Regensburg 2002 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 16: Neue und unkonventionelle Supraleiter II
TT 16.2: Vortrag
Mittwoch, 13. März 2002, 14:45–15:00, H18
Microwave surface impedance of magnesium diboride thin films — •Norbert Klein1, Biao-Bing Jin1, Sung-Ik Lee2, and W.N. Kang2 — 1Forschungszentrum Jülich, Institut für Schichten und Grenzflächen — 2Pohang University of Science and Technology, Pohang, Korea
We have measured the temperature dependence of the microwave surface impedance Zs = Rs + i ω µ0 λ of MgB2 films prepared by laser ablation at a frequency ω / 2 π of 18 GHz employing a dielectric resonator technique. We found that the temperature dependence of the magnetic field penetration depth exhibits a clear exponential behaviour over almost two orders of magnitude, which is a strong evidence for the s-wave nature of the order parameter. The absolute value of the reduced energy gap Δ / kTc was found to be 1.1 corresponding to an absolute gap value Δ of 3.5 meV (Tc= 39 K). The zero temperature values of the penetration depth determined from this fit were found to be between 140 and 190 nm. The temperature dependence of the microwave surface resitance below Tc/2 is consistent with the gap value determined from λ(T). For our best sample, the residual surface resistance (Rs[T → 0]) was found to be below the measurement accuracy of our cavity (10−4 Ω). These results are quite promising for microwave applications of MgB2.