Nernst effect in the cuprate superconductor YBa₂Cu ₃O_y: Broken rotational and translational symmetries

The Nernst coefficient of the cuprate superconductor YBa₂Cu ₃O_y was recently shown to become strongly anisotropic within the basal plane when cooled below the pseudogap temperature T, revealing that the pseudogap phase breaks the fourfold rotational symmetry of the CuO ₂ planes. Here we report on the evolution of this Nernst anisotropy at low temperature, once superconductivity is suppressed by a magnetic field. We find that the anisotropy drops rapidly below 80 K, to vanish in the T=0 limit. We show that this loss of anisotropy is due to the emergence of a small high-mobility electronlike pocket in the Fermi surface at low temperature, a reconstruction attributed to a low-temperature state that breaks the translational symmetry of the CuO₂ planes. We discuss the sequence of broken symmetries-first rotational, then translational-in terms of an electronic nematic-to-smectic transition such as could arise when unidirectional spin or charge modulations order. We compare YBa₂Cu ₃O_y with iron-pnictide superconductors where the process of (unidirectional) antiferromagnetic ordering gives rises to the same sequence of broken symmetries.