Shapiro steps on current-voltage curves of dc SQUIDs

We have studied in detail the current-voltage (I-V) curves of dc SQUIDs in the presence of rf fields in the frequency range 2-18 GHz. Strong voltage locking at half-integral as well as the usual integral Shapiro step spacing is observed when the applied dc-magnetic flux is close to half-integral numbers of flux quanta. Numerical simulations are in excellent agreement with the experimental I-V curves. The half-integral steps are found to be associated with the flip-flop between two adjacent fluxoid states of the SQUID synchronized to the rf field.