Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3$ in the range 0.5–5.6 eV at temperatures $50\le T\le 300\mathrm{K}$. A pronounced redistribution of spectral weight is found at the Néel temperature $T_N=140\mathrm{K}$. The anisotropy of the spectral weight transfer matches the magnetic ordering pattern. A superexchange model quantitatively describes spectral weight transfer induced by spin correlations. This analysis implies that the lowest-energy transitions around 2 eV are intersite $d$-$d$ transitions, and that ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3$ is a Mott-Hubbard insulator.

The temperature-dependent redistribution of the spectral weight of the CuO ₂ plane–derived conduction band of the YBa ₂ Cu ₃ O _6.9 high-temperature superconductor (superconducting transition temperature = 92.7 kelvin) was studied with wide-band (0.01– to 5.6–electron volt) spectroscopic ellipsometry. A superconductivity-induced transfer of the spectral weight involving a high-energy scale in excess of 1 electron volt was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states.

Using spectral ellipsometry, we measured the dielectric function of a ${\mathrm{N}\mathrm{a}}_{0.82(2)}{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ crystal that exhibits bulk antiferromagnetism with $T_N=19.8\mathrm{K}$. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and lattice responses that are indicative of charge ordering in the ${\mathrm{C}\mathrm{o}\mathrm{O}}_2$ layers. The second transition occurs around $T_N=19.8\mathrm{K}$ and reveals sizable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magnetopolarons due to a charge-induced spin-state transition of adjacent ${\mathrm{C}\mathrm{o}}^{3+}$ ions.

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric ${\text{LaMnO}}_3$ crystal in the spectral range 1.2–6.0 eV at temperatures $20\le T\le 300\text{K}$. The complex dielectric response along $b$ and $c$ axes of the $Pbnm$ orthorhombic unit cell, ${\tilde{\epsilon }}^b\left(\nu \right)$ and ${\tilde{\epsilon }}^c\left(\nu \right)$, is highly anisotropic over the spectral range covered in the experiment. The difference between ${\tilde{\epsilon }}^b\left(\nu \right)$ and ${\tilde{\epsilon }}^c\left(\nu \right)$ 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 $T_N=139.6\text{K}$. In this study we focus on the analysis of excitations observed at high energy $\left(\sim 4–5\text{eV}\right)$ and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at $\sim 2\text{eV}$. 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 $d^4{d}^4\rightleftharpoons d^3{d}^5$ 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 $\text{Mn}{e}_g$ 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 ${\text{LaMnO}}_3$, as well as to extract experimental information concerning the type of orbital order in ${\text{LaMnO}}_3$. This study identifies the lowest-energy optical transition at $\sim 2\text{eV}$ as an intersite $d\text{−}d$ transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.

We present the results of a systematic study of the c-axis lattice dynamics in single-layer ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_6$ (Bi2201), bilayer ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (Bi2212), and trilayer ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10}$ (Bi2223) cuprate superconductors. Our study is based on both experimental data obtained by spectral ellipsometry on single crystals and theoretical calculations. The calculations are carried out within the framework of a classical shell model, which includes long-range Coulomb interactions and short-range interactions of the Buckingham form in a system of polarizable ions. Using the same set of shell model parameters for Bi2201, Bi2212, and Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes of $A_{2u}$ symmetry and suggest the phonon mode eigenvector patterns. We achieve good agreement between the calculated $A_{2u}$ eigenfrequencies and the experimental values of the c-axis TO phonon frequencies which allows us to make a reliable phonon mode assignment for all three Bi-based cuprate superconductors. We also present the results of our shell model calculations for the $\Gamma$-point $A_{1g}$ symmetry modes in Bi2201, Bi2212, and Bi2223 and suggest an assignment that is based on published experimental Raman spectra. The superconductivity-induced phonon anomalies recently observed in the c-axis infrared and resonant Raman scattering spectra in trilayer Bi2223 are consistently explained with the suggested assignment.

Spectroscopic ellipsometry is used to determine the dielectric function of superconducting ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ ($T_c=27\mathrm{K}$) and undoped LaFeAsO polycrystalline samples in the wide range 0.01–6.5 eV at temperatures $10\le T\le 350\mathrm{K}$. The charge carrier response in both samples is heavily damped. The spectral weight transfer in LaFeAsO associated with an opening of the pseudogap at about 0.65 eV is restricted to energies below 2 eV. The spectra of superconducting ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ reveal a significant transfer of 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.