Stability of nodal quasiparticles in underdoped YBa₂Cu₃O_6+y probed by penetration depth and microwave spectroscopy

High-resolution measurements of superfluid density ρ_s(T) and broadband quasiparticle conductivity σ₁(Ω) have been used to probe the low-energy excitation spectrum of nodal quasiparticles in underdoped YBa₂Cu₃O_6+y. Penetration depth λ(T) is measured to temperatures as low as 0.05 K. σ₁(Ω) 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_x²−y² superconductivity, in particular, commensurate and incommensurate spin and charge-density waves; d_x²−y²+is and d_x²−y²+id_xy superconductivity; circulating current phases; and the BCS-BEC crossover. We conclude that the data are consistent with a pure d_x²−y² 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, T_d∼4 K. Commensurate spin and charge-density orders, however, are not expected to alter the nodal spectrum and therefore cannot be excluded.