Optical study of c-axis charge dynamics in YBa2Cu3Oy:nnCarrier self-confinement in the normal and the superconducting states

Abstract

The optical conductivity spectra of YBa2Cu3Oy crystals have been investigated with polarization E‖c over a wide doping range from the heavily underdoped to the overdoped regime, focusing on the charge dynamics in the c direction. For overdoped crystals, the conductivity spectra σc(ω) show a completely metallic T and ω dependence, which can be analyzed by an extended Drude model. However, the obtained quasiparticle scattering rate γc is one order larger than the in-plane value, which is 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-Tc cuprates and suggests unconventional charge dynamics along the c axis such as incoherent hopping. With decreasing oxygen content y, temperature dependence of the low-ω conductivity changes from a metallic to a semiconducting one, being consistent with the T dependence of the dc resistivity ρc. The conductivity for the underdoped crystals with dρc(T)/dT<0 are characterized by a suppression of the conductivity below ωsuppression(≈500–800ncm−1). This doping dependence of σc(ω,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 electron correlation. The carrier-confined state which is 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 Tc drop in the overdoped regime.

Source-reported materials — not catalogue approval

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