Spin Relaxation Mechanism Research Articles

Transient and steady state magneto-optical studies of the CsPbBr3 crystal

We have studied the magneto-optical properties of photoexcitations in $\mathrm{CsPb}{\mathrm{Br}}_{3}$ single crystal using the technique of picosecond time-resolved quantum beatings (QBs) in the circularly polarized photoinduced reflection as well as steady state magnetocircular dichroism (MCD) in $\mathrm{CsPb}{\mathrm{Br}}_{3}$ film. In the Voigt geometry at magnetic field strength $B$ > 0, we observed fast and slow QB oscillations that we attribute to the Larmor precession frequency of electrons and holes, respectively. From the linear frequency dependence on $B$, we extract the carrier anisotropic Land\'e $g$ factors for applied B along [010] and [001]; for electrons $|{g}_{[001]}^{\mathrm{e}}|=1.95\ifmmode\pm\else\textpm\fi{}0.04$ and $|{g}_{[010]}^{\mathrm{e}}|=1.82\ifmmode\pm\else\textpm\fi{}0.04$, whereas for holes $|{g}_{[001]}^{\mathrm{h}}|=0.69\ifmmode\pm\else\textpm\fi{}0.02$ and $|{g}_{[010]}^{\mathrm{h}}|=0.76\ifmmode\pm\else\textpm\fi{}0.02$. These values are in excellent agreement with a k\ifmmode\cdot\else\textperiodcentered\fi{}p model calculation applied to $\mathrm{CsPb}{\mathrm{Br}}_{3}$. Surprisingly, at $B=0$, we still observed QB oscillation of \ensuremath{\sim}500 MHz that we interpret as due to the Overhauser field that originates from the spin-aligned nuclei caused by the Knight field. From the measured MCD spectrum vs $B$, we obtained the $g$ factor of the bright excitons ${g}_{\mathrm{ex}}=2.18\ifmmode\pm\else\textpm\fi{}0.08$ showing that the $g$ value of holes in $\mathrm{CsPb}{\mathrm{Br}}_{3}$ is positive. We also measured the temperature and magnetic field dependencies of the electron and hole spin dephasing times which support the Elliot-Yafet spin-relaxation mechanism.

Tuning of spin relaxation and the Kondo effect in copper thin films by ionic gating

Spin relaxation length is a fundamental material parameter that influences all aspects of spin dependent transport. The ability to tune the spin relaxation length leads to novel spintronic phenomena and functionalities. We explore the tunability of the spin relaxation length in the mesoscopic Cu channels of nonlocal spin valves by using the ionic gating technique via a ${\mathrm{Li}}^{+}$ containing solid polymer electrolyte. At 5 K, the Cu spin relaxation length ${\ensuremath{\lambda}}_{\mathrm{Cu}}$ is tuned reversibly between 670 and 410 nm and the Cu resistivity ${\ensuremath{\rho}}_{\mathrm{Cu}}$ is tuned by 9%. The strength of the Kondo effect due to the Fe impurities in Cu is tuned by one order of magnitude. At 295 K, ${\ensuremath{\lambda}}_{\mathrm{Cu}}$ is tuned between 380 and 300 nm and ${\ensuremath{\rho}}_{\mathrm{Cu}}$ is tuned by 7%. A gradual amplification of the tuning ranges by repeated gate cycling is observed and clearly suggests an electrochemical origin. Tunable spin relaxation in simple metals enriches functionalities in metal-based spintronics and shines light on fundamental spin relaxation mechanisms.

Insights into the Spin Dynamics of Mononuclear Cerium(III) Single-Molecule Magnets.

