The center for Theoretical Physics (CPHT) at Ecole Polytechnique gathers research scientists working in diverse domains of fundamental and applied Physics. The overall coherence is assured by the corpus of common, transposable, mathematical and numerical methods.
CPHT is a joint research unit of CNRS and Ecole Polytechnique, and has a partnership with the Collège de France. His director is Jean-René Chazottes, Senior Researcher at CNRS.
CPHT is on the campus of Ecole Polytechnique, buildings 5 and 6. The reception offices are located in building 6 , offices 06.1046 and 06.1045.
 

Postal Address :
CPHT 
Ecole Polytechnique 
91128 Palaiseau cedex 
France

Secretary phone number : 01 69 33 42 01 (from abroad: +33 169 334 201)

Write an email to someone at CPHT :  : firstname.lastname@polytechnique.edu

 

 

Jan Thorben Schneider soutiendra publiquement ses travaux de thèse le 30 septembre 2022 à 15h00 au CPHT dans la salle de conférence Louis Michel.

Titre de la thèse : Far-from-equilibrium dynamics and entanglement in long-range quantum systems

Directeur de thèse : Laurent Sanchez-Palencia

Membres du jury de thèse

Grégoire Misguich, CEA Saclay, Rapporteur
Marco Schirò, Collège de France, Rapporteur
Karyn Le Hur, École Polytechnique, Examinatrice
Leonardo MazzaUniversité Paris-Saclay, Examinateur
Laurent Sanchez-Palenci, École polytechnique, Directeur de thèse

Abstract

In this thesis, we study the effects of long-range interactions on out-of-equilibrium and in-equilibrium features of lattice spin models by employing complementary analytical calculations and state-of-the-art tensor-network simulations while particularly focusing on the central and unique quantum feature of entanglement.
First, in the long-range transverse-field Ising model, we show the emergence of a weak form of causality characterised by non-universal dynamical exponents. On the one hand, local magnetisation and correlations have an emergent sub-ballistic causal cone while the marked features in the interior of it propagate super-ballistic or ballistic, respectively. On the other hand, the emergent causal cone for all entanglement entropies is shown to be ballistic irrespective of the interaction range and the interior is without marked features. Second, we determine the equilibrium quantum phase diagram of the long-range XXZ model in terms of the anisotropic coupling and the long-range interaction exponent through studying a representation of the spectrum of the reduced density matrix following a half-chain bipartition, the so-called entanglement spectrum.
We show it exhibits a remarkable self-similarity within the critical phase where the system is described by a Luttinger liquid while the self-similarity extends to the geometric entanglement and the Luttinger parameter.
The transition away from a Luttinger liquid is consistent with the breakdown of self-similarity and a renormalisation group analysis. The synergetic combination of the two latter allows us to locate the corresponding phase transitions which we corroborate by numerical simulations.
Furthermore, we show the entanglement Hamiltonian, the Hermitian operator whose spectrum is the entanglement spectrum, follows the form of the Bisognano--Wichmann theorem in large regions of the phases which include the short-range limit, while such a form can be excluded in the phase where genuinely long-ranged effects are relevant.
Our results shed new light through the lens of quantum entanglement on the out-of-equilibrium as well as ground state features of long-range interacting spin chains and pave the way for further experimental and theoretical studies.

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Balthazar de Vaulchier soutiendra publiquement ses travaux de thèse le 19 septembre 2022 à 14h00 à l'Ecole polytechnique dans l'Amphi Gregory.

