Raman spectroscopic study of carbon substitution in MgB₂

We study the effect of carbon doping on the Raman spectrum of the MgB ₂ superconductor. Out data show that significant changes in the Raman spectra of the MgB_2-xC_x compounds occur for carbon concentrations x>0.04. The E_2g mode at ∼580 cm^-1 hardens only moderately upon increased doping despite direct carbon substitution in the boron layers, while the dependence of its full width at half maximum (FWHM) on x reveals the competing effects of reduced electron-phonon coupling and increased disorder. The results are discussed in the framework of anisotropic lattice contraction and the position of the σ sub-bands with respect to the Fermi energy level. The relative intensity of the phonon peak at ∼770 cm^-1, associated with a peak in the phonon density of states, increases considerably and dominates for x = 0.08. Additionally, it hardens and broadens with increasing x in the range 0.04-0.08. These changes can be associated with carbon substitution-induced disorder in the investigated samples.