Searching for hidden order with muons

While many experimental techniques probe bulk or surface properties, insight into hidden orders can sometimes only be gained through probing local environments within a crystal. In muon spin resonance/rotation (µSR) experiments, a positive muon is implanted into a sample and the local magnetic fields at the muon stopping site can be inferred by measuring the precession of the muon’s spin.

Density functional theory (DFT) is increasingly being used to interpret µSR spectra, greatly enhancing the power and applicability of the technique. In particular, we try to answer four questions:

1. Where does the muon stop in the material?
2. How much does the muon perturb the local environment in which it stops?
3. How does the spin of the muon interact with electron spins in the material?
4. How much does the quantum nature of the muon affect the answer to the above questions?
   

Recently we have been exploring a variety of systems in which the µSR spectra can be explained through the interactions of muons and nearby small electron polarons [1,2]. Another novel phenomenon that we have been interested in is the possible appearance of magnetic monopoles at the surface of magnetoelectric materials [3].