Thermodynamic evidence for a nematic phase transition at the onset of pseudogap in YBCO and Hg1201

Abstract

A long-standing controversial issue in the quest to understand the superconductivity in cuprates is the nature of the enigmatic pseudogap region of the phase diagram. Especially important is whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature T*. Here we report torque-magnetometry measurements of anisotropic susceptibility within the ab planes in orthorhombic YBCO with exceptionally high precision. The in-plane anisotropy along [100] direction (Cu-O-Cu direction) displays a significant increase with a distinct kink at T*, showing a remarkable scaling behavior with respect to T/T* in a wide doping range. The analysis reveals that the rotational symmetry breaking (nematicity) sets in at T* in the limit where the effect of orthorhombicity is eliminated. We also performed the same measurements on simple tetragonal Hg1201 with single CuO2 layer. Two-fold susceptibility anisotropy emerges spontaneously at T*, providing direct evidence of the broken rotational symmetry. These firmly establish that the nematic phase transition is universal in high-Tc cuprates. Surprisingly, unlike YBCO, the diagonal nematicity along [110] direction develops in Hg1201. Furthermore, the nematicity in Hg1201 appears to compete with CDW.