Oscillatory decay of a two-component Bose-Einstein condensate

We study the decay of a two-component Bose-Einstein condensate with negative effective interaction energy. With a decreasing atom number due to losses, the atom-atom interaction becomes less important and the system undergoes a transition from a bistable Josephson regime to the monostable Rabi regime, displaying oscillations in phase and number. We study the equations of motion and derive an analytical expression for the oscillation amplitude. A quantum trajectory simulation reveals that the classical description fails for low emission rates, as expected from analytical considerations. Observation of the proposed effect will provide evidence for negative effective interaction.Comment: 4 pages, 3 figue

Probing Pseudogap by Josephson Tunneling

We propose here an experiment aimed to determine whether there are superconducting pairing fluctuations in the pseudogap regime of the high-$T_c$ materials. In the experimental setup, two samples above $T_c$ are brought into contact at a single point and the differential AC conductivity in the presence of a constant applied bias voltage between the samples, $V$, should be measured. We argue the the pairing fluctuations will produce randomly fluctuating Josephson current with zero mean, however the current-current correlator will have a characteristic frequency given by Josephson frequency $\omega_J = 2 e V /\hbar$. We predict that the differential AC conductivity should have a peak at the Josephson frequency with the width determined by the phase fluctuations time.Comment: 4 pages, 2 eps figure

Nonlinear Dynamics in Double Square Well Potential

Considering the coherent nonlinear dynamics in double square well potential we find the example of coexistence of Josephson oscillations with a self-trapping regime. This macroscopic bistability is explained by proving analytically the simultaneous existence of symmetric, antisymmetric and asymmetric stationary solutions of the associated Gross-Pitaevskii equation. The effect is illustrated and confirmed by numerical simulations. This property allows to make suggestions on possible experiments using Bose-Einstein condensates in engineered optical lattices or weakly coupled optical waveguide arrays

Collective Excitations of Bose-Einstein Condensates in a Double-Well Potential

We investigate collective excitations of Bose-Einstein condensates at absolute zero in a double-well trap. We solve the Bogoliubov equations with a double-well trap, and show that the crossover from the dipole mode to the Josephson plasma mode occurs in the lowest energy excitation. It is found that the anomalous tunneling property of low energy excitations is crucial to the crossover.Comment: 14 pages, 6 figure

Macroscopic Symmetry Group Describes Josephson Tunneling in Twinned Crystals

A macroscopic symmetry group describing the superconducting state of an orthorhombically twinned crystal of YBCO is introduced. This macroscopic symmetry group is different for different symmetries of twin boundaries. Josephson tunneling experiments performed on twinned crystals of YBCO determine this macroscopic symmetry group and hence determine the twin boundary symmetry (but do not experimentally determine whether the microscopic order parameter is primarily d- or s-wave). A consequence of the odd-symmetry twin boundaries in YBCO is the stability of vortices containing one half an elementary flux quantum at the intersection of a twin boundary and certain grain boundaries.Comment: 6 pages, to be published in the Proceedings of the MOS96 Conference in the Journal of Low Temperature Physic

Universal Magnetic Properties of La2−δSrδCuO4La_{2-\delta} Sr_{\delta} Cu O_4 at Intermediate Temperatures

We present the theory of two-dimensional, clean quantum antiferromagnets with a small, positive, zero temperature ($T$) stiffness $\rho_s$, but with the ratio $k_B T / \rho_s$ arbitrary. Universal scaling forms for the uniform susceptibility ($\chi_u$), correlation length($\xi$), and NMR relaxation rate ($1/T_1$) are proposed and computed in a $1/N$ expansion and by Mont\'{e}-Carlo simulations. For large $k_B T/\rho_s$, $\chi_u (T)/T$ and $T\xi(T)$ asymptote to universal values, while $1/T_{1}(T)$ is nearly $T$-independent. We find good quantitative agreement with experiments and some numerical studies on $La_{2-\delta} Sr_{\delta} Cu O_4$.Comment: 14 pages, REVTEX, 1 postscript figure appende

Critical fluctuations in superconductors and the magnetic field penetration depth

The superconducting transition is studied within the one-loop renormalization group in fixed dimension $D=3$ and at the critical point. A tricritical behavior is found, and for $\kappa > \kappa_c$, an attractive charged fixed point, distinct from that of a neutral superfluid. The critical exponents of the continuous transition are evaluated, and it is shown that the anomalous dimension of the gauge field equals unity. This implies the proportionality of the magnetic field penetration depth and the superconducting correlation length below the transition. The penetration depth exponent is nonclassical. We argue that it can not be extracted from the dual theory in a straightforward manner since it is not renormalized by fluctuations of the dual field.Comment: 12 pages, LaTex, two figures available upon reques

DC and AC Josephson effects with superfluid Fermi atoms across a Feshbach resonance

We show that both DC and AC Josephson effects with superfluid Fermi atoms in the BCS-BEC crossover can be described at zero temperature by a nonlinear Schrodinger equation (NLSE). By comparing our NLSE with mean-field extended BCS calculations, we find that the NLSE is reliable in the BEC side of the crossover up to the unitarity limit. The NLSE can be used for weakly-linked atomic superfluids also in the BCS side of the crossover by taking the tunneling energy as a phenomenological parameter.Comment: 8 pages, 4 figures, presented at the Scientific Seminar on Physics of Cold Trapped Atoms, 17th International Laser Physics Workshop (Trondheim, June 30 - July 4, 2008

Giant Josephson current through a single bound state in a superconducting tunnel junction

We study the microscopic structure of the Josephson current in a single-mode tunnel junction with a wide quasiclassical tunnel barrier. In such a junction each Andreev bound state carries a current of magnitude proportional to the {\em amplitude} of the normal electron transmission through the junction. Tremendous enhancement of the bound state current is caused by the resonance coupling of superconducting bound states at both superconductor-insulator interfaces of the junction. The possibility of experimental observation of the single bound state current is discussed.Comment: 11 pages, [aps,preprint]{revtex