Evidence for an unconventional superconducting order parameter in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$

We report measurements of the dynamic resistance ${\mathit{R}}_{\mathit{D}}$ of a dc SQUID formed by two contacts between two different edges of a single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ and a Nb block. The phase of the field-induced ${\mathit{R}}_{\mathit{D}}$ oscillations was compared with those of two adjacent SQUID’s, each formed by two Nb-Nb contacts. We find a phase difference of 160±30° between the oscillations of the composite SQUID and those of the SQUID’s formed by Nb-Nb junctions. A possible explanation for this reproducible effect is that this cuprate superconductor has an unconventional order parameter.