<?xml version="1.0" encoding="UTF-8"?><xml><records><record><source-app name="Biblio" version="7.x">Drupal-Biblio</source-app><ref-type>17</ref-type><contributors><authors><author><style face="normal" font="default" size="100%">Pissas, M.</style></author><author><style face="normal" font="default" size="100%">Stamopoulos, D.</style></author></authors></contributors><titles><title><style face="normal" font="default" size="100%">Evidence for geometrical barriers in an untwinned YBa2Cu3O7-delta single crystal</style></title><secondary-title><style face="normal" font="default" size="100%">PHYSICAL REVIEW B</style></secondary-title></titles><dates><year><style  face="normal" font="default" size="100%">2001</style></year><pub-dates><date><style  face="normal" font="default" size="100%">2001</style></date></pub-dates></dates><volume><style face="normal" font="default" size="100%">64</style></volume><isbn><style face="normal" font="default" size="100%">1098-01211550-235X</style></isbn><language><style face="normal" font="default" size="100%">eng</style></language><abstract><style face="normal" font="default" size="100%">The local ac and dc magnetic response in an untwinned YBa2Cu3O7-delta crystal has been measured using a microscopic Hall sensor. Due to the low pinning energy at elevated temperatures (near T-c), the ac response of the platelet-shaped crystal is governed by the geometrical barrier. From the field where the first vortex penetrates into the center of the crystal we estimated the first critical field (H-c1). The temperature variation of H-c1 near T-c is compatible with a strongly type-H superconductor, where the fluctuations in the order parameter (Psi) are those of an uncharged superfluid of the three dimensional XY model. In order to extract a reasonable value for the London penetration length at T=0 K, a value c(0)=3 is needed for the core contribution of the line energy of the vortex.</style></abstract><issue><style face="normal" font="default" size="100%">13</style></issue></record></records></xml>