Michele Governale

Persistent current in ballistic mesoscopic rings with Rashba spin-orbit coupling

The presence of spin-orbit coupling affects the spontaneously flowing persistent currents in mesoscopic conducting rings. Here we analyze their dependence on magnetic flux with emphasis on identifying possibilities to prove the presence and extract the strength of Rashba spin splitting in low-dimensional systems. Effects of disorder and mixing between quasi-one-dimensional ring subbands are considered. The spin-orbit coupling strength can be inferred from the values of flux where sign changes occur in the persistent charge current. As an important consequence of the presence of spin splitting, we identify a nontrivial persistent spin current that is not simply proportional to the charge current. The different flux dependences of persistent charge and spin currents are a unique signature of spin-orbit coupling affecting the electronic structure of the ring.

Spin accumulation in quantum wires with strong Rashba spin-orbit coupling

We present analytical and numerical results for the effect of Rashba spin-orbit coupling on band structure, transport, and interaction effects in quantum wires when the spin precession length is comparable to the wire width. The situation with only the lowest spin-split subbands occupied is particularly interesting because electrons close to Fermi points of the same chirality can have approximately parallel spins. We discuss consequences for spin-dependent transport and effective Tomonaga-Luttinger descriptions of interactions in the quantum wire.

Pumping spin with electrical fields

Spin currents can be obtained through adiabatic pumping by means of electrical gating only. This is possible by making use of the tunability of the Rashba spin-orbit coupling in semiconductor heterostructures. We demonstrate the principles of this effect by considering a single-channel wire with a constriction. We also consider realistic structures, consisting of several open channels where subband spin mixing and disorder are present, and we confirm our predictions. Two different ways to detect the spin-pumping effect, either using ferromagnetic leads or applying a magnetic field, are discussed.

Filtering spin with tunnel-coupled electron wave guides

We show how momentum-resolved tunneling between parallel electron wave guides can be used to observe and exploit lifting of spin degeneracy due to Rashba spin-orbit coupling. A device is proposed that achieves spin filtering without using ferromagnets or the Zeeman effect.

MODELING AND MANUFACTURABILITY ASSESSMENT OF BISTABLE QUANTUM-DOT CELLS

We have investigated the behavior of bistable cells made up of four quantum dots and occupied by two electrons, in the presence of realistic confinement potentials produced by depletion gates on top of a GaAs/AlGaAs heterostructure. Such a cell represents the basic building block for logic architectures based on the concept of quantum cellular automata (QCA) and of ground state computation, which have been proposed as an alternative to traditional transistor-based logic circuits. We have focused on the robustness of the operation of such cells with respect to asymmetries derived from fabrication tolerances. We have developed a two-dimensional model for the calculation of the electron density in a driven cell in response to the polarization state of a driver cell. Our method is based on the one-shot configuration-interaction technique, adapted from molecular chemistry. From the results of our simulations, we conclude that an implementation of QCA logic based on simple “hole arrays” is not feasible, because...

Quantum dots with Rashba spin-orbit coupling.

We present results on the effects of spin-orbit coupling on the electronic structure of few-electron interacting quantum dots. The ground-state properties as a function of the number of electrons in the dot N are calculated by means of spin-density functional theory. We find a suppression of Hunds rule due to the competition of the Rashba effect and exchange interaction. Introducing an in-plane Zeeman field leads to a paramagnetic behavior of the dot in a closed-shell configuration and to spin texture in space.

A Josephson quantum electron pump

Electron pumps usually deliver small numbers of electrons by using strong Coulomb blockade to limit their flow under an applied bias. By periodically modulating the wavefunction of the electrons in a hybrid superconducting device, they can be delivered without bias.

Magnetic barrier in confined two-dimensional electron gases: Nanomagnetometers and magnetic switches

We investigate the conductance properties of a hybrid ferromagnet-semiconductor structure consisting of a confined two-dimensional electron gas and a transverse ferromagnetic strip on top. Within the framework of the Landauer–Buttiker model, we develop an alternative way to consider magnetic fields. Our method describes devices ranging from a recently realized nanomagnetometer down to quasi-one-dimensional quantum wires. We provide a rigorous way to relate the measured resistance to the actual magnetization of the strip. Regarding the quasi-one-dimensional wires we propose a device application, a tunable magnetic switch.

Decoherence and dephasing in coupled Josephson-junction qubits

Abstract We investigate the decoherence and dephasing of two coupled Josephson qubits. With the interaction between the qubits being generated by current–current correlations, two different situations in which the qubits are coupled to the same bath, or to two independent baths, are considered. Upon focussing on dissipation being caused by the fluctuations of voltage sources, the relaxation and dephasing rates are explicitly evaluated. Analytical and numerical results for the coupled qubits dynamics are provided.

Adiabatic pumping through interacting quantum dots.

We present a general formalism to study adiabatic pumping through interacting quantum dots. We derive a formula that relates the pumped charge to the local, instantaneous Greens function of the dot. This formula is then applied to the infinite-U Anderson model for both weak and strong tunnel-coupling strengths.