Four novel CeIII mononuclear complexes of formulas [Ce(ntfa)3(MeOH)2] (1), [Ce(ntfa)3(5,5′-Me2bipy)] (2), [Ce(ntfa)3(terpy)] (3), and [Ce(ntfa)3(bipy)2] (4), where ntfa = 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dionato, 5,5′-Me2bipy = 5,5′-dimethyl-2,2′-dipyridyl, terpy = 2,2′:6′,2″-terpyridine, and bipy = 2,2′-bipyridine, have been synthesized and structurally characterized with CeIII displaying coordination numbers of 8, 8, 9, and 10, respectively. Magnetic measurements indicate that all the complexes show a field-induced single-ion magnet behavior under a small applied dc field. The magnetic analysis shows the relevance of the different spin relaxation mechanisms in the magnetic relaxation of the CeIII compounds, with special emphasis on the local-mode process. Multiconfigurational calculations were also performed to get more information on the axiality of the compounds.

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA2FAPbI7 using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control.

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.

Giant Spin Lifetime Anisotropy and Spin-Valley Locking in Silicene and Germanene from First-Principles Density-Matrix Dynamics.

Through first-principles real-time density-matrix (FPDM) dynamics simulations, we investigated spin relaxation due to electron-phonon and electron-impurity scatterings with spin-orbit coupling (SOC) in two-dimensional Dirac materials silicene and germanene at finite temperatures. We discussed the applicability of conventional descriptions of spin relaxation mechanisms by Elliott-Yafet (EY) and D'yakonov-Perel' (DP) compared to the FPDM method, which is determined by a complex interplay of intrinsic SOC, external fields, and scattering strength. For example, the electric field dependence of the spin lifetime by FPDM is close to the DP mechanism for silicene at room temperature but similar to the EY mechanism for germanene. Because of its stronger SOC strength and buckled structure in contrast to graphene, germanene has a giant spin lifetime anisotropy and spin-valley locking effect under nonzero Ez and low temperatures. More importantly, germanene has a long spin lifetime (∼100 ns at 50 K) and an ultrahigh carrier mobility, making it advantageous for spin-valleytronic applications.

Modeling of the photocurrent induced by inverse spin Hall effect under local circularly polarized photoexcitation

The inverse spin Hall effect (ISHE) has been recently demonstrated through the photoinduced inverse spin Hall effect (PISHE), where a focused laser normal to the device plane generates photocurrent that depends on the helicity of the photoexcitation. Here, we have employed a finite element method to rigorously simulate the helicity-dependent photocurrent (HDPC) under local helical photoexcitation, taking into account the complications of minority carriers, metal contact junctions, and spin relaxation. We found that the ISHE-induced electromotive force is inversely proportional to the doping level of the material, caused by the diffusion and drift current balance. Furthermore, the HDPC near the metal contact can either increase or decrease, because of the competing mechanisms of fast carrier recombination and spin relaxation at the contact. These simulation results provide insightful visualization of charge, spin, electric field, and current density distributions and deeper understanding of photoinduced ISHE.

Normal & reversed spin mobility in a diradical by electron-vibration coupling

π−conjugated radicals have great promise for use in organic spintronics, however, the mechanisms of spin relaxation and mobility related to radical structural flexibility remain unexplored. Here, we describe a dumbbell shape azobenzene diradical and correlate its solid-state flexibility with spin relaxation and mobility. We employ a combination of X-ray diffraction and Raman spectroscopy to determine the molecular changes with temperature. Heating leads to: i) a modulation of the spin distribution; and ii) a “normal” quinoidal → aromatic transformation at low temperatures driven by the intramolecular rotational vibrations of the azobenzene core and a “reversed” aromatic → quinoidal change at high temperatures activated by an azobenzene bicycle pedal motion amplified by anisotropic intermolecular interactions. Thermal excitation of these vibrational states modulates the diradical electronic and spin structures featuring vibronic coupling mechanisms that might be relevant for future design of high spin organic molecules with tunable magnetic properties for solid state spintronics.

