Abstract
We report a comprehensive muon spin rotation ($\mu$SR) study of the prototypical magnetoelectric antiferromagnet Cr$_2$O$_3$. We find the positively charged muon ($\mu^+$) 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 $\mu$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 $\mu^+$ into a linear magnetoelectric, and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.
Highlights
Charged muons implanted into semiconductors and insulators often form muonium (Mu 1⁄4 1⁄2μþe−), a hydrogenlike charge-neutral bound state
We presented strong evidence for the formation of a charge-neutral muon-polaron complex in Cr2O3 that, while not exhibiting signatures conventionally expected from neutral-charge states, significantly influences the muon behavior and contributes a well-resolved signal
The temperature dependence reveals a rich dynamic behavior that we explain in terms of a thermally activated transition between E2 and E1, and intra-E3 local muon hopping
Summary
Charged muons implanted into semiconductors and insulators often form muonium (Mu 1⁄4 1⁄2μþe−), a hydrogenlike charge-neutral bound state. Since the electronic structure of Mu in a solid is virtually identical to that of hydrogen, Mu has been studied extensively using muon spin spectroscopy (μSR) to learn about interstitial hydrogen, one of the most ubiquitous defects in semiconductors. Mu centers have been studied in a wide range of materials, providing direct information about the electronic structure of hydrogen defects as shallow or deeplevel dopants [2,3,4,5]. The study of such charge-neutral muon states has been limited to nonmagnetic materials, since paramagnetic Mu is widely assumed to be subject to strong depolarization in the presence of magnetic moments such as those on Cr3þ [3], and, with the exception of MnF2 [6], no Mu has been confirmed in magnetic materials
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