Effects of proton irradiation on the microwave properties of polycrystalline MgB2 films in the Meissner state

The critical current density (JC) and the upper critical field (HC2) of MgB2 in the mixed state appeared to be enhanced by proton irradiation due to existence of stronger pinning sites caused by increased disorder. We investigate the surface resistance (RS) of polycrystalline MgB2 films with the critical temperature (TC) of ~39 K in the Meissner state at ~8.5 GHz as a function of temperature before and after proton irradiation to a fluence of 4.8 X 1015 protons cm-2. Reduced TC was observed along with reduced intrinsic RS at temperatures below 24-26 K after proton irradiation. An increase of 9-14% in the pi-band gap energy was observed for proton-irradiated MgB2 films, which is attributable to enhanced interband scattering between the sigma-band and the pi-band caused by increased disorder. Two coherence peaks were observed at T/TC~0.56-0.59 and T/TC~0.90-0.93 in the temperature dependence of the microwave conductivity for pristine MgB2 films, reflecting the two-gap nature of MgB2. The peaks at T/TC~0.56-0.59 were changed to a plateau-like shape for the MgB2 films after proton irradiation. Our results for MgB2 films in the Meissner state suggest that modification in the gap energies might play a role in enhancing JC and HC2 for proton-irradiated MgB2 in the mixed state.