Observation of the Josephson plasma reflectivity edge in the infrared region in Bi-based superconducting cuprates

The far-infrared c-axis optical reflectivity spectra of a heavily Pb-doped Bi2212, Bi1.6Pb0.6Sr1.8CuCu2Oy, single crystal (T-c=65 K) were measured in the superconducting state. The crystal was found to be carrier overdoped as the optical phonons are weakly screened by the electronic excitations in the far-infrared region. Below T-c, a sharp reflectivity edge was observed, corresponding to the appearance of the Josephson plasma in the superconducting state. The plasma edge frequency reached 40 cm(-1) at 8 K, which is comparable with that observed for other cuprates with much less anisotropy. The c-axis penetration depth lambda(c) = 12.6 mu m was estimated from the unscreened plasma frequency w(ps)(c)=126.2 cm(-1). This lambda(c) value is one order of magnitude smaller than that of the optimally doped pure Bi2212, reflecting a drastic enhancement of the interlayer coupling in Pb-doped Bi2212.