Metal-insulator transition in YHx: scaling of the sub-THz conductivity
  The established scaling laws of the conductivity with temperature and doping are strong indications for the quantum nature of the metal-insulator transition in YH$_x$. Here we report the first results on the frequency scaling of the conductivity. Samples were brought from the insulating to the metallic phase by carrier doping via illumination. In the metallic phase, the sub-terahertz conductivity coincides with the dc data. These results do not agree with the simplest picture of a quantum-phase transition. 
I. G. Romijn, A. V. Pronin, H. B. Brom, A. F. Th. Hoekstra