Topological characterization of quantum phase transitions in a S=1/2 spin model

We have introduced a novel Majorana representation of S=1/2 spins using the Jordan-Wigner transformation and have shown that a generalized spin model of Kitaev defined on a brick-wall lattice is equivalent to a model of non-interacting Majorana fermions with Z_2 gauge fields without redundant degrees of freedom. The quantum phase transitions of the system at zero temperature are found to be of topological type and can be characterized by nonlocal string order parameters. In appropriate dual representations, these string order parameters become local order parameters and the basic concept of Landau theory of continuous phase transition can be applied.