[Dislocations et autres bizzareries topologiques]
Nous allons montrer que l'ouvrage Dislocations de Jacques Friedel, publié voici un demi-siècle, peut toujours être recommandé, ainsi que le voulait l'auteur, comme un manuel « pour des étudiants chercheurs universitaires et pour des élèves d'écoles d'ingénieurs, aussi bien que pour des ingénieurs chercheurs ». En effet, les dislocations sont connues aujourd'hui pour intervenir non seulement dans les cristaux solides, mais aussi dans de nombreux autres systèmes découverts plus récemment, tels que les cristaux colloïdaux ou des cristaux liquides avec des structures périodiques. De plus, le concept de dislocation constitue un excellent point de départ des exposés sur les défauts topologiques dans les systèmes munis de paramètres d'ordre résultant de symétries brisées : disinclinaisons dans des cristaux liquides nématiques ou hexatiques, dispirations dans les phases smectiques chirales ou disorientations dans les cristaux liquides lyotropes. La discussion sur les dislocations dans les phases bleues nous fournira une occasion pour nous souvenir de Sir Charles Frank, un ami de Jacques Friedel depuis ses années à Bristol, qui appelait ces mésophases éphémères « bizarreries topologiques ». Étant formées de réseaux de disclinations, les phases bleues sont similaires aux phases smectiques à joints de grains torsadés (Twist Grain Boundaries, TGB), qui sont constituées de réseaux de dislocations vis et dont l'existence a été prévue par de Gennes en 1972 à partir de l'analogie entre les phases smectiques et les supraconducteurs. Nous insisterons sur le fait que le livre de Jacques Friedel contient les germes de cette analogie.
We will show that the book Dislocations by Jacques Friedel, published half a century ago, can still be recommended, in agreement with the author's intention, as a textbook “for research students at University and for students at engineering schools as well as for research engineers”. Indeed, today dislocations are known to occur not only in solid crystals but also in many other systems discovered more recently such as colloidal crystals or liquid crystals having periodic structures. Moreover, the concept of dislocations is an excellent starting point for lectures on topological defects occurring in systems equipped with order parameters resulting from broken symmetries: disclinations in nematic or hexatic liquid crystals, dispirations in chiral smectics or disorientations in lyotropic liquid crystals. The discussion of dislocations in Blue Phases will give us an opportunity to call on mind Sir Charles Frank, friend of Jacques Friedel since his Bristol years, who called these ephemeral mesophases “topological oddities”. Being made of networks of disclinations, Blue Phases are similar to Twist Grain Boundary (TGB) smectic phases, which are made of networks of screw dislocations and whose existence was predicted by de Gennes in 1972 on the basis of the analogy between smectics and superconductors. We will stress that the book by Jacques Friedel contains seeds of this analogy.
Mot clés : Dislocations, Défauts topologiques, Cristaux liquides
@article{CRPHYS_2016__17_3-4_242_0,
author = {Pawel Pieranski},
title = {Dislocations and other topological oddities},
journal = {Comptes Rendus. Physique},
pages = {242--263},
publisher = {Elsevier},
volume = {17},
number = {3-4},
year = {2016},
doi = {10.1016/j.crhy.2015.12.002},
language = {en},
}
Pawel Pieranski. Dislocations and other topological oddities. Comptes Rendus. Physique, Volume 17 (2016) no. 3-4, pp. 242-263. doi : 10.1016/j.crhy.2015.12.002. https://comptes-rendus.academie-sciences.fr/physique/articles/10.1016/j.crhy.2015.12.002/
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