Stability of nodal quasiparticles in underdoped YBa2Cu3O6+y probed by penetration depth and microwave spectroscopy

TL;DR

This study uses high-resolution measurements of superfluid density and microwave conductivity to investigate the low-energy quasiparticle excitations in underdoped YBCO, supporting a pure d-wave superconductivity with some disorder.

Contribution

It provides experimental evidence favoring pure d-wave superconductivity in underdoped YBCO and rules out many competing states based on low-temperature quasiparticle data.

Findings

Data are consistent with pure d-wave superconductivity with strong scattering disorder.

Most competing states are ruled out or limited by the disorder energy scale (~4 K).

Spin and charge density orders do not significantly alter the nodal quasiparticle spectrum.

Abstract

High resolution measurements of superfluid density and broadband quasiparticle conductivity have been used to probe the low energy excitation spectrum of nodal quasiparticles in underdoped YBCO. Penetration depth is measured to temperatures as low as 0.05 K. Microwave conductivity is measured from 0.1 to 20 GHz and is a direct probe of zero-energy quasiparticles. The data are compared with predictions for a number of theoretical scenarios that compete with or otherwise modify pure d-wave superconductivity, in particular commensurate and incommensurate spin and charge density waves; d + i s and d + i d superconductivity; circulating current phases; and the BCS--BEC crossover. We conclude that the data are consistent with a pure d-wave state in the presence of a small amount of strong scattering disorder, and are able to rule out most candidate competing states either completely, or to a level set by the energy scale of the disorder, ~ 4 K. Commensurate spin and charge density orders, however, are not expected to alter the nodal spectrum and therefore cannot be excluded.