Divergent Nematic Susceptibility near the Pseudogap Critical Point in a Cuprate Superconductor

Abstract

Superconductivity is a quantum phenomenon caused by bound pairs of electrons.\nIn diverse families of strongly correlated electron systems, the electron pairs\nare not bound together by phonon exchange but instead by some other kind of\nbosonic fluctuations. In these systems, superconductivity is often found near a\nmagnetic quantum critical point (QCP) where a magnetic phase vanishes in the\nzero-temperature limit. Moreover, the maximum of superconducting transition\ntemperature Tc frequently locates near the magnetic QCP, suggesting that the\nproliferation of critical spin fluctuations emanating from the QCP plays an\nimportant role in Cooper pairing. In cuprate superconductors, however, the\nsuperconducting dome is usually separated from the antiferromagnetic phase and\nTc attains its maximum value near the verge of enigmatic pseudogap state that\nappears below doping-dependent temperature T*. Thus a clue to the pairing\nmechanism resides in the pseudogap and associated anomalous transport\nproperties. Recent experiments suggested a phase transition at T*, yet, most\nimportantly, relevant fluctuations associated with the pseudogap have not been\nidentified. Here we report on direct observations of enhanced nematic\nfluctuations in (Bi,Pb)2Sr2CaCu2O8+d by elastoresistance measurements, which\ncouple to twofold in-plane electronic anisotropy, i.e. electronic nematicity.\nThe nematic susceptibility shows Curie-Weiss-like temperature dependence above\nT*, and an anomaly at T* evidences a second-order transition with broken\nrotational symmetry. Near the pseudogap end point, where Tc is not far from its\npeak in the superconducting dome, nematic susceptibility becomes singular and\ndivergent, indicating the presence of a nematic QCP. This signifies quantum\ncritical fluctuations of a nematic order, which has emerging links to the\nhigh-Tc superconductivity and strange metallic behaviours in cuprates.\n