Raman spectroscopic study of carbon substitution in MgB2 Original Research Article

We study the effect of carbon doping on the Raman spectrum of the MgB2 superconductor. Out data show that significant changes in the Raman spectra of the MgB2−xCx compounds occur for carbon concentrations x>0.04. The E2g 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.