title: Onset of a Boson mode at the superconducting critical point of underdoped YBa2Cu3Oy.
abstract: The thermal conductivity kappa of underdoped YBa2Cu3Oy was measured in the T-->0 limit as a function of hole concentration p across the superconducting critical point at pSC identical with 5.0%. The evolution of bosonic and fermionic contributions to kappa was tracked as the doping level evolved continuously in each of our samples. For p< or =pSC, we observe a T3 component in kappa which we attribute to the boson excitations of a phase with long-range spin or charge order. Fermionic transport, observed as a T-linear term in kappa which persists unaltered through pSC, violates the Wiedemann-Franz law, since the electrical resistivity varies as log(1/T) and grows with decreasing p.
authors.affiliation: Département de physique and RQMP, Université de Sherbrooke, Sherbrooke, Canada.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Doiron-Leyraud
authors.forename: Mike
authors.initials: M
authors.lastname: Sutherland
authors.forename: S Y
authors.initials: SY
authors.lastname: Li
authors.forename: Louis
authors.initials: L
authors.lastname: Taillefer
authors.forename: Ruixing
authors.initials: R
authors.lastname: Liang
authors.forename: D A
authors.initials: DA
authors.lastname: Bonn
authors.forename: W N
authors.initials: WN
authors.lastname: Hardy