Microwave Response of Coaxial Cavities Made of Bulk Magnesium Diboride

We report on the microwave properties of coaxial cavities built by using bulk MgB₂ superconductor prepared by reactive liquid Mg infiltration technology. We have assembled a homogeneous cavity by using an outer MgB₂ cylinder and an inner MgB₂ rod and a hybrid cavity by using an outer copper cylinder and the same MgB₂ rod as inner conductor. By the analysis of the resonance curves, in the different resonant modes, we have determined the microwave surface resistance Rs of the MgB₂ materials as a function of the temperature and the frequency, in the absence of dc magnetic fields. At T=4.2 K and f ≈ 2.5 GHz, by an mw pulsed technique, we have determined the quality factor of the homogeneous cavity as a function of the input power up to a maximum level of about 40 dBm (corresponding to a maximum peak magnetic field of about 100 Oe). Contrary to what occurs in many films, Rs of the MgB₂ material used does not exhibit visible variations up to an input power level of about 10 dBm and varies less than a factor of 2 on further increasing the input power of 30 dB.