Conductivity of underdoped YBa(2)Cu(3)O(7-delta): evidence for incoherent pair correlations in the pseudogap regime.

A two-channel scenario for the conductivity of underdoped YBa 2Cu 3O (7-delta) is proposed. One is the single-particle excitations channel, which dominates in the optimally doped material, whose resistivity is linear as a function of temperature. The other one gives a contribution which merges the 3D Aslamazov-Larkin fluctuation conductivity at low temperature and obeys a power law at high temperature, depending on two superconductive parameters (T(c) and the zero temperature coherence length xi(c0)) and an energy scale Delta(*). This allows one to address the nature of the pseudogap in favor of incoherent pairing.