Normal-state reflectivity and superconducting energy-gap measurement ofLa2−xSrxCuO4

Abstract

We measure the reflectivity of pure and doped polycrystalline ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ in the gap region and up to 40 000 ${\mathrm{cm}}^{-1\mathrm{}}$ (≃5 eV). For the doped sample we analyze the reflectivity in terms of a superposition of a metallic $a-b$ plane component and a nearly insulating $c$-axis component with ordinary phonons. This approach removes several unphysical results obtained assuming a homogeneous surface and allows agreement between the measured infrared and dc properties, which has not been previously obtained. This approach further indicates that our infrared gap signature is dominated by the $c$-axis contribution and insensitive to the gap in the $a-b$ plane. We estimate the $c$-axis gap to be $2\Delta \simeq 2k_B{T}_c$, which is small compared to both the isotropic Bardeen-Cooper-Schrieffer prediction and tunneling measurements.