Heat capacity of untwinned YBa2Cu3O7-x in magnetic fields: Dimensional cross-over near Tc

The heat capacity of an untwinned single crystal of YBa2Cu3O7-x has been measured over a temperature range near Tc and in applied fields to 7 T. The sharp zero-field transition is significantly broadened by the field, but shifted to only slightly lower temperatures. The results are analyzed in the context of a cross-over from three dimensions to one dimensions, caused by the existence of the flux lattice. The data are consistent with finite-size analysis of the cross over, with a length scale that varies as B-12 and a logarithmic divergence of the infinite-length-scale heat capacity.