Capacitively coupled Josephson-junction chains: straight versus slanted coupling

We investigate analytically the quantum phase transition in a system of two chains of ultrasmall Josephson junctions, coupled capacitively with each other. Two different schemes of the coupling, straight and slanted, between the two chains are considered. In both coupling schemes, as the coupling capacitance is increased, the transport of particle-hole pairs is found to drive a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type from insulator to superconductor. A substantial discrepancy between the transition points in the two coupling schemes is observed, which reflects the difference between the transport mechanisms.