Nuclear magnetic resonance at pressures of up to 10.1 GPa detects an electronic topological transition in aluminum metal

We present high sensitivity $^{27}$Al nuclear magnetic resonance (NMR) measurements on metallic aluminum under high pressures of up to 10.1 GPa. The measured Knight shift and spin-lattice relaxation rate indicate an unexpected negative curvature in the pressure dependence of the electronic density of states (DOS) that violates a free electron behavior. Based on a careful analysis of the Fermiology of aluminum metal with numerical LDA calculations we attribute the observed change in the DOS to a pressure induced electronic topological transition. We discuss an unexpected increase of the NMR linewidth above 4.2 GPa that is not in agreement with the metal's cubic symmetry.