Persistent spin dynamics in candidate spin-liquid materials probed by muon-spin rotation

The technique of muon-spin rotation (μSR) has emerged as one of the most important spectroscopic techniques in condensed matter physics, used to study everything from superconductors to skyrmions. It is now possible to understand the interaction between the muon and its neighbouring spins in quantitative detail to probe entanglement and decoherence [1] and to characterise the muon site in many different materials with high accuracy [2]. An outstanding problem is that many magnetically frustrated systems exhibit what is known as persistent spin dynamics (PSD) in μSR experiments, the origin of which has remained mysterious since their discovery in the 1990s. As the temperature is lowered, the muon-spin relaxation rate rises (as would be expected for the slowing-down of spin fluctuations) but this rate then saturates at low temperature, the low-temperature fluctuations interpreted as PSD. To explain this phenomenon, I will describe how muons can couple to singlet states [3] and how this can be extended to understand the way muons couple to a variety of systems exhibiting highly frustrated magnetism, as well as to dynamical problems more generally. μSR experiments are usually carried out without resonance, but I will describe a new project which uses insights from magnetic resonance and include them into μSR, thereby extending the reach of the technique.