arXiv subjects

R. Raimondi

Publications and source records attributed to R. Raimondi.

At least 19 recordsLinked to original sources

arXiv2025

Quantum Kinetic Anatomy of Electron Angular Momenta Edge Accumulation

Controlling electron's spin and orbital degrees of freedom has been a major research focus over the past two decades, as it underpins the electrical manipulation of magnetization. Leveraging a recently introduced quantum kinetic theory of multiband systems [T. Valet and R. Raimondi, Phys. Rev. B 111, L041118 (2025)], we outline how the intrinsic angular momenta linear response is partitioned into intraband and interband contributions. Focusing on time reversal and inversion symmetric metals, we show that the spin and orbital Hall currents are purely intraband. We also reveal that the intrinsic edge densities originate partially, and in the orbital case probably mostly, from a new interband mechanism. We discuss how this profoundly impacts the interpretation of orbital edge accumulation observations, and has broader implications for current induced torques.

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arXiv2021

Spin drift-diffusion for two-subband quantum wells

Controlling the spin dynamics and spin lifetimes is one of the main challenges in spintronics. To this end, the study of the spin diffusion in two-dimensional electron gases (2DEGs) shows that when the Rashba and Dresselhaus spin-orbit couplings (SOC) are balanced, a persistent spin helix regime arises. There, a striped spin pattern shows a long lifetime, limited only by the cubic Dresselhaus SOC, and its dynamics can be controlled by in-plane drift fields. Here, we derive a spin diffusion equation for non-degenerate two-subbands 2DEGs. We show that the intersubband scattering rate, which is defined by the overlap of the subband densities, enters as a new nob to control the spin dynamics, and can be controlled by electric fields, being maximum for symmetric quantum wells. We find that for large intersubband couplings the dynamics follow an effective diffusion matrix given by approximately half of the subband-averaged matrices. This extra 1/2 factor arises from Matthiessen's rule summing over the intra- and intersubband scattering rates, and leads to a reduced diffusion constant and larger spin lifetimes. We illustrate our findings with numerical solutions of the diffusion equation with parameters extracted from realistic Schrödinger-Poisson calculations.

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arXiv2018

Role of phonon skew scattering in the spin Hall effect of platinum

We measure and analyze the effective spin Hall angle of platinum in the low residual resistivity regime by second harmonic measurements of the spin-orbit torques for a multilayer of Pt/Co/AlO$_x$. An angular dependent study of the torques allows us to extract the effective spin Hall angle responsible for the damping-like torque in the system. We observe a strikingly non-monotonic and reproducible temperature dependence of the torques. This behavior is compatible with recent theoretical predictions which include both intrinsic and extrinsic (impurities and phonons) contributions to the spin Hall effect at finite temperature.

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arXiv2017

Theory of the spin galvanic effect at oxide interfaces

The spin galvanic effect (SGE) describes the conversion of a non-equilibrium spin polarization into a transverse charge current. Recent experiments have demonstrated a large conversion efficiency for the two-dimensional electron gas formed at the interface between two insulating oxides, LaAlO$_3$ and SrTiO$_3$. Here we analyze the SGE for oxide interfaces within a three-band model for the Ti t$_{2g}$ orbitals which displays an interesting variety of effective spin-orbit couplings in the individual bands that contribute differently to the spin-charge conversion. Our analytical approach is supplemented by a numerical treatment where we also investigate the influence of disorder and temperature, which turns out to be crucial to provide an appropriate description of the experimental data.

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arXiv2017

Current-induced spin polarization in InGaAs and GaAs epilayers with varying doping densities

The current-induced spin polarization and momentum-dependent spin-orbit field were measured in In$_{x}$Ga$_{1-x}$As epilayers with varying indium concentrations and silicon doping densities. Samples with higher indium concentrations and carrier concentrations and lower mobilities were found to have larger electrical spin generation efficiencies. Furthermore, current-induced spin polarization was detected in GaAs epilayers despite the absence of measurable spin-orbit fields, indicating that the extrinsic contributions to the spin polarization mechanism must be considered. Theoretical calculations based on a model that includes extrinsic contributions to the spin dephasing and the spin Hall effect, in addition to the intrinsic Rashba and Dresselhaus spin-orbit coupling, are found to qualitatively agree with the experimental results.

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arXiv2017

Density inhomogeneities and Rashba spin-orbit coupling interplay in oxide interfaces

There is steadily increasing evidence that the two-dimensional electron gas (2DEG) formed at the interface of some insulating oxides like LaAlO3/SrTiO3 and LaTiO3/SrTiO3 is strongly inhomogeneous. The inhomogeneous distribution of electron density is accompanied by an inhomogeneous distribution of the (self-consistent) electric field confining the electrons at the interface. In turn this inhomogeneous transverse electric field induces an inhomogeneous Rashba spin-orbit coupling (RSOC). After an introductory summary on two mechanisms possibly giving rise to an electronic phase separation accounting for the above inhomogeneity,we introduce a phenomenological model to describe the density-dependent RSOC and its consequences. Besides being itself a possible source of inhomogeneity or charge-density waves, the density-dependent RSOC gives rise to interesting physical effects like the occurrence of inhomogeneous spin-current distributions and inhomogeneous quantum-Hall states with chiral "edge" states taking place in the bulk of the 2DEG. The inhomogeneous RSOC can also be exploited for spintronic devices since it can be used to produce a disorder-robust spin Hall effect.

