Zero-crossing Shapiro step in a three-junction SQUID magnetically coupled with two phase-shifted RF signals

We demonstrate a zero-crossing Shapiro step in a three-junction SQUID (3J-SQUID) coupled with two phase-shifted RF signals. The operation is based on periodic flux quantum transitions in the 3J-SQUID under the zero- and negatively-biasing conditions. A test circuit is fabricated using a Nb/AlO_x/Nb junction technology. When we feed RF signals up to 10.5 GHz, we observe a Shapiro step crossing the zero-current axis at the voltage position of the 1st order. The experimental current-voltage characteristics are quantitatively reproduced by simulation. Zero-crossing conditions for the DC offset flux and the RF power are obtained experimentally, which is also confirmed by simulation. A programmable Josephson voltage standard is discussed as one of the possible applications.