Wechselspiel von 3dXY-Fluktuationen und Vortices im Hochtemperatursupraleiter YBa₂Cu₃Oₓ

Abstract

In this work thermal expansion measurements have been performed on single crystals of YBa 2 Cu 3 O x with x=6.7-7.0 in magnetic fields up to 12 T. The superconducting fluctuations near T C , as well as the melting transitionof the vortex-lattice have been examined. It was possible to show that 3dXY-phase-fluctuations exist near and even well above T C , which are hardly suppressed by an applied field. No indications for 3dLLL-fluctuations in the amplitude of the order parameter have been found. Close to T C the divergence of the coherence length is limited by the distance between the field induced vortices and as a result superconducting phase coherence gets suppressed down to the vortex-lattice melting transition. The results of this analysis are in favor of a superconducting origin of the 'pseudo-gap', which appears mainly in underdoped samples well above T C . Strong hints were found, that the vortex-melting-transition is caused by the fluctuations in the phase of the superconducting order-parameter. In samples with some 'pinning centers' large irreversible anomalies have been found instead of a vortex-lattice melting-transition. These show a very similar behavior as the kinetic glass transition, which might appear in conventional supercooled liquids. These large anomalies allow a detailed analysis of the relaxations in the vortex-glass phases. In overdoped samples these relaxations show an extremely complex behavior which is believed to appear only in highly fragile glass forming liquids. As a cause of these complex relaxation-processes the phase-fluctuations are discussed.