NONADIABATIC ANALYSIS OF THE JOSEPHSON CRITICAL-CURRENT INFLUENCED BYQUANTUM PHASE FLUCTUATIONS

The influence of the electromagnetic environment on the dc Josephson c urrent is studied allowing for nonadiabatic motion of the phase differ ence across a junction. The critical current is evaluated nonperturbat ively within mean-field theory, fully taking into account the nonlocal kernels in the Ambegaokar-Eckern-Schon effective action. It turns out that the adiabatic approximation, which can be justified when the sup erconducting energy gap exceeds the charging energy, yields qualitativ ely correct results even for the opposite case, although quantitative deviations from the adiabatic approximation are found to be substantia l in some experimentally accessible regions.