•R. Lortz^{1,2}, C. Meingast^{1}, A.I. Rykov^{3},
T. Masui^{3} und S. Tajima^{3}
^{1}Forschungszentrum Karlsruhe, Institut für Festkörperphysik,
76021 Karlsruhe, Germany
^{2}Fakultät für Physik, Universität Karlsruhe,
76131 Karlsruhe, Germany
^{3}ISTEC, 10-13 Shinonome I-Chome, Koto-ku, Tokyo 135, Japan

There is presently a great deal of interest in the effect of high magnetic
fields on superconductivity, the pseudogap and on the AF-magnetic fluctuations
in high-Tc superconductors. We have studied the response of the superconducting
transition and the associated superconducting fluctuations in untwinned
YBa2Cu3Ox single crystals to magnetic fields up to 10 T using high-resolution
dilatometry, which was previously shown to be extremely useful in characterizing
the zero-field transition. We show that the broadening of the superconducting
transition in a magnetic field is a consequence of a finite-size scaling
effect due the additional vortex-vortex length scale, which prevents the
divergence of the coherence length at Tc. The behavior is analogous to
that of the superfluid transition in rotating superfluid 4He [1]. Apart
from this broadening, practically no field effect is observed on superconductivity
and on superconducting fluctuations, which we observe up to about 30 K
above Tc. [1] R. Haussmann, PRB 60, S. 12373 (1999).