  The recent detection of charge-density modulations in YBa2Cu3Oy and other
cuprate superconductors raises new questions about the normal state of
underdoped cuprates. In one class of theories, the modulations are intertwined
with pairing in a dual state, expected to persist up to high magnetic fields as
a vortex liquid. In support of such a state, specific heat and magnetisation
data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting
above the vortex-melting field Hvs at T = 0. Here we report high-field
measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54
that allow us to probe the Wiedemann-Franz law, a sensitive test of the
presence of superconductivity in a metal. In the T = 0 limit, we find that the
law is satisfied for fields immediately above Hvs. This rules out the existence
of a vortex liquid and it places strict constraints on the nature of the normal
state in underdoped cuprates.
