Elliott-Yafet Mechanism Research Articles

Carrier, spin and optical propagation properties of CsPbBr3 hexagonal nanocrystals

The shape of nanocrystals can effectively adjust their photo-physical properties, which becomes a route to improve the nanocrystal-based semiconductor device. Herein, we further dig the photo-physical properties of CsPbBr3 hexagonal nanocrystals (HNCs), in which CsPbBr3 cubic nanocrystals (CNCs) serves as a reference. It is found that the activity of carrier in CsPbBr3 HNCs is higher than that of CsPbBr3 CNCs, owing to the slow energy dissipation path related to the phonon energy, which slows down the carrier temperature cooling process and accelerate the carrier recombination behavior simultaneously. Moreover, the spin flip process may occur in the CsPbBr3 HNCs, whose lifetime is longer than that of CsPbBr3 CNCs owing to the weakening of phonon scattering of Elliott-Yafet mechanism. In addition, the photoluminescence test of CsPbBr3 HNCs in optical fiber microcavity shows that its light wave-guiding performance is determined by the self-absorption effect and the conventional light wave-guiding effect, which are both super to those of CsPbBr3 CNCs, benefiting from its high PL quantum yield. Our research suggests that the CsPbBr3 HNCs exhibits a huge potential in the field of optoelectronics, spintronics and photo-communication.

Electron Spin Transport in a Metal-Oxide-Semiconductor Si Two-Dimensional Inversion Channel: Effect of Hydrogen Annealing on Spin-Scattering Mechanism and Spin Lifetime

Electron spin transport in a two-dimensional $\mathrm{Si}$ inversion channel is experimentally and theoretically studied in terms of electron distribution in the subbands, electron momentum scattering processes, electron momentum lifetime, and spin lifetime. The electrical properties, electron charge transport, and spin transport are investigated by measuring a $\mathrm{Si}$-based spin metal-oxide-semiconductor field-effect transistor with a 10\penalty1000\-\allowhyphens{}-\ensuremath{\mu}m channel length under various bias and temperature conditions (4, 77, 150, and 295 K). In particular, in our unique procedure, the same device is measured before and after annealing to quantitatively clarify the change in the spin transport with lower and higher electron mobilities, by excluding device-to-device variability. Even when the distribution of electrons in the subbands, electron mobility, and temperature are significantly changed, the spin-flip probability, which is defined as the ratio of electron momentum lifetime to spin lifetime, is nearly constant at approximately 1/25 000, probably due to the Elliot-Yafet mechanism. The estimated spin conservation lengths at various temperatures are increased 2--50 times with increases in both the electron mobility and spin drift. With a high electron mobility of $3065\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 4 K, spin transport with a spin conservation efficiency of 94% is achieved through the 10-\ensuremath{\mu}m-long channel.

Many-body theory of phonon-induced spin relaxation and decoherence

First-principles calculations enable accurate predictions of electronic interactions and dynamics. However, computing the electron spin dynamics remains challenging. The spin-orbit interaction causes various dynamical phenomena that couple with phonons, such as spin precession and spin-flip e-ph scattering, which are difficult to describe with current first-principles calculations. In this work, we show a rigorous framework to study phonon-induced spin relaxation and decoherence, by computing the spin-spin correlation function and its vertex corrections due to e-ph interactions. We apply this approach to a model system and develop corresponding first-principles calculations of spin relaxation in GaAs. Our vertex-correction formalism is shown to capture the Elliott-Yafet, Dyakonov-Perel, and strong-precession mechanisms - three independent spin decoherence regimes with distinct physical origins - thereby unifying their theoretical treatment and calculation. Our method is general and enables quantitative studies of spin relaxation, decoherence, and transport in a wide range of materials and devices.

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.

