arXiv subjects

Michele Governale

Publications and source records attributed to Michele Governale.

At least 55 records · Page 3Linked to original sources

arXiv2009

Diagrammatic real-time approach to adiabatic pumping through metallic single-electron devices

We present a real-time diagrammatic formalism to study adiabatic pumping in chains of tunnel-coupled metallic islands. This approach is based on an expansion to linear order in the frequency of the time-dependent parameters and on a systematic perturbation expansion in the tunnel-coupling strength. We apply our formalism to a single-island and a double-island system. In the single-island setup, we find that the first-order contribution in the tunnel-coupling strength is purely due to the renormalization of the charging-energy gap. In the double-island system, we investigate the transition between weak and strong pumping.

cond-mat.mes-hall↗

arXiv2009

Adiabatic charge and spin pumping through quantum dots with ferromagnetic leads

We study adiabatic pumping of electrons through quantum dots attached to ferromagnetic leads. Hereby we make use of a real-time diagrammatic technique in the adiabatic limit that takes into account strong Coulomb interaction in the dot. We analyze the degree of spin polarization of electrons pumped from a ferromagnet through the dot to a nonmagnetic lead (N-dot-F) as well as the dependence of the pumped charge on the relative leads' magnetization orientations for a spin-valve (F-dot-F) structure. For the former case, we find that, depending on the relative coupling strength to the leads, spin and charge can, on average, be pumped in opposite directions. For the latter case, we find an angular dependence of the pumped charge, that becomes more and more anharmonic for large spin polarization in the leads.

cond-mat.mes-hall↗

arXiv2009

Non-local Andreev transport through an interacting quantum dot

We investigate sub-gap transport through a single-level quantum dot tunnel coupled to one superconducting and two normal-conducting leads. Despite the tendency of a large charging energy to suppress the equilibrium proximity effect, a finite Andreev current through the dot can be achieved in non-equilibrium situations. We propose two schemes to identify non-local Andreev transport. In one of them, the presence of strong Coulomb interaction leads to negative values of the non-local conductance as a clear signal of non-local Andreev transport.

cond-mat.supr-con↗

arXiv2008

Real-time diagrammatic approach to transport through interacting quantum dots with normal and superconducting leads

We present a real-time diagrammatic theory for transport through interacting quantum dots tunnel coupled to normal and superconducting leads. Our formulation describes both the equilibrium and non-equilibrium superconducting proximity effect in a quantum dot. We study a three-terminal transistor geometry, consisting of a single-level quantum dot tunnel coupled to two phase-biased superconducting leads and one voltage-biased normal lead. We compute both the Josephson current between the two superconductors and the Andreev current in the normal lead, and analyze their switching on and off as well as transitions between 0- and $π$-states as a function of gate and bias voltage. For the limit of large superconducting gaps in the leads, we describe the formation of Andreev bound states within an exact resummation of all orders in the tunnel coupling to the superconducting leads, and discuss their signature in the non-equilibrium Josephson- and Andreev- current and the quantum-dot charge.

cond-mat.supr-con↗

arXiv2007

Non-Equilibrium Josephson and Andreev Current through Interacting Quantum Dots

We present a theory of transport through interacting quantum dots coupled to normal and superconducting leads in the limit of weak tunnel coupling. A Josephson current between two superconducting leads, carried by first-order tunnel processes, can be established by non-equilibrium proximity effect. Both Andreev and Josephson current is suppressed for bias voltages below a threshold set by the Coulomb charging energy. A $π$-transition of the supercurrent can be driven by tuning gate or bias voltages.

cond-mat.supr-con↗

arXiv2007

Pumping through a quantum dot in the proximity of a superconductor

We study adiabatic pumping through a quantum dot tunnel-coupled to one normal and one superconducting lead. We generalize a formula which relates the pumped charge through a quantum dot with Coulomb interaction to the instantaneous local Green's function of the dot, to systems containing a superconducting lead. First, we apply this formula to the case of a non-interacting, single-level quantum dot in different temperature regimes and for different parameter choices, and we compare the results with the case of a system comprising only normal leads. Then we study the infinite-U Anderson model with a superconducting lead at zero temperature, and we discuss the effect of the proximity of the superconductor on the pumped charge.

cond-mat.mes-hall↗

arXiv2006

Adiabatic pumping through a quantum dot with coulomb interactions: A perturbation expansion in the tunnel coupling

We present a diagrammatic real-time approach to adiabatic pumping of electrons through interacting quantum dots. Performing a systematic perturbation expansion in the tunnel-coupling strength, we compute the charge pumped through a single-level quantum dot per pumping cycle. The combination of Coulomb interaction and quantum fluctuations, accounted for in contributions of higher order in the tunnel coupling, modifies the pumping characteristics via an interaction-dependent renormalization of the quantum-dot level. The latter is even responsible for the {\em dominant} contribution to the pumped charge when pumping via time-dependent tunnel-coupling strengths.

cond-mat.mes-hall↗

arXiv2005

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 Green function of the dot. This formula is then applied to the infinite-U Anderson model both for weak and strong tunnel-coupling strengths.

