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

**Vincent Démery (Gulliver-ESPCI)**

**Unbinding transition of probes in single-file systems**

Single-file transport, arising in quasi-one-dimensional geometries where particles cannot pass each other, is characterized by the anomalous dynamics of a probe, notably its response to an external force. I will present a simple hydrodynamic framework that allows to compute this response, and also the response of several probes to arbitrary external forces, where an unbinding transition can occur.

Séminaire commun avec le LPTHE. Horaire et salle exceptionnels: **bibilothèque du LPTHE (tours 13-14, 4ème étage) vendredi à 11h**

**Dynamics in quantum antiferromagnets**

**Maxime Dupont (U.C. Berkeley)**

Theoretically challenging, the understanding of the dynamical response in quantum magnets is of great interest, in particular for both inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments. In this talk, I will address this question for quasi-one-dimensional quantum magnets, e.g., weakly coupled spin chains for which many compounds are available in nature. In this class of systems, the dimensional crossover between a three-dimensional ordered regime at low temperature towards one-dimensional physics at higher temperature is a nontrivial issue, notably difficult concerning dynamical properties. I will present a comprehensive theoretical study based on both analytical calculations and numerical simulations which allows us to describe the full temperature crossover for the NMR relaxation rate 1/T1, from one-dimensional Tomonaga-Luttinger liquid physics to the three-dimensional ordered regime, as a function of interchain couplings.

**Compact packings of spheres**

**Thomas Fernique (LIPN, CNRS et U. Paris 13)**

It is well known that the best way to pack oranges in a (very large) box is to place them on a face-centered cubic lattice (also known as checkerboard), although this has been formally demonstrated only in 1998 (with difficulty). This talk focuses on what happens when the dimension or number of different spheres change. In particular, so-called compact packings (the term will be defined properly) seem good candidates to maximize density. In this tallk, we propose : a) a non-technical survey of the known mathematical results ; b) an overview of the underlying computer problems (interval arithmetic, resolution of systems of polynomial equations, combinatorial exploration) ; (c) a discussion of possible applications in chemistry, including self-assembly of supercrystals ; d) a discussion of numerical simulations that may be of interest in this context.

**Quantum spin liquids: an experimental view **

**Fabrice Bert (LPS Orsay)**

Spin liquids are fascinating states of matter where quantum fluctuations are strong enough to prevent any kind of magnetic ordering down to absolute zero temperature. Spin liquid physics has been for long a rich playground for theoreticians to discover novel quantum states and concepts, often relevant to other fields such as high Tc superconductivity, and there are now several materials realizing such ground states. I will present an experimental investigation with resonance techniques of two of them: a vanadium oxyfluoride compound where the V4+ ions form a unique S = ½ breathing kagome lattice which consists of alternating equilateral triangles, preserving the full frustration of the isotropic model, and the recently discovered Y-kapellasite which realizes an original spatially anisotropic kagome model.

**Continuous-time Quantum Walks**

**Kirone Mallick (IPhT CEA Saclay)**

Quantum analogs of classical random walks have been defined in quantum information theory as a useful concept to implement algorithms. Due to interference effects, statistical properties of quantum walks can drastically differ from their classical counterparts, leading to much faster computations.

In this talk, we shall discuss various statistical properties of continuous-time quantum walks on a lattice, such as: survival properties of quantum particles in the presence of traps (i.e. a quantum generalization of the Donsker-Varadhan stretched exponential law), the growth of a quantum population in the presence of a source, quantum return probabilities and Loschmidt echoes.