CHARGE-EXCITATION PICTURE OF CU NMR KNIGHT-SHIFT AND RELAXATION IN YBA2CU4O8 DEDUCED FROM A 3-BAND HUBBARD-MODEL

A simplified model of copper hole excitations in the singlet band of Y Ba2Cu4O8 is presented. The starting point is a three-band Hubbard mode l comprising a Kondo-like exchange between the copper and oxygen holes which gives rise to a new singlet correlated band. This band correspo nds to the motion of Zhang-Rice singlets through the antiferromagnetic background of localized copper spins and exhibits a well-developed sa ddle point of strong copper character. The main contribution to the su sceptibility comes from thermally activated copper holes in the extend ed saddle point of the singlet band. From an idealization of the corre sponding density of states we derive formulae for the spin-lattice rel axation time T-1 and the Knight shift K at the Cu(2) site.