Universality classes of order parameters composed of many body bound states

This theoretical paper discusses microscopic models giving rise to special types of order in which conduction electrons are bound together with localized spins to create composite order parameters. It is shown that composite order is related to the formation of a spin liquid with gapped excitations carrying quantum numbers which are a fraction of those of electron. These spin liquids are special in the sense that their formation necessarily involves spin degrees of freedom of both the conduction and the localized electrons. A detailed description of such spin liquid states is presented with a special emphasize on a demonstration of their robustness against local perturbations preserving the Lie group symmetry and the translational invariance.