Frequency and size dependence of ac Josephson effect in Nb/Au/YBCO heterojunctions

Abstract

High frequency dynamics of Nb/Au/YBaCuO heterojunctions on tilted NdGaO3 substrates have been studied. The both integer and non-integer Shapiro steps have been observed at mm-wave frequencies. Unconventional dependence of the critical current and the amplitudes of Shapiro steps vs. applied microwave power have been registered. Observed behavior deviates from existing theories of Josephson effect for junctions made from conventional or d-wave superconductors. Although the maximal size of the heterojunctions was smaller than the Josephson penetration depth, calculated from an averaged value of the critical current density, the experimental magnetic field dependences IC(H) deviate from the Fraunhofer pattern, pointing on non-uniform distribution of superconducting current density. Experimental results could be speculatively explained by origination of self-induced fractional magnetic vortices, which may take place in a junction where the amplitude and the phase of superconducting current alternate significantly over the junction area. Introducing a new lengthscale, which is much smaller than the Josephson penetration depth, the fractional vortices are considered, modifying the high frequency dynamics, namely the ac Josephson effect. Experimental results have been analyzed taking into account the second harmonic of superconducting current-phase relation and the influence of heterojunction capacitance.