Optical properties of the spin-ladder compoundSr14Cu24O41

We report the measurements of the pseudodielectric function, far-infrared reflectivity, and Raman scattering spectra in ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ single crystal. We study the lattice and the spin dynamics of the ${\mathrm{Cu}}_2{\mathrm{O}}_3$ spin ladders and ${\mathrm{CuO}}_2$ chains of this compound. The ellipsometric and the optical reflectivity measurements yield the gap values of 1.4, 1.86, 2.34 eV (2.5 eV) for the ladders (chains) along the c axis and 2.4 eV along the a axis. The electronic structure of the ${\mathrm{Cu}}_2{\mathrm{O}}_3$ ladders is analyzed using the tight-binding approach for the correlated electron systems. The correlation gap value of 1.4 eV is calculated with the transfer energy (hopping) parameters ${t=t}_0=0.26\mathrm{}\mathrm{eV},$ along and perpendicular to legs, $t_{\mathrm{xy}}=0.026\mathrm{}\mathrm{eV}$ (interladder hopping) and $U=2.1\mathrm{}\mathrm{eV},$ as a Coulomb repulsion. The optical parameters of the infrared-active phonons and plasmons are obtained by an oscillator fitting procedure of the reflectivity spectra. Raman scattering spectra are measured at different temperatures using different laser line energies. The two-magnon peak is observed at about 2880 ${\mathrm{cm}}^{\mathrm{-}1}$. At temperatures below 150 K the new infrared and Raman modes appear due to the charge ordering.