Depinning Transition of a Two Dimensional Vortex Lattice in a Commensurate Periodic Potential

We use Monte Carlo simulations of the 2D one component Coulomb gas on a triangular lattice, to study the depinning transition of a 2D vortex lattice in a commensurate periodic potential. A detailed finite size scaling analysis indicates this transition to be first order. No significant changes in behavior were found as vortex density was varied over a wide range.Comment: 5 pages, 8 figures. Revised discussion of correlation length exponent using a more accurate finite size scaling analysis. New figs. 5 and 6. Old figs. 6 and 7 now figs. 7 and

Flux lattice melting and depinning in the weakly frustrated 2D XY model

Monte Carlo simulations of the frustrated 2D XY model were carried out at small commensurate values of the frustration $f$. For $f=1/30$ a single transition was observed at which phase coherence (finite helicity modulus) and vortex lattice orientational order vanish together. For $f=1/56$ a new phase in which phase coherence is absent but orientational order persists was observed. Where comparison is possible, the results are in detailed agreement with the behavior of the lattice Coulomb gas model of vortices. It is argued that the helicity modulus of the frustrated 2D XY model vanishes for any finite temperature in the limit of weak frustration $f$.Comment: 4 pages, RevTeX, 3 figures in separate uuencoded file The manuscript will appear in Phys. Rev.

Fluctuation induced vortex pattern and its disordering in the fully frustrated XY model on a dice lattice

A highly degenerate family of states [proposed in PRB 63, 134503 (2001)] is proven to really minimize the Hamiltonian of the fully frustrated XY model on a dice lattice. The harmonic fluctuations are shown to be no consequence for the removal of the accidental degeneracy of these states, so a particular vortex pattern can be stabilized only by the anharmonic fluctuations. The structure of this pattern is found and the temperature of its disordering due to the proliferation of domain walls is estimated. The extreme smallness of the fluctuations induced free energy of domain walls leads to the anomalous prominence of the finite-size effects, which prevent the observation of vortex-pattern ordering in numerical simulations. In such a situation the loss of phase coherence may be related to the dissociation of fractional vortices with topological charges 1/8. In a physical situation the magnetic interaction of currents in a Josephson junction array will be a more important source for the stabilization of a particular vortex pattern than the anharmonic fluctuations.Comment: 20 pages, 7 figure

Transverse depinning and melting of a moving vortex lattice in driven periodic Josephson junction arrays

We study the effect of thermal fluctuations in a vortex lattice driven in the periodic pinning of a Josephson junction array. The phase diagram current ($I$) vs. temperature ($T$) is studied. Above the critical current $I_c(T)$ we find a moving vortex lattice (MVL) with anisotropic Bragg peaks. For large currents $I\gg I_c(T)$, there is a melting transition of the MVL at $T_M(I)$. When applying a small transverse current to the MVL, there is no dissipation at low $T$. We find an onset of transverse vortex motion at a transverse depinning temperature $T_{tr}(I)<T_M(I)$.Comment: 4 pages, 4 figures, Figure 2 changed, added new reference