Optical data are reported on a spectral weight transfer over a broad frequency range of Bi2Sr2CaCu2O8+delta, when this material became superconducting. Using spectroscopic ellipsometry, we observed the removal of a small amount of spectral weight in a broad frequency band from 10(4) cm(-1) to at least 2 x 10(4) cm(-1), due to the onset of superconductivity. We observed a blue shift of the ab-plane plasma frequency when the material became superconducting, indicating that the spectral weight was transferred to the infrared range. Our observations are in agreement with models in which superconductivity is accompanied by an increased charge carrier spectral weight. The measured spectral weight transfer is large enough to account for the condensation energy in these compounds.

Direct Two-Magnon Optical Absorption in α'-NaV2O5Damascelli, A.; van der Marel, D. ; Grüninger, M; Presura, C.; Palstra, T.T.M.; Jegoudez, J.; Revcolevschi, A.

Optical properties of single-crystal monoclinic CuO in the range 70 -6000 cm −1 were studied at temperatures from 7 to 300 K. Normal reflection spectra were obtained from the (001) and (010) crystal faces thus giving for the first time separate data for the Au and Bu phonon modes excited in the purely transverse way (TO modes). Mode parameters, including polarizations of the Bu modes not determined by the crystal symmetry, were extracted by the dispersion analysis of reflectivity curves as a function of temperature. Spectra of all the components of the optical conductivity tensor were obtained using the Kramers-Kronig method recently extended to the case of the low-symmetry crystals. The number of strong phonon modes is in agreement with the factor-group analysis for the crystal structure, currently accepted for the CuO. However, several "extra" modes of minor intensity are detected; some of them are observed in the whole studied temperature range, while existence of others becomes evident at low temperatures. Comparison of frequencies of "extra" modes with the available phonon dispersion curves points to possible "diagonal" doubling of the unit cell {a, b, c} → {a+c, b, a-c} and formation of the superlattice. The previously reported softening of the A 3 u mode (∼ 400 cm −1 ) with cooling at TN is found to be ∼ 10 % for the TO mode. The mode is very broad at high temperatures and strongly narrows in the AFM phase. We attribute this effect to strong resonance coupling of this mode to optical or acoustic bi-magnons and reconstruction of the magnetic excitations spectrum at the Néel point. A significant anisotropy of ǫ ∞ is observed: it was found to be 5.9 along the b-axis, 6.2 along the [101] chains and 7.8 the [101] chains. The "transverse" effective charge is more or less isotropic; its value is about 2 electrons.

Copper nitride films were grown by molecular-beam epitaxy of copper in the presence of nitrogen from a radio-frequency atomic source on (001) γ′-Fe4N/(001)MgO or directly on MgO substrates. The structural properties of the Cu3N films were found to be very dependent on the substrate and on the deposition temperature. At optimal growth conditions, the Cu3N films grow epitaxial on both substrates. The Cu3N films grown on MgO were characterized optically to be insulators with an energy gap of 1.65 eV. On γ′-Fe4N, Cu3N films with a thickness of only 6 nm, were grown as closed layers, epitaxial and rather smooth (root-mean-square roughness of 0.7 nm). This material has ideal properties to be used as a barrier in low resistance magnetic tunnel junctions.