Cotunneling transport and quantum phase transitions in coupled Josephson-Junction chains with charge frustration

We investigate the quantum phase transitions in two capacitively coupled chains of ultrasmall Josephson junctions, where the particle-hole symmetry-is broken by the gate voltage applied to each superconducting island. Near the maximal-frustration line, cotunneling of the particles along the two chains is shown to play a major role in the transport and to drive a quantum phase transition out of the charge-density-wave insulator, as the Josephson-coupling energy is increased. We also argue briefly that slightly off the symmetry line the universality class of the transition remains the same as that right on the line, being driven by the particle-hole pairs.