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.
Highlights
Magnesium diboride, MgB2, is a type-II superconductor with Tc = 39 K discovered in 2001 [1]
Carbon doping was achieved by chemical vapor deposition (CVD), a method which allows uniform distribution of carbon onto a boron precursor, using a purpose-built stainless-steel tube furnace for CVD of carbon on boron powder [7], which reacted with a nanoboron powder
As the temperature is lowered below Tc, 1/T1 deviates from Korringa’s law; the coherence peak [26] is not observed, which may be due to a pair-breaking mechanism due to the static magnetic field present, as hypothesized previously
Summary
If carbon atoms occupy a different chemical site than boron, the NMR line shape will differ between 13C and 11B NMR This is especially the case if C atoms truly act as flux-pinning centers; they will pin the normal conducting vortex core while the material is in the superconducting state. There is ambiguity in the 11B NMR data, possibly due to a peak-suppressing mechanism that may exist in the boron-graphite-like planes. Testing this hypothesis requires, in addition to 11B, yet another NMR active nucleus which resides in the planes, such as 10B or, if doped with carbon, 13C NMR. We determine NMR parameters such as the Knight shift and T1T for both 11B and 13C and compare our findings to ab initio calculations
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