On the flux phase conjecture at half-filling: An improved proof

We present a simplification of Lieb's proof of the flux phase conjecture for interacting fermion systems—such as the Hubbard model—at half-filling on a general class of graphs. The main ingredient is a procedure which transforms a class of fermionic Hamiltonians into reflection-positive form. The method can also be applied to other problems, which we briefly illustrate with two examples concerning thet–V model and an extended Falicov-Kimball model.