[en] When heated through the magnetic transition at T_C, La_0.7Ca_0.3MnO₃ changes from a band metal to a polaronic insulator. The Hall constant R_H, through its activated behavior and sign anomaly, provides key evidence for polaronic behavior. We use R_H and the Hall mobility to demonstrate the breakdown of the polaron phase. Above 1.4T_C, the polaron picture holds in detail, while below, the activation energies of both R_H and the mobility deviate strongly from their polaronic values. These changes reflect the presence of metallic, ferromagnetic fluctuations, in the volume of which the Hall effect develops additional contributions tied to quantal phases. (c) 2000 The American Physical Society

[en] Electron and hole transport in compensated InGaAsN (≅2% N) are examined through Hall mobility, photoconductivity, and solar cell photoresponse measurements. Short minority carrier diffusion lengths, photoconductive-response spectra, and doping dependent, thermally activated Hall mobilities reveal a broad distribution of localized states. At this stage of development, lateral carrier transport appears to be limited by large scale (>> mean free path) material inhomogeneities, not a random alloy-induced mobility edge. (c) 2000 American Institute of Physics

[en] A thick (260 nm) pseudomorphic metastable n-type Ge_0.06Si_0.94 layer grown by molecular beam epitaxy on an n-type Si(100) substrate was implanted at room temperature with 70 keV BF₂⁺ ions to a dose of 3x10¹³ cm^-2, so that a p-n junction was formed in the GeSi layers. The samples were subsequently annealed for 10 endash 40 s in a lamp furnace with a nitrogen ambient, or for 30 min in a vacuum-tube furnace. The samples were characterized by 2 MeV ⁴He backscattering/channeling spectrometry, double-crystal x-ray diffractometry, transmission electron microscopy, and by Hall effect measurements using the van der Pauw sample geometry. Samples annealed for either 40 s or 30 min at 800 degree C exhibit full electrical activation of the boron in the GeSi epilayer without losing their strain. The Hall mobility of the holes is lower than that of p-type Si doped under the same experimental conditions. These results can be attributed to the Hall factor of heavily doped p-type GeSi films which is less than unity while the Hall factor of a heavily doped p-type Si or n-type GeSi film is close to unity. When annealed at 900 degree C, the strain in both implanted and unimplanted layers is partly relaxed after 30 min, whereas it is not visibly relaxed after 40 s. copyright 1997 American Institute of Physics