## Fractionalized Charge Excitations in a Spin Liquid on Partially Filled Pyrochlore Lattices

We study the Mott transition from a metal to cluster Mott insulators in the 1/4– and 1/8-filled pyrochlore lattice systems. It is shown that such Mott transitions can arise due to charge localization in clusters or in tetrahedron units, driven by the nearest-neighbor repulsive interaction. The resulting cluster Mott insulator is a quantum spin liquid with a spinon Fermi surface, but at the same time a novel fractionalized charge liquid with charge excitations carrying half the electron charge. There exist two emergent U(1) gauge fields or “photons” that mediate interactions between spinons and charge excitations, and between fractionalized charge excitations themselves, respectively. In particular, it is suggested that the emergent photons associated with the fractionalized charge excitations can be measured in x-ray scattering experiments. Various other experimental signatures of the exotic cluster Mott insulator are discussed in light of candidate materials with partially filled bands on the pyrochlore lattice.

Gang Chen, Hae-Young Kee, Yong Baek Kim

## Generic spin model for the honeycomb iridiates beyond the Kitaev limit

Recently, realizations of Kitaev physics have been sought in the A2IrO3 family of honeycomb iridates, originating from oxygen-mediated exchange through edge-shared octahedra. However, for the j = 1/2 Mott insulator in these materials exchange from direct d-orbital overlap is relevant, and it was proposed that a Heisenberg term should be added to the Kitaev model. Here we provide the generic nearest-neighbour spin Hamiltonian when both oxygen-mediated and direct overlap are present, containing a bond dependent off-diagonal exchange in addition to Heisenberg and Kitaev terms. We analyze this complete model using a combination of classical techniques and exact diagonalization. Near the Kitaev limit, we find new magnetic phases, 120 degree and incommensurate spiral order, as well as extended regions of zigzag and stripy order. Possible applications to Na2IrO3 and Li2IrO3 are discussed.

Jeffrey G. Rau1, Eric Kin-Ho Lee1, Hae-Young Kee1,2,*

1Department of Physics, University of Toronto, Ontario M5S 1A7, Canada, 2Canadian Institute for Advanced Research, Toronto, Ontario, Canada

## Semimetal and Topological Insulator in Perovskite Iridates

The two-dimensional layered perovskite Sr_{2}IrO_{4} was proposed to be a spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified on a narrow J_{eff}=1/2band due to strong spin-orbit coupling. On the other hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO_{3} remains metallic. To understand the physical origin of the metallic state and possible transitions to insulating phases, we construct a tight-binding model for SrIrO_{3}. The band structure possesses a line node made ofJ_{eff}=1/2 bands below the Fermi level. As a consequence, instability toward magnetic ordering is suppressed, and the system remains metallic. This line node, originating from the underlying crystal structure, turns into a pair of three-dimensional nodal points on the introduction of a staggered potential or spin-orbit coupling strength between alternating layers. Increasing this potential beyond a critical strength induces a transition to a strong topological insulator, followed by another transition to a normal band insulator. We propose that materials constructed with alternating Ir- and Rh-oxide layers along the (001) direction, such as Sr_{2}IrRhO_{6}, are candidates for a strong topological insulator.

Jean-Michel Carter^{1}, V. Vijay Shankar^{1}, M. Ahsan Zeb^{2}, and Hae-Young Kee^{1,3,*}

^{1}Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7 Canada

^{2}Cavendish Laboratory, Cambridge University, Cambridge, United Kingdom

^{3}Canadian Institute for Advanced Research, Toronto, Ontario, Canada