Spin dynamics of the triangular lattice antiferromagnet ${\alpha}$-SrCr$_2$O$_4$*

We study the spin dynamics of the layered $S=3/2$ triangular lattice antiferromagnet ${\alpha}$-SrCr$_2$O$_4$ by means of inelastic neutron scattering on powder and single-crystal specimen. While the incommensurate long-range order observed below $T_{\rm N}$=43K resembles the usual 120$^\circ$-structure predicted for the perfect triangular lattice antiferromagnet, a spin-wave theory fit to the entire single-crystal dataset reveals strongly distorted exchange interactions. The extreme sensitivity of direct-exchange interactions to the small static Cr$^{3+}$-Cr$^{3+}$ distance variations reported by neutron diffraction, is quantitatively confirmed by {\it ab-initio} calculations that corroborate the spin-wave theory results.