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arXiv2016

Room-temperature transport properties of spin-orbit coupled Fermi systems: Spin thermoelectric effects, phonon skew scattering

In this note we summarize our recent results for the temperature dependence of transport coefficients of metallic films in the presence of spin-orbit coupling. Our focus is on (i) the spin Nernst and the thermal Edelstein effects, and (ii) the phonon skew scattering contribution to the spin Hall conductivity, which is relevant for the temperature dependence of the spin Hall angle. Depending on the parameters, the latter is expected to show a non-monotonous behavior.

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arXiv2015

Spin current swapping and Hanle spin Hall effect in the two dimensional electron gas

We analyze the effect known as "spin current swapping" (SCS) due to electron-impurity scattering in a uniform spin-polarized two-dimensional electron gas. In this effect a primary spin current $J_i^a$ (lower index for spatial direction, upper index for spin direction) generates a secondary spin current $J_a^i$ if $i \neq a$, or $J_j^j$, with $j\ne i$, if $i= a$. Contrary to naive expectation, the homogeneous spin current associated with the uniform drift of the spin polarization in the electron gas does not generate a swapped spin current by the SCS mechanism. Nevertheless, a swapped spin current will be generated, if a magnetic field is present, by a completely different mechanism, namely, the precession of the spin Hall spin current in the magnetic field. We refer to this second mechanism as Hanle spin Hall effect, and we notice that it can be observed in an experiment in which a homogeneous drift current is passed through a uniformly magnetized electron gas. In contrast to this, we show that an unambiguous observation of SCS requires inhomogeneous spin currents, such as those that are associated with spin diffusion in a metal, and no magnetic field. An experimental setup for the observation of the SCS is therefore proposed.

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arXiv2014

Theory of coupled spin-charge transport due to spin-orbit interaction in inhomogeneous two-dimensional electron liquids

Spin-orbit interactions in two-dimensional electron liquids are responsible for many interesting transport phenomena in which particle currents are converted to spin polarizations and spin currents and viceversa. Prime examples are the spin Hall effect, the Edelstein effect, and their inverses. By similar mechanisms it is also possible to partially convert an optically induced electron-hole density wave to a spin density wave and viceversa. In this paper we present a unified theoretical treatment of these effects based on quantum kinetic equations that include not only the intrinsic spin-orbit coupling from the band structure of the host material, but also the spin-orbit coupling due to an external electric field and a random impurity potential. The drift-diffusion equations we derive in the diffusive regime are applicable to a broad variety of experimental situations, both homogeneous and non-homogeneous, and include on equal footing "skew scattering" and "side-jump" from electron-impurity collisions. As a demonstration of the strength and usefulness of the theory we apply it to the study of several effects of current experimental interest: the inverse Edelstein effect, the spin-current swapping effect, and the partial conversion of an electron-hole density wave to a spin density wave in a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit couplings, subject to an electric field.

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arXiv2014

Spin Hall and Edelstein effects in metallic films: from 2D to 3D

A normal metallic film sandwiched between two insulators may have strong spin-orbit coupling near the metal-insulator interfaces, even if spin-orbit coupling is negligible in the bulk of the film. In this paper we study two technologically important and deeply interconnected effects that arise from interfacial spin-orbit coupling in metallic films. The first is the spin Hall effect, whereby a charge current in the plane of the film is partially converted into an orthogonal spin current in the same plane. The second is the Edelstein effect, in which a charge current produces an in-plane, transverse spin polarization. At variance with strictly two-dimensional Rashba systems, we find that the spin Hall conductivity has a finite value even if spin-orbit interaction with impurities is neglected and "vertex corrections" are properly taken into account. Even more remarkably, such finite value becomes "universal" in a certain configuration. This is a direct consequence of the spatial dependence of spin-orbit coupling on the third dimension, perpendicular to the film plane. The non-vanishing spin Hall conductivity has a profound influence on the Edelstein effect, which we show to consist of two terms, the first with the standard form valid in a strictly two-dimensional Rashba system, and a second arising from the presence of the third dimension. Whereas the standard term is proportional to the momentum relaxation time, the new one scales with the spin relaxation time. Our results, although derived in a specific model, should be valid rather generally, whenever a spatially dependent Rashba spin-orbit coupling is present and the electron motion is not strictly two-dimensional.

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arXiv2013

Inverse Edelstein Effect

We provide a precise microscopic definition of the recently observed "Inverse Edelstein Effect" (IEE), in which a non-equilibrium spin accumulation in the plane of a two-dimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The drift-diffusion equations that govern the effect are presented and applied to the interpretation of the experiments.

