Infrared spectroscopic study on Nb-ion-irradiated MgB2 thin films

Magnesium diboride (MgB₂) is a two-band superconductor with a high superconducting critical temperature (T_c) of approximately 39 K. Owing to the lack of vortex pinning centers, MgB₂ exhibits an abrupt decline in the critical current density (J_c) in an applied magnetic field. Here, we prepared 1 MeV Nb ion-irradiated MgB₂ thin-film samples with doses of $3\times {10}^{13}$, $7\times {10}^{13}$, and $9\times {10}^{13}$ ions/cm². Temperature-dependent magnetization and x-ray diffraction (XRD) measurements were performed to determine the T_c and c-axis lattice constant of each sample. Furthermore, a Fourier transform infrared (FTIR) spectroscopy was performed to obtain the infrared properties of the Nb-ion-irradiated MgB₂ thin-film samples. The optical conductivity of each sample in the low-energy region was fitted with two (narrow and broad) Drude modes. We found that the spectral weight redistribution from the low-to high-frequency regions and the broadening of the narrow Drude mode caused by irradiation are closely related to the reduction in T_c.