Dégénérescence (physique quantique)

Résumé

En physique quantique, la dégénérescence est le fait pour plusieurs états quantiques distincts de se retrouver au même niveau d'énergie. Un niveau d'énergie est dit dégénéré s'il correspond à plusieurs états distincts d'un atome, molécule ou autre système quantique. Le nombre d'états différents qui correspond à un niveau donné est dit son degré de dégénérescence. Mathématiquement, la dégénérescence est décrite par un opérateur hamiltonien ayant plusieurs fonctions propres avec la même valeur propre. Le phénomène est dû la plupart du temps à une symétrie entre les états. La brisure extérieure ou spontanée de la symétrie lève la dégénérescence. L'effet Zeeman ou la structure fine du spectre atomique sont des exemples de levée de dégénérescence. La dégénérescence joue aussi un rôle dans la physique statistique. Au cas d'un système de N particules en trois dimensions, un niveau d'énergie peut correspondre à plusieurs fonctions d'onde ou états physiques. Ces états dégénérés possèdent to

Source officielle

https://fr.wikipedia.org/wiki/D%C3%A9g%C3%A9n%C3%A9rescence_(physique_quantique)

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A Novel Method to Quantify Self-Healing Capabilities of Fiber-Reinforced Polymers

Amaël Maximilien Cohades, Véronique Michaud, Cecilia Scazzoli, Robin Samuel Trigueira

The present work investigates a novel and practical method to evaluate the healing efficiency of carbon-reinforced polymer composites. The method should be representative of damage occurring during the lifetime of a composite part, should tend to damage the healable matrix mostly and yet be simple and cost-effective to set up. Thus, the capacity to recover low-velocity impact damage has been evaluated via three-point bending flexural tests. Carbon-reinforced composite laminates were produced using HealTech (TM) T300-TW200-42RW-1250, a commercially healable resin pre-impregnated Torayca T300 3K twill 2 x 2 fabric with an aerial weight of 200 g/m(2). Fibers were oriented at +/- 45 degrees or at 0 degrees-90 degrees, and the laminates were impacted at different energy levels. Flexural properties of undamaged, damaged, and healed samples were compared, and the healing efficiency was calculated as the ratio of healed and undamaged ultimate flexural strength or modulus. Since matrix healing efficiency is the value to characterize, it was shown that +/- 45 degrees laminates could be tested without major fiber damage and, thus, provide the best matrix healing efficiency results. Such a method proved to be 1) representative of early-stage damage of composite FRPs often occurring in the form of delamination or matrix microcracking, and 2) a fast and reliable characterization technique requiring the use of a limited amount of material.

FRONTIERS MEDIA SA2022

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Flux Tube S-Matrix Bootstrap

Andrea Guerrieri, Aditya Hebbar

We bootstrap the S matrix of massless particles in unitary, relativistic two dimensional quantum field theories. We find that the low energy expansion of such S matrices is strongly constrained by the existence of a UV completion. In the context of flux tube (FT) physics, this allows us to constrain several terms in the S matrix low energy expansion or-equivalently-on Wilson coefficients of several irrelevant operators showing up in the FT effective action. These bounds have direct implications for other physical quantities; for instance, they allow us to further bound the ground state energy as well as the level splitting of degenerate energy levels of large FTs. We find that the S matrices living at the boundary of the allowed space exhibit an intricate pattern of resonances with one sharper resonance whose quantum numbers, mass, and width are precisely those of the world-sheet axion proposed by Athenodorou, Bringoltz, and Teper and Dubovsky, Flauger, and Gorbenko. The general method proposed here should be extendable to massless S matrices in higher dimensions and should lead to new quantitative bounds on irrelevant operators in theories of Goldstones and, also, in gauge and gravity theories.

AMER PHYSICAL SOC2019

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Two-dimensional systems with C2T (PT) symmetry exhibit the Euler class topology E is an element of Z in each two-band subspace realizing a fragile topology beyond the symmetry indicators. By systematically studying the energy levels of Euler insulating phases in the presence of an external magnetic field, we reveal the robust gaplessness of the Hofstadter butterfly spectrum in the flat-band limit, while for the dispersive bands the gapping of the Landau levels is controlled by a hidden symmetry. We also find that the Euler class E of a two-band subspace gives a lower bound for the Chern numbers of the magnetic subgaps. Our study provides new fundamental insights into the fragile topology of flat-band systems going beyond the special case of E = 1 as, e.g., in twisted bilayer graphene, thus opening the way to a very rich, still mainly unexplored, topological landscape with higher Euler classes.

AMER PHYSICAL SOC2022

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