Many-body quantum muon effects and quadrupolar coupling in solids

• Published in: Communications physics 2023-06, Vol.6 (1), p.142-9, Article 142
• Main Authors: Gomilšek, Matjaž, Pratt, Francis L., Cottrell, Stephen P., Clark, Stewart J., Lancaster, Tom
• Format: Article
• Language: English
• Subjects: 639/301/1034; 639/766/119/995; 639/766/483/1139; Accuracy; Anharmonicity; Approximation; Atoms & subatomic particles; Density functional theory; Dipoles; Light; Molecular dynamics; Muons; Nitrogen; Nuclei; Physics; Physics and Astronomy; Quantum entanglement; Solid nitrogen
• ISSN: 2399-3650; 2399-3650
• DOI: 10.1038/s42005-023-01260-7

Strong quantum zero-point motion (ZPM) of light nuclei and other particles is a crucial aspect of many state-of-the-art quantum materials. However, it has only recently begun to be explored from an ab initio perspective, through several competing approximations. Here we develop a unified description of muon and light nucleus ZPM and establish the regimes of anharmonicity and positional quantum entanglement where different approximation schemes apply. Via density functional theory and path-integral molecular dynamics simulations we demonstrate that in solid nitrogen, α –N 2 , muon ZPM is both strongly anharmonic and many-body in character, with the muon forming an extended electric-dipole polaron around a central, quantum-entangled [N 2 – μ –N 2 ] + complex. By combining this quantitative description of quantum muon ZPM with precision muon quadrupolar level-crossing resonance experiments, we independently determine the static 14 N nuclear quadrupolar coupling constant of pristine α –N 2 to be –5.36(2) MHz, a significant improvement in accuracy over the previously-accepted value of –5.39(5) MHz, and a validation of our unified description of light-particle ZPM. Quantum entanglement and uncertainty in the positions of light nuclei and implanted particles can crucially impact our understanding of advanced materials. This paper develops a unified theoretical description of these effects and applies it to muon spectroscopy measurements of a material constant to significantly improve their accuracy.