Non equilibrium phase diagrams of current driven Josephson junction arrays.

V. I. MARCONI; D. DOMINGUEZ

Josephson Junction and Superconductivity Research.

Nova Science Publisher

Año: 2007; p. 63 - 110

We  present a review  of our previous numerical studies onnon equilibrium  vortex dynamics in Josephson Junction arrays (JJA)driven by a   dc current. Dynamical phase diagrams for differentmagnetic 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 atemperature lower than the melting transition.  More interestingly, forlarge currents (well above the critical current)there is an analogous sequence of transitions but forthe transverse response of a fast moving VL. Thereis a {it transverse depinning temperature} below the meltingtransition of the moving VL.We also discuss the dependence with the direction of the applied dccurrent of the transport properties of diluted vortex arrays on a  square JJA at low temperatures.  We show that  {itorientational pinning} phenomenon leads to a  finite transversecritical current when the bias current  is applied in the  directionsof 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 transportproperties  of  square  JJA with a dc current applied in the ``diagonaldirection´´ ([11] direction) is analyzed and a finite transverse voltageis  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 transversesuperconducting coherence (continuous $U(1)$ symmetry).