We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.
@article{arxiv.cond-mat/0303283,
title = {Microwave performance of high-density bulk MgB2},
author = {A. T. Findikoglu and A. Serquis and L. Civale and X. Z. Liao and Y. T. Zhu and M. E. Hawley and F. M. Mueller and V. F. Nesterenko and Y. Gu},
journal= {arXiv preprint arXiv:cond-mat/0303283},
year = {2009}
}