Phase transitions in Josephson-junction ladders in a magnetic field

A model of a Josephson-junction ladder in a perpendicular magnetic field with f0=p/q flux quanta per plaquette is considered. It is shown that the topological features of the zero-temperature phase diagram, as a function of charging energy and small deviations f-f0 from commensurability of the vortex lattice, are strongly dependent on q. In addition to a superconductor-insulator transition, a commensurate-incommensurate transition is also possible within the superconducting phase for q≥2. For q>3, an intermediate incommensurate phase occurs for f→f0