Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices

We consider ultracold bosons in a 2D square optical lattice described by the Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is applied to the system, which shakes the lattice along one of the diagonals. The effect of the shaking is to renormalize the nearest-neighbor hopping coefficients, which can be arbitrarily reduced, can vanish, or can even change sign, depending on the shaking parameter. It is therefore necessary to account for higher-order hopping terms, which are renormalized differently by the shaking, and introduce anisotropy into the problem. We show that the competition between these different hopping terms leads to finite-momentum condensates, with a momentum that may be tuned via the strength of the shaking. We calculate the boundaries between the Mott-insulator and the different superfluid phases, and present the time-of-flight images expected to be observed experimentally. Our results open up new possibilities for the realization of bosonic analogs of the FFLO phase describing inhomogeneous superconductivity.Comment: 7 pages, 7 figure

Macroscopic quantum tunneling in "small" Josephson junctions in magnetic field

We study the phenomenon of macroscopic quantum tunneling (MQT) in small Josephson junctions (JJ) with an externally applied magnetic field. The latter results in the appearance of the Fraunhofer type modulation of the current density along the barrier. The problem of MQT for a point-like JJ is reduced to the motion of the quantum particle in the washboard potential. In the case of a finite size JJ under consideration, this problem corresponds to a MQT in potential which itself, besides the phase, depends on space variables. Finally, the general expression for the crossover temperature T_0 between thermally activated and macroscopic quantum tunneling regimes and the escaping time tau_esc have been calculated

Effect of magnetic pair breaking on Andreev bound states and resonant supercurrent in quantum dot Josephson junctions

We propose a model for resonant Josephson tunneling through quantum dots that accounts for Cooper pair-breaking processes in the superconducting leads caused by a magnetic field or spin-flip scattering. The pair-breaking effect on the critical supercurrent $I_c$ and the Josephson current-phase relation $I(\phi)$ is largely due to the modification of the spectrum of Andreev bound states below the reduced (Abrikosov-Gorkov) quasiparticle gap. For a quantum dot formed in a quasi-one-dimensional channel, both $I_c$ and $I(\phi)$ can show a significant magnetic field dependence induced by pair breaking despite the suppression of the orbital magnetic field effect in the channel. This case is relevant to recent experiments on quantum dot Josephson junctions in carbon nanotubes. Pair-breaking processes are taken into account via the relation between the Andreev scattering matrix and the quasiclassical Green functions of the superconductors in the Usadel limit.Comment: 5 pages, 6 eps figures, new results adde

Chromomagnetic instability in two-flavor quark matter at nonzero temperature

We calculate the effective potential of the 2SC/g2SC phases including vector condensates ($$and$$) and study the gluonic phase and the single plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state at nonzero temperature. Our analysis is performed within the framework of the gauged Nambu--Jona-Lasinio model. We compute potential curvatures with respect to the vector condensates and investigate the temperature dependence of the Meissner masses squared of gluons of color 4--7 and 8 in the neutral 2SC/g2SC phases. The phase diagram is presented in the plane of temperature and coupling strength. The unstable regions for gluons 4--7 and 8 are mapped out on the phase diagram. We find that, apart from the case of strong coupling, the 2SC/g2SC phases at low temperatures are unstable against the vector condensation until the temperature reaches tens of MeV.Comment: 10 pages, 10 figures, revisions to text, published in Phys. Rev.

Decoherence processes in a current biased dc SQUID

A current bias dc SQUID behaves as an anharmonic quantum oscillator controlled by a bias current and an applied magnetic flux. We consider here its two level limit consisting of the two lower energy states | 0 \right> and | 1 \right>. We have measured energy relaxation times and microwave absorption for different bias currents and fluxes in the low microwave power limit. Decoherence times are extracted. The low frequency flux and current noise have been measured independently by analyzing the probability of current switching from the superconducting to the finite voltage state, as a function of applied flux. The high frequency part of the current noise is derived from the electromagnetic environment of the circuit. The decoherence of this quantum circuit can be fully accounted by these current and flux noise sources.Comment: 4 pages, 4 figure

Coherent oscillations in a superconducting multi-level quantum system

We have observed coherent time evolution of states in a multi-level quantum system, formed by a current-biased dc SQUID. The manipulation of the quantum states is achieved by resonant microwave pulses of flux. The number of quantum states participating in the coherent oscillations increases with increasing microwave power. Quantum measurement is performed by a nanosecond flux pulse which projects the final state onto one of two different voltage states of the dc SQUID, which can be read out

Dynamics of two-dimensional electron gas in non-uniform magnetic field

We have theoretically studied dynamics of the two-dimensional electron system (2DES) placed in a strong laterally non-uniform magnetic field, which appears due to ferromagnetic film on the top of heterostructure. It is shown that lateral inhomogeneity of a strong magnetic field allows itself "magnetic gradient" or special magnetic-edge magnetoplasmons. This mechanism is different from usual "density gradient" edge magnetoplasmons. We have solved self-consistently Poisson equation for non-uniform density distribution of the 2DES for realistic heterostructure together with hydrodynamic equation of 2D Fermi liquid. As a result eigen value problem has been obtained that corresponds to the motion of charge density wave perpendicular to magnetic gradient. It is shown that for non-monotonic distribution of magnetic field "magnetic gradient" magnetoplasmons may move in both directions. To solve eigen value problem we have compared two types of numerical approaches: first is grid method that diagonalizes large Hermitian matrix and second is semi-analytical approach that expand each eigen mode on the set of orthogonal functions.Fundação para a Ciência e a Tecnologia (FCT

Phases of a fermionic model with chiral condensates and Cooper pairs in 1+1 dimensions

We study the phase structure of a 4-fermi model with three bare coupling constants, which potentially has three types of bound states. This model is a generalization of the model discussed previously by A. Chodos et al. [Phys. Rev. D 61, 045011 (2000)], which contained both chiral condensates and Cooper pairs. For this generalization we find that there are two independent renormalized coupling constants which determine the phase structure at finite density and temperature. We find that the vacuum can be in one of three distinct phases depending on the value of these two renormalized coupling constants

Nanosecond quantum state detection in a current biased dc SQUID

This article presents our procedure to measure the quantum state of a dc SQUID within a few nanoseconds, using an adiabatic dc flux pulse. Detection of the ground state is governed by standard macroscopic quantum theory (MQT), with a small correction due to residual noise in the bias current. In the two level limit, where the SQUID constitutes a phase qubit, an observed contrast of 0.54 indicates a significant loss in contrast compared to the MQT prediction. It is attributed to spurious depolarization (loss of excited state occupancy) during the leading edge of the adiabatic flux measurement pulse. We give a simple phenomenological relaxation model which is able to predict the observed contrast of multilevel Rabi oscillations for various microwave amplitudes.Comment: 10 pages, 8 figure