Charged and spin-excitation gaps in half-filled strongly correlated electron systems: A rigorous result

By exploiting the particle-hole symmetries of the Hubbard model, the periodic Anderson model, and the Kondo lattice model at half-filling and by applying a generalized version of Lieb's spin-reflection positivity method, we show that the char,oed gaps of these models are always larger than their spin-excitation gaps. This theorem confirms the previous results derived by either the variational approach or the density-matrix renormalization-group approach. [S0163-1829(98)07336-6].Physics, Condensed MatterSCI(E)13ARTICLE127612-76185