Non equilibrium phase diagrams of current driven Josephson junction arrays

VERONICA I. MARCONI; DANIEL DOMINGUEZ

Josephson Junction and Superconductivity Research

Nova Science Publishers Inc.

Año: 2006; p. 63 - 110

We present a review of our previous numerical studies on non equilibrium vortex dynamics in Josephson Junction arrays (JJA) driven by a dc current. Dynamical phase diagrams for different magnetic fields, current directions and varying temperature are discussed and compared. First, the effect of thermal fluctuations in a current driven diluted vortex lattice (VL) is analyzed. The case of $f=1/25$ (where $f$ is the fraction of flux quanta per plaquette in the array) is considered and the phase diagram as a function of the driving current and temperature is analyzed. In equilibrium, this system has a weakly first-order melting transition of the vortex lattice, which coincides with a depinning transition. When a low current is applied, the ``longitudinal´´ depinning transition occurs at a temperature lower than the melting transition. More interestingly, for large currents (well above the critical current) there is an analogous sequence of transitions but for the transverse response of a fast moving VL. There is a {it transverse depinning temperature} below the melting transition of the moving VL. We also discuss the dependence with the direction of the applied dc current of the transport properties of diluted vortex arrays on a square JJA at low temperatures. We show that {it orientational pinning} phenomenon leads to a finite transverse critical current when the bias current is applied in the directions of high symmetry and it leads to an {it anomalous transverse voltage} when vortices are driven away from the favorable directions. In addition, the effect of disorder in the transport properties of square JJA with a dc current applied in the ``diagonal direction´´ ([11] direction) is analyzed and a finite transverse voltage is also observed in this case. The case of a fully frustrated square JJA, corresponding to $f=1/2$, driven by a dc current and with thermal fluctuations is also discussed. In equilibrium, the low temperature phase has two broken symmetries: the U(1) symmetry, corresponding to superconducting coherence, and the $Z_2$ symmetry corresponding to the periodic order of the VL, which forms a ``checkerboard pattern". At high currents (well above the critical current) two well separated transitions are observed. The order of the checkerboard vortex lattice (discrete $Z_2$ symmetry) is destroyed at a much lower temperature than the transverse superconducting coherence (continuous $U(1)$ symmetry).