Microscopic evidence for the intra-unit-cell electronic nematicity inside the pseudogap phase in YBa2Cu4O8

Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-Tsuperconductors.Here,we reportO nuclear magnetic resonance(NMR) studies on two planar oxygen sites in stoichio?metric cuprate YBaCuOto investigate the symmetry breaking inside the pseudogap phase.We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below T～185 K,which is close to the pseudogap temperature T*.Our result provides a microscopic evidence for intra-unit-cell electronic nematicity.The difference in quadrupole resonance frequency between the two oxygen sites is unchanged below T,which suggests that the observed nematicity does not directly stem from the local charge density modulation.Furthermore,a short-range charge density wave(CDW) order is observed below T(?)150 K.The additional broadening in theO-NMR spectra because of this CDW order is determined to be inequivalent for the two oxygen sites,which is similar to that observed in case of nematicity.These results suggest a possible connection between nematicity,CDW order,and pseudogap.