de Haas-van Alphen effect investigation of the electronic structure of   Al substituted MgB_2
  We report a de Haas-van Alphen (dHvA) study of the electronic structure of Al doped crystals of MgB$_2$. We have measured crystals with $\sim 7.5$% Al which have a $T_c$ of 33.6 K, ($\sim 14$% lower than pure MgB$_2$). dHvA frequencies for the $\sigma$ tube orbits in the doped samples are lower than in pure MgB$_2$, implying a $16\pm2%$ reduction in the number of holes in this sheet of Fermi surface. The mass of the quasiparticles on the larger $\sigma$ orbit is lighter than the pure case indicating a reduction in electron-phonon coupling constant $\lambda$. These observations are compared with band structure calculations, and found to be in excellent agreement. 
A. Carrington, J.D. Fletcher, J.R. Cooper, O.J. Taylor, L. Balicas,   N.D. Zhigadlo, S.M. Kazakov, J. Karpinski, J.P.H. Char...