arXiv subjects

Ion Garate

Publications and source records attributed to Ion Garate.

49 records · Page 3Linked to original sources

arXiv2012

Weak Localization and Antilocalization in Topological Insulator Thin Films with Coherent Bulk-Surface Coupling

We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional (3D) topological insulator (TI). We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case.

cond-mat.mes-hall↗

arXiv2012

Noninvasive Probe of Charge Fractionalization in Quantum Spin-Hall Insulators

When an electron with well-defined momentum tunnels into a nonchiral Luttinger liquid, it breaks up into two separate wave packets that carry fractional charges and move in opposite directions. A direct observation of this phenomenon has proven elusive, mainly due to single-particle and plasmon backscattering caused by measurement probes. This paper theoretically introduces two topological insulator devices that are naturally suited for detecting fractional charges and their velocities directly and in a noninvasive fashion.

cond-mat.mes-hall↗

arXiv2011

Charge-Kondo Effect in Mesoscopic Superconductors Coupled to Normal Metals

We develop a theoretical proposal for the charge Kondo effect in mesoscopic normal-superconductor-normal heterostructures, where the superconducting gap exceeds the electrostatic charging energy. Charge-Kondo correlations in these devices alter the conventional temperature-dependence of Andreev reflection and electron cotunneling. We predict typical Kondo temperatures of $\gtrsim 10 {\rm mK}$, and suggest experimental architectures that combine superconducting charge-qubits with semiconducting nanowires at cryogenic temperatures.

cond-mat.mes-hall↗

arXiv2011

Excitons and Optical Absorption on the Surface of a Strong Topological Insulator with a Magnetic Energy Gap

We present a theoretical study of interacting electron-hole pairs located on a magnetized surface of a strong topological insulator (TI). The excitonic energy levels and the optical absorption on such surface display unique and potentially measurable features such as (i) an enhanced binding energy for excitons whose total angular momentum is aligned with the magnetic exchange field, (ii) a stark dependence of the optical absorption on the direction of the magnetic exchange field as well as on the chirality of the incident light, and (iii) a tunable center-of-mass motion of spinful excitons induced by particle-hole asymmetry in the exchange field. Our predictions are relevant to surfaces of magnetically doped TIs or surfaces coated with magnetic films, in addition to TI nanowires placed under longitudinal magnetic fields.

cond-mat.str-el↗

arXiv2011

Kondo Temperature in Multilevel Quantum Dots

We develop a general method to evaluate the Kondo temperature in a multilevel quantum dot that is weakly coupled to conducting leads. Our theory reveals that the Kondo temperature is strongly enhanced when the intradot energy-level spacing is comparable to or smaller than the charging energy. We propose an experiment to test our result, which consists of measuring the size-dependence of the Kondo temperature.

cond-mat.str-el↗

arXiv2010

Magnetoelectric Response of the Time-Reversal Invariant Helical Metal

We derive compact analytical expressions for the coupled spin-charge susceptibility of a clean helical metal at the surface of a three dimensional topological insulator (TI). These expressions lead to unconventional non-collinear RKKY interactions between two impurity magnetic moments placed on the surface of a TI, and predict the generation of electric currents by time-dependent magnetic moments. We determine the influence of gate and bias voltages on the interlayer exchange coupling between two single-domain ferromagnetic monolayers deposited on top of a TI.

cond-mat.str-el↗

arXiv2010

Interplay between Symmetric Exchange Anisotropy, Uniform Dzyaloshinskii-Moriya Interaction and Magnetic Fields in the Phase Diagram of Quantum Magnets and Superconductors

We theoretically study the joint influence of uniform Dzyaloshinskii-Moriya (DM) interactions, symmetric exchange anisotropy (with its axis parallel to the DM vector) and arbitrarily oriented magnetic fields on one-dimensional spin 1/2 antiferromagnets. We show that the zero-temperature phase diagram contains three competing phases: (i) an antiferromagnet with Neel vector in the plane spanned by the DM vector and the magnetic field, (ii) a {\em dimerized} antiferromagnet with Neel vector perpendicular to both the DM vector and the magnetic field, and (iii) a gapless Luttinger liquid. Phase (i) is destroyed by a small magnetic field component along the DM vector and is furthermore unstable beyond a critical value of easy-plane anisotropy, which we estimate using Abelian and non-Abelian bosonization along with perturbative renormalization group. We propose a mathematical equivalent of the spin model in a one-dimensional Josephson junction (JJ) array located in proximity to a bulk superconductor. We discuss the analogues of the magnetic phases in the superconducting context and comment on their experimental viability.

