Phase Diagram of the Spin-1/2 Kitaev-Gamma Chain and Emergent SU(2) Symmetry

We study the phase diagram of a one-dimensional version of the Kitaev spin-1/2 model with an extra T term, using analytical, density matrix renormalization group and exact diagonalization methods. Two intriguing phases are found. In the gapless phase, although the exact symmetry group of the system is discrete, the low energy theory is described by an emergent SU(2)(1) Wess-Zumino-Witten (WZW) model. On the other hand, the spin-spin correlation functions exhibit SU(2) breaking prefactors, even though the exponents and the logarithmic corrections are consistent with the SU(2)(1) predictions. A modified non-Abelian bosonization formula is proposed to capture such exotic emergent partial SU(2) symmetry. In the ordered phase, there is numerical evidence for an O-h -> D-4 spontaneous symmetry breaking.