NMR analysis of the electronic structure of superconducting yttrium carbide halides

Nuclear magnetic resonance and relaxation are reported for 13C, 79/81Br and 127I in the layered carbide halides Y2C2Br2, Y2C2Br0.5I1.5, Y2C2I2 and Y2CBr2 for natural isotope abundances and for 13C-enriched samples. Field and temperature dependences of 13C spin-lattice relaxation are derived. The data analysis supports p-like character of the conduction electrons at the C2 dumbbell sites in the superconducting dicarbides. Nonlinear temperature dependence of the 13C Korringa rate in Y2C2Br2 indicates that the Fermi energy falls in a region of positive curvature of the energy-dependent density of states.