We report a detailed study of the ${}^{11}\mathrm{B}$ and ${}^{27}\mathrm{Al}$ NMR spin-lattice relaxation rates ${(1/T}_1)$ and the ${}^{27}\mathrm{Al}$ Knight shift (K) in ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2,$ $0<~x<~1.$ The evolution of ${(1/T}_{1}T)$ and K with x is in excellent agreement with the prediction of ab initio calculations of a highly anisotropic Fermi surface, consisting mainly of hole-type two-dimensional (2D) cylindrical sheets from bonding ${2p}_{x,y}$ boron orbitals. The density of states at the Fermi level also decreases sharply on Al doping and the 2D sheets collapse at $x\approx 0.55,$ where the superconducting phase disappears.

• Received 23 September 2002

DOI:https://doi.org/10.1103/PhysRevB.66.140514