EXPERIMENTAL-DETERMINATION OF THE SYMMETRY OF THE ORDER-PARAMETER IN YBCO

At present, the symmetry of the order parameter in the high temperatur e superconductor YBCO is quite controversial. Recent experiments using SQUIDs and Josephson junctions appear to support competing theories, with some experiments supporting a d(x)2-(y)2 pairing symmetry for the order parameter and others a s-wave pairing symmetry. We note that a number of factors such as trapped flux, magnetic field gradients and S QUID asymmetries could lead such measurements astray. We use a Scannin g SQUID Microscope and a time-reversal invariance test to resolve thes e experimental problems. We find the order parameter in YBCO has a tim e-reversal invariant d(x)2-(y)2 symmetric component. We estimate the a mplitude of any imaginary s-wave symmetric component to be less than 4 % and any real s-wave component to be less than 82%.