Progress in ultrafast spintronics research

Ultrafast spintronics is one of the most promising fields for information processing technology innovation. When compared with traditional electronic devices, new spin-based devices have advantages such as low power consumption, high speed, and nonvolatility. Exploring novel physical phenomena and mechanisms relating to electronic spins significantly promotes the development of next-generation spintronic devices. In this review, we outline a sequence of advances in the fields of ultrafast magnetics and spintronics. First, a quick overview of spintronics and the discovery of ultrafast demagnetization induced by a femtosecond laser is provided. The proposed physical mechanisms and theories of ultrafast spin dynamics are then summarized, including the phenomenological three-temperature model, computations based on the time-dependent density functional theory, the Elliott-Yafet spin-flip mechanism, and the nonlocal superdiffusive spin transport model. Following that, the most recent experimental and theoretical studies on all-optical switching of ferrimagnetic and ferromagnetic metals are thoroughly reviewed. Furthermore, the discovery of hot-electron spin transport induced by ultrashort lasers as well as its influence on ultrafast magnetization dynamics, ultrafast spin transfer torque, spin injection into semiconductors, and spin accumulation and dissipation in hot-electron transport are described. The advancements in the optimization and manipulation of spintronics-based terahertz emitters are reviewed and analyzed. Finally, we discuss the current challenges in ultrafast spintronics research as well as possible future studies.

D’yakonov–Perel and Elliot–Yafet spin relaxation rates in InGaAs/InAlAs multiple quantum wells at room temperature

Room-temperature spin relaxation rates resulting from the D’yakonov–Perel (DP) and Elliot–Yafet (EY) mechanisms were determined by combining microscopic and macroscopic Kerr rotation measurements in narrow-gap (001) InGaAs/InAlAs quantum wells (QWs). Two samples with different DP relaxation rate were compared and we found that EY relaxation rate () was approximately double the value of the average DP relaxation rate even in the asymmetric QWs with large spin–orbit parameters. We also found that the anisotropy of spin relaxation time owing to the DP mechanism is weakened significantly in the system in which the EY mechanism dominates the spin relaxation.

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.

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.

Determination of spin diffusion length in Bi [formula omitted]-doped Cu films by weak antilocalization measurements

Working with the weak antilocalization measurements, we investigate the spin diffusion length in Bi δ-doped Cu films with the fine-tuned Bi concentration. By exploiting the long spin diffusion length of Cu and the strong spin-orbit coupling of Bi with the δ-doping technique, the spin diffusion length can be tuned artificially. Furthermore, it is found that the spin relaxation is dominated by elastic scatterings from Bi impurities in Bi δ-doped Cu systems, which follows the Elliott-Yafet mechanism.

Spin diffusion length in Au wires and films investigated by weak antilocalization measurements

Working with weak antilocalization measurements on Au wires and films grown by molecular beam epitaxy and dc magnetron sputtering with fine-tuned electron scatterings, we investigated the spin diffusion length in Au mesoscopic structures in quasi-1D and quasi-2D regimes. The result shows that the spin relaxations in both Au quasi-1D wires and quasi-2D films follow the Elliott–Yafet mechanism regardless of the deposition method. Furthermore, we observed a dimensionality behavior of spin diffusion between the quasi-1D and quasi-2D cases following the generalized Einstein's equation on Brownian motion.

Robust Spin Interconnect with Isotropic Spin Dynamics in Chemical Vapor Deposited Graphene Layers and Boundaries.

The utilization of large-area graphene grown by chemical vapor deposition (CVD) is crucial for the development of scalable spin interconnects in all-spin-based memory and logic circuits. However, the fundamental influence of the presence of multilayer graphene patches and their boundaries on spin dynamics has not been addressed yet, which is necessary for basic understanding and application of robust spin interconnects. Here, we report universal spin transport and dynamic properties in specially devised single layer, bilayer, and trilayer graphene channels and their layer boundaries and folds that are usually present in CVD graphene samples. We observe uniform spin lifetime with isotropic spin relaxation for spins with different orientations in graphene layers and their boundaries at room temperature. In all of the inhomogeneous graphene channels, the spin lifetime anisotropy ratios for spins polarized out-of-plane and in-plane are measured to be close to unity. Our analysis shows the importance of both Elliott-Yafet and D’yakonov-Perel’ mechanisms with an increasing role of the latter mechanism in multilayer channels. These results of universal and isotropic spin transport on large-area inhomogeneous CVD graphene with multilayer patches and their boundaries and folds at room temperature prove its outstanding spin interconnect functionality, which is beneficial for the development of scalable spintronic circuits.

