NMR spectroscopy of the normal and superconducting states of MgB2 and comparison to AlB2

Abstract

11B NMR measurements have been performed in 11B enriched MgB2 powder samples in external fields of 0.813, 1.55, 4.7, and 7.2 T both in the normal phase and in the superconducting phase. A previously unreported dipolar Pake doublet has been observed in the quadrupole perturbed NMR spectrum. The Knight shift can thus be accurately determined by narrowing the line with the magic angle spinning (MAS) technique. Results of Knight shift (K) and relaxation rates (1/T1) for both 11B and 27Al nuclei are reported also for AlB2. The comparison of the data in the two compounds shows the dramatic drop of the density of states at the boron site in AlB2 with respect to MgB2. The experimental values for K and 1/T1 are in most cases in good agreement with the theoretical values obtained from first principles calculations. The recovery of the nuclear magnetization below Tc in random powder samples is nonexponential due to the anisotropy of the upper critical field. The exponential drop of 1/T1 in the superconducting phase observed by Kotegawa et al. is confirmed here but not the coherence peak.

Source-reported materials — not catalogue approval

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