Bosonic Dirac materials are testbeds for dissipationless spin-based electronics. In the quasi two-dimensional honeycomb lattice of CrX$_{ 3}$ (X=Cl, Br, I), Dirac magnons have been predicted at the crossing of acoustical and optical spin waves, analogous to Dirac fermions in graphene. Here we show that, distinct from CrBr$_{ 3}$ and CrI$_{ 3}$, gapless Dirac magnons are present in bulk CrCl$_{ 3}$, with inelastic neutron scattering intensity at low temperatures approaching zero at the Dirac $K$ point. Upon warming, magnon-magnon interactions induce strong renormalization and decreased lifetimes, with a ~25% softening of the upper magnon branch intensity from 5 to 50 K, though magnon features persist well above T$_{ N}$. Moreover, an unusual negative thermal expansion (NTE) of the $a$-axis lattice constant and anomalous phonon behavior are observed below 50 K, indicating magnetoelastic and spin-phonon coupling arising from an increase in the in-plane spin correlations that begins tens of Kelvin above T$_{ N}$.
Categories: cond-mat.mtrl-sci