Spin transport in multilayer graphene away from the charge neutrality point

Graphene is considered as a promising material in spintronics due to its long spin relaxation time and long spin relaxation length. However, its spin transport properties have been studied at low carrier density only, beyond which much is still unknown. In this study, we explore the spin transport and spin precession properties in multilayer graphene at high carrier density using ionic liquid gating. We find that the spin relaxation time is directly proportional to the momentum relaxation time, indicating that the Elliott-Yafet mechanism still dominates the spin relaxation in multilayer graphene away from the charge neutrality point.

Spin Polarization Dynamics of Free Charge Carriers in CsPbI3 Nanocrystals.

Lead halide perovskites (LHPs) exhibit large spin-orbit coupling (SOC), leading to only twofold-degenerate valence and conduction bands and therefore allowing for efficient optical orientation. This makes them ideal materials to study charge carrier spins. With this study we elucidate the spin dynamics of photoexcited charge carriers and the underlying spin relaxation mechanisms in CsPbI3 nanocrystals by employing time-resolved differential transmission spectroscopy (DTS). We find that the photoinduced spin polarization significantly diminishes during thermalization and cooling toward the energetically favorable band edge. Temperature-dependent DTS reveals a decay in spin polarization that is more than 1 order of magnitude faster at room temperature (3 ps) than at cryogenic temperatures (32 ps). We propose that spin relaxation of free charge carriers in large-SOC materials like LHPs occurs as a result of carrier-phonon scattering, as described by the Elliott-Yafet mechanism.

Abrupt enhancement of spin–orbit scattering time in ultrathin semimetallic SrIrO3 close to the metal–insulator transition

We report a magnetotransport study of spin relaxation in 1.4–21.2 nm epitaxial SrIrO3 thin films coherently strained on SrTiO3 substrates. Fully charge compensated semimetallic transport has been observed in SrIrO3 films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm2/V s to 150 cm2/V s with decreasing film thickness. In the two-dimensional regime, the charge dephasing and spin–orbit scattering lengths extracted from the weak localization/anti-localization effects show power-law dependence on temperature, pointing to the important role of electron–electron interaction. The spin–orbit scattering time τso exhibits an Elliott–Yafet mechanism dominated quasi-linear dependence on the momentum relaxation time τp. Ultrathin films approaching the critical thickness of metal–insulator transition show an abrupt enhancement in τso, with the corresponding τso/τp about 7.6 times of the value for thicker films. A likely origin for such unusual enhancement is the onset of strong electron correlation, which leads to charge gap formation and suppresses spin scattering.

Spin-phonon relaxation times in centrosymmetric materials from first principles

We present a first-principles approach for computing the phonon-limited $T_1$ spin relaxation time due to the Elliot-Yafet mechanism. Our scheme combines fully-relativistic spin-flip electron-phonon interactions with an approach to compute the effective spin of band electrons in materials with inversion symmetry. We apply our method to silicon and diamond, for which we compute the temperature dependence of the spin relaxation times and analyze the contributions to spin relaxation from different phonons and valley processes. The computed spin relaxation times in silicon are in excellent agreement with experiment in the 50$-$300 K temperature range. In diamond, we predict intrinsic spin relaxation times of 540 $\mu$s at 77 K and 2.3 $\mu$s at 300 K. Our work enables precise predictions of spin-phonon relaxation times in a wide range of materials, providing microscopic insight into spin relaxation and guiding the development of spin-based quantum technologies.

