We study
the metal-insulator transition (MIT) in
magnetically doped semiconductors. This
MIT is a type of Quantum Phase Transition, a phase transition which is
achieved
by tuning some other parameter in the ground state Hamiltonian other
than
temperature (electronic concentration, pressure or magnetic
field). Proximal to this Quantum Phase Transition a
quantum critical scaling is expected for the various physical
observables of
the system.

We have observed that the dc conductivity,
σ_{DC}(H,T,x),
of Gd-doped amorphous silicon, a-Gd_{x}Si_{1-x},
a system which
undergoes both a concentration tuned, as well as a magnetic field tuned
MIT,
obeys scaling consistent with quantum critical behavior. In fact we have been able
to show scaling for
both concentration tuning (top panel) and for magnetic field tuning for
a
single sample (lower panel). Critical
exponents agree with a disorder-driven transition in both cases.

Furthermore, for Quantum Phase
Transitions, the temperature
T and frequency ω should be interchangeable as seen in the non-magnetic
analog
system a-NbSi. We however find no T-ω scaling for a-GdSi down to 0.1
THz. We observe
that the σ_{DC}(H,T)
converge at T>50K whereas the real part of the complex frequency
dependent
optical conductivity σ_{1}(H,ω) do not converge
until nearly 1 eV. We
suggest that this is due to magnetic
interactions providing a separate length scale for this problem.