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-T_c superconductors. Here, we report ¹⁷O nuclear magnetic resonance (NMR) studies on two planar oxygen sites in stoichiometric cuprate YBa₂Cu₄O₈ 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.