Dynamics of Exciton Formation at the Si(100)<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>c</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>4</mml:mn><mml:mo>×</mml:mo><mml:mn>2</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math>Surface

Weinelt, Martin; Kutschera, Michael; Fauster, Thomas; Rohlfing, Michael

doi:10.1103/physrevlett.92.126801

Cited by 129 publications

(106 citation statements)

References 43 publications

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“…The formation of Ps via this mechanism depends on the availability of surface electrons, which may be produced either thermally [35] or by laser irradiation [36]. The former process involves the thermal activation of electrons to surface states, the existence of which is dependent on the presence of a positron, which means that the temperature dependence of Ps emission from Si looks almost identical to thermal activation curves obtained from metal samples [37].…”

Section: Introductionmentioning

confidence: 54%

“…In the case of Si, the surface reconstruction changes from (2×1) to (4×2) below ~ 150 K [83]; while we might not expect this to significantly affect the formation of Ps it should be checked, especially since the electronic surface exciton observed by Wienelt et al [36] was only observed for the (4×2) surface [84]. If there is a thermal contribution to the energy spread of emitted Ps then photoemission from a cold target would provide a narrow beam.…”

Section: Discussionmentioning

confidence: 99%

“…Lifetime and laser spectroscopy has revealed that the emission of Ps from the surface of p-Si(100) does not, for the most part, occur via the usual channels for metals or semiconductors [35] described above (i.e., direct capture of electrons by energetic positrons or thermal desorption of Ps from a positronic surface state [24]). Rather, Ps from this material is mostly emitted via the formation of an intermediate surface electron-positron bound state that appears to be analogous to the surface exciton (that is, an electron bound to a hole in the surface band) observed by Weinelt et al [36]. The surface excitons are formed when electrons are excited from the valence to the conduction band, and then become captured in dangling bond surface states, from whence they form surface excitons by becoming bound to surface holes [36].…”

Section: Introductionmentioning

confidence: 99%

“…Rather, Ps from this material is mostly emitted via the formation of an intermediate surface electron-positron bound state that appears to be analogous to the surface exciton (that is, an electron bound to a hole in the surface band) observed by Weinelt et al [36]. The surface excitons are formed when electrons are excited from the valence to the conduction band, and then become captured in dangling bond surface states, from whence they form surface excitons by becoming bound to surface holes [36]. However, if there is a positron present on the surface, as may be the case just after the implantation of a positron pulse, then these electrons may form positronium via a similar mechanism.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Introductionmentioning

confidence: 54%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Positronium formation via excitonlike states on Si and Ge surfaces

et al. 2011

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“…Applications cover the ultrafast dynamics in image potential states, [1][2][3][4][5] electron relaxation in metals, 6-9 semiconductors 10-13 and more recently topological insulators, [14][15][16][17][18][19] charge transfer processes at solid state interfaces [20][21][22][23][24] and in adsorbate on surfaces [25][26][27][28][29][30][31] , and the formation and dynamics of excitons. 10,11,[32][33][34][35] The theoretical description of excitons constitutes the main focus of the present work.…”

Section: Introductionmentioning

confidence: 99%

First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra

et al. 2016

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We show that any quasi-particle or GW approximation to the self-energy does not capture excitonic features in time-resolved (TR) photoemission spectroscopy. In this work we put forward a first-principles approach and propose a feasible diagrammatic approximation to solve this problem. We also derive an alternative formula for the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the relaxed regime characterized by the presence of quasi-stationary excitons and vanishing polarization. The main distinctive feature of the theory is that the diagrams must be evaluated using excited Green's functions. As this is not standard the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated ab initio. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR and angle-resolved photoemission.

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Electron Dynamics in Image Potential States at Metal Surfaces

Fauster

2010

Dynamics at Solid State Surfaces and Interfaces

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Electron dynamics describes the temporal evolution of an electronic state due to interactions with particles or quasiparticles. The ground state of the system is characterized by the stationary states whose time dependence is determined by the energy of the state. In the present context, we consider an excited electron at the surface, for example, in a surface or adsorbate state. On its relaxation path to the ground state (or thermal equilibrium) it is going to be scattered and the possible processes can be divided into the following categories:. Electron-electron scattering: This most important process for energy relaxation leads to decay into bulk or surface states at lower energy with simultaneous creation of an electron-hole pair. The coupling to the electronic system can also include other quasiparticles such as excitons, plasmons, and magnons that are not covered in this chapter. . Electron-phonon scattering: This scattering process mainly changes the direction of the electron motion and embraces both scattering to bulk bands and scattering within the surface state band. The energy loss is usually rather small compared to electron-electron scattering. Temperature is a convenient control of electron-phonon scattering. . Electron defect scattering: Real surfaces contain always a nonnegligible amount of defects, such as steps or impurity atoms. The associated electron defect scattering mainly changes the electron momentum leaving the energy almost unchanged. In many aspects, it is similar to electron-phonon scattering. Electron defect scattering can be identified by controlling the defect density.Image potential states [1] are a special class of surface states that exist on many metal surfaces (see chapter 3 of volume 2). The small binding energies relative to the vacuum level point to a weak coupling to the metal. Correspondingly, image potential states can have relatively long lifetimes (>10 fs) and can conveniently be studied with

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Dynamics of Exciton Formation at the Si(100)c(4×2)Surface

Cited by 129 publications

References 43 publications

Positronium formation via excitonlike states on Si and Ge surfaces

Positronium formation via excitonlike states on Si and Ge surfaces

First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra

Electron Dynamics in Image Potential States at Metal Surfaces

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