Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La(1-x)Ca(x)MnO(3) (with x of about 0.3), which exhibit colossal magnetoresistance. The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a lateral scale of the order of 10 nanometers. Above the bulk transition temperature T(c), the images show mostly insulating behavior. Below T(c), a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T(c). These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.

We have performed electron tunneling spectroscopy on FeSi single crystals in the temperature range 4-300 K by using a scanning tunneling microscope. The differential conductance (dI/dV), when corrected for Schottky barrier effects, exhibits two strongly temperature-dependent peaks on either side of the Fermi level that emerge below Ϸ200 K and that are separated by a ͑pseudo͒gap of Ϸ50 meV. Our observations can be ascribed to the formation of quasiparticle density of states caused by d-electron correlation. The tunneling spectra are in good agreement with photoemission spectroscopy as both techniques probe the correlated d-electron density of states ͑DOS͒. Our results are also consistent with optical reflectivity data and Raman spectroscopy, which, in contrast, are sensitive to the conduction (c) electron DOS. ͓S0163-1829͑98͒08248-4͔

We have performed electron tunneling spectroscopy on FeSi single crystals in the temperature range 4-300 K by using a scanning tunneling microscope. The differential conductance (dI/dV), when corrected for Schottky barrier effects, exhibits two strongly temperature-dependent peaks on either side of the Fermi level that emerge below Ϸ200 K and that are separated by a ͑pseudo͒gap of Ϸ50 meV. Our observations can be ascribed to the formation of quasiparticle density of states caused by d-electron correlation. The tunneling spectra are in good agreement with photoemission spectroscopy as both techniques probe the correlated d-electron density of states ͑DOS͒. Our results are also consistent with optical reflectivity data and Raman spectroscopy, which, in contrast, are sensitive to the conduction (c) electron DOS. ͓S0163-1829͑98͒08248-4͔

Optical properties of the weakly helimagnetic metal MnSi have been determined in the photon energy range from 2 meV to 4.5 eV using the combination of grazing incidence reflectance at 80 • (2 meV to 0.8 eV ) and ellipsometry (0.8 to 4.5 eV). As the sample is cooled below 100 K the effective mass becomes strongly frequency dependent at low frequencies, while the scattering rate developes a linear frequency dependence. The complex optical conductivity can be described by the phenomenological relation σ(ω, T ) ∝ (Γ(T ) + iω) −0.54 used for cuprates and ruthenates.

We measured the re ectivity of a single crystal of FeSi from the far-infrared to the visible region (50-20 000 cm 1 ), varying the temperature between 4 and 300 K. The optical conductivity function was obtained via Kramers-Kronig analysis. We observed a dirty metal-like behavior at room temperature and the opening of a gap of 70 meV at low temperature. Of the ve optical phonons expected from group theory analysis for this material only four have been observed in the far-infrared region.