Single-electron and Cooper-pair tunneling in ultrasmall Josephson junctions. Coulomb and environmental effects

Abstract

The author studies the I-V curve of a small capacitance Josephson tunnel junction with a zero subgap conductance. In the limit of large external impedance and at low temperature, single electron tunneling across the junction is suppressed for eV < 2{Delta} + E{sub c} due to a combination of superconducting and Coulomb gaps and the process of Cooper pair tunneling (CPT) plays the main role. For E{sub c} {approx gt} 2{Delta} Coulomb interaction between electrons of a tunneling Cooper pair strongly modifies the effective Josephson coupling energy and the CPT current can substantially deviate from its standard value. Very close to the point eV {approx equal} 2{Delta} the effective tunneling time of a Cooper pair becomes very large and the CPT process is not adiabatic.