Physics (Internal)

Topological spin excitations in a three-dimensional antiferromagnet

This event is part of the Biophysics/Condensed Matter Seminar Series.

The recent discovery of topological semimetals, which possess distinct electron-band crossing in the bulk and novel surface states, has stimulated intense research interest. By extending the notion of symmetry-protected band crossing into one of the simplest magnetic groups, namely by including the symmetry of time-reversal followed by space-inversion, we predict the existence of topological magnon-band crossing in three-dimensional antiferromagnets [1]. The crossing may take the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered-moment direction. In a concrete example of a Heisenberg spin model for a “spin-web” compound, Cu3TeO6, we theoretically demonstrate the presence of Dirac magnons over a wide parameter range using linear spin-wave approximation. Inelastic neutron scattering experiments have been carried out to detect the bulk magnon-band crossing in a single-crystal sample [2]. The highly interconnected nature of the spin-1/2 network suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data with very good agreement with the theoretical calculations. The predicted topological Dirac points are confirmed. Further studies will be discussed, including determination of non-collinear spin canting in the magnetic ground state of Cu3TeO6 and search for the topological magnon surface states.

[1] K. Li et al., PRL 119, 247202 (2017). [2] W. Yao et al., Nature Physics 14, 1011 (2018).