Two-mode behavior for the E2g broad band in Mg(B1−xCx)2

A systematic Raman study has been carried out on single crystals of Mg(B1−xCx)2 with carbon concentrations in the range of x = 0.0-0.15. We have found remarkable differentiations of the Raman spectra with doping. Even with small amounts of carbon substitution (x = 0.027) the E2g-symmetry broad band at ∼600 cm−1 (for pure MgB2) diminishes and new modes appear of the approximate A1g/B1g (∼750 cm−1) and E2g (∼850 cm−1) symmetries, though the transition temperature is only slightly reduced (<2 K). The broad band does not evolve continuously into the mode of same symmetry at ∼850 cm−1, as assumed by other investigators. The practical disappearance of the original ∼600 cm−1 mode at x ≃ 0.08 coincides with the fast reduction of Tc. The doping dependence of the phonons indicates a two-mode behavior, which could be driven by the 2D-3D electronic topological transition and associated with a possible nanoscale phase separation.