### Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle 523 du LPTMC - Tour 12-13

**Xia-qing Shi (Soochow university & SPEC, CEA Saclay)**

**Recent results on dense bacterial suspensions**

This talk will show that bacterial suspensions, beyond their intrinsic, dominating importance in biology, are also excellent systems to explore and test theoretical results on active matter, I will present recent experimental results on dense bacterial suspensions obtained in the groups of Masaki Sano (University of Tokyo), Yilin Wu (Chinese University of Hong Kong), and Hepeng Zhang (Shanghai Jiaotong University). I will put them in context, situating them within our current knowledge of active matter, stressing differences and similarities. Particular attention will be paid to the modeling efforts already deployed or to be developed in order to understand the fascinating large-scale phenomena observed by these 3 groups.

**Raphaël Chétrite (Laboratoire J. Dieudonné, Nice)**

**On Gibbs-Shannon Entropy **

This talk will focus on the question of the physical contents of the Gibbs-Shannon entropy outside equilibrium.

Article : Gavrilov-Chetrite-Bechhoeffer : Direct measurement of weakly nonequilibrium system entropy is consistent with Gibbs-Shannon form. PNAS 2017.

**Grégory Schehr (LPTMS Orsay)**

**Finite temperature free fermions and the Kardar-Parisi-Zhang equation at finite time**

I will consider a system of N one-dimensional free fermions confined by a harmonic well. At zero temperature (T=0), this system is intimately connected to random matrices belonging to the Gaussian Unitary Ensemble. In particular, the density of fermions has, for large N, a finite support and it is given by the Wigner semi-circular law. Besides, close to the edges of the support, the quantum fluctuations are described by the so-called Airy-Kernel (which plays an important role in random matrix theory). What happens at finite temperature T ? I will show that at finite but low temperature, the fluctuations close to the edge, are described by a generalization of the Airy kernel, which depends continuously on temperature. Remarkably, exactly the same kernel arises in the exact solution of the Kardar-Parisi-Zhang (KPZ) equation in 1+1 dimensions at finite time.

I will also discuss recent results for fermions in higher dimensions.

**André Thiaville (LPS Orsay)**

**Topology in magnetism**

After recalling what topology is, the seminal work by Ernst Feldtkeller [1] introducing topology in magnetism will be described. His arguments were generalized and systematized, first by Maurice Kléman and coworkers [2], into what is now known as the topological theory of defects, for all types of order parameter and samples’ dimensionality and topology [3]. The tools of this theory can be also used to define topologically stable continuous magnetic structures, also called topological solitons, prominent examples being the vortex and the skyrmion. The associated topological number will be recalled and its signification discussed through examples. The distinct role of chirality, compared to topology, will be stressed. We shall then describe the links of topology with magnetization dynamics. The electrical transport through magnetic textures with non-trivial topology will also be discussed. Finally, the energy cost of “breaking the topology” will be discussed, through the following questions: what is the energy of the singular Bloch point, how large is the energy barrier to reverse a vortex or to delete a magnetic skyrmion ?

[1] E. Feldtkeller, Mikromagnetisch stetige und unstetige Magnetisierungskonfigurationen, Z. angew. Phys. 19, 530-536 (1965). English translation IEEE Trans. Magn. 53, 0700308 (2017).

[2] G. Toulouse, M. Kléman, Principles of a classification of defects in ordered media, J. Physique Lett. 37, L149-151 (1976).

[3] N.D. Mermin, The topological theory of defects in ordered media, Rev. Mod. Phys. 51, 591-648 (1979); M. Kléman, Points, lines and walls (Wiley, New York, 1983).

**André Thiaville (LPS Orsay)**

**Magnétisme et topologie [titre provisoire]**