C-Axis Spectra of La_2-x Ba_xCuO₄ in the Normal and Superconducting State

Reflectance spectra of superconducting La_2-xBa_xCuO₄ have been studied for polarization parallel to the c-axis in a wide frequency range from 20-35,000 cm^-1, at doping from 0≤x≤0.155 and temperatures above and below T_c. The samples are single crystals grown using the traveling solvent floating zone method. These samples were wet polished with alumina fine powder to make a good reflective surface. The c-axis spectra showed insulating behavior, with only lattice vibrations below the charge-transfer gap. We observed below T_c no evidence for a Josephson plasma edge in the far infrared, despite seeing good superconducting behavior for the a-axis spectra. The results of annealing in an oxygen atmosphere will be discussed.