## 抄録

We report the results of nuclear spin-lattice relaxation time, T_{1} of ^{11}B in MgB_{2}, Al-doped MgB_{2} and NbB_{2}. A T_{1}T = constant behavior was observed in the normal state, indicating the absence of strong magnetic correlation. In the superconducting (SC) state, 1/T_{1} shows a tiny coherence peak just below T_{c} and decreases exponentially, demonstrating an s-wave superconductivity. The magnitude of SC gap is estimated as 2Δ/k_{B}T_{c}∼5, which is quite larger than the weak-coupling value in the BCS theory, 2Δ/k_{B}T_{c} = 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 T_{1} measurement on a bulk polycrystalline sample. In NbB_{2} with a low value of T_{c} = 5 K, the T_{1} measurement revealed the distinct coherence peak just below T_{c}, followed by an exponential decrease with a magnitude of SC gap 2Δ/k_{B}T_{c} = 3.1. These data are consistent with the weak-coupling BCS theory. As Al^{3+} is substituted for Mg^{2+}, 1/T_{1}T, which is proportional to the square of the density of states at the Fermi level N(E_{F}), decreases. The variation in T_{c} against the relative changes in N(E_{F}) deduced from 1/T_{1}T 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-T_{c} superconductivity in MgB_{2} is shown to occur through the strong coupling with high frequency phonons.

本文言語 | English |
---|---|

ページ（範囲） | 25-32 |

ページ数 | 8 |

ジャーナル | Physica C: Superconductivity and its applications |

巻 | 378-381 |

号 | PART 1 |

DOI | |

出版ステータス | Published - 10月 1 2002 |

外部発表 | はい |

## 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

## フィンガープリント

「Evidence for strong-coupling s-wave superconductivity in MgB_{2}:

^{11}B-NMR study of MgB

_{2}and the related materials」の研究トピックを掘り下げます。これらがまとまってユニークなフィンガープリントを構成します。