SUSCEPTIBILITY AND KNIGHT-SHIFT ANOMALIES IN CUPRATE SUPERCONDUCTORS

The unconventional temperature variation of the static susceptibility χ(T) that has been discovered in various copper oxide superconductors is explained in terms of a model density of states that has a step shape at an energy threshold E0 along with a logarithmic Van Hove singularity at the same E0. Calculations of χ(T) and the Knight shift above the superconducting transition temperature Tc yield good fits to the YBCO, BSCCO, and LSCO data by adjusting only the Fermi energy μ in correspondence to the oxygen or Sr content, respectively. When μ is right on or slightly below the Van Hove singularity, an upturn in χ occurs as the temperature T is lowered. By contrast, when μ is slightly above the threshold energy E0, a downturn in χ is achieved as T is lowered. A correlation of these phenomena with experimental data provides insight into the proximity of the Van Hove singularity to μ in several cuprate superconductors. The YBCO and TBCO cuprates with the higher Tc values exhibit a nearly constant susceptibility that suggests a Fermi energy well removed from the Van Hove singularity. The sensitivity of Tc as well as the susceptibility to chemical changes may provide tests of electronic mechanisms of electron pairing as well as the BCS theory. © 1995 The American Physical Society.