Evidence of a critical hole concentration in underdoped YBa2Cu3Oy single crystals revealed by 63Cu NMR

We report a Cu-63 NMR investigation in detwinned YBa2Cu3Oy 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 Cu-63 uncover the emergence of static order at a well-defined onset temperature T-0 with an as yet unknown order parameter. 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 -> 0. Our data indicate the presence of a critical hole doping p(c) similar to 0.1, which may control both the static order at p < p(c) and the spin pseudogap at p > p(c). DOI: 10.1103/PhysRevB.86.220504