Physics

Dr. Darshan Joshi
Max Plank Institute for Solid State Research ,Stuttgart, Germany

Abstract:
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  
Z_2 TQP in the presence of spin-orbit coupling. The Z_2 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 Z_2 invariant vanishes, to a different topological quantum  
paramagnet. 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.