Sapphire α−Al2O3 puzzle: Joint μSR and density functional theory study

Abstract

Sapphire (α-Al2O3) has been investigated by the muon spin rotation (µSR) method in several experiments in the past. The main µSR component is a diamagnetic-like signal with a fast relaxation. Because of this diamagnetic-like behavior, the signal was assigned to either positively charged muonium (Mu+) or negatively charged muonium (Mu−), but neither of the two assignments was satisfactory (the so-called “sapphire puzzle”). We have proposed that the signal is due to a weakly-paramagnetic muonium configuration (transition state) which is formed during the reaction of muonium with the host lattice. In the present paper we report new experimental data on Al2O3 and discuss these and earlier data in the Mu− and in the transition state model. Calculations based on density-functional theory were also performed with detailed findings on the energetics of the different muonium configurations and their migration energies. We conclude that the transition state model is more plausible than the Mu− model, but the Mu− interpretation cannot be excluded completely. In addition, evidence is presented that the bare muon performs local motion but no long-range diffusion below room temperature in the microsecond time range.