Dissipation-relaxation dynamics of a spin- 12 particle with a Rashba-type spin-orbit coupling in an ohmic heat bath

Spin-orbit coupling (SOC), which is inherent to a Dirac particle that moves under the influence of electromagnetic fields, manifests itself in a variety of physical systems including non-relativistic ones. For instance, it plays an essential role in spintronics developed in the past few decades, particularly by controlling spin current generation and relaxation. In the present work, by using an extended Caldeira-Leggett model, we elucidate how the interplay between spin relaxation and momentum dissipation of an open system of a single spin-$1/2$ particle with a Rashba type SOC is induced by the interactions with a spinless, three-dimensional environment. Staring from the path integral formulation for the reduced density matrix of the system, we have derived a set of coupled nonlinear equations that consists of a quasi-classical Langevin equation for the momentum with a frictional term and a spin precession equation. The spin precesses around the effective magnetic field generated by both the SOC and the frictional term. It is found from analytical and numerical solutions to these equations that a spin torque effect included in the effective magnetic field causes a spin relaxation and that the spin and momentum orientations after a long time evolution are largely controlled by the Rashba coupling strength. Such a spin relaxation mechanism is qualitatively different from, e.g., the one encountered in semiconductors where essentially no momentum dissipation occurs due to the Pauli blocking.

Tuning the spintronic properties of graphene with atomically precise Au clusters

Spin relaxation is investigated in lateral nonlocal graphene spin valves with increasing densities of soft-landed Au3 and Au6 clusters. It is found that both gold clusters scatter spins via the Elliot–Yafet mechanism. The induced spin–orbit coupling strength is a few meV for both clusters, with the value for Au3 being roughly twice as large as that of Au6. A gradual increase of the deposited cluster density (up to clusters cm−2) decreases the spin and momentum lifetime of the graphene channel, with Au6 clusters affecting both spin and momentum lifetime more strongly than the Au3 clusters. Density functional theory calculations provide insights into the spin relaxation mechanism. The dependence of graphene’s electronic and spintronic properties on the density and the exact cluster size indicates the importance of the microscopic details for graphene functionalisation towards spintronic applications.

Hyperfine-Induced Electron-Spin Dephasing in Negatively Charged Colloidal Quantum Dots: A Survey of Size Dependence.

The electron spin relaxation processes are complicated in semiconductor quantum dots. Different spin relaxation mechanisms may result in an increased or decreased spin relaxation rate with the size. The information on size-dependent spin dynamics helps to clarify and better understand the underlying spin relaxation processes. We investigate the size dependence of the electron spin dynamics in negatively photocharged CdSe and CdS colloidal quantum dots by time-resolved ellipticity spectroscopy. It is revealed that the electron spin dephasings of photodoped electron in zero or weak magnetic fields are dominated by the electron-nuclear hyperfine interaction for all measured samples. The hyperfine-induced electron spin dephasing time is ∼1-2 ns at room temperature and decreases with decreasing the size D. In addition to a size-dependent dephasing time that is directly proportional to D3/2, our measurements also show a size-independent time component, likely due to the laser-induced nuclear spin ordering.

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D'yakonov-Perel' spin relaxation mechanism in wurtzite GaN.

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.

Experimental extraction of donor-driven spin relaxation in n -type nondegenerate germanium

Using pure spin current transport measurements in lateral spin-valve devices, we study the spin relaxation in an $n$-type nondegenerate Ge layer, which is moderately doped Ge (P: $\ensuremath{\sim}{10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$). The obtained spin diffusion length $({\ensuremath{\lambda}}_{\mathrm{Ge}})$ of the nondegenerate Ge is two to three times greater than that of heavily doped degenerate Ge (P: $\ensuremath{\sim}{10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) in the temperature range from 8 to 100 K. We find that the electron spin lifetime ($\ensuremath{\tau}$) for the nondegenerate Ge is monotonically increased with decreasing temperature $(T)$. The increase in $\ensuremath{\tau}$ at temperatures less than 50 K can be interpreted in terms of the donor-driven spin relaxation mechanism including the 1/$\sqrt{T}$ behavior in multivalley semiconductors, proposed by Song et al. [Y. Song, O. Chalaev, and H. Dery, Phys. Rev. Lett. 113, 167201 (2014)]. We note that it is important for the $\ensuremath{\tau}$ of the moderately doped nondegenerate Ge to partly consider the $T$-independent component of spin relaxation in addition to the 1/$\sqrt{T}$ component.

