Scale invariant transfer matrices and Hamiltionians

Given a direct system of Hilbert spaces s↦Hs (with isometric inclusion maps ιst:Hs→Ht for s⩽t) corresponding to quantum systems on scales s, we define notions of scale invariant and weakly scale invariant operators. In some cases of quantum spin chains we find conditions for transfer matrices and nearest neighbour Hamiltonians to be scale invariant or weakly so. Scale invariance forces spatial inhomogeneity of the spectral parameter. But weakly scale invariant transfer matrices may be spatially homogeneous in which case the change of spectral parameter from one scale to another is governed by a classical dynamical system exhibiting fractal behaviour.