Muonium

µSR -- here

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...wherein the reader is introduced to the muonium (Mu) atom, a light isotope of the hydrogen (H) atom, consisting of a positive muon (µ⁺) orbited by an electron (e^-). (Technically, according to strict naming conventions, it should really be called "muium", but who can pronounce that?)

In solids, any neutral bound state between a µ⁺ and an e^- is considered muonium, and is written Mu⁰ to emphasize its neutrality, since the muon also appears in its "bare" positively charged state (Mu⁺) and (in some cases) bound to two electrons, forming Mu^-. A comprehensive taxonomy of "muonium centers" will be attempted here.