•R. Lortz^1,2, C. Meingast¹, A.I. Rykov³, T. Masui³ und S. Tajima^3
1Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe, Germany
²Fakultät für Physik, Universität Karlsruhe, 76131 Karlsruhe, Germany
³ISTEC, 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).