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.