Theory of optical conductivity in doped manganites

The optical conductivity of the double exchange and the orbital degenerate Kondo lattice Hamiltonian (orbital t-J model) are invetigated by means of finite temperature diagonalization for different doping concentrations. The temperature dependence of sum rules, kinetic energy and Drude weight are studied. In particular it is shown that the double exchange model cannot explain the anomalous absorption in the uniform ferromagnetic phase of manganites discovered Okimoto et al. [...

We discuss the role of orbital degeneracy in the transport properties of perovskite manganites, focusing in particular on the optical conductivity in the metallic ferromagnetic phase at low temperatures. Orbital degeneracy and strong correlations are described by an orbital t-J model which we treat in a slave-boson approach. Employing the memory-function formalism we calculate the optical conductivity, which is found to exhibit a broad incoherent component extending up to bar...

We report measurements of the resistivity in the ferromagnetic state of epitaxial thin films of La_{1-x}Ca_{x}MnO_{3} and the low temperature specific heat of a polycrystalline La_{0.8}Ca_{0.2}MnO_{3}. The resistivity below 100 K can be well fitted by \rho - \rho_{o} = E \omega_{s}/sinh^{2}(\hbar \omega_{s}/2k_{B}T) with \hbar \omega_{s}/k_{B} \simeq 100 K and E being a constant. Such behavior is consistent with small-polaron coherent motion which involves a relaxation due to...

Transport, magnetic, magneto-optical (Kerr effect) and optical (light absorption) properties have been studied in an oriented polycrystalline La_{0.67}Ca_{0.33}MnO_{3} film which shows colossal magneto-resistance. The correlations between these properties are presented. A giant change in IR light transmission (more than a 1000-fold decrease) is observed on crossing the Curie temperature (about 270 K) from high to low temperature. Large changes in transmittance in a magnetic f...

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result i...

A new mechanism is proposed to explain the colossal magnetoresistance and related phenomena. Moving electrons accompanied by Jahn-Teller phonon and spin-wave clouds may form composite polarons in ferromagnetic narrow-band manganites. The ground-state and finite-temperature properties of such composite polarons are studied in the present paper. By using a variational method, it is shown that the energy of the system at zero temperature decreases with the formation of composite...

We investigated the temperature ($T=$ 15 $\sim $ 290 K) and the magnetic field ($H=$ 0 $\sim $ 17 T) dependent optical conductivity spectra of a charge/orbital ordered manganite, Nd$_{1/2}$Sr$_{1/2}$MnO$_3$. With variation of $T$ and $H$, large spectral weight changes were observed up to 4.0 eV. These spectral weight changes could be explained using the polaron picture. Interestingly, our results suggested that some local ordered state might remain above the charge ordering t...

We present a small-but-sizeable magnetic polaron picture where transport at high temperatures is activated while at low temperatures it is band-like. We show that both double exchange and finite bandwidth effects are important to understand colossal magnetoresistance as well as the coincidence of the metal-insulator and the ferromagnetic transitions in manganites. The magnetic transition is explained using band-like motion of the polarons.

Optical reflectivity spectra were measured on cleaved surfaces of La1-xSrxMnO3 single crystals (x=0.175, T_C=283 K) over a temperature range 10-295 K. The optical conductivity $sigma(\omega)$ shows, keeping single-component nature, incoherent-to-coherent crossover with increase of electrical conductivity. The $\sigma(\omega)$ spectrum of low-temperature ferromagnetic-metallic phase (10 K) exhibits a pronounced Drude-like component with large spectral weight, contrary to the p...

We present the optical reflectance and conductivity spectra for non-oxide antiperovskite superconductor $MgCNi_{3}$ at different temperatures. The reflectance drops gradually over a large energy scale up to 33,000 cm$^{-1}$, with the presence of several wiggles. The reflectance has slight temperature dependence at low frequency but becomes temperature independent at high frequency. The optical conductivity shows a Drude response at low frequencies and four broad absorption fe...