27Al and 11B NMR Study of the Two-Band Superconductor Mg1-xAlxB2

We have performed magnetization and Al-27- and B-11-NMR measurements of Mg1-xAlxB2 with x = 0, 0.16, 0.22. 0.30, and 0.45 in both the normal and superconducting (SC) states. In the normal state, the temperature-independent values of 1/T1T of Al-27 and B-11 decrease with increasing x. The 1/T-1 of Al-27 decreases exponentially with temperature in the SC state followed by the appearance of a tiny coherence peak just below T-c. The 1/T-1 of B-11 for x >= 0.22 shows no gap behavior below T-c, indicating the large reduction in superconducting gap with x at the B site. The temperature dependence of I/T1T in the SC state is quantitatively analyzed using a s wave SC model with two gaps of 2 Delta/k(B)T(c) similar to 1.5 and 4. The present results show that Delta(sigma)., is markedlly deceases with increasing x, namely, with sigma band filling by electron doping, providing convincing evidence for two SC gaps with spin singlet s-wave pairing in Mg1-xAlxB2