Effect of a normal-state pseudogap on optical conductivity in underdoped cuprate superconductors

We calculate the c-axis infrared conductivity sigma(c)(omega) in underdoped cuprate superconductors for spin fluctuation exchange scattering within th e CuO2 planes including a phenomenological cl-wave pseudogap of amplitude E g. For temperatures decreasing below a temperature T*similar to E-g/2, a ga p for omega<2E(g) develops in sigma(c)(omega) in the incoherent (diffuse) t ransmission limit. The resistivity shows "semiconducting" behavior, i.e., i t increases for low temperatures above the constant behavior for E-g = 0. W e find that the pseudogap structure in the in-plane optical conductivity is about twice as big as in the interplane conductivity a,(w), in qualitative agreement with experiment. This is a consequence of the fact that the spin fluctuation exchange interaction is suppressed at low frequencies as a res ult of the opening of the pseudogap. While the c-axis conductivity in the u nderdoped regime is described best by incoherent transmission, in the overd oped regime coherent conductance gives a better description. [S0163-1829(99 )02842-8].