Evidence of a critical hole concentration in underdoped YBa$_2$Cu$_3$O$_y$ single crystals revealed by $^{63}$Cu NMR

TL;DR

This study uses $^{63}$Cu NMR to identify a critical hole doping level in underdoped YBa$_2$Cu$_3$O$_y$ that influences static order and the spin pseudogap, revealing a phase boundary at $p_c\,\sim\,0.1$.

Contribution

It provides evidence for a critical hole concentration in underdoped YBa$_2$Cu$_3$O$_y$ that governs static order and pseudogap phenomena, a novel insight into the phase diagram.

Findings

Identification of a static order onset temperature $T_0$.

Detection of a critical doping level $p_c\sim 0.1$.

Correlation between static order and the spin pseudogap.

Abstract

We report a $^{63}$Cu NMR investigation in detwinned YBa$_2$Cu$_3$O$_y$ single crystals, focusing on the highly underdoped regime (y=6.35-6.6). Measurements of both the spectra and the spin-lattice relaxation rates of $^{63}$Cu uncover the emergence of static order at a well-defined onset temperature $T_0$ without a known order parameter as yet. While $T_0$ is rapidly suppressed with increasing hole doping concentration $p$, the spin pseudogap was identified only near and above the doping content at which $T_0\rightarrow 0$. Our data indicate the presence of a critical hole doping $p_c\sim 0.1$, which may control both the static order at $p<p_c$ and the spin pseudogap at $p>p_c$.