Titre de la thèse : String theory and aspects of quantum or induced gravity

Directeur de thèse : Hervé Partouche

Membres du jury de thèse

M. Hervé PARTOUCHE École polytechnique Directeur de thèse
M. Iosif BENA CEA/Saclay Rapporteur
M. Jan TROOST Ecole Normale Supérieure Rapporteur
M. Carlo ANGELANTONJ Université de Turin Examinateur
M. Antoniadis IGNATIOS Sorbonne Université Examinateur
M. Guillaume BOSSARD Ecole polytechnique Examinateur
M. Francesco NITTI Université de Paris Examinateur

Résumé

Cette thèse explore divers perspectives visant à obtenir une théorie de gravitation quantique. Elle s’articule autour de trois axes majeurs, à savoir la théorie des cordes, la gravitation induite, et la fonction d’onde de l’Univers ; chacun d’entre eux est susceptible d’apporter des réponses à l’unification de la gravitation avec la mécanique quantique. Nous commençons ainsi par une présentation de la théorie des cordes, en particulier de la corde hétérotique, avant de s’intéresser à des modèles de compactification appliqués à cette corde. Le premier est un mécanisme de Scherk-Schwarz brisant la supersymétrie : une instabilité se produit alors dans des régions de l’espace de module, qui engendre une transition de phase que nous caractérisons. Le deuxième considère des modèles d’orbifold que nous unifions avec le formalisme des lignes de Wilson, afin notamment de décrire des actions sur 8 fermions au lieu de 16, et obtenir ainsi des groupes de jauges SO(2n+1). Ensuite, nous présentons un modèle de gravitation induite dépourvu de divergence ultraviolette, en attribuant à chaque champ de matière une infinité d’états de Kaluza-Klein. Les constantes induites de la théorie effective sont alors calculées à une boucle, et un choix spécifique de champ permet d’éliminer les divergences. Enfin, la troisième partie s’attache à la fonction d’onde de l’Univers telle que définie par Hartle et Hawking. Nous calculons dans un premier temps l’intégrale de chemin avec un formalisme invariant de jauge, ce qui faisait défaut jusqu’à présent. Puis nous prouvons que la probabilité d’amplitude est bien invariante sous une redéfinition des champs, en reliant du même coup ces redéfinitions de champ à l’ambiguïté résiduelle dans la formulation de l’équation de Wheeler-DeWitt. Enfin, nous donnons des pistes pour aborder ce problème avec une intégrale de chemin Lorentzienne et non plus Euclidienne.

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Seminar on Condensed matter and Topology 
Thursday 15th of September
2pm-5pm
Workshop to take place at CPHT, room "Louis Michel" (Batiment 6, niveau 0, CPHT, École Polytechnique)
Related to the PhD Thesis of Julian Legendre (10am-1pm)

Chairperson: Alexandru Petrescu, INRIA Ecole des Mines Paris (PhD Yale & CPHT, 2015)

2pm-2:25pm(+5 minutes questions)
Walter Hofstetter, Goethe University Frankfurt (DFG FOR2414 Speaker)
Strong Correlations and Topology in Multiflavor Quantum Gases

2:30pm-2:55pm(+5 minutes questions)
Guillaume Roux, LPTMS University Orsay
Kinetic pairing and two-fluid phase in a spinless fermions chain

3pm-3:30pm Cafes/Encas (Break/Cakes)

3:30pm-3:55pm(+5 minutes questions)
Ulrich Schneider, University of Cambridge
Ultracold atoms in optical quasicrystals – from fractality to localisation

4pm-4:25pm(+5 minutes questions)
Pierre Pujol, University Paul Sabatier Toulouse
A skyrmion fluid and bimeron glass emerging from a chiral spin liquid

4:30pm-4:55pm(+5 minutes questions)
Karyn Le Hur, CPHT Ecole Polytechnique and CNRS
Correlated Entangled matter, Light and Fractional Topology

 
The PhD defense of Julian Legendre and the seminar will also be accessible remotely via the following zoom link:

 

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Antoine Georges, Gabriel Kotliar and Dieter Vollhardt have been jointly awarded the 2022 Feenberg Memorial Medal for established work that has significantly advanced the field of many-body physics. The award is for the Dynamical Mean Field Theory (DMFT) method, now an important tool in the Many-Body community. The award is for the Dynamical Mean Field Theory (DMFT) method, now an important tool in the Many-Body community.
See: https://tarheels.live/rpmbt21/2022/08/05/feenberg-memorial-medal-awardees-announced/

