Superconductor-insulator transition of Josephson-junction arrays on a honeycomb lattice in a magnetic field

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We study the superconductor to insulator transition at zero temperature in a Josephson-junction array model on a honeycomb lattice with f flux quantum per plaquette. The path integral representation of the model corresponds to a (2+1)-dimensional classical model, which is used to investigate the critical behavior by extensive Monte Carlo simulations on large system sizes. In contrast to the model on a square lattice, the transition is found to be first order for f=1/3 and continuous for f=1/2 but in a different universality class. The correlation-length critical exponent is estimated from finite-size scaling of vortex correlations. The estimated universal conductivity at the transition is approximately four times its value for f=0. The results are compared with experimental observations on ultrathin superconducting films with a triangular lattice of nanoholes in a transverse magnetic field.