3D X-Y SCALING OF THE SPECIFIC-HEAT OF YBA2CU3O7-DELTA SINGLE-CRYSTALS

The specific heat of a single crystal sample of YBa2Cu3O7-delta (YBCO) has been measured with magnetic fields up to 8 Tesla applied parallel to the c-axis of the crystal. We analyse the data in terms of the Low est Landau Level (LLL) and critical scaling theories, Recent measureme nts of magnetisation and resistivity have been unable to distinguish b etween the critical scaling and LLL regimes. We find that the data doe s got scale using the LLL theory and therefore conclude that the LLL t heory is not applicable in magnetic fields less than 8 Tesla. However, we find that the data does scale using the 3-dimensional X-Y model. T his provides strong evidence for the existence of a critical regime wi thin which there is scaling behaviour characteristic of the 3-dimensio nal X-Y model with critical exponents consistent with those observed i n superfluid He-4.