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(32) Production(s) de l'année 2024
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Measurement of near-field thermal radiation between multilayered metamaterials
Auteur(s): Zhang Sen, Dang Yongdi, Li Xinran, Iqbal Naeem, Jin Yi, Choudhury Pankaj k, Antezza M., Xu Jianbin, Yungui Ma
(Article) Publié:
Physical Review Applied, vol. 21 p.024054 (2024)
DOI: 10.1103/PhysRevApplied.21.024054
Résumé: The near-field radiative heat transfer (NFRHT) between one-dimensional metamaterials comprised of phonon dielectric multilayers was investigated experimentally. Large-size (1 × 1
cm
2
) near-field samples were fabricated using
Si
C
,
Si
O
2
, and
Ge
layers at a certain gap distance, and the effects of layer-stacking order and phonon-resonance quality on NFRHT were examined. The measured results show good agreement with the theoretical results obtained by employing the transmission-matrix method. Super-Planckian thermal radiation was observed between emitters and receivers with identical structures. The failure of effective-medium theory (EMT) at predicting the near-field heat flux has been evidenced by measurements, particularly in the presence of bounded surface modes, such as the epsilon-near-zero mode. Additionally, analyses have shown that, in specific scenarios, the EMT can offer reasonable physical insights into the underlying coupling process from the perspective of homogenized media. Furthermore, the conditions for applying the EMT in the near-field regime were also touched upon.
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BPS counting in string compactifications
Auteur(s): Alexandrov S.
Conférence invité: Fields & Strings 2024 (Moscou, RU, 2024-02-05)
Résumé: I'll review the known results about BPS indices, which encode in particular the entropy of BPS black holes, appearing in string compactifications down to four dimensions with various number of supersymmetries. First, I'll recall the well-known results about BPS states in N=8 and N=4 compactifications, and then present what is known about them in the N=2 case. Depending on time, I hope to cover some recent advances where an important role was played by (mock) modular symmetry.
Commentaires: presentation by Zoom
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Collective Relaxation Dynamics in a Three-Dimensional Lattice Glass Model
Auteur(s): Nishikawa Y., Berthier L.
(Article) Publié:
Physical Review Letters, vol. 132 p.067101 (2024)
Texte intégral en Openaccess :
Ref HAL: hal-04453684_v1
Ref Arxiv: 2307.08110
DOI: 10.1103/PhysRevLett.132.067101
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé: We numerically elucidate the microscopic mechanisms controlling the relaxation dynamics of a three-dimensional lattice glass model that has static properties compatible with the approach to a random first-order transition. At low temperatures, the relaxation is triggered by a small population of particles with low-energy barriers forming mobile clusters. These emerging quasiparticles act as facilitating defects responsible for the spatially heterogeneous dynamics of the system, whose characteristic lengthscales remain strongly coupled to thermodynamic fluctuations. We compare our findings both with existing theoretical models and atomistic simulations of glass-formers.
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A new approach combining microwave heat pulse and infrared thermography for non-invasive portable sap flow velocity measurement
Auteur(s): Louche Hervé, Pénarier Annick, Clair Bruno, Nouvel Philippe, Coillot C., Do Frederic
(Article) Publié:
Agricultural And Forest Meteorology, vol. 347 p.109896 (2024)
Texte intégral en Openaccess :
Ref HAL: hal-04445925_v1
DOI: 10.1016/j.agrformet.2024.109896
Exporter : BibTex | endNote
Résumé: Xylem sap flow measurement is a key method to quantify plant water use and assess the responses to environmental conditions and climatic change. However, available methods are generally invasive and of limited portability. This paper presents a non-invasive approach called TIMFLOW that combines microwave heat pulse and infrared thermography, while having a high portability and versatility potential. The methodology was tested in laboratory conditions for black poplar (Populus nigra) stems of various diameters (10–45 mm) and for the known sap flow velocity range (10–100 cm h−1). The heat pulse was generated by microwaves with a power amplifier supplying a bi-quad antenna at 2.45 GHz frequency located near the stem. The scene was filmed using a relatively low-cost light and compact InfraRed (IR) thermography camera. A stem temperature map was used to determine the heat pulse propagation velocity. The calculated heat velocity was highly correlated with the applied flow velocity with a unique relationship regardless of the diameter. The latter result confirms the equation of Marshall (1958) which links the sap velocity to the heat velocity with a vessel fraction of around 25 % within samples. The feasibility of outdoor measurements was also successfully tested. The assumed potentials and limitations of the proposed methodology are discussed. In summary, the study demonstrates the concept and validates, in woody stems, this new methodology for non-invasive portable sap flow velocity measurement.
