### Attention : désormais les séminaires auront lieu tous les lundis à 11h00 en salle 523 du LPTMC - Tour 12-13

**
Karim Essafi**

**Abstract:**Competing interactions in frustrated magnets prevent ordering down to very low temperatures and stabilize exotic highly degenerate phases where strong correlations coexist with fluctuations. We study a very general nearest-neighbour Heisenberg spin model Hamiltonian on the kagome lattice which consist of Dzyaloshinskii-Moriya, ferro- and antiferromagnetic interactions. We present a three-fold mapping which transforms the well-known Heisenberg antiferromagnet (HAF) and XXZ model onto two lines of time-reversal Hamiltonians. The mapping is exact for both classical and quantum spins, i.e. preserves the energy spectrums of the HAF and XXZ model. As a consequence, our three-fold mapping gives rise to a connected network of quantum spin liquids centered around the Ising antiferromagnet. We show that this quantum disorder spreads over an extended region of the phase diagram at linear order in spin wave theory, which overlaps with the parameter region of Herbertsmithite ZnCu3(OH)6Cl2. At the classical level, all the phases have an extensively degenerate ground-state which present a variety of properties such as ferromagnetically induced pinch points in the structure factor and spontaneous scalar chirality which was absent in the original HAF and XXZ models.

**16h - Bibliothèque du LPTHE (couloir 13-14, 4ème étage)**

**Shamik Gupta**

Max Planck Institute for the Physics of Complex Systems,

**Synchronization in coupled oscillator systems**

Collective synchronization involves a large population of coupled oscillators of diverse frequencies spontaneously synchronizing to oscillate at a common frequency. Common examples are synchronized firings of heart cells, phase synchronization in electrical power networks, rhythmic applause in concert halls, etc. The Kuramoto model serves as a prototype to study collective synchronization. The model comprises oscillators with distributed natural frequencies interacting through a mean-field coupling. Interpreting the dynamics as that of a long-range interacting system driven out of equilibrium by quench disordered external torques, I will discuss the rich out-of-equilibrium phenomena the model exhibits, and in particular, its complete phase diagram for unimodal frequency distributions.

**Shamik Gupta**

Max Planck Institute for the Physics of Complex Systems,

**Synchronization in coupled oscillator systems**

Abstract: Collective synchronization involves a large population of coupled oscillators of diverse frequencies spontaneously synchronizing to oscillate at a common frequency. Common examples are synchronized firings of heart cells, phase synchronization in electrical power networks, rhythmic applause in concert halls, etc. The Kuramoto model serves as a prototype to study collective synchronization. The model comprises oscillators with distributed natural frequencies interacting through a mean-field coupling. Interpreting the dynamics as that of a long-range interacting system driven out of equilibrium by quench disordered external torques, I will discuss the rich out-of-equilibrium phenomena the model exhibits, and in particular, its complete phase diagram for unimodal frequency distributions.

**Andrej Mesaros**

Cornell university

** Short-ranged charge modulations in cuprates: r-space or k-space?**

Electronic spatial charge modulations (CM) have been recently established as integral to the phase diagram of high-transition-temperature (high-Tc) cuprate superconductors. It is not yet understood if CM are involved in establishing high-Tc superconductivity, or are detrimental to it. The answer hinges on determining, in face of disorder, the relationship between periodicities of electronic CM and underlying lattice. Here we determine that CM are commensurate in the entire pseudogap region of Bi2Sr2CaCu2O8 phase diagram. The found commensurability points towards a strong coupling perspective on electrons, likely implying cooperation with superconductivity. We introduce a general analysis that exploits phase-sensitive measurements to find the periodicity of inhomogeneous modulations, and apply it to scanning tunneling microscopy images.

**Simona Cocco**

LPS, ENS

**Inference of interaction networks from the correlated activity of a population of variables.**

I will present some inverse statistical physics approaches to extract models of the activity of a population of variables from their recorded correlated activity. I will detail two applications of these inference techniques in the domain of neuroscience and of bioinformatics: the analysis of multi-electode recordings of a population of neurons and the modeling of protein structure and functions from sequences collected in data bases.