Conductivity of Underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{7-\delta }$: Evidence for Incoherent Pair Correlations in the Pseudogap Regime

A two-channel scenario for the conductivity of underdoped $\mathrm{YBa}{}_2\mathrm{Cu}{}_3\mathrm{O}{}_{7-\delta }$ 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 ${\xi }_{c0\mathrm{}}$) and an energy scale ${\Delta }^{*}$. This allows one to address the nature of the pseudogap in favor of incoherent pairing.