Decay of correlations in 2D quantum systems with continuous symmetry
  We study a large class of models of two-dimensional quantum lattice systems with continuous symmetries, and we prove a general McBryan-Spencer-Koma-Tasaki theorem concerning algebraic decay of correlations. We present applications of our main result to the Heisenberg-, Hubbard-, and t-J models, and to certain models of random loops. 
Costanza Benassi, Juerg Froehlich, Daniel Ueltschi
math-ph math.MP