ac Josephson effect for asymmetric superconducting junctions

We calculate the ac and dc current-voltage (I-V) relation when a tunnel barrier is placed in the normal region of a superconductor-normal-metal-superconductor (S1NS2) junction, where the two superconducting contacts have different gaps (Delta(2) greater than or equal to Delta(1) greater than or equal to 0). The voltages at which subharmonic gap structure (SGS) appears in the zero-temperature I-V are the same as previously found for the SGS in ballistic S1NS2 junctions at a finite temperature. Some of the SGS's are strong enough to produce negative differential resistance. We determine the variation of the ac Josephson current with bias, junction transmissivity, gap ratio Delta(1)/Delta(2), and junction length L. The ac Josephson current through a ballistic SNS junction can also be used to measure the superconducting phase difference phi.