Origin of the unusual dependence of Raman D band on excitation wavelength in graphite-like materials

We have revisited the still unresolved puzzle of the dispersion of the Rama n disorder-induced D band as a function of laser excitation photon energy E -L in graphite-like materials. We propose that the D mode is a combination of an optic phonon at the K point in the Brillioun zone and an acoustic pho non whose momentum is determined uniquely by the double resonance condition . The fit of the experimental data with the double-resonance model yields t he reduced effective mass of 0.025 m(e) for the electron-hole pairs corresp onding to the A(2) transition, in agreement with other experiments. The mod el can also explain the difference between omega (S) and omega (AS) for D a nd D* modes, and predicts its dependence on the Raman excitation frequency. (C) 2001 American Institute of Physics.