Tunneling and Transport Measurements at the Metal-Insulator Transition of Amorphous Nb: Si

Tunneling and conductivity measurements through the metal-insulator transition in amorphous ${\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$ are reported. The authors observe the correlation gap $\Delta$ which varies with resistivity and have related this to the metal-insulator transition as observed in the conductivity. The samples were prepared by a process which allows precise control of the Nb concentration. The results as a function of voltage, temperature, and concentration are compared with current theoretical predictions.