Spin-orbit coupling in elemental two-dimensional materials

The fundamental spin-orbit coupling and spin mixing in graphene and rippled honeycomb lattice materials silicene, germanene, stanene, blue phosphorene, arsenene, antimonene, and bismuthene is investigated from first principles. The intrinsic spin-orbit coupling in graphene is revisited using multi-band $k\cdot p$ theory, showing the presence of non-zero spin mixing in graphene despite the mirror symmetry. However, the spin mixing itself does not lead to the the Elliott-Yafet spin relaxation mechanism, unless the mirror symmetry is broken by external factors. For other aforementioned elemental materials we present the spin-orbit splittings at relevant symmetry points, as well as the spin admixture $b^2$ as a function of energy close to the band extrema or Fermi levels. We find that spin-orbit coupling scales as the square of the atomic number Z, as expected for valence electrons in atoms. For isolated bands, it is found that $b^2\sim Z^4$. The spin-mixing parameter also exhibits giant anisotropy which, to a large extent, can be controlled by tuning the Fermi level. Our results for $b^2$ can be directly transferred to spin relaxation time due to the Elliott-Yafet mechanism, and therefore provide an estimate of the upper limit for spin lifetimes in materials with space inversion center.

Electron spin and momentum lifetimes in two-dimensional Si accumulation channels: Demonstration of Schottky-barrier spin metal-oxide-semiconductor field-effect transistors at room temperature

We have investigated the electron spin lifetime ${\ensuremath{\tau}}_{\mathrm{S}}$ and momentum lifetime $\ensuremath{\tau}$ in a two-dimensional (2D) accumulation channel of Schottky-barrier spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs). The spin MOSFETs examined in this study have Fe/Mg/MgO/Si Schottky-tunnel junctions at the source/drain and a 15-nm-thick nondegenerated Si channel with a phosphorus donor doping concentration ${N}_{\mathrm{D}}$ of $1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. We estimated $\ensuremath{\tau}$ and the electron diffusion coefficient ${D}_{e}$ in the 2D accumulation channel from experimental results of a Hall-bar-type MOSFET device and self-consistent calculations using Poisson's and Schr\odinger's equations. The spin MOSFETs with various channel lengths ${L}_{\mathrm{ch}}\phantom{\rule{0.28em}{0ex}}(=0.3\ensuremath{-}10\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m})$ exhibited transistor characteristics with a high on/off ratio of $\ensuremath{\sim}{10}^{6}$ as well as clear spin-valve signals at 295 K. From the spin-valve signals measured with various gate electric fields $(2\ensuremath{-}5\phantom{\rule{0.16em}{0ex}}\mathrm{MV}/\mathrm{cm}), {\ensuremath{\tau}}_{\mathrm{S}}$ and the electron spin diffusion length ${\ensuremath{\lambda}}_{\mathrm{S}}$ were estimated. We found that the spin-flip rate per one momentum scattering event $\ensuremath{\tau}/{\ensuremath{\tau}}_{\mathrm{S}}$ is $\ensuremath{\sim}1/14\phantom{\rule{0.16em}{0ex}}000$, which is almost unchanged by the gate electric field. The proportionality between ${\ensuremath{\tau}}_{\mathrm{S}}$ and $\ensuremath{\tau}$ indicates that the Elliott-Yafet mechanism is dominant in the Si 2D electron accumulation channel, and that the spin-flip rate per one phonon scattering event and that per one surface roughness scattering event are the same. Based on the Elliott-Yafet theory, there is a possibility that the spin-orbit coupling in the Si 2D accumulation channel is almost twice as strong as that in bulk Si materials.

Calculation of the electron spin relaxation time in GaAs by an acoustic piezoelectric phonon scattering

The electron spin relaxation times due to acoustic piezoelectric phonon scattering in GaAs are calculated based on a magnetic susceptibility formula derived using the projection-reduction method in the linear response scheme. The temperature, magnetic field, and electron density dependencies of the relaxation time are investigated. The result shows that the relaxation of the electron spin can be explained by the Elliott–Yafet mechanism and acoustic piezoelectric phonon scattering at a low temperature by comparison with the experimental result.

