DECOUPLING IN THE ONE-DIMENSIONAL FRUSTRATED QUANTUM XY MODEL AND JOSEPHSON-JUNCTION LADDERS - ISING CRITICAL-BEHAVIOR

A generalization of the one-dimensional frustrated quantum XY model is considered in which the interchain and intrachain coupling constants of the two infinite XY (planar rotor) chains have different strengths. The model can describe the superconductor to insulator transition due to charging effects in a ladder of Josephson junctions in a magnetic field with half a flux quantum per plaquette. From a fluctuation-effec tive action, this transition is expected to be in the universality cla ss of the two-dimensional classical XY-Ising model. The critical behav ior is studied using a Monte Carlo transfer matrix applied to the path -integral representation of the model and a finite-size-scaling analys is of data on small system sizes. It is found that, unlike the previou s studied case of equal interchain and intrachain coupling constants, the XY and Ising-like excitations of the quantum model decouple for la rge interchain coupling, giving rise to pure Ising model critical beha vior for the chirality order parameter and a superconductor-insulator transition in the universality class of the two-dimensional classical XY model.