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

We report the results of nuclear spin-lattice relaxation time, T-1 of B-11 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/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 2Delta/k(B)T(c) similar to 5, which is quite larger than the weak-coupling value in the BCS theory, 2Delta/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, measurement on a bulk polycrystalline sample. In NbB2 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 2Delta/k(B)T(c) = 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(E-F), decreases. The variation in T-c 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 omega - 700 K and an electron-phonon coupling constant lambda similar to 0.87. Thus obtained values are in good agreement with the values suggested by the theoretical works. The high-T-c superconductivity in MgB2 is shown to occur through the strong coupling with high frequency phonons. (C) 2002 Elsevier Science B.V. All rights reserved.