Extrinsic spin-charge coupling in diffusive superconducting systems

We present a theoretical study of diffusive superconducting systems with extrinsic spin-orbit coupling and arbitrarily strong impurity potential. We derive from a microscopic Hamiltonian a diffusion equation for the quasi-classical Green function, and demonstrate that all mechanisms related to the spin-orbit coupling are expressed in terms of three kinetic coefficients: the spin Hall angle, the spin current swapping coefficient, and the spin relaxation rate due to Elliott-Yafet mechanism. The derived diffusion equation contains a hitherto unknown term describing a spin-orbit torque that appears exclusively in the superconducting state. As an example, we provide a qualitative description of a magnetic vortex in a superconductor with triplet correlations, and show that the novel term describes a spin torque proportional to the vector product between the spectral angular momentum of the condensate and the triplet vector. Our equation opens up the possibility to explore spintronic effects in superconductors with no counterparts in the normal metallic state.

(Keynote) Physics, Materials and Applications of High-Mobility Organic Transistor Circuits

Development of functional materials and understanding of the microscopic mechanisms mutually benefit through their close interaction. To accelerate development of organic semiconductor devices for industrial application to flexible and printed electronics, it has been essential to understand the mechanism of charge transport in conjunction with molecular-scale charge transfer. We examine the idea that high-mobility charge transport in newly developed solution-crystalized organic transistors is caused by band-like charge transport, using several different methods to probe coherence in the electronic charge. These materials, being essentially different from conventional organic semiconductors with basically incoherent hopping-like transport, often show high values of mobility exceeding 10 cm2/Vs so that high-speed organic transistor circuits are realized. We first employ Hall-effect measurement which differentiates the coherent band transport from site-to-site hopping. Rubrene single crystal transistors were the first to show the Hall voltage obviously indicating the band transport with the mobility exceeding 10 cm2/Vs, whereas the result of pentacene single crystal transistors with the mobility around 1 cm2/Vs suggested somewhat intermediate between coherent and incoherent charge transport. An interesting crossover from hopping-like to band-like transport is also found by gradually changing temperature and external pressure, clearly indicating involvement of intermolecular phonons. Stimulated by the experiments in the early stage, such materials as decyldinaphthobenzo-dithiophene derivatives (Cn-DNBDT) are newly synthesized to stabilize the molecular position in the crystal form by introducing steric hindrance in their p-conjugated cores. With the development of solution-crystallization method, arrays of the single crystal transistors are formed to the size of wafer scale on plastic substrates. The mobility is as high as 20 cm2/Vs for p-type compound and 4 cm2/Vs for an n-type compound. We measured spin relaxation time of the electric-field induced carriers down to 4.2 K using the large-area ultra-thin single-crystal transistors, so that electron-spin relaxiation is precisely investigated. We found that the spin-relaxation follows the Elliott-Yafet mechanism so that the spin life time is consistently elongated at low temperatures due to reduced phonon scattering via spin-orbit coupling. The result suggest that charge carrier mobility can as high as 650 cm2/Vs with minimized phonon scattering at the low temperature. This presentation also focuses on recent development of key technologies for printed LSIs which can provide future low-cost platforms for RFID tags, AD converters, data processors, and sensing circuitries. Such prospect bears increasing reality because of recent research innovations in the field of material chemistry, charge transport physics, and solution processes of printable organic semiconductors. With excellent chemical and thermal stability in recently developed new materials, we are developing simple integrated devices based on CMOS using p-type and n-type printed organic FETs. Particularly important are new processing technologies for continuous growth of the organic single-crystalline semiconductor “wafers” from solution and for lithographical patterning of semiconductors and metal electrodes. Successful rectification and identification are demonstrated at 13.56 MHz with printed organic CMOS circuits.