The Hae-Young Kee Group | University of Toronto

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.

Hidden spin liquid in an antiferromagnet: Applications to FeCrAs

July 7, 2012 2011, Physical Review B, Publications

The recently studied material FeCrAs exhibits a surprising combination of experimental signatures, with metallic, Fermi liquid like specic heat but resistivity showing strong non-metallic character. The Cr sublattice posseses local magnetic moments, in the form of stacked (distorted) Kagome lattices. Despite the high degree of magnetic frustration, anti-ferromagnetic order develops below TN 125K suggesting the non-magnetic Fe sublattice may play a role in stabilizing the ordering. From the material properties we propose a microscopic Hamiltonian for the low energy degrees of freedom, including the non-magnetic Fe sublattice, and study its properties using slaverotor mean eld theory. Using this approach we nd a spin liquid phase on the Fe sublattice, which survives even in the presence of the magnetic Cr sublattice. Finally, we suggest that the features of FeCrAs can be qualitatively explained by critical flutuations in the non-magnetic sublattice Fe due to proximity to a metal-insulator transition.

Jerey G. Rau and Hae-Young Kee
Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada

Semimetal and Topological Insulator in Perovskite Iridates

March 7, 2012 2012, Physical Review B, Publications

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 J_eff=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 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 Sr₂IrRhO₆, are candidates for a strong topological insulator.

Jean-Michel Carter¹, V. Vijay Shankar¹, M. Ahsan Zeb², and Hae-Young Kee^1,3,*
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

Hidden and antiferromagnetic order as a rank-5 superspin in URu2Si2

March 5, 2012 2012, Publications

We propose a candidate for the hidden order in URu2Si2: a rank-5 E type spin density wave between Uranium 5f crystal field doublets breaking time reversal and lattice tetragonal symmetry in a manner consistent with recent torque measurements [R. Okazaki et al, Science 331, 439 (2011)]. We argue that coupling of this order parameter to magnetic probes can be hidden by crystal field effects, while still having significant effects on transport, thermodynamics and magnetic susceptibilities. In a simple tight-binding model for the heavy quasiparticles, we show the connection between the hidden order and antiferromagnetic phases arises since they form different components of this single rank-5 pseudo-spin vector. Using a phenomenological theory, we show the experimental pressure-temperature phase diagram can be qualitatively reproduced by tuning terms which break pseudo-spin rotational symmetry. As a test of our proposal, we predict the presence of small magnetic moments in the basal plane oriented in the [110] direction ordered at the wave-vector (0,0,1).

Jerey G. Rau1 and Hae-Young Kee1, 2
1Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
2Canadian Institute for Advanced Research/Quantum Materials Program, Toronto, Ontario MSG 1Z8, Canada