Special Condensed Matter Seminar, Donna Sheng, Cal State U., "Chiral spin liquid and quantum phase diagram for extended Kagome Heisenberg model”

Wed, Dec 10, 2014, 1:30 pm to 2:30 pm
PCTS Seminar Room
The kagome spin-1/2 model with dominant nearest neighboring (J1) antiferromagnetic coupling had been proposed to host an exotic gapped Z2 spin liquid based on density matrix renormalization group study. Here we report a new finding that small perturbations from the second (J2) and third neighboring (J3) exchange couplings will lead to a time reversal symmetry breaking chiral spin liquid. Searching for the microscopic understanding of the emerging and collapsing of these phases, we study the quantum phase diagram and the interplay of J1-J2-J3 couplings in the kagome lattice model. For SU2 invariant model, we establish a rich phase diagram where a chiral spin liquid phase emerges between the magnetically ordered antiferromagnetic phase known as $q=(0,0)$ state and a complex non-coplanar ordered state with spins forming the vertices of a cuboctahedron known as a cuboc1 phase. We characterize the spontaneous time-reversal symmetry breaking chiral spin liquid as the Laughlin nu=1/2 bosonic fractional quantum Hall state proposed 20 years ago, based on topological Chern number and modular matrices of the state. The robustness of the chiral spin liquid persists into spin anisotropic model, including the pure XY model (where all the spin exchange interactions are XY interactions). We explore the nature of quantum phase transitions from chiral spin liquid to time-reversal invariant spin liquid and other magnetic ordered phases, and point to the possibility of the novel continuous transitions in such systems. We also reveal that there may be a gapless spin liquid state for nearest neighboring dominant spin anisotropic model, with the low energy singlet excitations as magneto-roton minimum of the system. We will also discuss the possible indications of the theoretical results to the experimental relevant frustrated kagome magnets.