cond-mat.mes-hall↗

arXiv2005

Signatures of spin-related phases in transport through regular polygons

We address the subject of transport in one-dimensional ballistic polygon loops subject to Rashba spin-orbit coupling. We identify the role played by the polygon vertices in the accumulation of spin-related phases by studying interference effects as a function of the spin-orbit coupling strength. We find that the vertices act as strong spin-scattering centers that hinder the developing of Aharovov-Casher and Berry phases. In particular, we show that the oscillation frequency of interference pattern can be doubled by modifying the shape of the loop from a square to a circle.

cond-mat.mes-hall↗

arXiv2005

Rashba effect in quantum networks

We present a formalism to study quantum networks made up by single-channel quantum wires in the presence of Rashba spin-orbit coupling and magnetic field. In particular, linear transport through one-dimensional and two-dimensional finite-size networks is studied by means of the scattering formalism. In some particular quantum networks, the action of the magnetic field or of the Rashba spin-orbit coupling induces localization of the electron wave function. This phenomenon, which relies on both the quantum-mechanical interference and the geometry of the network, is manifested through the suppression of the conductance for specific values of the spin-orbit-coupling strength or of the magnetic field. Furthermore, the interplay of the Aharonov-Bohm phases and of the non-Abelian phases introduced by spin-orbit coupling, is discussed in a number of cases.

cond-mat.mes-hall↗

arXiv2004

Rashba spin precession in quantum Hall edge channels

Quasi--one dimensional edge channels are formed at the boundary of a two-dimensional electron system subject to a strong perpendicular magnetic field. We consider the effect of Rashba spin--orbit coupling, induced by structural inversion asymmetry, on their electronic and transport properties. Both our analytical and numerical results show that spin--split quantum--Hall edge channels exhibit properties analogous to that of Rashba--split quantum wires. Suppressed backscattering and a long spin life time render these edge channels an ideal system for observing voltage--controlled spin precession. Based on the latter, we propose a magnet--less spin--dependent electron interferometer.

cond-mat.mes-hall↗

arXiv2004

Rashba effect induced localization in quantum networks

We study a quantum network extending in one-dimension (chain of square loops connected at one vertex) made up of quantum wires with Rashba spin-orbit coupling. We show that the Rashba effect may give rise to an electron localization phenomenon similar to the one induced by magnetic field. This localization effect can be attributed to the spin precession due to the Rashba effect. We present results both for the spectral properties of the infinite chain, and for linear transport through a finite-size chain connected to leads. Furthermore, we study the effect of disorder on the transport properties of this network.

cond-mat.mes-hall↗

arXiv2004

Andreev interference in adiabatic pumping

Within the scattering approach, we develop a model for adiabatic quantum pumping in hybrid normal/superconductor systems where several superconducting leads are present. This is exploited to study Andreev-interference effects on adiabatically pumped charge in a 3-arm beam splitter attached to one normal and two superconducting leads with different phases of the order parameters. We derive expressions for the pumped charge through the normal lead for different parameters for the scattering region, and elucidate the effects due to Andreev interference. In contrast to what happens for voltage-driven transport, Andreev interference does not yield in general a pumped current which is a symmetric function of the superconducting-phase difference.

cond-mat.mes-hall↗

arXiv2004

Universal Rashba Spin Precession of Two-Dimensional Electrons and Holes

We study spin precession due to Rashba spin splitting of electrons and holes in semiconductor quantum wells. Based on a simple analytical expression that we derive for the current modulation in a broad class of experimental situations of ferromagnet/nonmagnetic semiconductor/ferromagnet hybrid structures, we conclude that the Datta-Das spin transistor (i) is feasible with holes and (ii) its functionality is not affected by integration over injection angles. The current modulation shows a universal oscillation period, irrespective of the different forms of the Rashba Hamiltonian for electrons and holes. The analytic formulas approximate extremely well exact numerical calculations of a more elaborate Kohn--Luttinger model.

cond-mat.mes-hall↗

arXiv2004

Two-dimensional hole precession in an all-semiconductor spin field effect transistor

We present a theoretical study of a spin field-effect transistor realized in a quantum well formed in a p--doped ferromagnetic-semiconductor- nonmagnetic-semiconductor-ferromagnetic-semiconductor hybrid structure. Based on an envelope-function approach for the hole bands in the various regions of the transistor, we derive the complete theory of coherent transport through the device, which includes both heavy- and light-hole subbands, proper modeling of the mode matching at interfaces, integration over injection angles, Rashba spin precession, interference effects due to multiple reflections, and gate-voltage dependences. Numerical results for the device current as a function of externally tunable parameters are in excellent agreement with approximate analytical formulae.

cond-mat.mes-hall↗

arXiv2003

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-onedimensional 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.

cond-mat.mes-hall↗

arXiv2001

Mesoscopic effects in tunneling between parallel quantum wires

We consider a phase-coherent system of two parallel quantum wires that are coupled via a tunneling barrier of finite length. The usual perturbative treatment of tunneling fails in this case, even in the diffusive limit, once the length L of the coupling region exceeds a characteristic length scale L_t set by tunneling. Exact solution of the scattering problem posed by the extended tunneling barrier allows us to compute tunneling conductances as a function of applied voltage and magnetic field. We take into account charging effects in the quantum wires due to applied voltages and find that these are important for 1D-to-1D tunneling transport.

cond-mat.mes-hall↗

arXiv2001

Decoherence and dephasing in coupled Josephson-junction qubits

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.

cond-mat↗