Infrared properties of La${}_{2-x}$(Ca,Sr)${}_x$CaCu${}_2$O${}_{6+\hat{I}´}$ single crystals

Abstract

The in-plane optical properties of two crystals of the bilayer cuprate \LSxCCO, one with excess Ca and $x=0.10$ and the other with Sr and $x=0.15$, were investigated over the frequency range of 45--25000 cm${}^{-1}$. The optical conductivity has been derived from Kramers-Kronig transformation. Each crystal exhibits a peak at around 15000 cm${}^{-1}$ which corresponds to the charge-transfer gap of the parent insulator. With increasing carrier density, spectral weight shifts from the CT excitation to the low-$\ddot{I}‰$ region. For the superconducting sample ($x=0.15$), the optical conductivity displays a peak in the far-infrared region, which shifts toward zero frequency with decreasing temperature. The temperature-dependent behavior favors a dynamical localization picture. A ``pseudogap'' feature is observed in the low-frequency reflectance and the scattering rate spectra. Both the energy scale and the temperature dependence of the ``pseudogap'' are similar to other bilayer cuprates.

5 pages, 3 figures