Microwave surface resistance in MgB2: Effect of Te and cubic-BN addition on flux flow and pinning

Abstract

MgB${}_2$ is a perspective superconductor for many power applications. How this potential refers also to microwave or radiofrequency applications is still to be determined. Although its ultimate surface resistance in zero field is not competitive with conventional metallic superconductors, its strong pinning properties can favor RF applications in a dc magnetic field. Nonetheless, the RF response in the vortex state has been relatively less studied, as well as the effect of artificial pinning centers on the microwave surface resistance in the mixed state. In this paper we study the surface resistance of spark-plasma-sintered MgB${}_2$, with and without Te and cubic-BN (cBN) addition, in a dc magnetic field up to 1.2 T. We summarize previous results on pure MgB${}_2$, and we present new data on Te- and cBN- added MgB${}_2$. We use a two-tone dielectric-loaded resonator to measure the field-dependent surface resistance at 16.5 and 26.7 GHz in the temperature range from 10 K to $T_c$. By exploiting the simultaneous measurements at two frequencies, we extract the flux-flow resistivity, the pinning constant $k_p$ and the depinning frequency $f_p$. The two-band nature of MgB${}_2$ affects the field dependence of the flux-flow resistivity. The microscopic superconducting state is not affected by the addition of artificial pinning centers, indicating that Te and cBN do not affect interband or intraband scattering. Pinning shows a measurable trend towards an increase in the Te- and cBN- added samples at higher temperatures and fields. We finally compare the results to those obtained in bulk Nb${}_3$Sn, also in view of possible in-field RF applications such as microwave cavity-based haloscopes.