Two-dimensional angular correlation study of delocalized positronium in alkali halides

Nagai, Yasuyoshi; Saito, Haruo; Nagashima, Yasuyuki; Kobayashi, Nobuhiko; Hyodo, Toshio

doi:10.1088/0953-8984/9/50/024

Cited by 2 publications

(4 citation statements)

References 12 publications

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“…Such an assumption looks quite natural for the Ps atom in an ionic crystal in view of the experimental fact that Ps in alkali halides hardly distinguishes the anion and cation and only ''sees'' the simple cubic lattice. 8,10 The factor F a (q) in the third term is equal for longitudinal acoustic phonons to…”

Section: The Energy Of the Positronium As A Function Of Its Couplmentioning

confidence: 99%

“…3,4 The formation of Bloch-type positronium 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 crystal͒ in the momentum distribution of the photons resulting from the 2␥-decay of Ps upon irradiating the crystals by low-energy positrons at sufficiently low temperatures ͑typically less than a few tens K͒. [5][6][7][8][9][10] Such a shape of the momentum distribution is possible only in the case in which the wave function of the annihilating positronium is a Bloch function. 2 The positronium atom in this case is delocalized throughout the volume of the entire crystal.…”

Section: Introductionmentioning

confidence: 99%

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

Bondarev

1998

Phys. Rev. B

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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.

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Section: The Energy Of the Positronium As A Function Of Its Couplmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Bondarev

1998

Phys. Rev. B

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“…In contrast to the case of the exciton, 26 such an assumption appears to be fairly natural for Ps, since the experiment shows that Ps in alkali halides hardly distinguishes the anion and the cation and only ''sees'' the simple cubic lattice. 8,10 The second term in Eq. ͑2͒ is the phonon Hamiltonian.…”

Section: ͑2͒mentioning

confidence: 99%

“…3,4 The formation of the Bloch-type positronium is confirmed by observing very narrow peaks ͑the central peak and the side peaks appearing at the momentum corresponding to the reciprocal lattice vectors of the sample crystal͒ in the momentum distribution of the annihilation radiation at sufficiently low temperatures ͑typically less than a few tens K͒. [5][6][7][8][9][10] This type of positronium is completely delocalized.…”

Section: Introductionmentioning

confidence: 99%

Positronium in alkali halides: Tunneling from the delocalized to the self-trapped state

Bondarev

Hyodo

1998

Phys. Rev. B

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The tunnel transition of positronium ͑Ps͒ from its stable delocalized state to the metastable localized state in alkali halides is investigated theoretically using the formalism developed by Nasu and Toyozawa for excitons ͓J. Phys. Soc. Jpn. 50, 235 ͑1981͔͒. The tunneling rate between these two states is found to have the exponential dependence on the parameter B/ប (B is half of the Ps bandwidth, is the averaged frequency of the longitudinal acoustic phonons͒, whereas the potential barrier height between the two states is mainly determined by the bandwidth B and is proportional to B. A fitting procedure taking into account the quantum tunneling of Ps between the delocalized and the self-trapped state is proposed for the theoretical interpretation of the experimental data on Ps self-trapping. The procedure yields the numerical values of the parameters of Ps self-trapping ͑the tunneling rate and the potential barrier height͒ in qualitative agreement with those calculated theoretically. ͓S0163-1829͑98͒06218-3͔

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Two-dimensional angular correlation study of delocalized positronium in alkali halides

Cited by 2 publications

References 12 publications

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

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

Positronium in alkali halides: Tunneling from the delocalized to the self-trapped state

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