NMR in ``underdoped'' and ``overdoped'' YBaCuO compounds: fermi-liquid approach

We examine the NMR experimental data for the normal state of YBa$_2$Cu$_3$O$_{7-\delta}$ within the Fermi-liquid approach. We show that the observed temperature dependence of the Knight shift and $^{17}$O ($^{89}$Y) relaxation rate in ``underdoped'' ( $T_c=60\,$K ) compound can be interpreted as resulting from a peak in the density of states near the Fermi level. The possible origin of such peak is the quasi-one-dimensional van Hove singularity in the fermionic spectrum, conjectured by Abrikosov, Gofron and Campuzano. The proposed spectrum allows us to account for the qualitatively different NMR data for ``underdoped'' and ``overdoped''( $T_c=90\,$K ) compounds by varying the value of chemical potential only.