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

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We have performed magnetization and 27Al- and 11B-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 $1/T_{1}T$ of 27Al and 11B decrease with increasing $x$. The $1/T_{1}$ of 27Al decreases exponentially with temperature in the SC state followed by the appearance of a tiny coherence peak just below $T_{\text{c}}$. The $1/T_{1}$ of 11B for $x \geq 0.22$ shows no gap behavior below $T_{\text{c}}$, indicating the large reduction in superconducting gap with $x$ at the B site. The temperature dependence of $1/T_{1}T$ in the SC state is quantitatively analyzed using a $s$ wave SC model with two gaps of $2\Delta/k_{\text{B}}T_{\text{c}} \sim 1.5$ 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.
2010-12-15