cond-mat.str-el↗

arXiv2009

Inverse Spin-Galvanic Effect in a Topological-Insulator/Ferromagnet Interface

When a ferromagnet is deposited on the surface of a topological insulator the topologically protected surface state develops a gap and becomes a 2-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a large effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.

cond-mat.mtrl-sci↗

arXiv2009

Influence of a Transport Current on Magnetic Anisotropy in Gyrotropic Ferromagnets

Current-induced torques are commonly used to manipulate non-collinear magnetization configurations. In this article we discuss current-induced torques present in a certain class of collinear magnetic systems, relating them to current-induced changes in magnetic anisotropy energy. We present a quantitative estimate of their characteristics in uniform strained ferromagnetic (Ga,Mn)As.

cond-mat.mtrl-sci↗

arXiv2008

Non-Adiabatic Spin Transfer Torque in Real Materials

The motion of simple domain walls and of more complex magnetic textures in the presence of a transport current is described by the Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations depend sensitively on the ratio between the dimensionless material parameter $β$ which characterizes non-adiabatic spin-transfer torques and the Gilbert damping parameter $α$. This ratio has been variously estimated to be close to 0, close to 1, and large compared to 1. By identifying $β$ as the influence of a transport current on $α$, we derive a concise, explicit and relatively simple expression which relates $β$ to the band structure and Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate that intrinsic spin-orbit interactions lead to intra-band contributions to $β$ which are often dominant and can be (i) estimated with some confidence and (ii) interpreted using the "breathing Fermi surface" model.

cond-mat.mtrl-sci↗

arXiv2008

Theory of Weak Localization in Ferromagnetic (Ga,Mn)As

We study quantum interference corrections to the conductivity in (Ga,Mn)As ferromagnetic semiconductors using a model with disordered valence band holes coupled to localized Mn moments through a p-d kinetic-exchange interaction. We find that at Mn concentrations above 1% quantum interference corrections lead to negative magnetoresistance, i.e. to weak localization (WL) rather than weak antilocalization (WAL). Our work highlights key qualitative differences between (Ga,Mn)As and previously studied toy model systems, and pinpoints the mechanism by which exchange splitting in the ferromagnetic state converts valence band WAL into WL. We comment on recent experimental studies and theoretical analyses of low-temperature magnetoresistance in (Ga,Mn)As which have been variously interpreted as implying both WL and WAL and as requiring an impurity-band interpretation of transport in metallic (Ga,Mn)As.

cond-mat.mtrl-sci↗

arXiv2008

Gilbert Damping in Conducting Ferromagnets II: Model Tests of the Torque-Correlation Formula

We report on a study of Gilbert damping due to particle-hole pair excitations in conducting ferromagnets. We focus on a toy two-band model and on a four-band spherical model which provides an approximate description of ferromagnetic (Ga,Mn)As. These models are sufficiently simple that disorder-ladder-sum vertex corrections to the long-wavelength spin-spin response function can be summed to all orders. An important objective of this study is to assess the reliability of practical approximate expressions which can be combined with electronic structure calculations to estimate Gilbert damping in more complex systems.

cond-mat.mtrl-sci↗

arXiv2008

Gilbert Damping in Conducting Ferromagnets I: Kohn-Sham Theory and Atomic-Scale Inhomogeneity

We derive an approximate expression for the Gilbert damping coefficient α_G of itinerant electron ferromagnets which is based on their description in terms of spin-density-functional-theory (SDFT) and Kohn-Sham quasiparticle orbitals. We argue for an expression in which the coupling of magnetization fluctuations to particle-hole transitions is weighted by the spin-dependent part of the theory's exchange-correlation potential, a quantity which has large spatial variations on an atomic length scale. Our SDFT result for α_G is closely related to the previously proposed spin-torque correlation-function expression.

cond-mat.mes-hall↗