Ref HAL: hal-04244864_v1
Ref Arxiv: 2310.14682
DOI: 10.1140/epje/s10189-023-00306-6
Ref. & Cit.: NASA ADS
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Small-angle scattering is a commonly used tool to analyze the dispersion of nanoparticles in all kinds of matrices. Besides some obvious cases, the associated structure factor is often complex and cannot be reduced to a simple interparticle interaction, like excluded volume only. In recent experiments, we have encountered a surprising absence of structure factors (S(q) = 1) in scattering from rather concentrated polymer nanocomposites [A.-C. Genix et al, ACS Appl. Mater. Interfaces 11 (2019) 17863]. In this case, quite pure form factor scattering is observed. This somewhat “ideal” structure is further investigated here making use of reverse Monte Carlo simulations in order to shed light on the corresponding nanoparticle structure in space. By fixing the target “experimental” apparent structure factor to one over a given q-range in these simulations, we show that it is possible to find dispersions with this property. The influence of nanoparticle volume fraction and polydispersity has been investigated, and it was found that for high concentrations only a high polydispersity allows reaching a state of S = 1. The underlying structure in real space is discussed in terms of the pair-correlation function, which evidences the importance of attractive interactions between polydisperse nanoparticles. The calculation of partial structure factors shows that there is no specific ordering of large or small particles, but that the presence of attractive interactions together with polydispersity allows reaching an almost “structureless” state.

Résumé:

BPS indices encoding entropy of supersymmetric black holes in compactifications of Type II string theory on compact Calabi-Yau threefolds coincide with generalized Donaldson-Thomas invariants whose computation represents an outstanding problem. I'll show how this problem can be solved for a set of one-parameter threefolds by combining a direct integration of topological string, modular properties of rank 0 DT invariants counting D4-D2-D0 BPS states, and wall-crossing relations between rank 1 and rank 0 DT invariants. In particular, one obtains explicit (mock) modular functions encoding infinite sets of D4-D2-D0 BPS indices and new boundary conditions for the holomorphic anomaly equation allowing to overcome the limitations of the direct integration method.

Résumé:

BPS indices encoding entropy of supersymmetric black holes in compactifications of Type II string theory on compact Calabi-Yau threefolds coincide with generalized Donaldson-Thomas invariants whose computation represents an outstanding problem. I'll show how this problem can be solved for a set of one-parameter threefolds by combining a direct integration of topological string, modular properties of rank 0 DT invariants counting D4-D2-D0 BPS states, and wall-crossing relations between rank 1 and rank 0 DT invariants. In particular, one obtains explicit (mock) modular functions encoding infinite sets of D4-D2-D0 BPS indices and new boundary conditions for the holomorphic anomaly equation allowing to overcome the limitations of the direct integration method.

Ref HAL: hal-04251349_v2
Ref Arxiv: 2304.07135
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Exciton energy structure and population dynamics in a wide CdTe/CdZnTe quantum well are studied by spectrally-resolved pump-probe spectroscopy. Multiple excitonic resonances in reflectance spectra are observed and identified by solving numerically three-dimensional Schrodinger equation. The dynamics of the pump-probe reflectivity signal is shown to be dominated by the photoinduced nonradiative broadening of the excitonic resonances, while pump-induced exciton energy shift and reduction of the oscillator strength appear to be negligible. This broadening is induced by the reservoir of dark excitons with large in-plane wave vector, which are coupled to the bright excitons' states. The dynamics of the pump-induced nonradiative broadening observed experimentally is characterized by three components: signal build up on the scale of tens of picoseconds (i) and bi-exponential decay on the scale of one nanosecond (ii) and ten nanoseconds (iii). Possible mechanisms of the reservoir population and depletion responsible for this behavior are discussed.

Commentaires: 8 pages, 3 figures

Ref HAL: hal-04251300_v1
Ref Arxiv: 2306.04404
DOI: 10.1103/PhysRevB.108.125421
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Excitons hosted by GaN/(Al,Ga)N quantum wells (QWs) are spatially indirect due to the giant built-in electricfield that separates electrons and holes along the growth direction. This electric field, and thus exciton energy,can be reduced by depositing metallic layers on the sample surface. Using spatially resolved microphotolu-minescence spectroscopy we compare the effects of two different materials, nickel/gold (NiAu) and few-layergraphene (FLG), on the potential landscape experienced by the excitons. We are able to (i) determine the potentialbarriers imposed on QW excitons by deposition of FLG and NiAu to be 14 and 82 meV, respectively, and(ii) to evince their impact on the exciton transport at appropriate densities. Optical losses and inhomogeneousbroadening induced by deposition of NiAu and FLG layers are similar, and their joined implementation constitutea promising tool for electrostatic modulation of exciton densities even in the absence of any applied electric bias

Ref HAL: hal-04244898_v1
DOI: 10.3389/frsfm.2023.1260211
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In the context of a more sustainable economy, bio-surfactants become increasingly important, due to their independence of petrol-based chemistry, their usually mild synthesis conditions, and in certain cases their pharmacological activity. We have recently discussed self-assembly studies in binary systems of bio-surfactants of microbial origin, or saponins extracted from plants (Hellweg et al., Frontiers in Soft Matter, 2023, 2). In the present review, we focus on the formation of microemulsions based on these molecules. We review the formation and structure of microemulsion systems formed by oil, water, and biosurfactants, with a particular focus on Quillaja saponins and rhamnolipids.

Ref HAL: hal-04248760_v1
DOI: 10.1088/1361-6528/acc3a2
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Nanoporous GaN layers were fabricated using selective area sublimation through a self-organized AlN nanomask in a molecular beam epitaxy reactor. The obtained pore morphology, density and size were measured using plan-view and cross-section scanning electron microscopy experiments. It was found that the porosity of the GaN layers could be adjusted from 0.04 to 0.9 by changing the AlN nanomask thickness and sublimation conditions. The room temperature photoluminescence properties as a function of the porosity were analysed. In particular, a strong improvement (>100) of the room temperature photoluminescence intensity was observed for porous GaN layers with a porosity in the 0.4–0.65 range. The characteristics of these porous layers were compared to those obtained with a Si$_x$ N$_y$ nanomask. Furthermore, the regrowth of p-type GaN on light emitting diode structures made porous by using either an AlN or a Si$_x$ N$_y$ nanomask were compared.