Critical behavior of Josephson-junction arrays at f=1/2

The critical behavior of frustrated Josephson-junction arrays at $f=1/2$ flux quantum per plaquette is considered. Results from Monte Carlo simulations and transfer matrix computations support the identification of the critical behavior of the square and triangular classical arrays and the one-dimensional quantum ladder with the universality class of the XY-Ising model. In the quantum ladder, the transition can happen either as a simultaneous ordering of the $Z_2$ and $U(1)$ order parameters or in two separate stages, depending on the ratio between interchain and intrachain Josephson couplings. For the classical arrays, weak random plaquette disorder acts like a random field and positional disorder as random bonds on the $Z_2$ variables. Increasing positional disorder decouples the $Z_2$ and $U(1)$ variables leading to the same critical behavior as for integer $f$.Comment: 9 pages, Latex, workshop on JJA, to appear in Physica

Phase-coherence transition in granular superconductors with π\pi junctions

We study the three-dimensional XY-spin glass as a model for the resistive behavior of granular superconductors containing a random distribution of $\pi$ junctions, as in high-$T_c$ superconducting materials with d-wave symmetry. The $\pi$ junctions leads to quenched in circulating currents (chiralities) and to a chiral-glass state at low temperatures, even in the absence of an external magnetic field. Dynamical simulations in the phase representation are used to determine the nonlinear current-voltage characteristics as a function of temperature. Based on dynamic scaling analysis, we find a phase-coherence transition at finite temperature below which the linear resistivity should vanish and determine the corresponding critical exponents. The results suggest that the phase and chiralities may order simultaneously for decreasing temperatures into a superconducting chiral-glass state.Comment: 5 pages, 1 figure, Proc. of ICM 2000, to appear in J. Magn. Magn. Mate

Phase transitions in the one-dimensional frustrated quantum XY model and Josephson-junction ladders

A one-dimensional quantum version of the frustrated XY (planar rotor) model is considered which can be physically realized as a ladder of Josephson-junctions at half a flux quantum per plaquette. This system undergoes a superconductor to insulator transition at zero temperature as a function of charging energy. The critical behavior is studied using a Monte Carlo transfer matrix applied to the path-integral representation of the model and a finite-size-scaling analysis of data on small system sizes. Depending on the ratio between the interchain and intrachain couplings the system can have single or double transitions which is consistent with the prediction that its critical behavior should be described by the two-dimensional classical XY-Ising model.Comment: 13 pages, Revtex, J. Appl. Phys. (to appear), Inpe-las-00

Phase and vortex correlations in Josephson-junction arrays at irrational frustration

Phase coherence and vortex order in a Josephson-junction array at irrational frustration are studied by extensive Monte Carlo simulations using the parallel tempering method. A scaling analysis of the correlation length of phase variables in the full equilibrated system shows that the critical temperature vanishes with a power-law divergent correlation length and critical exponent $\nu_{ph}$, in agreement with recent results from resistivity scaling analysis. A similar scaling analysis for vortex variables reveals a different critical exponent $\nu_{v}$, suggesting that there are two distinct correlation lengths associated with a decoupled zero-temperature phase transition.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let