Laboratoire de Physique Théorique

de la Matière Condensée



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


Karim Essafi
Okinawa Institute of Science and Technology <>

A Kagome Map of Spin Liquids

*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.

Séminaire du LPTMC du mardi 24/10 à 11h à la bibliothèque du LPTHE (couloir 13-14 4ème étage).

David Carpentier (ENS Lyon)

The Chiral Anomaly and Ballistic Transport of Weyl Fermions

The chiral anomaly is a unique property of massless relativistic particles in three dimensions. While the chiral nature of particles was expected to be a conserved property, the breaking of this symmetry was a bizarre property of the associated field theory. The recent focus on materials in which low energy electrons behave as massless relativistic Weyl  particles has opened the possibility to experimentally access consequences on transport of this field theory property. Indeed, an anomalous  contribution to the conductance in the presence of a magnetic field was associated to the presence of the chiral anomaly. In this talk, I will focus on the different regime of ballistic transport of Weyl fermions in small mesoscopic conductors.

I will show how the anomalous transport can be traced back to the presence of anomalous Landau Levels in such a junction, and to the chiral anomaly. I will finally comment on the relation between diffusive and ballistic regimes.


Vittore Scolari (Institut Pasteur, Paris)

Kinetic signature of cooperativity in the irreversible collapse of a polymer

We investigated the kinetics of a polymer collapse due to the formation of irreversible crosslinks between its monomers. We use the contact probability as a function of the monomeric distance as a scale dependent order parameter and show in simulations the emergence of acooperative pearling instability. This produces a sharp conformational transition in time, inducing a crossover between short and long distance behaviour due to the formation of pearls. The size of pearls and the transition time depends on the equilibrium dynamics of the polymer and the rate at which cross links are formed. We finally confirm experimentally the existence of this transition using a chromosome conformation capture experiment.

Julien Cividini (Weizmann institute, Israël)

Driven tracer with absolute negative mobility

Instances of negative mobility, where a system responds to a perturbation in a way opposite to naive expectation, have been studied theoretically and experimentally in numerous nonequilibrium systems. After reviewing part of the literature on the topic, we will consider a simple one-dimensional lattice model of a driven tracer in bath. We will show that contrary to previous expectations, Absolute Negative Mobility (ANM), whereby current is produced in a direction opposite to the drive, occurs around an equilibrium state. We derive analytical predictions for the mobility in the linear response regime. The high density regime will help us elucidate the mechanism leading to ANM. The lattice model can be seen as a toy model for hard Brownian discs in a narrow planar channel. Molecular dynamics studies show that the hard discs model exhibits Negative Differential Mobility (NDM), but no ANM.


Nicolas Wschebor (Université de la République d'Uruguay, Montevideo)

Proving conformal invariance in critical scalar theories in any dimension

Conformal invariance in three dimension has a tremendous renewed interest due to the surprisingly good results obtained by using the “conformal bootstrap” in last five years. In this talk, the interest of this symmetry is reviewed and its existence in critical (scale invariant) theories in any dimension is discussed. In particular, using Wilson renormalization group, we show that if no integrated vector operator of scaling dimension −1 exists in a given model, then scale invariance implies conformal invariance. By using the Lebowitz inequalities, we prove that this necessary condition (or another similar necessary condition proposed by Polchinski many years ago) is fulfilled in all dimensions less than four for the Ising universality class. This shows, in particular, that scale invariance implies conformal invariance for the three-dimensional Ising model. Finally, the extension of this result to other critical systems is discussed.