Microscopic evidence for the intra-unit-cell electronic nematicity inside the pseudogap phase in YBa${}_2$Cu${}_4$O${}_8$

Abstract

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 ${}^{17}$O nuclear magnetic resonance (NMR) studies on two planar oxygen sites in stoichiometric 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 $T_{\mathrm{nem}}\sim$ 185 K, which is close to the pseudogap temperature $T^{\ast }$. 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 $T_{\mathrm{nem}}$, 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 $T\simeq$ 150 K. The additional broadening in the ${}^{17}$O-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.

14 pages, 5 figures