Infrared anisotropy and plasmons in high-temperature superconductors

We show that from measurements of reflectivity of a uniaxial medium taken at a finite incidence angle with s- and p-polarized light it is possible to determine the dielectric function both parallel and perpendicular to the optical axis. When applied to a layered compound with its surface parallel to the layers, this technique allows for an accurate determination of the loss function perpendicular to the layers. This is demonstrated for the case of c-axis-oriented films of the high-temperature superconductor Tl₂Ba₂Ca₂Cu₃O₁₀, on which we carried out polarized reflectivity measurements at 45° incidence angle above and below T_c. We also studied the infrared response of La_1.85Sr_0.15CuO₄ single crystals with the light polarized parallel and perpendicular to the c-axis as a function of temperature. We confirm the results reported by Tamasaku [Phys. Rev. Lett. 69 (1992) 1455]: in the superconducting state a plasma branch occurs below the gap.