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.

Topological crystalline metal in orthorhombic perovskite iridates

Since topological insulators were theoretically predicted and experimentally observed in semiconductors with strong spin–orbit coupling, increasing attention has been drawn to topological materials that host exotic surface states. These surface excitations are stable against perturbations since they are protected by global or spatial/lattice symmetries. Following the success in achieving various topological insulators, a tempting challenge now is to search for metallic materials with novel topological properties. Here we predict that orthorhombic perovskite iridates realize a new class of metals dubbed topological crystalline metals, which support zero-energy surface states protected by certain lattice symmetry. These surface states can be probed by photoemission and tunnelling experiments. Furthermore, we show that by applying magnetic fields, the topological crystalline metal can be driven into other topological metallic phases, with different topological properties and surface states.

Yige Chen, Yuan-Ming Lu, Hae-Young Kee

Odd-Parity Triplet Superconducting Phase in Multiorbital Materials with a Strong Spin-Orbit Coupling: Application to Doped Sr2IrO4

We explore possible superconducting states in t2g multiorbital correlated electron systems with strong spin-orbit coupling (SOC). In order to study such systems in a controlled manner, we employ large-scale dynamical mean-field theory (DMFT) simulations with the hybridization expansion continuous-time quantum Monte Carlo (CTQMC) impurity solver. To determine the pairing symmetry, we go beyond the local DMFT formalism using parquet equations to introduce the momentum dependence in the two-particle vertex and correlation functions. In the strong SOC limit, a singlet, d-wave pairing state in the electron-doped side of the phase diagram is observed at weak Hund’s coupling, which is triggered by antiferromagnetic fluctuations. When the Hund’s coupling is comparable to SOC, a twofold degenerate, triplet p-wave pairing state with relatively high transition temperature emerges in the hole-doped side of the phase diagram, which is associated with enhanced charge fluctuations. Experimental implications to doped Sr2IrO4 are discussed.

Zi Yang Meng, Yong Baek Kim, Hae-Young Kee

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