Abstract
In contrast to the conventional picture the upper critical fields measured by magnetoresistance in high-Tc superconductors including cubic Ba1−xKxBiO3, reveal a positive curvature in the temperature dependence. Several explanations have been proposed, but in magnetoresistance measurements flux-flow phenomena can play a role. We address the problem by tunneling spectroscopy, where we measure the superconducting density of states. The elastic tunneling current has been measured in the whole temperature and magnetic-field range up to the critical values. The obtained Hc2(T) dependence is in agreement with the Werthamer-Helfand-Hohenberg prediction.
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