2018/09/30 by Darshan G. Joshi, Andreas P. Schnyder
Mathematics · Physics and Astronomy · #Advanced Condensed Matter Physics #Combinatorics #Mathematics #Paramagnetism #Physics #Quantum many-body systems #Quantum mechanics #Topological Materials and Phenomena #Topological index #Topological insulator #Topology (electrical circuits) #cond-mat.str-el
paper · pdf · doi:10.1103/physrevb.100.020407
published as Phys. Rev. B 100, 020407 (2019) · 10.5 pages; (v2) minor changes
openalex created_date 2018/09/27 · arxiv created 2019/03/11 · openalex publication_date 2019/07/26 · arxiv updated 2019/07/31 · openalex updated_date 2026/08/05
Topological quantum paramagnets are exotic states of matter, whose magnetic excitations have a topological band structure, while the ground state is topologically trivial. Here we show that a simple model of quantum spins on a honeycomb bilayer hosts a time-reversal-symmetry protected ℤ2 topological quantum paramagnet (topological triplon insulator) in the presence of spin-orbit coupling. The excitation spectrum of this quantum paramagnet consists of three triplon bands, two of which carry a nontrivial ℤ2 index. As a consequence, there appear two counterpropagating triplon excitation modes at the edge of the system. We compute the triplon edge state spectrum and the ℤ2 index for various parameter choices. We further show that upon making one of the Heisenberg couplings stronger, the system undergoes a topological quantum phase transition, where the ℤ2 index vanishes, to a different topological quantum paramagnet. In this case the counterpopagating triplon edge modes are disconnected from the bulk excitations and are protected by a chiral and a unitary symmetry. We discuss possible realizations of our model in real materials, in particular d4 Mott insulators, and their potential applications.