Strong coupling s-wave superconductivity in M_gB₂: ₁₁111111B-NMR Study _of MgB2 and Al-do_pedMgB2

We have measured nuclear spin-lattice relaxation time, T₁ of ₁₁B in M_gB₂ and Al-doped M_gB ₂-T₁T = const, behavior is observed in the normal state, indicating the absence of strong magnetic correlation. In the superconducting (SC) state of M_gB_g, 1/T₁ shows a tiny coherence peak just below T_g and decreases exponentially with a large SC gap of 2A/k_BT_c ∼ 5. These results suggest that M _gB₂ is a strong coupling s-wave superconductor. As Al ₃₊ is doped into Mg₂₊ site, 1/T₁T which is proportional to the square of the density of states (DOS) decreases. The variation of T_c against the DOS deduced from 1/T₁T is fitted by McMillan equation, and the Debye frequency ω and the electron-phonon coupling constant A were estimated as ∼ 700 K and ∼ 0.87, respectively. Both are in good agreement with the values from theoretical works. The high-T_c superconductivity in M_gB₂ is considered to occur through strong coupling between electrons and high frequency-phonon.