Topological Spin Excitations Without Spin-Orbit Coupling

The​ concept of topology in condensed matter physics ​has ​won the 2016 noble prize​ in physics​.​ Usually, topological band theory is attributed to electronic systems only. ​  Recently, researchers have extended ​the concept of topological condensed matter​  to non-electronic systems such as spin waves  in quantum magnets​ (magnon)​ and mechanical waves​ ​in solids​ (phonon)​. In this talk, I will give a general exposition of topological magnon insulators and magnon Hall effect in quantum magnetic systems, in which the Dzyaloshinskii-Moriya interaction (DMI) is the driving force similar to  spin-orbit coupling (SOC) in electronic systems.  I will also discuss recent experimental realizations of this phenomenon. In the last part of the talk, I will show that the DMI​ or SOC​ is not the primary​ source​ of topological spin excitations in magnetic systems with spin frustrations. ​This interaction can be zero and topological spin excitation still persists. This is in sharp contrast to previous ​experimental reports in ​insulating ​ferromagnet​s​​  Lu2V2O7 and Cu(1-3, bdc).