$^{11}$B and $^{27}$Al NMR spin-lattice relaxation and Knight shift study of Mg$_{1-x}$Al$_x$B$_2$. Evidence for anisotropic Fermi surface

TL;DR

This study uses NMR techniques to investigate the electronic structure of Mg$_{1-x}$Al$_x$B$_2$, confirming the anisotropic Fermi surface and linking the collapse of 2-D sheets to the loss of superconductivity.

Contribution

It provides experimental evidence for the anisotropic Fermi surface in Mg$_{1-x}$Al$_x$B$_2$ and correlates Fermi surface changes with superconductivity suppression.

Findings

Fermi surface is highly anisotropic with 2-D cylindrical sheets.

Density of states decreases sharply with Al doping.

Superconductivity disappears when 2-D sheets collapse at x≈0.55.

Abstract

We report a detailed study of $^{11}$B and $^{27}$Al NMR spin-lattice relaxation rates ($1/T_1$), as well as of $^{27}$Al Knight shift (K) of Mg$_{1-x}$Al$_x$B$_2$, $0\leq x\leq 1$. The obtained ($1/T_1T$) and K vs. x plots are in excellent agreement with ab initio calculations. This asserts experimentally the prediction that the Fermi surface is highly anisotropic, consisting mainly of hole-type 2-D cylindrical sheets from bonding $2p_{x,y}$ boron orbitals. It is also shown that the density of states at the Fermi level decreases sharply on Al doping and the 2-D sheets collapse at $x\approx 0.55$, where the superconductive phase disappears.