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arXiv2013

Intrinsic Spin Hall Effect at Oxide Interfaces: a Simple Model

An asymmetric triangular potential well provides the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO_3 and SrTiO_3 (LAO/STO). These electrons have been recently shown to exhibit a large spin-orbit coupling of the Rashba type, i.e., linear in the in-plane momentum. In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. Besides splitting each subband into two branches of opposite helicity, the spin-orbit interaction causes the wave function in the direction perpendicular to the plane of the quantum well (the z direction) to depend on the in-plane wave vector k. In contrast to the extreme asymmetric case, i.e., the wedge-shaped quantum well, for which the intrinsic spin Hall effect vanishes due to vertex corrections, we find that the asymmetric well supports a non-vanishing intrinsic spin Hall conductivity, which increases in magnitude as the well becomes more symmetric. The spin Hall conductivity is found to be proportional to the square of the spin-orbit coupling constant and, in the limit of low carrier density, depends only on the effective mass renormalization associated with the k-dependence of the wave functions in the z direction. Its origin lies in the non-vanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well.

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arXiv2013

Spin thermoelectrics in a disordered Fermi gas

We study the connection between the spin-heat and spin-charge response in a disordered Fermi gas with spin-orbit coupling. It is shown that the ratio between the above responses can be expressed as the thermopower $S=-(πk_B)^2Tσ'/3eσ$ times a number $R_s$ which depends on the strength and type of the spin-orbit couplings considered. The general results are illustrated by examining different two-dimensional electron or hole systems with different and competing spin-orbit mechanisms, and we conclude that a metallic system could prove much more efficient as a heat-to-spin than as a heat-to-charge converter.

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arXiv2012

Onsager relations in a two-dimensional electron gas with spin-orbit coupling

Theory predicts for the two-dimensional electrons gas with only Rashba spin-orbit interaction a vanishing spin Hall conductivity and at the same time a finite inverse spin Hall effect. We show how these seemingly contradictory results are compatible with the Onsager relations: the latter do hold for spin and particle (charge) currents in the two-dimensional electron gas, although (i) their form depends on the experimental setup and (ii) a vanishing bulk spin Hall conductivity does not necessarily imply a vanishing spin Hall effect. We also discuss the situation in which extrinsic spin orbit from impurities is present and the bulk spin Hall conductivity can be different from zero.

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arXiv2011

Spin-orbit interaction in a two-dimensional electron gas: a SU(2) formulation

Spin-orbit interaction is usefully classified as extrinsic or intrinsic depending on its origin: the potential due to random impurities (extrinsic), or the crystalline potential associated with the band or device structure (intrinsic). In this paper we will show how by using a SU(2) formulation the two sources of spin-orbit interaction may be described in an elegant and unified way. As a result we obtain a simple description of the interplay of the two types of spin-orbit interaction, and a physically transparent explanation of the vanishing of the d.c. spin Hall conductivity in a Rashba two-dimensional electron gas when spin relaxation is neglected, and its reinstatement when spin relaxation is allowed. Furthermore, we obtain an explicit formula for the transverse spin polarization created by an electric current, which generalizes the standard formula obtained by Edelstein and Aronov and Lyanda-Geller by including extrinsic spin-orbit interaction and spin relaxation.

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arXiv2011

Spin-Charge Locking and Tunneling into a Helical Metal

We derive a kinetic equation for the electrons moving on the surface of a three-dimensional topological insulator. Due to the helical nature of the excitations backward scattering is suppressed in the collision integral, and the spin dynamics is entirely constrained by that of the charge. We further analyze the tunneling between the helical and a conventional metal or ferromagnet. We find that the tunnel resistance strongly depends on the angle between the magnetization in the ferromagnet and the current in the helical metal. A nonmagnetic layer on top of the helical metal amplifies the current-induced spin polarization.

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arXiv2010

Non-Abelian gauge fields in the gradient expansion: generalized Boltzmann and Eilenberger equations

We present a microscopic derivation of the generalized Boltzmann and Eilenberger equations in the presence of non-Abelian gauges, for the case of a non-relativistic disordered Fermi gas. A unified and symmetric treatment of the charge $[U(1)]$ and spin $[SU(2)]$ degrees of freedom is achieved. Within this framework, just as the $U(1)$ Lorentz force generates the Hall effect, so does its $SU(2)$ counterpart give rise to the spin Hall effect. Considering elastic and spin-independent disorder we obtain diffusion equations for charge and spin densities and show how the interplay between an in-plane magnetic field and a time dependent Rashba term generates in-plane charge currents.

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arXiv2010

Inverse Spin Hall Effect and Anomalous Hall Effect in a Two-Dimensional Electron Gas

We study the coupled dynamics of spin and charge currents in a two-dimensional electron gas in the transport diffusive regime. For systems with inversion symmetry there are established relations between the spin Hall effect, the anomalous Hall effect and the inverse spin Hall effect. However, in two-dimensional electron gases of semiconductors like GaAs, inversion symmetry is broken so that the standard arguments do not apply. We demonstrate that in the presence of a Rashba type of spin-orbit coupling (broken structural inversion symmetry) the anomalous Hall effect, the spin Hall and inverse spin Hall effect are substantially different effects. Furthermore we discuss the inverse spin Hall effect for a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit coupling; our results agree with a recent experiment.

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