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.
- Al-doping effect
- s-wave superconductivity
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering