Synthesis and search for superconductivity in LiBC

Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al [1], we have attempted to synthesise Li deficient LixBC (x = 1, 0.8, 0.6 and 0.4) and look for superconductivity in this system. Our synthesis procedure, following the recipe for MgB2, involves reaction of elemental components in a Ta crucible at 900o C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P63/mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, was observed in the temperature range of 300 to 4 K. This is possibly related to the presence of disorder in the B-C stacking; evidence for which is suggested from a study of the vibrational modes of LixBC through infrared spectroscopy.Comment: 7 pages, 6 figures 1 tabl

Superconductivity in MgB<SUB>2</SUB>: phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB<SUB>2</SUB>, and superconducting transition in Carbon doped MgB<SUB>2</SUB> are presented. The superconducting transition temperature in MgB<SUB>2-x</SUB>C<SUB>x</SUB> 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<SUB>c </SUB>. The observed variation of T<SUB>c</SUB> 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 MgB<SUB>2</SUB>, through infrared absorption measurements indicate three modes at 410,475 and 560 cm<SUP>&#8722;1</SUP>. 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<SUP>&#8722;1</SUP> shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity