Thermodynamic evidence for nematic phase transition at the onset of pseudogap in cuprates

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_pg. 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_pg, showing a remarkable scaling behavior with respect to T/T_pg in a wide doping range. The analysis reveals that the rotational symmetry breaking (nematicity) sets in at T_pg 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_pg , providing direct evidence of the broken rotational symmetry. These firmly establish that the nematic phase transition is universal in high-T_c cuprates. Surprisingly, unlike YBCO, the diagonal nematicity along [110] direction develops in Hg1201. Furthermore, the development of the diagonal nematicity is suppressed below the CDW order, implying that the nematicity competes with CDW.

In collaboration with Y. Sato, S. Kasahara, H. Murayama, Y. Kasahara (Kyoto Univ.), T. Shibauchi (Univ. of Tokyo), E.G. Moon (KAIST), T. Nishizaki (Kyushu Sangyo Univ.), T. Loew, J. Porra , B. Keimer (Max Planck), Y. Uchiyama (JASRI), and A. Yamamoto (RIKEN).