Existence of free and self-trapped positronium states in alkali halide crystals: Theoretical analysis and comparison with experiment

Abstract

A variational calculation is performed of the eigenstates of a positronium atom coupled with a field of longitudinal acoustic phonons in ionic crystals at finite temperatures. On the basis of this calculation a theoretical analysis is made of the possibility of self-localization ͑self-trapping͒ of positronium. The self-trapped states of positronium in NaF, NaCl, KCl, and KI crystals are found to be metastable with the energy higher by ϳ0.01-0.1 eV with respect to the stable delocalized ͑free͒ states. The self-trapped states of positronium in MgF 2 and ␣-SiO 2 crystals are unstable at absolute zero temperature and become metastable with an increase in temperature for TϾϳ300 K. The difference in the energies of such ''high-temperature'' self-trapped states and the free states of positronium in MgF 2 and ␣-SiO 2 is found to be at least one order of magnitude larger than that in the other alkali halides, explaining theoretically experimental evidence for the nonexistence of self-trapped positronium in these crystals. The basic characteristics ͑energy, effective mass, mean number of surrounding phonons, and localization radius͒ of the self-trapped and free states as well as the deformation potential constants are calculated for positronium in the crystals above. The results obtained are in good agreement with known experimental data.

Sensitivity of Positronium Momentum Distribution to Phase Transitions in Crystalline Dielectrics

“…Such an effect of a thermally activated selflocalization (self-trapping) of Ps was observed in many ionic crystals and was analyzed theoretically in Refs. [6,7]. Fig.…”

Analysis of Possible Scattering Processes for Bloch Positronium in Ionic Crystals at Elevated Temperatures

“…Such an e˜ect of a therm al l y acti vated self-l ocali zati on of Ps wa s observed i n m any i oni c crysta l s and wa s anal yzed theoreti cal ly in Refs. [2,3]. The onl y excepti ons presentl y kno wn are the (3 37) I .V .…”

“…As i s shown i n recent exp eri ments [4,5], the Ps ato m i n these crysta l s remai ns del ocal i zed up to tem peratures ¿ 7 0 0 K (thi s exp eri menta l fact was Ùrst expl ai ned theo reti cal l y in R ef. [3]). The centra l and satel l ite Ps p eaks i n Mg F 2 are observed to b e dra sti cal l y bro adened at tem p eratures hi gher tha n 2 0 0 K. Thi s dra sti c bro adeni ng was fai l ed to b e expl ai ned i n term s of Ps scatteri ng on l ongi tudi nal acousti c pho no ns.…”

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].…”

“…Such an effect of a thermally activated selflocalization (self-trapping) of Ps was observed in a number of dielectric crystals and was studied theoretically in Refs. [11,12] in terms of the interaction of Ps with the field of a short-range acoustic deformation-potential (long-wavelength longitudinal acoustic phonons).…”