Optical studies of the metal-nonmetal transition in a metalrare-gas disordered system

In this paper we report the results of an experimental study of the optical properties in the energy range 0.65-3.80 eV of binary mixtures of Hg and Xe at 6 K over the concentration range X=1.0-0.47 atomic fraction Hg. A new optical criterion for the specification of the metal-nonmetal transition (MNMT) is advanced which predicts that the real part of the low-frequency dielectric function should decrease abruptly with increase of the metal concentration beyond the MNMT. This prediction is borne out for the Hg-Xe system and was used to specify the concentration XM=0.80±0.02 for the MNMT. Subsequently, we have analyzed the frequency-dependent conductivity of the Hg-Xe films in terms of the random-phase model, which was extended to handle a system with two overlapping bands. The theoretical results were used to fit simultaneously the dc conductivity and the optical properties with a model density of states in the concentration range 0.47>~X>~0.88. The marked overestimate of the dc conductivity calculated from the fit of the random-phase model for mercury concentrations X<0.80 was interpreted in terms of the termination of the strong-scattering metallic regime where the states at the Fermi energy become localized. The composition XM=0.80± 0.02, making the MNMT obtained from the onset of localization, is in excellent agreement with the independent estimate based on an optical criterion and with the onset of a positive temperature coefficient of the dc conductivity. Our analysis provides an unambiguous identification of the MNMT in this disordered material and draws a distinction between the MNMT (XM=0.80±0.02) and the conductivity onset (XC=0.69±0.01) in these low-temperature binary mixtures. We propose that the topological percolation threshold marks the conductivity transition and that in the composition range XC<~X<~XM thermally activated hopping prevails, while a Mott-Anderson MNMT is exhibited at X=XM.