The Nernst coefficient of the cuprate superconductor YBa2Cu3Oy was recentlyshown to become strongly anisotropic within the basal plane when cooled belowthe pseudogap temperature T*, revealing that the pseudogap phase breaks thefour-fold rotational symmetry of the CuO2 planes. Here we report on theevolution of this Nernst anisotropy at low temperature, once superconductivityis suppressed by a magnetic field. We find that the anisotropy drops rapidlybelow 80 K, to vanish in the T=0 limit. We show that this loss of anisotropy isdue to the emergence of a small high-mobility electron-like pocket in the Fermisurface at low temperature, a reconstruction attributed to a low-temperaturestate that breaks the translational symmetry of the CuO2 planes. We discuss thesequence of broken symmetries - first rotational, then translational - in termsof an electronic nematic-to-smectic transition such as could arise whenunidirectional spin or charge modulations order. We compare YBa2Cu3Oy withiron-pnictide superconductors where the process of (unidirectional)antiferromagnetic ordering gives rises to the same sequence of brokensymmetries.
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