Hall coefficients and optical properties of La2-xSrxCuO4 single-crystal thin films.

Abstract

The low-field Hall coefficient ${R}_{H}$, optical reflectance and transmittance of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system with various Sr concentrations from $x=0 \mathrm{to} 0.36$ are systematically studied using single-crystal thin films epitaxially grown on (100) face SrTi${\mathrm{O}}_{3}$ substrates with the $c$ axis normal to the film surface. For the $x$ range measured, ${R}_{H}$ is positive and decreases more rapidly than that expected from the Sr concentration but more slowly than reported earlier for polycrystalline specimens, indicating anisotropy of ${R}_{H}$. Furthermore, the $x$ dependence indicates deviation from that expected from a simple band model. Within the superconducting composition range, ${R}_{H}$ exhibits characteristic temperature dependence. The optical reflectance spectrum changes from that of a semiconductor at $x=0$ to a typical metallic one characterized by the Drude model for $x>0.1$, indicating the development of itinerant holes in the Cu-O planes. In the optical transmission spectra, an anomalous absorption band is seen in addition to the fundamental absorption corresponding to an energy gap of about 2 eV. This band, which develops with Sr doping, implies an enhancement of the density of states near the Fermi level. Taking these observations into account, the normal-state transport properties are explained with a qualitative consistence.