Onset of a boson mode at superconducting critical point of underdoped YBa2Cu3Oy

TL;DR

This study measures the low-temperature thermal conductivity of underdoped YBa2Cu3Oy to investigate the emergence of bosonic excitations at the superconducting critical point, revealing a phase with long-range order and a thermal metallic state.

Contribution

It provides direct experimental evidence of a bosonic mode onset at the superconducting critical point in underdoped cuprates, highlighting a phase with long-range order and a violation of the Wiedemann-Franz law.

Findings

Observation of a T^3 contribution to thermal conductivity indicating bosonic excitations.

Persistence of fermionic thermal transport across the critical point, indicating a thermal metal.

Violation of the Wiedemann-Franz law in the phase below the critical doping.

Abstract

The thermal conductivity $\kappa$ of underdoped \Y was measured in the $T \to 0$ limit as a function of hole concentration $p$ across the superconducting critical point at $p_{SC}$ = 5.0%. ``Time doping'' was used to resolve the evolution of bosonic and fermionic contributions with high accuracy. For $p \leqslant p_{SC}$, we observe an additional $T^3$ contribution to $\kappa$ which we attribute to the boson excitations of a phase with long-range spin or charge order. Fermionic transport, manifest as a linear term in $\kappa$, is seen to persist unaltered through $p_{SC}$, showing that the state just below $p_{SC}$ is a thermal metal. In this state, the electrical resistivity varies as log$(1/T)$ and the Wiedemann-Franz law is violated.