Optical study of c-axis charge dynamics in YBa{sub 2}Cu{sub 3}O{sub y}: Carrier self-confinement in the normal and the superconducting states

The optical conductivity spectra of YBa{sub 2}Cu{sub 3}O{sub y} crystals have been investigated with polarization E{parallel}c over a wide doping range from the heavily underdoped to the overdoped regime. For overdoped crystals, the conductivity spectra {sigma}{sub c}({omega}) show a completely metallic T and {omega} dependence, which can be analyzed by an extended Drude model. However, the obtained quasiparticle scattering rate {gamma}{sub c} is one order larger than the in-plane value, quite different from the case of a conventional low-dimensional material with a strong mass anisotropy. It is evidence for a non-Fermi-liquid state in high-T{sub c} cuprates and suggests unconventional charge dynamics along the c axis such as incoherent hopping. With decreasing oxygen content y, temperature dependence of the low-{omega} conductivity changes from a metallic to a semiconducting one, consistent with the T dependence of the dc resistivity {rho}{sub c}. The conductivity for the underdoped crystals with d{rho}{sub c}(T)/dT{lt}0 are characterized by a suppression of the conductivity below {omega}{sub suppression}({approx}500{endash}800 cm{sup {minus}1}). This doping dependence of {sigma}{sub c}({omega},T) indicates a change from a strongly confined regime for low doping to a weakly confined regime for high doping, and suggests that the mechanism of carrier confinement is closely related to strong electronmore » correlation. The carrier-confined state symbolized by a large scattering rate leads to a dirty limit behavior for the c direction in the superconducting state, whereas it is in a clean limit in the plane direction. There are a couple of experimental facts which suggest a d-wave gap. A huge amount of unpaired carriers and no decrease of maximum gap amplitude were found for the overdoped crystals, which does not support the mean-field theory for explanation of the T{sub c} drop in the overdoped regime. {copyright} {ital 1997} {ital The American Physical Society}« less