arXiv subjects

David Sanchez

Publications and source records attributed to David Sanchez.

At least 109 records · Page 6Linked to original sources

arXiv2007

Evanescent states in quantum wires with Rashba spin-orbit coupling

We discuss the calculation of evanescent states in quasi-one-dimensional quantum wires in the presence of Rashba spin-orbit interaction. We suggest a computational algorithm devised for cases in which longitudinal and transverse motions are coupled. The dispersion relations are given for some selected cases, illustrating the feasibility of the proposed computational method. As a practical application, we discuss the solutions for a wire containing a potential step.

cond-mat.mes-hall↗

arXiv2007

Spin-polarized transport in II-VI magnetic resonant tunneling devices

We investigate electronic transport through II-VI semiconductor resonant tunneling structures containing diluted magnetic impurities. Due to the exchange interaction between the conduction electrons and the impurities, there arises a giant Zeeman splitting in the presence of a moderately low magnetic field. As a consequence, when the quantum well is magnetically doped the current-voltage characteristics shows two peaks corresponding to transport for each spin channel. This behavior is experimentally observed and can be reproduced with a simple tunneling model. The model thus allows to analyze other configurations. First, we further increase the magnetic field, which leads to a spin polarization of the electronic current injected from the leads, thus giving rise to a relative change in the current amplitude. We demonstrate that the spin polarization in the emitter can be determined from such a change. Furthermore, in the case of a magnetically doped injector our model shows a large increase in peak amplitude and a shift of the resonance to higher voltages as the external field increases. We find that this effect arises from a combination of giant Zeeman splitting, 3-D incident distribution and broad resonance linewidth.

cond-mat.mes-hall↗

arXiv2006

The Fano-Rashba effect

We analyze the linear conductance of a semiconductor quantum wire containing a region where a local Rashba spin-orbit interaction is present. We show that Fano lineshapes appear in the conductance due to the formation of quasi bound states which interfere with the direct transmission along the wire, a mechanism that we term the Fano-Rashba effect. We obtain the numerical solution of the full Schrödinger equation using the quantum-transmitting-boundary method. The theoretical analysis is performed using the coupled-channel model, finding an analytical solution by ansatz. The complete numerical solution of the coupled-channel equations is also discussed, showing the validity of the ansatz approach.

cond-mat.mes-hall↗

arXiv2006

Fano-Rashba effect in a quantum wire

We predict the occurrence of Fano lineshapes in a semiconductor quantum wire with local spin-orbit (Rashba) coupling. We show that the Rashba interaction acts in a strictly one dimensional channel as an attractive impurity, leading to the formation of purely bound states. In a quasi-one dimensional system these bound states couple to the conduction ones through the Rashba intersubband mixing, giving rise to pronounced dips in the linear conductance plateaus. We give exact numerical results and propose an approximate model capturing the main ingredients of the effect.

cond-mat.mes-hall↗

arXiv2005

Rashba interaction in quantum wires with in-plane magnetic fields

We analyze the spectral and transport properties of ballistic quasi one-dimensional systems in the presence of spin-orbit coupling and in-plane agnetic fields. Our results demonstrate that Rashba precession and intersubband coupling must be treated on equal footing for wavevectors near the magnetic field induced gaps. We find that intersubband coupling limits the occurrence of negative effective masses at the gap edges and modifies the linear conductance curves in the strong coupling limit. The effect of the magnetic field on the spin textured orientation of the wire magnetization is discussed.

cond-mat.mes-hall↗

arXiv2005

Chirality in Coulomb-blockaded quantum dots

We investigate the two-terminal nonlinear conductance of a Coulomb-blockaded quantum dot attached to chiral edge states. Reversal of the applied magnetic field inverts the system chirality and leads to a different polarization charge. As a result, the current--voltage characteristic is not an even function of the magnetic field. We show that the corresponding magnetic-field asymmetry arises from single-charge effects and vanishes in the limit of high temperature.

cond-mat.mes-hall↗

arXiv2005

Interaction induced magnetic field asymmetry of nonlinear mesoscopic electrical transport

We demonstrate that the nonlinear I-V characteristics of a two probe conductor is not an even function of magnetic field. While the conductance of a two-probe conductor is even in magnetic field, we find that already the contributions to the current which are second order in voltage, are in general not even. This implies a departure from the Onsager microreversibility principle already in the weakly nonlinear regime. Interestingly, the effect that we find is due to the Coulomb interaction. A measurement of the magnetic field asymmetry can be used to determine the effective interaction strength. As a generic example, we discuss the I-V characteristics of a chaotic quantum dot. The ensemble averaged I-V of such a cavity is linear: nonlinearities are due to quantum interference. Consequently, phase-breaking reduces the asymmetry. We support this statement with a calculation which treats inelastic scattering with the help of a voltage probe.

cond-mat.mes-hall↗

arXiv2005

Spintronic transport and Kondo effect in quantum dots

We investigate the spin-dependent transport properties of quantum-dot based structures where Kondo correlations dominate the electronic dynamics. The coupling to ferromagnetic leads with parallel magnetizations is known to give rise to nontrivial effects in the local density of states of a single quantum dot. We show that this influence strongly depends on whether charge fluctuations are present or absent in the dot. This result is confirmed with numerical renormalization group calculations and perturbation theory in the on-site interaction. In the Fermi-liquid fixed point, we determine the correlations of the electric current at zero temperature (shot noise) and demonstrate that the Fano factor is suppressed below the Poissonian limit for the symmetric point of the Anderson Hamiltonian even for nonzero lead magnetizations. We discuss possible avenues of future research in this field: coupling to the low energy excitations of the ferromagnets (magnons), extension to double quantum dot systems with interdot antiferromagnetic interaction and effect of spin-polarized currents on higher symmetry Kondo states such as SU(4).

cond-mat.mes-hall↗

arXiv2005

Probing spin and orbital Kondo effects with a mesoscopic interferometer

We investigate theoretically the transport properties of a closed Aharonov-Bohm interferometer containing two quantum dots in the strong coupling regime. We find two distinct physical scenarios depending on the strength of the interdot Coulomb interaction. When the interdot Coulomb interaction is negligible only spin fluctuations are important and each dot develops a Kondo resonance at the Fermi level independently of the applied magnetic flux. The transport is characterized by the interference of these two independent Kondo resonances. On the contrary, for large interdot interaction, only one electron can be accommodated onto the double dot system. In this situation, not only the spin can fluctuate but also the orbital degree of freedom (the pseudo-spin). As a result, we find different ground states depending on the value of the applied flux. When $ϕ=π$ (mod $2π$) ($ϕ=2πΦ/Φ_0$, where $Φ$ is applied flux, and $Φ_0=h/e$ the flux quantum) the electronic transport can take place via simultaneous correlations in the spin and pseudo-spin sectors, leading to the highly symmetric SU(4) Kondo state. Nevertheless, we find situations with $ϕ>0$ (mod $2π$) where the pseudo-spin quantum number is not conserved during tunneling events, giving rise to the common SU(2) Kondo state with an enhanced Kondo temperature. We investigate the crossover between both ground states and discuss possible experimental signatures of this physics as a function of the applied magnetic flux.

cond-mat.mes-hall↗

arXiv2005

Three-terminal transport through a quantum dot in the Kondo regime: Conductance, dephasing, and current-current correlations

We investigate the nonequilibrium transport properties of a three-terminal quantum dot in the strongly interacting limit. At low temperatures, a Kondo resonance arises from the antiferromagnetic coupling between the localized electron in the quantum dot and the conduction electrons in source and drain leads. It is known that the local density of states is accessible through the differential conductance measured at the (weakly coupled) third lead. Here, we consider the multiterminal current-current correlations (shot noise and cross correlations measured at two different terminals). We discuss the dependence of the current correlations on a number of external parameters: bias voltage, magnetic field and magnetization of the leads. When the Kondo resonance is split by fixing the voltage bias between two leads, the shot noise shows a nontrivial dependence on the voltage applied to the third lead. We show that the cross correlations of the current are more sensitive than the conductance to the appearance of an external magnetic field. When the leads are ferromagnetic and their magnetizations point along opposite directions, we find a reduction of the cross correlations. Moreover, we report on the effect of dephasing in the Kondo state for a two-terminal geometry when the third lead plays the role of a fictitious voltage probe.

