Fingerprints of Mott-Hubbard physics in optical spectra of antiferromagnetic LaTiO3

Magnetic properties of Mott-Hubbard insulators are determined by superexchange interactions mediated by the high-spin (HS) and low-spin (LS) intersite d-d charge excitations, which can be associated with the HS- and LS-Hubbard subbands in optical experiments. To explore the Mott-Hubbard physics in orthorhombic LaTiO_3 crystal exhibiting the G-type antiferromagnetic order at the Neel temperature T_N = 146 K, we present a comprehensive spectroscopic ellipsometry study in the spectral range 0.5-5.6 eV at temperatures 10 K < T < 300 K. We found that the complex dielectric function spectra of LaTiO_3 crystal are almost featureless, nearly isotropic, and weakly temperature dependent in the range of d-d optical transitions. Nonetheless, analyzing the difference spectra below the T_N, we have identified the LS-state d^1d^1 - d^2d^0 excitations at ~3.7 and ~5.15 eV and estimated values of the on-site Coulomb repulsion U ~4.2 eV and Hund's exchange constant J_H ~0.5 eV, which define the energy of the HS-state d^1d^1 - d^2d^0 excitation at ~2.7 eV. In addition, we discovered that the pronounced lowest-energy 1.3 eV optical band displays the critical intensity behavior and anomalous broadening with decreasing temperature below the T_N. The discovered properties indicate that the 1.3 eV band in LaTiO_3 can be associated with a Mott-Hubbard exciton.Comment: 8 pages, 8 figures, 1 tabl

Giant phonon anomalies in the pseudo-gap phase of TiOCl

We report infrared and Raman spectroscopy results of the spin-1/2 quantum magnet TiOCl. Giant anomalies are found in the temperature dependence of the phonon spectrum, which hint to unusual coupling of the electronic degrees of freedom to the lattice. These anomalies develop over a broad temperature interval, suggesting the presence of an extended fluctuation regime. This defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a dimensionality cross-over leads to a dimerized ground state with a global spin-gap of about 2$\Delta_{spin}\approx$~430 K.Comment: 4 pages, 3 figures, for further information see http://www.peter-lemmens.d

In-Plane Spectral Weight Shift of Charge Carriers in YBa2Cu3O6.9YBa_2Cu_3O_{6.9}

The temperature dependent redistribution of the spectral weight of the $CuO_2$ plane derived conduction band of the $YBa_2Cu_3O_{6.9}$ high temperature superconductor (T_c = 92.7 K) was studied with wide-band (from 0.01 to 5.6 eV) spectroscopic ellipsometry. A superconductivity - induced transfer of the spectral weight involving a high energy scale in excess of 1 eV 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

Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study

Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors.Comment: 30 pages, 6 Figure

Characteristics of the immune response of scots pine during infection caused by Heterobasidion annsum s. s.

Heterobasidion annosum is a causative agent of the root and butt and appears to be one of the most economically important conifer pathogens, which causes devastation in boreal forest at northern hemisphere

Charge ordering and magneto-polarons in Na0.82_{0.82}CoO2_2

Using spectral ellipsometry, we have measured the dielectric function of a Na$_{0.82(2)}$CoO$_2$ crystal that exhibits bulk antiferromagnetism with T$_{N}$=19.8 K. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and the lattice response that are indicative of charge ordering in the CoO$_{2}$ layers. The second transition coincides with T$_{N}$=19.8 K and reveals a sizeable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magneto-polarons due to a charge-induced spin-state transition of adjacent Co$^{3+}$ ions

Signatures of Electronic Correlations in Optical Properties of LaFeAsO1−x_{1-x}Fx_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.Comment: 4 pages, 4 figures, units in Fig.2 adde