Raman scattering study of lattice and magnetic excitations in CrAs

We report on the lattice and spin dynamics in monopnictide CrAs using polarized Raman scattering. This system exhibits a first-order magnetostructural phase transition at ${T}_{N}\ensuremath{\sim}265\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which the magnetic moments of Cr form an incommensurate double helical magnetic structure while the unit-cell volume abruptly expands. Across the transition, the frequencies of the fully symmetric ${A}_{g}$ phonons strongly renormalize, which along with the results of first-principles calculations suggests the presence of a sizeable coupling of the phonons to the magnetic degrees of freedom in this compound. In addition, we observe two broad modes at around 350 and $1700\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in the magnetic phase, which we associate with two-magnon Raman scattering. This work provides one of the few examples of magnetic Raman scattering from a noncollinear magnet.