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Water-Driven Sol–Gel Transition in Native Cellulose/1-Ethyl-3-methylimidazolium Acetate Solutions
Auteur(s): Mohamed Yunus Roshan Akdar, Koch Marcus, Dieudonne-George P., Truzzolillo D., Colby Ralph, Parisi Daniele
(Article) Publié:
Acs Macro Letters, vol. 13 p.219-226 (2024)
Ref HAL: hal-04426657_v1
DOI: 10.1021/acsmacrolett.3c00710
Exporter : BibTex | endNote
Résumé: The addition of water to native cellulose/1-ethyl-3methylimidazolium acetate solutions catalyzes the formation of gels, where polymer chain-chain intermolecular associations act as cross-links. However, the relationship between water content (Wc), polymer concentration (Cp), and gel strength is still missing. This study provides the fundamentals to design water-induced gels. First, the sol-gel transition occurs exclusively in entangled solutions, while in unentangled ones, intramolecular associations hamper interchain cross-linking, preventing the gel formation. In entangled systems, the addition of water has a dual impact: at low water concentrations, the gel modulus is water-independent and controlled by entanglements. As water increases, more cross-links per chain than entanglements emerge, causing the modulus of the gel to scale as Gp ∼ C p^2 Wc^3.0±0.2. Immersing the solutions in water yields hydrogels with noncrystalline, aggregate-rich structures. Such water-ionic liquid exchange is examined via Raman, FTIR, and WAXS. Our findings provide avenues for designing biogels with desired rheological properties.
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Colloidal Self-Assembly of Silver Nanoparticle Clusters for Optical Metasurfaces
Auteur(s): Lafitte Maeva, Dwivedi Ranjeet, Elancheliyan R., Lagugné-Labarthet François, Buisson Lionel, Ly Isabelle, Barois Philippe, Baron Alexandre, Mondain-Monval Olivier, Ponsinet Virginie
(Article) Publié:
Langmuir, vol. 40 p.2601-2615 (2024)
Ref HAL: hal-04423467_v1
DOI: 10.1021/acs.langmuir.3c02900
Exporter : BibTex | endNote
Résumé: Optical metasurfaces are two-dimensional assemblies of nanoscale optical resonators and could constitute the next-generation of ultra-thin optical components. The development of methods to manufacture those nanostructures on a large scale is still a challenge, while most performance demonstrations were obtained with lithographically fabricated metasurfaces, that are restricted to small scales. Self-assembly fabrication routes are scattering of the nanoresonators in a meta-fluid, and show that they present strong optical magnetic resonances and directional forward scattering patterns, with scattering efficiencies of up to 4. The metasurfaces consist in homogeneous films, of variable surface density, of colloidal clusters which have the same extinction properties on the surface and in the fluid. This experimental approach allows for the large scale production of metasurfaces.
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Impact of Single-Walled Carbon Nanotube Functionalization on Ion and Water Molecule Transport at the Nanoscale
Auteur(s): Mejri Alia, Arroyo Nicolas, Herlem Guillaume, Palmeri J., Manghi Manoel, Henn F., Picaud Fabien
(Article) Publié:
Nanomaterials, vol. 14 p.117 (2024)
Ref HAL: hal-04411217_v1
DOI: 10.3390/nano14010117
Exporter : BibTex | endNote
Résumé: Nanofluidics has a very promising future owing to its numerous applications in many domains. It remains, however, very difficult to understand the basic physico-chemical principles that control the behavior of solvents confined in nanometric channels. Here, water and ion transport in carbon nanotubes is investigated using classical force field molecular dynamics simulations. By combining one single walled carbon nanotube (uniformly charged or not) with two perforated graphene sheets, we mimic single nanopore devices similar to experimental ones. The graphitic edges delimit two reservoirs of water and ions in the simulation cell from which a voltage is imposed through the application of an external electric field. By analyzing the evolution of the electrolyte conductivity, the role of the carbon nanotube geometric parameters (radius and chirality) and of the functionalization of the carbon nanotube entrances with OH or COO− groups is investigated for different concentrations of group functions.
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