Superconducting properties of MgB$_2$ probed by radiation-induced disordering

The electrical resistivity $\rho$ and upper critical field $H_{c2}$ were measured in MgB$_2$ disordered by nuclear reactor neutrons. We found that superconducting temperature $T_c$ decreases under irradiation from 40 to 5 K. Despite strong disordering (more than 10 dpa (displacements per atom)), the initial crystal structure is preserved. The residual resistivity $\rho_0$ increases from 0.35 to 2 m$\Omega$ cm while $-dH_{c2}/dT$ remains approximately unchanged ($-dH_{c2}/dT \approx 0.5$ T/K), and such behavior may be interpreted as decrease in the density of electronic states.