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The infrared reflectivity of a La_(0.67)Ca_(0.33)MnO_3 single crystal is studied over a broad range of temperatures (78–340 K), magnetic fields (0–16 T), and wave numbers (20–9000cm^(-1)). The optical conductivity gradually changes from a Drude-like behavior to a broad peak feature near 5000cm-1 in the ferromagnetic state below the Curie temperature T_C=307K. Various features of the optical conductivity bear striking resemblance to recent theoretical predictions based on the interplay between the double exchange interaction and the Jahn-Teller electron-phonon coupling. A large optical magnetoconductivity is observed near T_C.

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO 3 crystal in the spectral range 1.2–6.0 eV at temperatures 20≤T≤300 K . The complex dielectric response along b and c axes of the Pbnm orthorhombic unit cell, e b (ν) and e c (ν) , is highly anisotropic over the spectral range covered in the experiment. The difference between e b (ν) and e c (ν) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A -type antiferromagnetic long-range order at T N =139.6 K . In this study we focus on the analysis of excitations observed at high energy (∼4–5 eV) and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at ∼2 eV . We used a classical dispersion analysis to quantitatively determine the temperature-dependent optical spectral-weights shifts between low- and high-energy optical bands. Based on the observation of a pronounced spectral-weight transfer between both features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d 4 d 4 ⇌d 3 d 5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral-weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn e g electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between e g orbitals in LaMnO 3 , as well as to extract experimental information concerning the type of orbital order in LaMnO 3 . This study identifies the lowest-energy optical transition at ∼2 eV as an intersite d-d transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.

A comprehensive ellipsometric study was performed on ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ single crystals in the spectral range from 0.01 to 6.2 eV. Direct and indirect band gaps of 73 and 10 meV, respectively, were observed in FeSi at 7 K. One of four infrared-active phonons that is energetically close to the direct absorption edge is coupled both to the electrons and to the low-energy phonon. This is evident from asymmetry in the phonon line shape and a reduction in its frequency when the absorption edge shifts across the phonon energy due to the temperature dependence of the direct band gap. As the temperature increases, the indirect gap changes sign, which manifests as a transition from a semiconductor to a semimetal. The corresponding gain of the spectral weight at low energies was recovered within an energy range of several eV. The present findings strongly support the model indicating that ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ can be well described in an itinerant picture, taking into account self-energy corrections.

The anisotropic complex dielectric response was accurately extracted from spectroscopic ellipsometry measurements at phonon frequencies for the three principal crystallographic directions of an orthorhombic $(Pbnm)$ ${\text{YTiO}}_{3}$ single crystal. We identify all 25 infrared-active phonon modes allowed by symmetry $7{B}_{1u}$, $9{B}_{2u}$, and $9{B}_{3u}$ polarized along the $c$, $b$, and $a$ axes, respectively. From a classical dispersion analysis of the complex dielectric functions $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$ and their inverses $\ensuremath{-}1/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$, we define the resonant frequencies, widths, and oscillator strengths of the transverse-optical (TO) and longitudinal-optical phonon modes. We calculate eigenfrequencies and eigenvectors of ${B}_{1u}$, ${B}_{2u}$, and ${B}_{3u}$ normal modes and suggest assignments of the TO phonon modes observed in our ellipsometry spectra by comparing their frequencies and oscillator strengths with those resulting from the present lattice-dynamics study. Based on these assignments, we estimate dynamical effective charges of the atoms in the ${\text{YTiO}}_{3}$ lattice. We find that in general, the dynamical effective charges in ${\text{YTiO}}_{3}$ lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric ${\text{BaTiO}}_{3}$, the dynamical effective charge of oxygen related to a displacement along the $c$ axis does not show the anomalously large value. At the same time, the dynamical effective charges of Y and $ab$ plane oxygen exhibit anisotropy, indicating a strong hybridization along the $a$ axis.

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.

Giant anomalies are found in the temperature dependence of Raman-active phonons in the quantum magnet $\mathrm{TiOCl}$, suggesting the presence of an extended fluctuation regime. This regime coincides with a pseudogap phase identified in earlier NMR experiments. The observed large, local spin-gap is proposed to origin from coupled spin∕lattice fluctuations. Below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in the long-range crystallographically distorted phase, a dimerized ground state with a smaller, global spin-gap of about $2{\ensuremath{\Delta}}_{spin}\ensuremath{\approx}430\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ exists. This transition also marks a dimensionality cross-over of the system.

We observe a large and anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}^{16,18}{\mathrm{O}}_{6.9}.$ For ${}^{18}\mathrm{O}$ a pronounced low-energy electronic mode (LEM) appears around $240{\mathrm{cm}}^{\ensuremath{-}1}.$ This a-axis LEM exhibits a clear aging effect, after 1 y it is shifted to $190{\mathrm{cm}}^{\ensuremath{-}1}.$ For ${}^{16}\mathrm{O}$ we cannot resolve a corresponding a-axis LEM above $120{\mathrm{cm}}^{\ensuremath{-}1}.$ We interpret the LEM in terms of a collective electronic mode that is pinned by ``isotopic defects,'' i.e., by the residual ${}^{16}\mathrm{O}$ in the matrix of ${}^{18}\mathrm{O}.$

Critical behavior of the infrared reflectivity of RuSr_2GdCu_2O_8 ceramics is observed near the superconducting T_{SC} = 45 K and magnetic T_M = 133 K transition temperatures. The optical conductivity reveals the typical features of the c-axis optical conductivity of strongly underdoped multilayer superconducting cuprates. The transformation of the Cu-O bending mode at 288 cm^{-1} to a broad absorption peak at the temperatures between T^* = 90 K and T_{SC} is clearly observed, and is accompanied by the suppression of spectral weight at low frequencies. The correlated shifts to lower frequencies of the Ru-related phonon mode at 190 cm^{-1} and the mid-IR band at 4800 cm^{-1} on decreasing temperature below T_M are observed. It provides experimental evidence in favor of strong electron-phonon coupling of the charge carriers in the Ru-O layers which critically depends on the Ru core spin alignment. The underdoped character of the superconductor is explained by strong hole depletion of the CuO_2 planes caused by the charge carrier self-trapping at the Ru moments.