HUBBARD-MODEL PREDICTIONS FOR THE KNIGHT-SHIFT IN CUPRATES

We investigate with Quantum Monte Carlo (QMC) simulations supplemented by Hartree-Fock (HF) calculations the static spin susceptibilities in a 2D tight-binding Hubbard model with attractive pairing interaction as the simplest lattice superconductor. The susceptibility is experime ntally accessible in the high-T-c, cuprates via measurements of the sp in part of the Knight shift K-s,. Whereas K-s is quite understood and similar to conventional expectations at the normal state - superconduc tor transition in case of the overdoped and optimally doped cuprates, the behavior of K-s is highly unconventional in the underdoped materia ls. A maximum in the susceptibility is observed at T-c in the overdope d materials, and a strong decrease below T-c, usually associated with a spin singlet - i.e. pair - formation. The position of this maximum s hifts to increasingly higher temperatures far above the superconductin g transition temperature T-c in case of the underdoped materials. We i nvestigate K-s in the frame of this simple lattice superconductor, and a short discussion on consequences for several other experimentally a ccessible quantities is given.