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arXiv · 1702.04889

Nonreciprocal magnons and symmetry-breaking in the noncentrosymmetric antiferromagnet

G. Gitgeatpong·Y. Zhao·P. Piyawongwatthana·Y. Qiu·L. W. Hariger·N. P. Butch·T. J. Sato·K. Matan
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Abstract

Magnons, the spin-wave quanta, are disturbances that embody a wave propagating through a background medium formed by ordered magnetic moments. In an isotropic Heisenberg system, these disturbances vary in a continuous manner around an ordered spin structure, thus requiring infinitesimal energy as a wavevector approaches a magnetic zone centre. However, competing anisotropic interactions arising from broken symmetry can favour a distinct static and dynamic spin state causing a shift of the minimum point of the magnon dispersion to a nonreciprocal wavevector. Here we report the first direct evidence of these nonreciprocal magnons in an antiferromagnet. In the antiferromagnet we investigated, namely, noncentrosymmetric $\alpha$-Cu$_2$V$_2$O$_7$, they are caused by the incompatibility between anisotropic exchange and antisymmetric Dzyaloshinskii-Moriya interactions resulting in competing collinear and helical spin structures, respectively. The nonreciprocity introduces the difference in the phase velocity of the counter-rotating modes, causing the opposite spontaneous magnonic Faraday rotation of the left- and right-propagating spin-waves. The breaking of spatial inversion and time reversal symmetry is revealed as a magnetic-field-induced asymmetric energy shift, which provides a test for the detailed balance relation.

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G. Gitgeatpong, Y. Zhao, P. Piyawongwatthana, Y. Qiu, L. W. Hariger, N. P. Butch, T. J. Sato, K. Matan. 2017-02-16. Nonreciprocal magnons and symmetry-breaking in the noncentrosymmetric antiferromagnet. https://doi.org/10.1103/physrevlett.119.047201

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