Microscopic evidence for the intra-unit-cell electronic nematicity inside the pseudogap phase in YBa_(2)Cu_(4)O_(8)

Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-T_(c) superconductors.Here,we report 17O nuclear magnetic resonance(NMR) studies on two planar oxygen sites in stoichio?metric cuprate YBa_(2)Cu_(4)O_(8) to 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_(nem) ~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_(nem),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 the 17O-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.