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Frances HELLOGO.GIFellman Group
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Magnetic Semiconductors

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.
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We have observed that the dc conductivity, σDC(H,T,x), of Gd-doped amorphous silicon, a-GdxSi1-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 σDC(H,T) converge at T>50K whereas the real part of the complex frequency dependent optical conductivity σ1(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.