Wiedemann-Franz law in the underdoped cuprate superconductor YBa2Cu3 Oy
The electrical and thermal Hall conductivities of the cuprate superconductor YBa2Cu3Oy, Ïƒxy and Îºxy, were measured in a magnetic field up to 35 T, at a hole concentration (doping) p=0.11. In the T=0 limit, we find that the Wiedemann-Franz law, Îºxy/T=(Ï€2/3)(kB/e)2Ïƒxy, is satisfied for fields immediately above the vortex-melting field Hvs. This rules out the existence of a vortex liquid at T=0 and it puts a clear constraint on the nature of the normal state in underdoped cuprates, in a region of the doping phase diagram where charge-density-wave order is known to exist. As the temperature is raised, the Lorenz ratio, Lxy=Îºxy/(ÏƒxyT), decreases rapidly, indicating that strong small-q scattering processes are involved. Â© 2016 American Physical Society.