Study of B11 and C13 NMR on doped MgB2 in the normal and in the superconducting state

Abstract

We have studied carbon-doped magnesium diboride nanoparticles using $^{13}\mathrm{C}$ and $^{11}\mathrm{B}$ NMR in the normal and superconducting states. Measurements of the line shape reveal the role of carbon as a flux-pinning center and, combined with Knight shift measurements, suggest the doping procedure favors the chemical substitution scenario. We perform ab initio calculations on a structure with a single carbon-boron substitution which yield results that match the experimental data. The $^{13}\mathrm{C}$ and $^{11}\mathrm{B}$ Knight shift data are used to extract the spin susceptibility, which indicates a BCS pairing mechanism; however, we do not observe the Hebel-Slichter coherence peak from 1/${T}_{1}$ data, which we hypothesize is due to a pair-breaking mechanism present in the boron planes.