Fractional flux quanta in high-T_c/low-T_c superconducting structures

When the conventional superconductor niobium and the cuprate superconductor YBa₂Cu₃O7-δ (YBCO) are connected in a ring-shaped geometry, the ground state is characterized by a spontaneously generated current corresponding to a half-integer magnetic flux quantum in the large inductance limit. The spontaneously generated currents result from a π-phase shift originating from the difference in order parameter symmetry for niobium (s-wave) and YBCO (d_x²-_y²-wave). The fractional magnetic flux quanta in high-T_c/low-T_c superconducting structures have been used for an angle-resolved phase sensitive study to the order parameter symmetry in YBCO, which was found to contain an s-wave admixture of at least 9%. Moreover, the experiments set an upper limit of a few percent on any complex admixture. The fractional flux quanta have also been used for the realization of a novel superconducting logic circuit element: a toggle flip-flop with integrated π-rings. The d_x²-_y²-wave-induced π-phase shifts alleviate the need for bias current lines and improve design-, fabrication- and operation margins. The π-ring-based toggle flip-flop has been integrated in a test circuit and its correct operation was demonstrated. Besides in π-rings, the fractional magnetic flux quanta have also been studied in a so-called corner geometry. The first on-chip manipulation and read-out of half-integer flux quanta in a 0-π-0 junction is presented. This is an important step forward if corner junctions are to be integrated in superconducting electronics.