The metal-insulator transition in amorphous Si_{1-x}Ni_x: Evidence for Mott's minimum metallic conductivity

We study the metal-insulator transition in two sets of amorphous Si_{1-x}Ni_x films. The sets were prepared by different, electron-beam-evaporation-based technologies: evaporation of the alloy, and gradient deposition from separate Ni and Si crucibles. The characterization included electron and scanning tunneling microscopy, glow discharge optical emission spectroscopy, and Rutherford back scattering. Investigating the logarithmic temperature derivative of the conductivity, w = d ln sigma / d ln T, we observe that, for insulating samples, w(T) shows a minimum increasing at both low and high T. Both the minimum value of w and the corresponding temperature seem to tend to zero as the transition is approached. The analysis of this feature of w(T,x) leads to the conclusion that the transition in Si_{1-x}Ni_x is very likely discontinuous at zero temperature in agreement with Mott's original views.