Signatures of Electronic Correlations in Optical Properties of LaFeAsO$_{1-x}$F$_x$

Spectroscopic ellipsometry is used to determine the dielectric function of the superconducting LaFeAsO$_{0.9}$F$_{0.1}$ ($T_c$ = 27 K) and undoped LaFeAsO polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 $leq T leq$ 350 K. The free charge carrier response in both samples is heavily damped with the effective carrier density as low as 0.040$pm$0.005 electrons per unit cell. The spectral weight transfer in the undoped LaFeAsO associated with opening of the pseudogap at about 0.65 eV is restricted at energies below 2 eV. The spectra of superconducting LaFeAsO$_{0.9}$F$_{0.1}$ reveal a significant transfer of the spectral weight to a broad optical band above 4 eV with increasing temperature. Our data may imply that the electronic states near the Fermi surface are strongly renormalized due to electron-phonon and/or electron-electron interactions.

Optical response of ferromagnetic YTiO_3 studied by spectral ellipsometry

We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric YTiO_3 single crystal with ferromagnetic Curie temperature T_C = 30 K. The optical response exhibits a weak but noticeable anisotropy. Using a classical dispersion analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV. Although the optical conductivity spectra are only weakly temperature dependent below 300 K, we are able to distinguish high- and low-temperature regimes with a distinct crossover point around 100 K. The low-temperature regime in the optical response coincides with the temperature range in which significant deviations from Curie-Weiss mean field behavior are observed in the magnetization. Using an analysis based on a simple superexchange model, the spectral weight rearrangement can be attributed to intersite d_i^1d_j^1 longrightarrow d_i^2d_j^0 optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around 2.9 eV can be associated with the high-spin-state (^3T_1) optical transition. This indicates that other mechanisms, such as weakly dipole-allowed p-d transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at 2 eV. The recorded optical spectral weight gain of 2.9 eV optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbit-lattice phenomena near ferromagnetic ordering temperature in YTiO_3.

Spectroscopic distinction between the normal state pseudogap and the superconducting gap of cuprate high T_{c} superconductors

We report on broad-band infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa_{2}Cu_{3}O_{7-d} (R=Y, Nd, and La) single crystals. Our data provide a detailed account of the spectral weight (SW) redistributions due to the normal state pseudogap (PG) and the superconducting (SC) gap. They show that these phenomena involve different energy scales, exhibit distinct doping dependencies and thus are likely of different origin. In particular, the SW redistribution in the PG state closely resembles the one of a conventional charge- or spin density wave (CDW or SDW) system.