  The low temperature specific heat C(B,T) of an YBa2Cu3O7.00 single crystal is
measured from 1.2 to 10 K in magnetic fields up to 14 T. The anisotropic
component Caniso(T,B)=C(T,B//c)-C(T,B//ab) is a pure vortex quantity obtained
directly from experiment. It follows a scaling relation predicted recently for
line nodes characteristic of d-wave vortices. Our experimental field and
temperature range corresponds to a crossover region where the limit
Caniso(T,B)is proportional to T*sqrt(B) does not strictly apply. The variation
of the entropy caused by the magnetic field at low T is thermodynamically
compatible with measurements near Tc.
