Evidence for strong-coupling s-wave superconductivity in MgB2: 11B-NMR study of MgB2 and the related materials

H. Kotegawa, K. Ishida, Y. Kitaoka, T. Muranaka, N. Nakagawa, H. Takagiwa, J. Akimitsu


28 被引用数 (Scopus)


We report the results of nuclear spin-lattice relaxation time, T1 of 11B in MgB2, Al-doped MgB2 and NbB2. A T1T = constant behavior was observed in the normal state, indicating the absence of strong magnetic correlation. In the superconducting (SC) state, 1/T1 shows a tiny coherence peak just below Tc and decreases exponentially, demonstrating an s-wave superconductivity. The magnitude of SC gap is estimated as 2Δ/kBTc∼5, which is quite larger than the weak-coupling value in the BCS theory, 2Δ/kBTc = 3.5. This rules out a possibility for the multiple SC gaps that was suggested in many literatures. The anisotropy in the upper critical field was also corroborated by the T1 measurement on a bulk polycrystalline sample. In NbB2 with a low value of Tc = 5 K, the T1 measurement revealed the distinct coherence peak just below Tc, followed by an exponential decrease with a magnitude of SC gap 2Δ/kBTc = 3.1. These data are consistent with the weak-coupling BCS theory. As Al3+ is substituted for Mg2+, 1/T1T, which is proportional to the square of the density of states at the Fermi level N(EF), decreases. The variation in Tc against the relative changes in N(EF) deduced from 1/T1T is well fitted in terms of the McMillan equation by assuming a characteristic phonon frequency ω ∼ 700 K and an electron-phonon coupling constant λ ∼ 0.87. Thus obtained values are in good agreement with the values suggested by the theoretical works. The high-Tc superconductivity in MgB2 is shown to occur through the strong coupling with high frequency phonons.

ジャーナルPhysica C: Superconductivity and its applications
出版ステータスPublished - 10月 1 2002

ASJC Scopus subject areas

  • 電子材料、光学材料、および磁性材料
  • 凝縮系物理学
  • エネルギー工学および電力技術
  • 電子工学および電気工学


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