Magnetic-field-induced finite-size effect in the high-temperature superconductor YBa2Cu3O7-a comparison to rotating superfluid 4 He

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.