Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 34: Superconductivity - Vortex Dynamics, Vortex Phases, Pinning

TT 34.3: Talk

Friday, March 30, 2007, 10:45–11:00, H19

Effect of dc magnetic field on the microwave losses in MgB₂ thin films — •Alexander Zaitsev, Rudolf Schneider, Roland Hott, Thorsten Schwarz, and Jochen Geerk — Forschungszentrum Karlsruhe, Institut für Festkörperphysik, P.O.B. 3640, D-76021 Karlsruhe, Germany

The microwave surface impedance (Z_s = R_s + iX_s) of in situ MgB₂ thin films was measured as a function of temperature and parallel dc magnetic field at several frequencies between 5.7 GHz 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-T_c cuprates. The films cooled in zero-field revealed a clear indication of the lower critical field, B_c1, with a small hysteresis around B ≤ B_c1. In higher fields (B > B_c1) the losses followed the Coffey-Clem and Brandt model, including the frequency dependences, whereas high-T_c Y-Ba-Cu-O films did not show a reasonable agreement with this model. Both the relatively high values of Δ X_s/Δ R_s ratio and their frequency dependence indicate a weak effect of the flux creep on the measured microwave loss in MgB₂ films. The temperature dependence of Δ X_s/Δ R_s ratio can be described by a microscopic pinning model for BCS-superconductors.