Spin-polarization dependence of the Rb-Xe spin-exchange optical pumping process

We experimentally study the Rb-Xe spin-exchange optical pumping process. The dependence of the spin-exchange rate on the intensity of the pumping light is measured at different temperatures under low magnetic fields. We demonstrate that the spin-exchange rate will decrease as the Rb spin polarization increases, which agrees well with the theoretical prediction. In our measurement, three spin exchange and spin relaxation mechanisms, namely, the spin exchange rates between Rb and Xe atoms caused by van der Waals molecule (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{vdW}}$) and binary collision (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{bin}}$) processes, and the spin relaxation rate due to the wall collision of Xe atoms (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{w}}$), have comparable magnitudes. These rates are extracted separately from the measured data with different Rb spin polarization at various temperatures. Our work provides a comprehensive confirmation of the physical picture of the spin-exchange optical pumping process.

Room-Temperature Spin-Transport Properties in an In0.5Ga0.5As Quantum Dot Spin-Polarized Light-Emitting Diode

An understanding of the spin-transport properties in semiconductor barriers is essential to improve the performance of spin-polarized light-emitting diodes (spin LEDs) for future optospintronics integration in information processing. Here, we report on the temperature and bias-voltage dependence of spin-transport properties in an ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dot (QD) spin LED using a combination of spin-dependent electroluminescence (EL) and time-resolved photoluminescence. The QD EL spin polarization increases with an increase in temperature above 125 K; this is attributed to the improved conversion efficiency from spin polarization of electrons to circular polarization of photons of the QDs. We find that both the electric field and temperature can enhance spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier above 200 K. At 298 K, the QD EL spin polarization decreases beyond 2.5 V; this is attributed to the enhanced D'yakonov Perel' spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier caused by the increase in electron temperature. This study provides valuable insights into the spin-relaxation mechanism in the semiconductor barrier during the room-temperature operation of the QD spin LED.

Diffusive Spin Transport in Narrow Two-Dimensional-Electron-gas Channels

Using the geometry of the spin field-effect transistor, we investigate the spin transport of in-plane spins in an array of narrow channels formed in a two-dimensional electron gas confined in an $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ quantum well. We demonstrate a clear enhancement of the spin-diffusion length with decreasing channel width, yielding up to 10 \textmu{}m for the narrowest channels. We discuss the observed enhancement in terms of the suppression of the Dyakonov-Perel mechanism of spin relaxation, which has been invoked in previous studies in the case of diffusion of out-of-plane oriented spins. We also show that the spin signal is significantly higher when injecting into an array of narrow channels, compared with injecting into a single narrow channel.

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}

Long-range spin jump diffusion revealed by dynamic light scattering

Spatiotemporal spin noise spectroscopy is combined with dynamic light scattering in order to reach spatial resolutions down to $\ensuremath{\sim}\ensuremath{\lambda}/10$. Applied to a system of localized electron spins, an insulating n-doped CdTe layer, this allows us to reveal long spin jump distances $\ensuremath{\ell}\ensuremath{\sim}2.7\phantom{\rule{4.pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. Spin noise spectra at large wave vectors $q$ ($q\ensuremath{\ell}\ensuremath{\gg}1$) provide a snapshot of the spin dynamics before jump (therefore not affected by spin motion), while at smaller $q$, spin motion sets in. This allows us to unravel the contributions of spin-orbit and hyperfine fields in the electron spin relaxation and to determine self-consistently all parameters relevant to the spin dynamics. We propose a phenomenological equation inspired by studies of atomic jump diffusion by neutron scattering, which includes the relevant spin relaxation mechanisms and the effect of time of flight of the spin fluctuation across the laser spot. This modeling reproduces all experimental results.

Electron Spin Relaxation Mechanisms of Atomic Hydrogen Trapped in Silsesquioxane Cages: The Role of Isotope Substitution

Encapsulated atomic hydrogen in silsesquioxane cages is a promising candidate for applications in emerging technologies like spin-based quantum computing, magnetic field sensing, and atomic clock d...