“…As mentioned above, the first rigorous proof of the properties of the Haldane phase [39,40] (in particular the existence of a spectral gap) was given by Affleck, Kennedy, Lieb and Tasaki in a particular spin-1 VBS chain [12,11], which is by now called the AKLT-model, and it is fair to say that it served as a paradigm for many of the subsequent studies on massive quantum spin chains. In that paper the authors also gave a detailed analysis of some other VBS models [41,42,43,44,45,46,47], which had been studied in the literature before, and thus introduced a new class of quantum spin Hamiltonians for which exact ground states with non-trivial properties can be constructed. Since then various other VBS models were introduced [15,37,48,49,50], including some interesting two-dimensional models [51,52,53,54,55,56,57].…”