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

TL;DR

This study provides microscopic evidence of intra-unit-cell electronic nematicity inside the pseudogap phase of YBa₂Cu₄O₈, revealing symmetry breaking and its relation to charge density wave order in high-temperature superconductors.

Contribution

The paper presents direct NMR evidence of intra-unit-cell nematicity in a cuprate superconductor, linking it to pseudogap phenomena and charge density wave order for the first time.

Findings

Identical Knight shifts at high T, diverging below T_nem~185 K

Nematicity not caused by local charge density modulation

Charge density wave order observed below 150 K

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 $T_{\rm nem} \sim$ 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 $T_{\rm nem}$, 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.