Superconductivity in MgB₂: phonon modes and influence of carbon doping

Abstract

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-xC_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39.4 K for x=0 to 26 K for x= 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 in T_c . The observed variation of T_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.