cond-mat.mes-hall↗

arXiv2004

Magnetic-field asymmetry of nonlinear mesoscopic transport

We investigate departures of the Onsager relations in the nonlinear regime of electronic transport through mesoscopic systems. We show that the nonlinear current--voltage characteristic is not an even function of the magnetic field due only to the magnetic-field dependence of the screening potential within the conductor. We illustrate this result for two types of conductors: A quantum Hall bar with an antidot and a chaotic cavity connected to quantum point contacts. For the chaotic cavity we obtain through random matrix theory an asymmetry in the fluctuations of the nonlinear conductance that vanishes rapidly with the size of the contacts.

cond-mat.mes-hall↗

arXiv2003

Kondo effect in a quantum dot coupled to ferromagnetic leads: A numerical renormalization group analysis

We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors. We also report on the influence of spin-flip processes.

cond-mat.mes-hall↗

arXiv2003

Andreev drag effect in ferromagnetic-normal-superconducting systems

We investigate conductances and current correlations in a system consisting of a normal multichannel conductor connected to one superconducting and two ferromagnetic electrodes. For antiparallel orientation of the ferromagnet polarizations, current injection from one ferromagnet can, due to Andreev reflection, lead to a net drag of current from the second ferromagnet toward the superconductor. We present the conditions for the Andreev drag in terms of the degree of lead polarizations, contact conductances and spin-flip scattering. Remarkably, both equilibrium and nonequilibrium zero-frequency current correlations between the ferromagnets become positive even in the presence of spin relaxation.

cond-mat.mes-hall↗

arXiv2003

Nonequilibrium spintronic transport through an artificial Kondo impurity: Conductance, magnetoresistance and shot noise

We investigate the nonequilibrum transport properties of a quantum dot when spin flip processes compete with the formation of a Kondo resonance in the presence of ferromagnetic leads. Based upon the Anderson Hamiltonian in the strongly interacting limit, we predict a splitting of the differential conductance when the spin flip scattering amplitude is of the order of the Kondo temperature. We discuss how the relative orientation of the lead magnetizations strongly influences the electronic current and the shot noise in a nontrivial way. Furthermore, we find that the zero-bias tunneling magnetoresistance becomes negative with increasing spin flip scattering amplitude.

cond-mat.mes-hall↗

arXiv2002

Spin-polarized current oscillations in diluted magnetic semiconductor multiple quantum wells

We study the spin and charge dynamics of electrons in n-doped II--VI semiconductor multiple quantum wells when one or more quantum wells are doped with Mn. The interplay between strongly nonlinear inter-well charge transport and the large tunable spin-splitting induced by exchange interactions with spin-polarized Mn ions produces interesting new spin dependent features. The tunneling current between quantum wells can be strongly spin polarized and, under certain conditions, can develop self sustained oscillations under a finite dc voltage. The spin polarization oscillates in both magnetic and nonmagnetic quantum wells and the time average in magnetic wells can differ from its zero-voltage value. Our numerical simulations demonstrate that the amplitude of the spin polarization oscillations depends on the distribution of magnetic wells within the sample. We discuss how the spin polarized current and the spin polarization of the quantum wells can be tailored experimentally.

cond-mat↗

arXiv2001

Field-domain spintronics in magnetic semiconductor multiple quantum wells

We develop a theory of non-linear growth direction transport in magnetically doped II-VI compound semiconductor multiple-quantum-well systems. We find that the formation of electric field domains can be controlled by manipulating the space dependence of the band electron spin-polarization, using its exchange coupling to local moments. We emphasize the importance of band electron spin relaxation in limiting the strength of these effects.

cond-mat↗

arXiv2001

Photoassisted dynamical transport in multiple quantum wells

We study the dynamical transport in weakly coupled superlattices in the presence of intense radiation in the THz regime. We derive a general model for the time dependent tunneling current within the Keldysh nonequilibrium-Green-function formalism. For the particular situation in which fast scattering procceses drive the system to local equilibrium in the wells drastic changes are found in the current vs. voltage curves.

cond-mat↗

arXiv2001

Temperature dependence of current self-oscillations and electric field domains in sequential tunneling doped superlattices

We examine how the current--voltage characteristics of a doped weakly coupled superlattice depends on temperature. The drift velocity of a discrete drift model of sequential tunneling in a doped GaAs/AlAs superlattice is calculated as a function of temperature. Numerical simulations and theoretical arguments show that increasing temperature favors the appearance of current self-oscillations at the expense of static electric field domain formation. Our findings agree with available experimental evidence.

cond-mat↗