*

The complex magnetic phase diagram^a of ZnCr₂Se₄ includes a quantum critical point^b separating spin-spiral (1.5 ≤ B ≤ 6.5 T) and field-polarized (B > 6.5 T) phases. We present thermal conductivity measurements at T ≤ T_N = 21 K in the spin-spiral phase which exhibit anisotropy at T < 2 K, for heat flow along and transverse to the spin-helix propagation direction, that is nonmonotonic in field. Calculated lattice and magnon thermal conductivities that employ a three-dimensional magnon dispersion inferred from neutron scattering studies^c,d favor a magnonic interpretation of the data.

^a A. S. Cameron et al., J. Phys.: Condens. Matter 28, 146001 (2016).
^b C. C. Gu et al., Phys. Rev. Lett. 120, 147204 (2018).
^c Y. V. Tymoshenko et al., Phys. Rev. X 7, 041049 (2017).
^d D. S. Inosov et al., arXiv:2009.13424v1