LOW-ENERGY EXCITATION SPECTRUM AND CU(2) SPIN-LATTICE RELAXATION FROMTHE SINGLET CORRELATED BAND IN YBA2CU4O8

We report the low energy excitation spectrum and the spin-lattice rela xation for the planar Cu of the high T-c superconductor YBa2Cu4O8. We use a three band Hubbard model. Its singlet band exhibits a well-estab lished saddle point of strong copper character. The susceptibility is governed by a Fermi-Liquid like contribution coming from thermally act ivated copper holes in this saddle point. By comparing our calculated and experimental results, we can conclude that in YBa2Cu4O8 the low en ergy physics is determined by a high density of charge excitations in the singlet impurity band together with spin excitations in the lower Hubbard band.