Effects of a nonlinear bath at low temperatures

We use the numerical flow-equation renormalization method to study a nonlinear bath at low temperatures. The model of our nonlinear bath consists of a single two-level system coupled to a linear oscillator bath. The effects of this nonlinear bath are analyzed by coupling it to a spin, whose relaxational dynamics under the action of the bath is studied by calculating spin-spin correlation functions. As a first result, we derive flow equations for a general four-level system coupled to an oscillator bath, valid at low temperatures. We then treat the two-level system coupled to our nonlinear bath as a special case of the dissipative four-level system. We compare the effects of the nonlinear bath with those obtained using an effective linear bath, and study the differences between the two cases at low temperatures.Comment: 15 pages, 7 figure

Spin filter using a semiconductor quantum ring side-coupled to a quantum wire

We introduce a new spin filter based on spin-resolved Fano resonances due to spin-split levels in a quantum ring (QR) side-coupled to a quantum wire (QW). Spin-orbit coupling inside the QR, together with external magnetic fields, induces spin splitting, and the Fano resonances due to the spin-split levels result in perfect or considerable suppression of the transport of either spin direction. Using the numerical renormalization group method, we find that the Coulomb interaction in the QR enhances the spin filter operation by widening the separation between dips in conductances for different spins and by allowing perfect blocking for one spin direction and perfect transmission for the other. The spin-filter effect persists as long as the temperature is less than the broadening of QR levels due to the QW-QR coupling. We discuss realistic conditions for the QR-based spin filter and its advantages to other similar devices.Comment: 5 pages, 4 figure

Perturbative corrections to the Gutzwiller mean-field solution of the Mott-Hubbard model

We study the Mott-insulator transition of bosonic atoms in optical lattices. Using perturbation theory, we analyze the deviations from the mean-field Gutzwiller ansatz, which become appreciable for intermediate values of the ratio between hopping amplitude and interaction energy. We discuss corrections to number fluctuations, order parameter, and compressibility. In particular, we improve the description of the short-range correlations in the one-particle density matrix. These corrections are important for experimentally observed expansion patterns, both for bulk lattices and in a confining trap potential.Comment: 10 pages, 10 figue

Anderson-type model for a molecule adsorbed on a metal surface

We investigate a modified Anderson model to study the local density of states (LDOS) of a molecular wire adsorbed on a metal. Using a self-consistent mean-field type approach we find an exponential decay of the LDOS along the molecule. A repulsive on-site interaction on the molecule suppresses the tunneling and decreases the characteristic decay length.Comment: 7 pages (using europhys.sty), 5 EPS figures, To appear in Europhys. Let

Aharonov-Bohm oscillations and resonant tunneling in strongly correlated quantum dots

We investigate Aharonov-Bohm oscillations of the current through a strongly correlated quantum dot embedded in an arbitrary scattering geometry. Resonant-tunneling processes lead to a flux-dependent renormalization of the dot level. As a consequence we obtain a fine structure of the current oscillations which is controlled by quantum fluctuations. Strong Coulomb repulsion leads to a continuous bias voltage dependent phase shift and, in the nonlinear response regime, destroys the symmetry of the differential conductance under a sign change of the external flux.Comment: RevTex, 5 pages, 3 PostScript figures. Accepted for publication in Phys. Rev. Let

Diamagnetic Response of Normal-metal -- Superconductor Double Layers

The magnetic response of a proximity-coupled superconductor-normal metal sandwich is studied within the framework of the quasiclassical theory. The magnetization is evaluated for finite values of the applied magnetic field (linear and nonlinear response) at arbitrary temperatures and is used to fit recent experimental low-temperature data. The hysteretic behavior predicted from a Ginzburg-Landau approach and observed in experiments is obtained within the quasiclassical theory and shown to exist also outside the Ginzburg-Landau region.Comment: RevTex, 11 pages, 9 PostScript figures include

Local Density of States in a Dirty Normal Metal connected to a Superconductor

A superconductor in contact with a normal metal not only induces superconducting correlations, known as proximity effect, but also modifies the density of states at some distance from the interface. These modifications can be resolved experimentally in microstructured systems. We, therefore, study the local density of states $N(E,x)$ of a superconductor - normal metal heterostructure. We find a suppression of $N(E,x)$ at small energies, which persists to large distances. If the normal metal forms a thin layer of thickness $L_n$, a minigap in the density of states appears which is of the order of the Thouless energy $\sim \hbar D/L_n^2$. A magnetic field suppresses the features. We find good agreement with recent experiments of Gu\'eron {\it et al.}Comment: 5 pages, RevTeX, 7 Figures (included), Submitted to PRB. Revised version: One figure changed, missprints correcte

Solid-State Quantum Communication With Josephson Arrays

Josephson junction arrays can be used as quantum channels to transfer quantum information between distant sites. In this work we discuss simple protocols to realize state transfer with high fidelity. The channels do not require complicate gating but use the natural dynamics of a properly designed array. We investigate the influence of static disorder both in the Josephson energies and in the coupling to the background gate charges, as well as the effect of dynamical noise. We also analyze the readout process, and its backaction on the state transfer

Signatures of tunable Majorana-fermion edge states

Chiral Majorana-fermion modes are shown to emerge as edge excitations in a superconductor--topological-insulator hybrid structure that is subject to a magnetic field. The velocity of this mode is tunable by changing the magnetic-field magnitude and/or the superconductor's chemical potential. We discuss how quantum-transport measurements can yield experimental signatures of these modes. A normal lead coupled to the Majorana-fermion edge state through electron tunneling induces resonant Andreev reflections from the lead to the grounded superconductor, resulting in a distinctive pattern of differential-conductance peaks.Comment: (13 pages, Accepted for publication in New Journal of Physics, an extension of and expansion on our previous work arXiv:1210.4057). arXiv admin note: text overlap with arXiv:1210.405

Spin-Dependent Josephson Current through Double Quantum Dots and Measurement of Entangled Electron States

We study a double quantum dot each dot of which is tunnel-coupled to superconducting leads. In the Coulomb blockade regime, a spin-dependent Josephson coupling between two superconductors is induced, as well as an antiferromagnetic Heisenberg exchange coupling between the spins on the double dot which can be tuned by the superconducting phase difference. We show that the correlated spin states-singlet or triplets-on the double dot can be probed via the Josephson current in a dc-SQUID setup.Comment: 4 pages, 4 figures; To appear in PRB; A few small changes including reference