Observation of a charge-neutral muon-polaron complex in antiferromagnetic Cr${}_2$O${}_3$

Abstract

We report a comprehensive muon spin rotation ($\hat{I}¼$SR) study of the prototypical magnetoelectric antiferromagnet Cr${}_2$O${}_3$. We find the positively charged muon ($\hat{I}{¼}^{+}$) occupies several distinct interstitial sites, and displays a rich dynamic behavior involving local hopping, thermally activated site transitions and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of $\hat{I}¼$SR spectra in a wide range of magnetic oxides, and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a $\hat{I}{¼}^{+}$ into a linear magnetoelectric, and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.

17 pages, 12 figures