Positronium in Alkali Halides

Sensitivity of Positronium Momentum Distribution to Phase Transitions in Crystalline Dielectrics

“…The formation of Bloch-type Ps in these crystals is confirmed by observing very narrow peaks (the central peak and satellite peaks appearing at the momentum corresponding to the reciprocal lattice vectors of the sample crystal) in the momentum distribution of the photons resulting from the 2γ-decay of Ps upon irradiating the crystals by low-energy positrons. As temperature increases, it is observed that the central Ps peak becomes drastically wider and the satellite peaks disappear, indicating the localization of Ps [5]. Such an effect of a thermally activated selflocalization (self-trapping) of Ps was observed in many ionic crystals and was analyzed theoretically in Refs.…”

“…Positronium has been found to form in a delocalized Bloch-type state in dielectric crystals with low enough concentration of defects (no more than 10 15 defects per cm 3 [4]) at sufficiently low temperatures (typically below a few tens K) [5]. The formation of Bloch-type Ps in these crystals is confirmed by observing very narrow peaks (the central peak and satellite peaks appearing at the momentum corresponding to the reciprocal lattice vectors of the sample crystal) in the momentum distribution of the photons resulting from the 2γ-decay of Ps upon irradiating the crystals by low-energy positrons.…”

Tunnel detrapping of self‐trapped positronium in SrF2 single crystal

“…The Ps atoms were observed to form in Bloch-like ("free") delocalized states at temperatures below a few tens K, and to tend to be ("self"-)trapped by acoustic lattice vibrations in a majority of crystals (except for MgF 2 and α-SiO 2 [6,7]) with increasing temperature [8]. Theoretical studies revealed the two-well structure of the potential energy surface of the Ps atom coupled to the field of acoustic phonons in a crystal [9].…”

Nonpolar optical scattering of positronium in magnesium fluoride

“…Physical nature of Ps states in crystalline dielectrics has been of a continued interest for more than three decades [6,7,8,9,10,11,12,13,14,15,16,17,18] ever since Brandt et al first identified the delocalized Ps state in the Angular Correlation of Annihilation Radiation (ACAR) spectrum of the single crystal of α-SiO 2 [7]. Positronium has been found to form in a delocalized Bloch-type state in dielectric crystals with low enough concentration of defects (no more than 10 15 defects per cm 3 [9]) at sufficiently low temperatures (typically below a few tens K) [9,10].…”

“…Positronium has been found to form in a delocalized Bloch-type state in dielectric crystals with low enough concentration of defects (no more than 10 15 defects per cm 3 [9]) at sufficiently low temperatures (typically below a few tens K) [9,10]. The formation of Blochtype Ps is confirmed by observing very narrow peaks (the central peak and satellite peaks appearing at the momentum corresponding to the reciprocal lattice vectors of the sample crystal) in the ACAR spectra resulting from the 2γ-decay of Ps and representing its momentum distribution in a crystal.…”