The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.

We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of Ga1-xMnxAs. We compare it with measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originate from Mn disorder and As antisite defects. The experimentally determined hopping energy is an excellent predictor of the Curie temperature, in agreement with the theory.

Epitaxial thin films of the n=1–5 members of the layered Srn+1RunO3n+1 oxide series were produced by reactive molecular-beam epitaxy. X-ray diffraction and high-resolution transmission electron microscopy confirm that these films are epitaxially oriented and nearly phase pure (>98%). The Sr2RuO4 (n=1) and Sr3Ru2O7 (n=2) samples show no ferromagnetic transition in the range from 5to300K, while the Sr4Ru3O10 (n=3), Sr5Ru4O13 (n=4), and Sr6Ru5O16 (n=5) samples show ferromagnetic transitions at 85, 95, and 130K, respectively.