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Understanding the nature of the mysterious pseudogap phenomenon is one of themost important issues associated with cuprate high-$T_c$ superconductors. Here,we report $^{17}$O nuclear magnetic resonance (NMR) studies on two planaroxygen sites in stoichiometric cuprate YBa$_2$Cu$_4$O$_8$ to investigate thesymmetry breaking inside the pseudogap phase. We observe that the Knight shiftsof the two oxygen sites are identical at high temperatures but different below$T_{\rm nem} \sim$ 185 K, which is close to the pseudogap temperature$T^{\ast}$. Our result provides a microscopic evidence for intra-unit-cellelectronic nematicity. The difference in quadrupole resonance frequency betweenthe two oxygen sites is unchanged below $T_{\rm nem}$, which suggests that theobserved nematicity does not directly stem from the local charge densitymodulation. Furthermore, a short-range charge density wave (CDW) order isobserved below $T \simeq$ 150 K. The additional broadening in the $^{17}$O-NMRspectra because of this CDW order is determined to be inequivalent for the twooxygen sites, which is similar to that observed in case of nematicity. Theseresults suggest a possible connection between nematicity, CDW order, andpseudogap.

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