Quantum Materials

Kee group’s research interests are Electronic Nematic Liquid, Topological Insulators, Frustrated Magnetic Systems, High Temperature Superconductors More »

Open Positions with Our Group

Open Positions for Postdoctoral, Graduate and Undergraduate Students. If interested contact: hykee@physics.utoronto.ca More »


Theory Group of Quantum Materials at the University of Toronto

Designing materials to achieve functional goals is one of the major challenges of modern condensed matter physics. To attain the ability to synthesize and control new materials, a careful consideration of how the different physical degrees of freedom such as charge, spin, orbital, and lattice, tune the properties of materials is required. The long term goal of our research is to achieve a theoretical understanding of the delicate balance among charge, spin, lattice and orbital degrees of freedom in complex materials. A few examples that we study include high temperature superconductors, topological insulators, electronic liquid crystalline materials, frustrated quantum magnets, and ultra-cold atom systems.

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 Sr2IrO4 was proposed to be a spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified on a narrow Jeff=1/2band due to strong spin-orbit coupling. On the other hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO3 remains metallic. To understand the physical origin of the metallic state and possible transitions to insulating phases, we construct a tight-binding model for SrIrO3. The band structure possesses a line node made ofJeff=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 Sr2IrRhO6, are candidates for a strong topological insulator.

Jean-Michel Carter1, V. Vijay Shankar1, M. Ahsan Zeb2, and Hae-Young Kee1,3,*
1Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7 Canada
2Cavendish Laboratory, Cambridge University, Cambridge, United Kingdom
3Canadian Institute for Advanced Research, Toronto, Ontario, Canada