cuprate superconductors

In cuprate high-T_c superconductors, enigmatic pseudogap state is considered to be a clue to its pairing mechanism. Recently electronic nematicity, characterized by four-fold rotational symmetry breaking in CuO₂ plane, has emerged as a key signature inside pseudogap region. Here we report on elastoresistance measurements in Pb-substituted Bi₂Sr₂CaCu₂O_8+δ, which allow us to evaluate the nematic susceptibility from the change in in-plane resistivity induced by uniaxial strain. The nematic susceptibility shows Curie-Weiss-like enhancement toward pseudogap temperature T^*, evidencing a second-order phase transition with rotational symmetry breaking. Near pseudogap critical point the Weiss temperature changes its sign and the magnitude of nematic susceptibility is strongly enhanced. This indicates the nematic quantum-critical fluctuations in cuprate superconductors.