No nematicity at the onset temperature of the pseudogap phase in the cuprate superconductor YBCO

TL;DR

This study finds no evidence of nematicity at the pseudogap onset temperature in YBCO cuprates, but detects nematicity at lower temperatures linked to charge order development.

Contribution

The paper provides direct transport measurements showing the absence of nematicity at the pseudogap onset, clarifying its relation to the phase diagram of cuprate superconductors.

Findings

No additional in-plane anisotropy at pseudogap onset

Strong anisotropy appears at lower temperatures

Nematicity is associated with charge order, not pseudogap onset

Abstract

Electronic nematicity is the spontaneous loss of rotational symmetry in a metal, without breaking translational symmetry. In the cuprate superconductors, there is experimental evidence for nematicity, but its origin remains unclear. Here we investigate the onset of nematicity in the transport of charge by means of electric and thermoelectric measurements in underdoped YBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm y}$, performed by passing the current (electrical or thermal) first along the $a$ axis then the $b$ axis of the orthorhombic structure in the same crystal, with a hole doping $p = 0.12$. Upon cooling, we observe no additional in-plane anisotropy -- beyond the background anisotropy due to the CuO chains -- in either the resistivity $\rho$ or the Seebeck coefficient $S$ as the temperature $T^{\star}$~for the onset of the pseudogap phase is crossed. We conclude that the pseudogap phase of cuprates is not nematic. However, at temperatures much lower than $T^{\star}$, a strong additional anisotropy is observed, most clearly in the Peltier coefficient $\alpha = S / \rho$. We interpret it as nematicity associated with the development of charge order.