Magnetic pair - breaking in superconducting Ba 1 − x K x BiO 3 investigated by magnetotunneling

The de Gennes and Maki theory of gapless superconductivity for dirty superconductors is used to interpret the tunneling measurements on the strongly type-II high-Tc oxide-superconductor Ba1−xKxBiO3 in high magnetic fields up to 30 Tesla. We show that this theory is applicable at all temperatures and in a wide range of magnetic fields starting from 50 percent of the upper critical field Bc2. In this magnetic field range the measured superconducting density of states (DOS) has the simple energy dependence as predicted by de Gennes from which the temperature dependence of the pair-breaking parameter α(T ), orBc2(T ), has been obtained. The deduced temperature dependence of Bc2(T ) follows the Werthamer-Helfand-Hohenberg prediction for classical type-II superconductors in agreement with our previous direct determination. The amplitudes of the deviations in the DOS depend on the magnetic field via the spatially averaged superconducting order parameter which has a squareroot dependence on the magnetic field. Finally, the second Ginzburg-Landau parameter κ2(T ) has been determined from the experimental data. PACS numbers: 74.60.Ec, 74.25.Dw, 74.50.+r. Typeset using REVTEX