title: Topological characterization of quantum phase transitions in a spin-1/2 model.
abstract: 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 noninteracting Majorana fermions with Z2 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 (SOP). In appropriate dual representations, these SOP become local order parameters and the basic concept of Landau theory of continuous phase transition can be applied.
authors.affiliation: Institute of Theoretical Physics, Chinese Academy of Sciences, P.O. Box 2735, Beijing 100080, China.
authors.forename: Xiao-Yong
authors.initials: XY
authors.lastname: Feng
authors.forename: Guang-Ming
authors.initials: GM
authors.lastname: Zhang
authors.forename: Tao
authors.initials: T
authors.lastname: Xiang