MAGNETIC FIELD DEPENDENCE OF THE MICROWAVE SURFACE RESISTANCE IN MgB2 SUPERCONDUCTORS

We report experimental results on microwave surface resistance in MgB2 superconductors. The results have been obtained using the cavity perturbation method. Two samples have been investigated: one was made up of commercial Alfa-Aesar powder and the other one was obtained by direct reaction from elements. The temperature and dc magnetic field dependence of the surface resistance has been investigated down to 4.2 K and up to 10 kOe, respectively. Our results show that microwave losses are strongly affected by the magnetic field even by a relatively weak one. In particular, an unusually enhanced field dependence of the low-T residual surface resistance has been detected in both samples.