Topological quantum paramagnet in quantum spin systems
by Dr. Darshan Joshi (MPI Stuttgart)
Thursday, November 30, 2017 from to (Asia/Kolkata)
Topological quantum paramagnets are exotic states of matter with trivial paramagnetic ground states hosting topological excitations. We show that in the presence of spin-orbit coupling a quantum spin ladder hosts a topological quantum paramagnet (TQP). This phase has fractional excitations localized at the ends of the ladder. The excitation spectrum is topologically non-trivial and is characterized by a non-zero winding number. We discuss possible experimental signatures to detect this exotic phase. As another example, we show that a simple model of quantum spins on a honeycomb bilayer hosts a Z2 TQP in the presence of spin-orbit coupling. The Z2 invariant is the same as that in the case of the fermionic quantum spin Hall state. We further show that upon making one of the Heisenberg couplings stronger the system undergoes a topological quantum phase transition, where the Z2 invariant vanishes, to a different TQP. In this case the edge states are disconnected from the bulk excitations and the phase is characterized by a different topological invariant. This physics is amenable to experiments, where an anisotropic coupling can be induced under pressure.