Anharmonic quantum muon effects of light particles in a spin liquid material
The quantum behavior of light nuclei and other particles in materials challenges classical intuition and introduces novel phenomena. Here we demonstrate that muon spin spectroscopy ( $\mu$SR) is a powerful tool for exploring the quantum effects of light particles, such as the muon, in condensed matter. The muon's quantum nature is profoundly influenced by the surrounding, offering a unique probe for understanding the role of light atoms and their role in shaping local electronic environments. In Zn-barlowite, a candidate quantum spin liquid, we show that standard density functional theory (DFT) methods, which treat the muon as a classical point-like particle, fail to capture its strong quantum anharmonic effects. Only by modeling the muon as a spatially extended quantum particle, thus accounting for the anharmonicity, can the experimental $\mu$SR data be understood. This approach not only improves the interpretation of $\mu$SR results but also opens the door to studying the quantum effects of other light particles, like hydrogen and lithium nuclei, which can greatly influence material properties.