  The effect of strong magnetic fields (11 T) on superconductivity in
YBa2Cu3O7-d is investigated using high-resolution thermal expansion. We show
that the field-induced broadening of the superconducting transition is due to a
finite-size effect resulting from the field-induced vortex-vortex length scale.
The physics of this broadening has recently been elucidated for the closely
related case of rotating superfluid 4 He [R. Haussmann, Phys. Rev. B60, 12373
(1999)]. Our results imply that the primary effect of magnetic fields of the
order of 10 T is to destroy the phase coherence; the pairing, on the other
hand, appears to be quite insensitive to these fields.
