Three Short Stories on Chiral Structures in Condensed Matter

Abstract

Chirality plays a crucial role in many instances in condensed matter physics. The three examples of this role that I present pose well defined questions and answers to these questions will be proposed. An unifying thema of these stories is perhaps that the chiral information may be transported from the molecular to the (quasi) macroscopic level. The first example concerns the observation of crystals growing in the form of spirals on Langmuir monolayers. I argue that this might be due to an uneven distribution of impurities in the growing crystal, this one being itself related to the molecular structures through the lack of symmetry of the Wulff’s plot for crystals of chiral material. The second example (a joint work with J. Lega) is related to the helical structures observed by N. Mendelson on strings of a mutant of Bacterium Subtillis. It is possible to explain this helicity as resulting from a buckling of the cell wall under forces depending on the helical structure of the molecules. This provides a way for transferring the genetic information on chirality from molecular to large scales. The third story (a joint work with P. Coullet and J. Lega) is about nonvariational effect in the dynamics of Bloch walls in ferromagnets. Those Bloch walls have an helicity depending on their preparation, but with a definite sign, if the magnetic interaction is not too anisotropic. Then under some conveniently chosen external stress (a rotating magnetic field) it is possible to move those Bloch walls in a direction depending on the sign of their helicity. This is a typical nonequilibrium phenomenon, because for gradient flow systems, the dynamics of the wall between two phases (here the two orientations of the magnetization) is independent on the wall structure and is a function of the (free) energy difference between the two sides of the wall only.