Topological triply-degenerate points in ultracold atoms and photonic hyperbolic metamaterials*

Topological states of matter provide a fertile ground for discovering new quasiparticles in condensed matter physics, such as Weyl and Dirac fermions, which were originally predicted in high-energy physics and recently observed in solid-state materials. In topological semimetals, Weyl and Dirac points correspond to two- and fourfold degenerate linear band crossing points, hallmarks of relativistic particles with half-integer spins. Remarkably, the recent discovery of triply degenerate points (TDPs) in semimetals has opened an avenue for exploring new types of quasiparticles that have no analog in quantum field theory. Such TDPs possess effective integer spins while preserving Fermi statistics and linear dispersions. In previous studies, the emergence of such protected nodes was often attributed to spin-vector-momentum couplings. In this talk, I will discuss three new types of TDPs that are classified by different monopole charges (C = +/- 2, 1, 0) and induced by the interplay between spin-tensor- and spin-vector-momentum couplings. I will discuss how to realize these new types of TDPs using ultracold atoms in optical lattices and photons in non-Hermitian hyperbolic metamaterials. These proposed atomic and photonic systems may provide highly controllable platforms for exploring and engineering novel quasiparticles without counterparts in quantum field theory.

References:
[1] X.-W. Luo, K. Sun, C. Zhang, Spin-tensor--momentum-coupled Bose-Einstein condensates, Phys. Rev. Lett. 119, 193001 (2017).
[2] H. Hu, J. Hou, F. Zhang, C. Zhang, Topological Triply-Degenerate Points Induced by Spin-Tensor-Momentum Couplings, Phys. Rev. Lett. 120, 240401 (2018).
[3] H. Hu, C. Zhang, Spin-1 Topological Monopoles in Parameter Space of Ultracold Atoms, Phys. Rev. A 98, 013627 (2018).
[4] J. Hou, Z. Li, X.-W. Luo, Q. Gu, C. Zhang, Topological bands and triply-degenerate points in non-Hermitian hyperbolic metamaterials, arXiv:1808.06972