Superconductivity in MgB2: Phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB₂, and superconducting transition in carbon doped MgB₂ are presented. The superconducting transition temperature in MgB_{2 x}C_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39-4 K forx = 0 to 26 K forx = 0.5. It is shown the changes in lattice volume, as obtained from x-ray diffraction measurements, can account only partially for the observed decrease inT _c . The observed variation ofT _c with carbon content is seen to correlate with the Debye temperatures, obtained from an analysis of the resistivity data. Investigation of the phonon modes in MgB₂, through infrared absorption measurements indicate three modes at 410,475 and 560 cm^-1. The former two are associated with the infrared active modes, and the third component is associated with the Raman mode, that gets activated due to disorder. A study of the temperature dependence of these modes indicates no changes across the superconducting transition. The mode at 560 cm^-1 shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity.