Raman scattering investigation of electron-phonon coupling in carbon substituted MgB₂

Abstract

Room temperature Raman scattering measurements have been carried out on well characterized samples of MgB_2-xC_x. The Raman line corresponding to the E_2g phonon mode shows progressive hardening from 620 cm⁻¹ in pristine MgB₂ to 775 cm⁻¹ in the sample with carbon fraction x = 0.2. The corresponding line width on the other hand increases from a value of about 220 to 286 cm⁻¹ in samples with x = 0.1, beyond which it decreases to a value of 167 cm⁻¹ for x = 0.2. From the average mode frequency and the line width obtained from Raman measurements and taking the values of N(0) obtained from the calculated variation in σ-hole density of states in MgB_2-xC_x, the electron-phonon coupling strength to the E_2g phonon, λ_2g, is evaluated using Allen's formula. This remains large for low C fraction, but shows a rapid decrease for x>0.10. Using this value of λ_2gappropriately weighted, T_C is obtained from McMillan's equation. These values are in good agreement with the experimentally measured T_C variation in MgB_2-xC_x.