The low-field Hall coefficient $R_H$, optical reflectance and transmittance of the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ system with various Sr concentrations from $x=0\mathrm{} \mathrm{to} 0.36$ 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 $x=0\mathrm{}$ to a typical metallic one characterized by the Drude model for $x>\mathrm{}0.1\mathrm{}$, 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.

• Received 26 September 1988

DOI:https://doi.org/10.1103/PhysRevB.39.2312