Raman Scattering Investigation of Electron Phonon Coupling in Carbon substituted MgB2

Room temperature Raman scattering measurements have been carried out on well characterized samples of MgB2-xCx. The Raman line corresponding to the E2g 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 cm-1 to 286 cm-1 in samples with x = 0.1, beyond which it decreases to a value 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 s hole density of states in MgB2-xCx, the electron phonon coupling strength to the E2g phonon, l2g, is evaluated using Allen’s formula. This remains large for low C fraction, but shows rapid decrease for x > 0.10. Using this value of l2g appropriately weighted, TC is obtained from McMillan’s equation. These values are in good agreement with the experimentally measured TC variation in MgB2-xCx.