Scaling analysis was performed on families of DC and AC conductivity curves falling on the metallic and insulating sides of the metal-insulator transition in the amorphous magnetically doped semiconductor, $a\text{−}{\mathrm{Gd}}_x{\mathrm{Si}}_{1-x}$. The transport curves were obtained both as a function of discretely varying both the gadolinium dopant concentration, $x$, and separately by changing an applied magnetic field, $H$. Both tuning parameters result in correlation length exponents of $\nu =1$ and dynamical scaling exponents of $z=2$. Temperature-frequency results differ markedly as compared to previous work on the nonmagnetic analog $a\text{−}{\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$. Our data also indicate a broader than predicted parameter space showing quantum critical behavior, and a phenomenologically determined quantum critical line in the zero temperature $x\text{−}H$ plane is presented. The results are explained in terms of a single tunable parameter, namely disorder.

• Received 16 November 2005

DOI:https://doi.org/10.1103/PhysRevB.73.155201