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Symmetry plays a crucial role in condensed matter physics. While the magnetic ordering breaks the time-reversal symmetry, the lack of the spatial inversion symmetry in magnetic materials gives rise the antisymmetric coupling. In nearly all cases, this antisymmetric coupling can be described as Dzyaloshinskii–Moriya (DM) interaction. However, in α-Cu₂V₂O₇, a large splitting of the nonreciprocal magnons has been observed in the first time. The detailed inelastic neutron scattering study clearly demonstrated the splitting related to the opposite magnon circular [1]. The spin wave calculation shows an extremely large antisymmetric coupling (~ 2.79 meV). This value is comparable to exchange J and is 1-2 order of magnitude larger than normal DM interaction. This is indicative of either an enormously large spin-orbital coupling (Δg / g ~ 1), or is a new type of the antisymmetric magnetic interaction which its true origin is still unknown. The last but not the least, the result is the first reported direct observation of the magnon circular birefringence.
[1] G. Gitgeatpong, Y. Zhao, P. Piyawongwatthana, Y. Qiu, L. W. Harriger, N. P. Butch, T. J. Sato, and K. Matan, Phys. Rev. Lett. 119, 47201 (2017).