Broken rotational symmetry in the pseudogap phase of a high-Tc superconductor

The nature of the pseudogap phase is a central problem in the quest to understand high-$T_c$ cuprate superconductors. A fundamental question is what symmetries are broken when that phase sets in below a temperature $T^*$. Here we report the observation of a large in-plane anisotropy of the Nernst effect in YBa$_2$Cu$_3$O$_y$ that sets in precisely at $T^*$, throughout the doping phase diagram [1]. We show that the CuO chains of the orthorhombic lattice are not responsible for this anisotropy, which is therefore an intrinsic property of the CuO$_2$ planes. We conclude that the pseudogap phase is an electronic state which strongly breaks four-fold rotational symmetry.\\[4pt] [1] R. Daou et al., arXiv:0909.4430