Muonium spectroscopy in ZnSe: Metastability and conversion

Abstract

High-precision spectroscopic information is obtained on the muonium states in ZnSe by high-field transverse $\ensuremath{\mu}\mathrm{SR}$ measurements. At low temperatures, two muonium states ${\mathrm{Mu}}_{I}$ and ${\mathrm{Mu}}_{II}$ are observed with isotropic hyperfine parameters of ${\mathrm{A}}_{I}=3283.63\ifmmode\pm\else\textpm\fi{}0.51\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and ${\mathrm{A}}_{II}=3454.26\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ (74% and 77% of the vacuum value, respectively). State I is thermally unstable and converts to state II at approximately $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. State II is stable up to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at least. We assign ${\mathrm{Mu}}_{II}$ to the cation interstitial tetrahedral site and discuss the possibility that ${\mathrm{Mu}}_{I}$ may correspond either to muonium at the same site but in the unrelaxed lattice or to the anion interstitial tetrahedral site. The temperature dependence of the hyperfine interaction was fitted with a local vibrational model giving an oscillator energy of approximately $8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The amplitudes and the depolarization rates are measured over the entire temperature range and are discussed in the text.