Wiktionary
n. (context physics chemistry English) an exotic atom formed when a positively charged muon (an anti-muon) and an electron are bound by their mutual electrical attraction; chemically equivalent to a light, short-lived isotope of hydrogen
Wikipedia
Muonium is an exotic atom made up of an antimuon and an electron, which was discovered in 1960 and is given the chemical symbol . During the muon's lifetime, muonium can enter into compounds such as muonium chloride or sodium muonide . Due to the mass difference between the antimuon and the electron, muonium is more similar to atomic hydrogen than positronium . Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium, and thus it can usefully be considered as an exotic light isotope of hydrogen.
Although muonium is short-lived, physical chemists study it using muon spin spectroscopy (μSR), a magnetic resonance technique analogous to nuclear magnetic resonance (NMR) or electron spin resonance (ESR) spectroscopy. Like ESR, μSR is useful for the analysis of chemical transformations and the structure of compounds with novel or potentially valuable electronic properties. Muonium is usually studied by muon spin rotation, in which the Mu atom's spin precesses in a magnetic field applied transverse to the muon spin direction (since muons are typically produced in a spin-polarized state from the decay of pions), and by avoided level crossing (ALC), which is also called level crossing resonance (LCR). The latter employs a magnetic field applied longitudinally to the polarization direction, and monitors the relaxation of muon spins caused by "flip/flop" transitions with other magnetic nuclei.
Because the muon is a lepton, the atomic energy levels of muonium can be calculated with great precision from quantum electrodynamics (QED), unlike in the case of hydrogen, where the precision is limited by uncertainties related to the internal structure of the proton. For this reason, muonium is an ideal system for studying bound-state QED and also for searching for physics beyond the standard model.
Usage examples of "muonium".
In addition, because the muon spends a good part of its time close to the proton of the muonium atom, there is a fair probability of spontaneous proton-proton fusion.
Positronium, like muonium, has been made in the lab, but it too is unstable.
Unfortunately the muonium cable is not without its problems, quite apart from the difficulty of making it in solid form.