Evidence for electron-phonon interaction in Fe${}_{1-x}$M${}_x$Sb${}_2$ (M=Co, Cr) single crystals

We have measured polarized Raman scattering spectra of the Fe${}_{1-x}$Co${}_x$Sb${}_2$ and Fe${}_{1-x}$Cr${}_x$Sb${}_2$ (0$\le x\le$0.5) single crystals in the temperature range between 15 K and 300 K. The highest energy $B_{1g}$ symmetry mode shows significant line asymmetry due to phonon mode coupling width electronic background. The coupling constant achieves the highest value at about 40 K and after that it remains temperature independent. Origin of additional mode broadening is pure anharmonic. Below 40 K the coupling is drastically reduced, in agreement with transport properties measurements. Alloying of FeSb${}_2$ with Co and Cr produces the B${}_{1g}$ mode narrowing, i.e. weakening of the electron-phonon interaction. In the case of A${}_g$ symmetry modes we have found a significant mode mixing.