Berlin 2008 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 15: Superconductivity: Poster Session
TT 15.43: Poster
Dienstag, 26. Februar 2008, 14:00–18:00, Poster B
Effect of a dc magnetic field on the microwave losses in MgB2 thin films — •Roland Hott1, Alexander G. Zaitsev1, Rudolf Schneider1, Thorsten Schwarz2, and Jochen Geerk1 — 1Forschungszentrum Karlsruhe, Institut für Festkörperphysik, P.O. Box 3640, D-76021 Karlsruhe, Germany — 2Forschungszentrum Karlsruhe, Institut für Synchrotronstrahlung, P.O. Box 3640, D-76021 Karlsruhe, Germany
The microwave surface impedance (Zs = Rs + i Xs) of in situ MgB2 thin films was measured as a function of temperature and parallel dc magnetic field at several frequencies between 5.7 and 18.5 GHz using a dielectric resonator technique. The results are consistent with the expectations for a classical type-II superconductor and, consequently, quite different from those of the high-Tc cuprates. The films cooled in zero field revealed a clear indication of the lower critical field Bc1, with a small hysteresis around B≤ Bc1. In higher fields (B > Bc1), the losses followed the Coffey-Clem and Brandt model, including the frequency dependences, whereas high-Tc Y-Ba-Cu-O films did not show a reasonable agreement with this model. Both the relatively high values of Δ Xs/Δ Rs ratio and their frequency dependence indicate a weak effect of flux creep on the measured microwave loss in MgB2 films. The temperature dependence of Δ Xs /Δ Rs ratio can be described by a microscopic pinning model for BCS superconductors.