Llorenç Serra

Fano-Rashba effect in a quantum wire

We predict the occurrence of Fano line shapes 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 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.

Thermoelectric transport of mesoscopic conductors coupled to voltage and thermal probes

We investigate the basic properties of the thermopower (Seebeck coefficient) of phase-coherent conductors under the influence of dephasing and inelastic processes. Transport across the system is caused by a voltage bias or a thermal gradient applied between two terminals. Inelastic scattering is modeled with the aid of an additional probe acting as an ideal potentiometer and thermometer. We find that inelastic scattering reduces the conductors thermopower and, more unexpectedly, generates a magnetic field asymmetry in the Seebeck coefficient. The latter effect is shown to be a higher-order effect in the Sommerfeld expansion. We discuss our result by using two illustrative examples. First, we consider a generic mesoscopic system described within random matrix theory and demonstrate that thermopower fluctuations disappear quickly as the number of probe modes increases. Second, the asymmetry is explicitly calculated in the quantum limit of a ballistic microjunction. We find that asymmetric scattering strongly enhances the effect and discuss its dependence on temperature and Fermi energy.

From Coulomb blockade to the Kondo regime in a Rashba dot

We investigate the electronic transport in a quantum wire with localized Rashba interaction. The Rashba field forms quasibound states that couple to the continuum states with an opposite spin direction. The presence of this Rashba dot causes Fano-like antiresonances and dips in the wire’s linear conductance. The Fano line shape arises from the interference between the direct transmission channel along the wire and the hopping through the Rashba dot. Due to the confinement, we predict the observation of large charging energies in the local Rashba region, which lead to Coulomb-blockade effects in the transport properties of the wire. Importantly, the Kondo regime can be achieved with a proper tuning of the Rashba interaction, giving rise to an oscillating linear conductance for a fixed occupation of the Rashba dot.

Strongly modulated transmission of a spin-split quantum wire with local Rashba interaction

We investigate the transport properties of ballistic quantum wires in the presence of Zeeman spin splittings and a spatially inhomogeneous Rashba interaction. The Zeeman interaction is extended along the wire and produces gaps in the energy spectrum, which allow electron propagation only for spinors lying along a certain direction. For spins in the opposite direction, the waves are evanescent far away from the Rashba region, which plays the role of the scattering center. The most interesting case occurs when the magnetic field is perpendicular to the Rashba field. Then, the spins of the asymptotic wave functions are not eigenfunctions of the Rashba Hamiltonian, and the resulting coupling between spins in the Rashba region gives rise to sudden changes of the transmission probability when the Fermi energy is swept along the gap. After briefly examining the energy spectrum and eigenfunctions of a wire with extended Rashba coupling, we analyze the transmission through a region of localized Rashba interaction, in which a double interface separates a region of constant Rashba interaction from wire leads free from spin-orbit coupling. For energies slightly above the propagation threshold, we find the ubiquitous occurrence of transmission zeros antiresonances, which are analyzed by matching methods in the one-dimensional limit. We find that a minimal tight-binding model yields analytical transmission line shapes of Fano antiresonance type. The general angular dependence of the external magnetic field is treated within projected Schrodinger equations with the nondiagonal Hamiltonian matrix elements mixing different wave function components. Finally, we consider a realistic quantum wire where the energy subbands are coupled via the Rashba intersubband coupling term and discuss its effect on the transmission zeros. We find that the antiresonances are robust against intersubband mixing, magnetic field changes, and smooth variations of the wire interfaces, which paves the way for possible applications of spin-split Rashba wires as spintronic current modulators.

Spin-orbit effects in GaAs quantum wells: Interplay between Rashba, Dresselhaus, and Zeeman interactions

12 pages.-- PACS numbers: 73.21.Fg, 73.22.Dj, 73.22.Lp.-- Final full-text version of the paper available at: http://dx.doi.org/10.1103/PhysRevB.74.115303.

Roto-vibrational spectrum and Wigner crystallization in two-electron parabolic quantum dots

We provide a quantitative determination of the crystallization onset for two electrons in a parabolic two-dimensional confinement. This system is shown to be well described by a rotovibrational model, Wigner crystallization occurring when the rotational motion gets decoupled from the vibrational one. The Wigner molecule thus formed is characterized by its moment of inertia and by the corresponding sequence of rotational excited states. The role of a vertical magnetic field is also considered. Additional support to the analysis is given by the Hartree-Fock phase diagram for the ground state and by the random-phase approximation for the moment of inertia and vibron excitations.

Ground state and far-infrared absorption of two-electron rings in a magnetic field

Motivated by recent experiments [A. Lorke et al., Phys. Rev. Lett. 84, 2223 (2000)] an analysis of the ground state and far-infrared absorption of two electrons confined in a quantum ring is presented. The height of the repulsive central barrier in the confining potential is shown to influence, in an important way, the ring properties. The experiments are best explained assuming the presence of both high- and low-barrier quantum rings in the sample. The formation of a two-electron Wigner molecule in certain cases is discussed.

The static polarisability of metal clusters and spheres in an improved Thomas-Fermi approximation

The authors have used an approximate Thomas-Fermi-Dirac-Weizsacker functional to evaluate the static polarisability of clusters and spheres of metallic atoms for excitation operators of the kind rLYL0. Calculations for Na and Ag systems made of up to N=1000 atoms have been carried out in the spherical jellium approximation for multipolarities L=1 to 15. These results have been combined with others obtained previously by the authors to estimate the width of the surface resonance and its evolution with L.

Symmetry breaking and the random-phase approximation in small quantum dots

The random-phase approximation has been used to compute the properties of parabolic two-dimensional quantum dots beyond the mean-field approximation. Special emphasis is put on the ground state correlation energy, the symmetry restoration, and the role of the spurious modes within the random-phase approximation. A systematics with the Coulombic interaction strength is presented for the 2-electron dot, while for the 6- and 12-electron dots selected cases are discussed. The validity of the random-phase approximation is assessed by comparison with available exact results.

Electron localization and optical absorption of polygonal quantum rings

This work was financially supported by the Research Fund of the University of Iceland, the Nordic network NANOCONTROL, project No.: P-13053, and by MINECO-Spain (Grant No. FIS2011-23526).