Ref HAL: hal-03929003_v1
PMID 36611023
Ref Arxiv: 2203.14604
DOI: 10.1038/s41467-023-35812-w
Ref. & Cit.: NASA ADS
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A recent breakthrough in glass science has been the synthesis of ultrastable glasses via physical vapor deposition techniques. These samples display enhanced thermodynamic, kinetic and mechanical stability, with important implications for fundamental science and technological applications. However, the vapor deposition technique is limited to atomic, polymer and organic glass-formers and is only able to produce thin film samples. Here, we propose a novel approach to generate ultrastable glassy configurations in the bulk, via random particle bonding, and using computer simulations we show that this method does indeed allow for the production of ultrastable glasses. Our technique is in principle applicable to any molecular or soft matter system, such as colloidal particles with tunable bonding interactions, thus opening the way to the design of a large class of ultrastable glasses.

Ref HAL: hal-03927602_v1
Ref Arxiv: 2212.11918
Ref INSPIRE: 2617479
DOI: 10.21468/SciPostPhys.15.4.177
Ref. & Cit.: NASA ADS
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A dark sector with non-abelian gauge symmetry provides a sound framework to justify stable dark matter (DM) candidates. We consider scalar fields charged under a $SU(N)$ gauge group, and show that the centre of $SU(N)$, the discrete subgroup $Z_N$ also known as $N$-ality, can ensure the stability of scalar DM particles. We analyse in some details two minimal DM models of this class, based on $SU(2)$ and $SU(3)$, respectively. These models have non-trivial patterns of spontaneous symmetry breaking, leading to distinctive phenomenological implications. For the $SU(2)$ model these include a specific interplay of two DM states, with the same interactions but different masses, and several complementary DM annihilation regimes, either within the dark sector or through the Higgs portal. The $SU(3)$ model predicts dark radiation made of a pair of dark photons with a unique gauge coupling, as well as regimes where DM semi-annihilations become dominant and testable.

Ref HAL: hal-03926750_v1
DOI: 10.1021/acs.energyfuels.2c03434
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The development of hybrid nanomaterials that preserve and combine the properties of their constituents is a central issue of nanosciences. Herein, we describe the polymerization via CuAAC (copper-catalyzed azide-alkyne cycloaddition) of cobalt(III) corroles around conductive carbon nanotubes to produce chemically robust hybrid catalysts for Oxygen Reduction Reaction (ORR). A combination of techniques including UV-Vis-NIR absorption, Raman and X-ray Photoelectron Spectroscopy (XPS) as well as Scanning Electron Microscopy (SEM) were used to characterize the assembly of the two parts of the functional hybrid system for which the activity and the selectivity toward the ORR process in acidic media are enlightened by a combination of Rotating Disk Electrode (RDE) and Rotating Ring Disk Electrode (RRDE) measurements. The polymerized hybrid (click MWNT-CoCorr) exhibits an overpotential of ca. 230 mV compared to a reference platinum ink; the number of electrons involved in the reduction of oxygen is close to 3 in acidic media demonstrating that the corrole cobalt centers in the hybrids reduce oxygen via a mix of 2 and 4 electrons pathways.

Ref HAL: hal-03922297_v1
Ref Arxiv: 2212.07954
Ref INSPIRE: 2614853
DOI: 10.1103/PhysRevD.108.015021
Ref. & Cit.: NASA ADS
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Within the bottom-up approach to holography, we construct a class of six-dimensional gravity models, and discuss solutions that can be interpreted, asymptotically in the far UV, in terms of dual five-dimensional conformal field theories deformed by a single scalar operator. We treat the scaling dimension of such operator, related to the mass of the one scalar field in the gravity theory, as a free parameter. One dimension in the regular geometry is compactified on a shrinking circle, hence mimicking confinement in the resulting dual four-dimensional theories. We study the mass spectrum of bosonic states. The lightest state in this spectrum is a scalar particle. Along the regular (confining) branch of solutions, we find the presence of a tachyonic instability in part of the parameter space, reached by a smooth deformation of the mass spectrum, as a function of the boundary value of the background scalar field in the gravity theory. In a region of parameter space nearby the tachyonic one, the lightest scalar particle can be interpreted as an approximate dilaton, sourced by the trace of the stress-energy tensor, and its mass is parametrically suppressed. We also compute the free energy, along several branches of gravity solutions. We find that both the dilatonic and tachyonic regions of parameter space, identified along the branch of confining solutions, are hidden behind a first-order phase transition, so that they are not realised as stable solutions, irrespectively of the scaling dimension of the deforming field-theory operator. The (approximate) dilaton, in particular, appears in metastable solutions. Yet, the mass of the lightest state, computed close to the phase transition, is (mildly) suppressed. This feature is amplified when the (free) parameter controlling the scaling dimension of the deformation is 5/2, half the dimension of space-time in the field theory.

Ref HAL: hal-03920514_v1
Ref Arxiv: 2209.09579
DOI: 10.1063/5.0128820
Ref. & Cit.: NASA ADS
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The low-temperature quasi-universal behavior of amorphous solids has been attributed to the existence of spatially localized tunneling defects found in the low-energy regions of the potential energy landscape. Computational models of glasses can be studied to elucidate the microscopic nature of these defects. Recent simulation work has demonstrated the means of generating stable glassy configurations for models that mimic metallic glasses using the swap Monte Carlo algorithm. Building on these studies, we present an extensive exploration of the glassy metabasins of the potential energy landscape of a variant of the most widely used model of metallic glasses. We carefully identify tunneling defects and reveal their depletion with increased glass stability. The density of tunneling defects near the experimental glass transition temperature appears to be in good agreement with experimental measurements.

Ref HAL: hal-03917632_v2
Ref Arxiv: 2208.09887
DOI: 10.1038/s41524-023-01140-2
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Dimensionality provides a clear fingerprint on the dispersion of infrared-active, polar-optical phonons. For these phonons, the local dipoles parametrized by the Born effective charges drive the LO-TO splitting of bulk materials; this splitting actually breaks down in two-dimensional materials. Here, we extend the existing theory to the one-dimensional (1D) case. Combining an analytical model with the implementation of density-functional perturbation theory in 1D boundary conditions, we show that the dielectric splitting in the dispersion relations collapses logarithmically at the zone center. The dielectric properties and the radius of the 1D materials are linked by the present work to these red shifts, opening novel IR and Raman characterization avenues.

Ref HAL: hal-03917628_v1
Ref Arxiv: 2212.12237
DOI: 10.1103/PhysRevB.107.094308
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Electron-phonon interaction and phonon frequencies of doped polar semiconductors are sensitive to long-range Coulomb forces and can be strongly affected by screening effects of free carriers, the latter changing significantly when approaching the two-dimensional limit. We tackle this problem within a linear-response dielectric-matrix formalism, where screening effects can be properly taken into account by generalized effective charge functions and the inverse scalar dielectric function, allowing for controlled approximations in relevant limits. We propose complementary computational methods to evaluate from first principles both effective charges -- encompassing all multipolar components beyond dynamical dipoles and quadrupoles -- and the static dielectric function of doped two-dimensional semiconductors, and provide analytical expressions for the long-range part of the dynamical matrix and the electron-phonon interaction in the long-wavelength limit. As a representative example, we apply our approach to study the impact of doping in disproportionated graphene, showing that optical Fr\"ohlich and acoustic piezoelectric couplings, as well as the slope of optical longitudinal modes, are strongly reduced, with a potential impact on the electronic/intrinsic scattering rates and related transport properties.

Commentaires: 28 pages, 11 figures