NMR and thermodynamic studies of cuprate high-T_c superconductors

Full text: NMR Knight shift and electronic specific heat measurements are presented for YBa₂Cu₃O_7-d and YBa₂Cu₄O₈ along with several other cuprates, both underdoped and overdoped. The remarkable quantitative correspondence between the spin susceptibility and the entropy shows that the thermodynamic properties are those of a weakly interacting Fermion system despite the obvious presence of strong correlations and other competing correlations such as the pseudogap and charge order. This places strong constraints on the as-yet unresolved theory of cuprate superconductivity. These measurements enable the phase diagram to be mapped in considerable detail and the features that emerge play an exceedingly important role in practical applications. (author)