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 E-2g phonon mode shows progressive hardening from 620cm(-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 a-hole density of states in MgB2-xCx, the electron-phonon coupling strength to the E-2g phonon, lambda(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 lambda(2g) appropriately weighted, T-C is obtained from McMillan ' s equation. These values are in good agreement with the experimentally measured TC variation in MgB2-xCx.