Thermal Conductivity across the Phase Diagram of Cuprates: Low-Energy Quasiparticles and Doping Dependence of the Superconducting Gap

TL;DR

This study investigates heat transport in cuprate superconductors across different doping levels, revealing that low-energy quasiparticles and the superconducting gap maintain a consistent d-wave form, closely tracking the pseudogap, and showing minimal doping dependence.

Contribution

It provides comprehensive doping-dependent measurements of thermal conductivity, demonstrating the persistent d-wave gap structure and its relation to the pseudogap in cuprates.

Findings

Residual linear thermal conductivity decreases with underdoping.

Superconducting gap scales with the pseudogap.

Quasiparticle effective charge remains near unity across doping.

Abstract

Heat transport in the cuprate superconductors YBa$_2$Cu$_3$O$_{y}$ and La$_{2-x}$Sr$_x$CuO$_4$ was measured at low temperatures as a function of doping. A residual linear term kappa_{0}/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from kappa_{0}/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.