EVIDENCE FOR AN UNCONVENTIONAL SUPERCONDUCTING ORDER-PARAMETER IN YBA2CU3O6.9

We report measurements of the dynamic resistance R(D) of a dc SQUID fo rmed by two contacts between two different edges of a single crystal o f YBa2Cu3O6.9 and a Nb block. The phase of the field-induced R(D) osci llations was compared with those of two adjacent SQUID's, each formed by two Nb-Nb contacts. We find a phase difference of 160 +/- 30-degree s between the oscillations of the composite SQUID and those of the SQU ID's formed by Nb-Nb junctions. A possible explanation for this reprod ucible effect is that this cuprate superconductor has an unconventiona l order parameter.