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 YBa2Cu4O8 is presented. The starting point is a three-band Hubbard model comprising a Kondo-like exchange between the copper and oxygen holes which gives rise to a new singlet correlated band. This band corresponds to the motion of Zhang-Rice singlets through the antiferromagnetic background of localized copper spins and exhibits a well-developed saddle point of strong copper character. The main contribution to the susceptibility comes from thermally activated copper holes in the extended saddle point of the singlet band. From an idealization of the corresponding density of states we derive formulae for the spin-lattice relaxation time T1 and the Knight shift K at the Cu(2) site. © 1994