M. Governale

Finite-frequency noise in a quantum dot with normal and superconducting leads

We consider a single-level quantum dot tunnel-coupled to one normal and one superconducting lead. We employ a diagrammatic real-time approach to calculate the finite-frequency current noise for subgap transport. The noise spectrum gives direct access to the internal dynamics of the dot. In particular, the noise spectrum shows sharp dips at the frequency of the coherent oscillations of Cooper pairs between the dot and the superconductor. This feature is most pronounced when the superconducting correlation is maximal. Furthermore, in the quantum-noise regime, omega > k(B)T, mu(N), the noise spectrum exhibits steps at frequencies equal to the Andreev addition energies. The height of these steps is related to the effective coupling strength of the excitations. The finite-frequency noise spectrum hence provides a full spectroscopy of the system.

Unconventional superconductivity from magnetism in transition-metal dichalcogenides

We investigate proximity-induced superconductivity in monolayers of transition metal dichalcogenides (TMDs) in the presence of an externally generated exchange field. A variety of superconducting order parameters is found to emerge from the interplay of magnetism and superconductivity, covering the entire spectrum of possibilities to be symmetric or antisymmetric with respect to the valley and spin degrees of freedom, as well as even or odd in frequency. More specifically, when a conventional emph{s}-wave superconductor with singlet Copper pairs is tunnel-coupled to the TMD layer, both spin-singlet and triplet pairings between electrons from the same and opposite valleys arise due to the combined effects of intrinsic spin-orbit coupling and a magnetic-substrate-induced exchange field. As a key finding, we reveal the existence of an exotic even-frequency triplet pairing between equal-spin electrons from different valleys, which arises whenever the spin orientations in the two valleys are noncollinear. All types of superconducting order turn out to be highly tunable via straightforward manipulation of the external exchange field.

Finite-time full counting statistics and factorial cumulants for transport through a quantum dot with normal and superconducting leads.

We study the finite-time full counting statistics for subgap transport through a single-level quantum dot tunnel-coupled to one normal and one superconducting lead. In particular, we determine the factorial and the ordinary cumulants both for finite times and in the long-time limit. We find that the factorial cumulants violate the sign criterion, indicating a non-binomial distribution, even in absence of Coulomb repulsion due to the presence of superconducting correlations. At short times the cumulants exhibit oscillations which are a signature of the coherent transfer of Cooper pairs between the dot and the superconductor.

Suppression of Coulomb exchange energy in quasi-two-dimensional hole systems

We have calculated the exchange-energy contribution to the total energy of quasi-two-dimensional hole systems realized by a hard-wall quantum-well confinement of valence-band states in typical semiconductors. The magnitude of the exchange energy turns out to be suppressed from the value expected for analogous conduction-band systems whenever the mixing between heavy-hole and light-hole components is strong. Our results are obtained using a very general formalism for calculating the exchange energy of many-particle systems where single-particle states are spinors. We have applied this formalism to obtain analytical results for spin-3/2 hole systems in limiting cases.

Coulomb-exchange effects in nanowires with spin splitting due to a radial electric field

We present a theoretical study of Coulomb-exchange interaction for electrons confined in a cylindrical quantum wire and subject to a Rashba-type spin-orbit coupling with radial electric field. The effect of spin splitting on the single-particle band dispersions, the quasiparticle effective mass, and the systems total exchange energy per particle are discussed. Exchange interaction generally suppresses the quasiparticle effective mass in the lowest nanowire sub-band, and a finite spin splitting is found to significantly increase the magnitude of the quasiparticle-mass suppression (by up to 15% in the experimentally relevant parameter regime). In contrast, spin-orbit coupling causes a modest (1%-level) reduction of the magnitude of the exchange energy per particle. Our results shed light on the interplay of spin-orbit coupling and Coulomb interaction in quantum-confined systems, including those that are expected to host exotic quasiparticle excitations.

Exporting superconductivity across the gap: Proximity effect for semiconductor valence-band states due to contact with a simple-metal superconductor

The proximity effect refers to the phenomenon whereby superconducting properties are induced in a normal conductor that is in contact with an intrinsically superconducting material. In particular, the combination of nano-structured semiconductors with bulk superconductors is of interest because these systems can host unconventional electronic excitations such as Majorana fermions when the semiconductors charge carriers are subject to a large spin-orbit coupling. The latter requirement generally favors the use of hole-doped semiconductors. On the other hand, basic symmetry considerations imply that states from typical simple-metal superconductors will predominantly couple to a semiconductors conduction-band states and, therefore, in the first instance generate a proximity effect for band electrons rather than holes. In this article, we show how the superconducting correlations in the conduction band are transferred also to hole states in the valence band by virtue of inter-band coupling. A general theory of the superconducting proximity effect for bulk and low-dimensional hole systems is presented. The interplay of inter-band coupling and quantum confinement is found to result in unusual wave-vector dependencies of the induced superconducting gap parameters. One particularly appealing consequence is the density tunability of the proximity effect in hole quantum wells and nanowires, which creates new possibilities for manipulating the transition to nontrivial topological phases in these systems.

Effect of Valence-Band Mixing on Density Oscillations in 2D Hole Systems

We have calculated the spatial profile of density modulations generated in two-dimensional (2D) hole gases in response to an impurity potential. Both short-range and Coulombic point impurities are considered. The density response of hole systems turns out to be generally different from that seen in conduction-electron systems. Our results point to the importance of valence-band-mixing effects, especially in the regime of higher hole sheet densities.

Charge transport by modulating spin-orbit gauge fields for quasi-one-dimensional holes

We present a theoretical study of ac charge transport arising from adiabatic temporal variation of zero-field spin splitting in a quasi-one-dimensional hole system (realized, e.g., in a quantum wire or point contact). As in conduction-electron systems, part of the current results from spin-dependent electromotive forces. We find that the magnitude of this current contribution is two orders of magnitude larger for holes and exhibits parametric dependences that make it more easily accessible experimentally. Our results suggest hole structures to be good candidates for realizing devices where spin currents are pumped by time-varying electric fields.

Problems and Perspectives in Quantum-Dot Based Computation

A review of recent modeling work on quantum cellular automata is presented. Detailed simulations at the single cell level, based on the Configuration-Interaction method, allow to analyze cell bistable behavior for different material systems and geometrical structures. In addition, they allow to evaluate the maximum allowed fabrication tolerances required for correct cell operation. Simulations at the circuit level, based on a simplified physical model of each cell, allow to analyze operation of single logic gates and more complex combinatorial logic networks. A simulation method based on simulated annealing is presented, which allows to compute the ground state and low-lying excited states of the QCA system without the need of exploring the whole configuration space.