Abstract
We examine the NMR experimental data for the normal state of YBa 2Cu 3O 7−σ within the Fermi-liquid approach. We explain the temperaturer dependence of the knight shift as well as 17O ( 89Y) relaxation rate in “underdoped” ( T c = 60 K) compound as stemming from a strong variation of the density of states (DOS) near the Fermi level. We extract the small-energy DOS structure from the data on the Knight shift. The proposed origin of this structure is the van Hove singularity resulted from the extended saddle point in fermionic spectrum. Such 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. We argue that the AF fluctuations, present at copper sites, should be regarded as an almost independent source of relaxation and exist on the Fermi-liquid background.
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