Observation of quantum corrections to conductivity up to optical frequencies

It is well known that the conductivity of disordered metals is suppressed in the limit of low frequencies and temperatures by quantum corrections. Although predicted by theory to exist up to much higher energies, such corrections have so far been experimentally proven only for $\ensuremath{\lesssim}80$ meV. Here, by a combination of transport and optical studies, we demonstrate that the quantum corrections are present in the strongly disordered conductor MoC up to at least $\ensuremath{\sim}4$ eV, thereby extending the experimental window where such corrections were found by a factor of 50. The knowledge of both the real and imaginary parts of conductivity enables us to identify the microscopic parameters of the conduction electron fluid. We find that the conduction electron density of strongly disordered MoC is surprisingly high and we argue that this should be considered a generic property of metals on the verge of a disorder-induced localization transition.