Origins of positronium emitted from<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">SiO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Nagashima, Yasuyuki; Morinaka, Yuta; Kurihara, Toshikazu; Nagai, Yasuyoshi; Hyodo, Toshio; Shidara, T.; Nakahara, K.

doi:10.1103/physrevb.58.12676

Cited by 123 publications

(89 citation statements)

References 12 publications

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“…Furthermore, the unknown binding energy of Ps at the MgO surface ͑MgO is too dense a material for Ps to be formed in the bulk͒ and the uncertainty about whether valence or conduction electrons participate in Ps formation obscure a direct translation to the positron affinity for MgO. 29,30 Summarizing, determining positronium formation potentials at the surface of insulating oxides such as MgO is experimentally difficult, which results in the different values reported in the literature. Therefore, we turn to ab initio calculations to determine the positron affinities.…”

Section: Theoretical Methods and Conceptsmentioning

confidence: 78%

Positron confinement in embedded lithium nanoclusters

et al. 2002

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Quantum confinement of positrons in nanoclusters offers the opportunity to obtain detailed information on the electronic structure of nanoclusters by application of positron annihilation spectroscopy techniques. In this work, positron confinement is investigated in lithium nanoclusters embedded in monocrystalline MgO. These nanoclusters were created by means of ion implantation and subsequent annealing. It was found from the results of Doppler broadening positron beam analysis that approximately 92% of the implanted positrons annihilate in lithium nanoclusters rather than in the embedding MgO, while the local fraction of lithium at the implantation depth is only 1.3 at. %. The results of two-dimensional angular correlation of annihilation radiation confirm the presence of crystalline bulk lithium. The confinement of positrons is ascribed to the difference in positron affinity between lithium and MgO. The nanocluster acts as a potential well for positrons, where the depth of the potential well is equal to the difference in the positron affinities of lithium and MgO. These affinities were calculated using the linear muffin-tin orbital atomic sphere approximation method. This yields a positronic potential step at the MgOʈLi interface of 1.8 eV using the generalized gradient approximation and 2.8 eV using the insulator model.

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Section: Theoretical Methods and Conceptsmentioning

confidence: 78%

Positron confinement in embedded lithium nanoclusters

et al. 2002

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“…The positronium atoms are emitted into the voids of such materials from the bulk by work-function emission [19], which is largely independent of the sample temperature. However, this does mean that such Ps will be initially hot (~ 1 eV [20]) and must cool via collisions with the internal surfaces, which can lead to annihilation. Thus, the colder the emitted Ps, the less there will be, and the longer it will take to leave the sample [29].…”

Section: Introductionmentioning

confidence: 99%

“…In an insulator this may occur in the bulk material [19,20], while in conductors or semiconductors Ps may only be formed on surfaces; bulk Ps is unbound because of screening effects in metals [21], or likely exists as a weakly bound exciton-like state in some semiconductors [22].…”

Section: Introductionmentioning

confidence: 99%

Positronium formation via excitonlike states on Si and Ge surfaces

et al. 2011

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Recent experiments combining lifetime and laser spectroscopy of positronium (Ps) show that these atoms are emitted from p-Si(100) at a rate that depends on the sample temperature, suggesting a thermal activation process, but with an energy that does not, precluding direct thermal activation as the emission mechanism. Moreover, the amount of Ps emitted is substantially increased if the target is irradiated with 532 nm laser light just prior to the implantation of the positrons. Our interpretation of these data was that the Ps was emitted via an exciton-like positron-electron surface state, not dissimilar to an electronic surface exciton observed on Si in two-photon photoemission measurements. The hypothesis that this state may be populated by electrons from one of the occupied electronic surface states, either thermally or by laser excitation, is consistent with our observations and suggests that one should expect a high Ps yield at room temperature from an n-doped Si(100) sample, since in this case the same surface states will already be occupied. Here we present data obtained with an n-Si(100) target that supports our model, and also reveals the unexpected result that the kinetic energy of Ps emitted from this material actually decreases when it is heated, an effect we attribute to shifts in the surface energy levels due to the presence of a high density of thermally generated electrons. We show data obtained with a Ge(100) sample that corroborate the idea that the effects we observe are related to surface electron states, and hence should occur for any indirect band gap semiconductor with dangling bond states. Our model is further confirmed by the observation that the Ps yield from p-Si depends linearly on the surface density of the implanted positrons due to the effects of electron-hole pairs created as the positrons slow down.

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“…The delay of signal is equal to the time of flight of Ps and for given base length allows to determine the velocity and the kinetic energy of o-Ps. The inset on the right of figure shows the results of TOF measurements by Nagashima et al [62] for KJ crystal. The values of U found in the experiments of this kind lie in the range 1÷3 eV.…”

Section: The Ore Model and Blob Modelmentioning

confidence: 99%

“…The η parameter diminishes with deepening the potential well. For the case of silica the TOF measurements [62] give U ≈ 3 eV; thus for the ground state in the well η should be 0.084 nm. In Fig.…”

Section: The Role Of Excited Statesmentioning

confidence: 99%