Numerical renormalization group study of the one-dimensional Kondo insulator

We have studied the one-dimensional Kondo chain at half filling using a density matrix formulation of the numerical renormalization group. The charge gap is larger than the spin gap for all antiferromagnetic values of the exhange coupling J. A new type of excitation, a neutral spin singlet, consists of a particle and a hole which are repulsive for J{}5t and attractive for J{}5t. As J{→}0, RKKY interactions become more important and the staggered susceptibility {χ}(q=2kF) diverges. We have also studied {χ}(q) and the dispersion of the low-lying spin excitations as a function of wave vector q.