Frequency dependence of the microwave surface resistance of MgB₂ by coaxial cavity resonator

Highlights: • We investigate the microwave properties of a bulk MgB₂ rod 94.3 mm long. • The MgB₂ rod is used as inner conductor of a coaxial cavity. • The mw surface resistance vs. frequency is studied in the range 1–9 GHz. • R_s vs. f curves follow a fⁿ law, with n decreasing with the temperature. • Deviations from the quadratic law are highlighted at relatively low temperatures. - Abstract: We report on the microwave (mw) properties of a cylindrical MgB₂ rod prepared by the reactive liquid Mg infiltration technology. The MgB₂ rod, 94.3 mm long, is used as inner conductor of a coaxial cavity having a Cu tube as external conductor. By analyzing the resonance curves of the cavity in the different resonant modes and at different temperatures, we have determined the temperature dependence of the mw surface resistance, R_s, of the MgB₂ material, at fixed frequencies, and the frequency dependence of R_s, at fixed temperatures. Our results show that the R_s(f) curves follow a fⁿ law, where n decreases on increasing the temperature, starting from n≈2, at T=4.2K, down to n≈0.7 at T⩾T_c. The double-gap nature of MgB₂ manifests itself in the presence of a wide low-T tail in the R_s(T) curves, which can be ascribed to the quasiparticles thermally excited through the π gap even at relatively low temperatures