Temperature dependence of the microwave surface impedance measured on different kinds of MgB${}_2$ samples

Abstract

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB${}_2$. One film has $T_c=30$ K and is not textured, the other is partially c-axis orientated with $T_c=38$ K. These samples show different types of temperature dependence of the field penetration depth: linear for the powder sample, exponential with $\Delta /k{T}_c<1.76$ (film with $T_c=30$ K) and strong coupling behaviour with $\Delta /k{T}_c\sim 2.25$ (film with $T_c=38$ K). The results are well described in terms of an anisotropic gap model or presence of a slightly deficient MgB${}_2$ phase. The data set for all samples - taken as a whole cannot be fitted into a two gap scenario.

4 pages, 3 figures