the regime where a dark dipolar quantum liquid was reported recently on a similar sample.

Strong electron-hole symmetric Rashba spin-orbit coupling in graphene/monolayer transition metal dichalcogenide heterostructures

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 interfacial interaction between TMD and graphene as evidenced by quenching of the TMD photoluminescence. We observe strong induced graphene SOC that manifests itself in a pronounced weak anti-localization (WAL) effect in the graphene magnetoconductance. The spin relaxation rate extracted from the WAL analysis varies linearly with the momentum scattering time and is independent of the carrier type. This indicates a dominantly Dyakonov-Perel spin relaxation mechanism caused by the induced Rashba SOC. Our analysis yields a Rashba SOC energy of ~1.5 meV in graphene/WSe2 and ~0.9 meV in graphene/MoS2, respectively. The nearly electron-hole symmetric nature of the induced Rashba SOC provides a clue to possible underlying SOC mechanisms.

Impurity-Driven Two-Dimensional Spin Relaxation Induced by Intervalley Spin-Flip Scattering in Silicon

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 SiGe heterostructures are a natural platform to test and utilize this spin relaxation mechanism due to the valley splitting near the interface and its tunability by electrical gating or applied stress. Our proposed new spin relaxation mechanism may explain a part of the spin relaxation contribution to Si-based 2DEG systems.

Control of the electron spin relaxation by the built-in piezoelectric field in InGaAs quantum wells

The electron spin dynamics is studied by time-resolved optical orientation experiments in strained InGaAs/GaAs quantum wells (QWs) grown on (111) or (001) substrates. For a given well width, the electron spin relaxation time in (111) InGaAs QWs decreases by an order of magnitude when the indium fraction in the well varies only from 4% to 12%. In contrast, the electron spin relaxation time depends weakly on the indium fraction in similar InGaAs quantum wells grown on (001) substrates. The strong variation of the electron spin relaxation time in (111) strained quantum well can be well interpreted by the Dyakonov-Perel spin relaxation mechanism where the conduction band spin-orbit splitting is dominated by the structural inversion asymmetry (Rashba term) induced by the piezoelectric field. In (001) QWs, due to the absence of piezoelectric field, the electron spin relaxation time is solely controlled by the Dresselhaus term. These results demonstrate the possibility to engineer the electron spin relaxation time in (111)-oriented quantum wells by the piezoelectric field induced by the built-in strain.

Unusual narrowing of the ESR line width in ordered structures with linear chains of Ge/Si quantum dots

Electron states in ordered Ge/Si heterostructures with linear chains of quantum dots (QDs) were studied by the electron spin resonance (ESR) method. A new ESR signal with principal g-factor values g zz = 1.9993 ± 0.0001, g xx = g yy = 1.9990 ± 0.0001 was detected. Unlike disordered QD structures, where ESR line broadening is usually observed (evidence of Dyakonov–Perel mechanism efficiency), the structures under study demonstrate the narrowing of ESR line when the external magnetic field deviates from the growth direction. The ESR line width is ΔH = 1.2 Oe for perpendicular magnetic field (along the growth direction) and ΔH = 0.8 Oe for in-plane magnetic field. The narrowing of ESR line can be explained by combination of two mechanisms. The first one is suppression of Dyakonov–Perel spin relaxation due to a settled direction of electron motion and finiteness of QD chains. The second one is cancelation of the wavefunction shrinking effect with decreasing the perpendicular component of the magnetic field.

Strain dependent electron spin dynamics in bulk cubic GaN

The electron spin dynamics under variable uniaxial strain is investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. Spin relaxation is found to be approximately independent of the applied strain, in complete agreement with estimates for Dyakonov-Perel spin relaxation. Our findings clearly exclude strain-induced relaxation as an effective mechanism for spin relaxation in cubic GaN.

Spin-dependent phenomena in semiconductors in strong electric fields

We develop a theory of spin-dependent phenomena in the streaming regime characterized by ballistic acceleration of electrons in the moderate electric field until they achieve the optical phonon energy and abruptly emit phonons. It is shown that the Dyakonov–Perel spin relaxation is drastically modified in this regime, the current-induced spin orientation remarkably increases, reaches a high value ≈2% in the electric field ∼1 kV cm−1 and falls with further increase in the field. The spin polarization enhancement is caused by squeezing of the electron momentum distribution in the direction of drift. We also predict field-induced oscillatory dynamics of spin polarization of the photocarriers excited into the conduction band by a short circularly polarized optical pulse.