English

 

Julian Legendre will publicly defend his thesis work on September 15, 2022 at 10:00 a.m. at the CPHT in the Louis Michel conference room

Title: Topological phases, light response and kagome lattice

Advisor: Karyn Le Hur

Jury :

Walter Hofstetter (Président du Jury) Professeur, Goethe Universität Frankfurt (Institut für Theoretische Physik)
Grégoire Misguich (Rapporteur) Chercheur en physique théorique et ingénieur CEA (Institut de Physique Théorique)
Guillaume Roux (Rapporteur) Maître de Conférences, Université Paris-Saclay (Laboratoire de Physique Théorique et Modèles Statistiques)
Pierre Pujol (Examinateur) Professeur, Université Paul Sabatier Toulouse (Laboratoire de Physique Théorique)
Ulrich Schneider (Examinateur) Profeseur, University of Cambridge (Cavendish Laboratory)
Karyn Le Hur (Directrice de thèse) Directrice de Recherche au CNRS et Professeure PCC, École Polytechnique (Centre de Physique Théorique)

Abstract: We theoretically study topological lattice models relevant to current experimental solid- state and artificial systems. We develop an explicit analytical computation of the Chern number in the systems we study and we compare it with other computation methods of this topological invariant. We propose a protocol, based on the local response to a light input, to probe the topological properties of a Haldane boson model in a photonic system. On the kagome lattice, we investigate (i) a magnetic and topological phase transition for a two-channel model, in relation with recently discovered quantum materials, and (ii) a time- reversal topological model with flux, Rashba spin-orbit coupling and Hubbard interactions, relevant for realization in cold-atom gases.

After the reception, we planned a seminar on Condensed matter and Topology

 
The PhD defense of Julian Legendre and the seminar will also be accessible remotely via the following zoom link:

 

English

 

Sabine Harribey soutiendra publiquement ses travaux de thèse le 17 juin 2022 à l'INRIA (bâtiment Alan Turing), salle Gilles Kahn.

Titre de la thèse : Renormalization in tensor field theory and the melonic fixed point

Participer à la réunion Zoom pour la soutenance de thèse :
https://ecolepolytechnique.zoom.us/j/89946552020
ID de réunion : 899 4655 2020

Co-directeurs de thèse : Razvan Gurau, Dario Benedetti et Christoph Kopper

Membres du jury :
- Holger Gies (Université Friedrich-Schiller de Jena) Rapporteur
- Grigory Tarnopolsky (Carnegie Mellon University) Rapporteur
- Matthias Bartelmann (Université d’Heidelberg)
- Lauriane Chomaz (Université d’Heidelberg)
- Vincent Rivasseau (Université Paris-Saclay)

Abstract: This thesis focuses on the study of the renormalization group flow in tensor field theories. Its first part considers a quartic tensor model with O(N)^3 symmetry and long-range propagator. The existence of a non-perturbative fixed point in any d at large N is established. We found four lines of fixed points parametrized by the so-called tetrahedral coupling. One of them is infrared attractive, strongly interacting and gives rise to a new kind of conformal field theories, called melonic CFTs. This melonic CFT is then studied in more details. We first compute dimensions of bilinears and operator product expansion coefficients at the fixed point. The results are consistent with a unitary CFT at large N. We then compute 1/N corrections to the fixed point. At next-to-leading order, the line of fixed points collapses to one fixed point. However, the corrections are complex and unitarity is broken at next-to-leading order. Finally, the F-theorem is investigated for this model. This theorem states that the free energy of a CFT on the sphere in dimension 3 decreases along the renormalization group flow. We show that our model respects this theorem. The next part of the thesis investigates sextic tensor field theories in rank 3 and 5. In rank 3, we found two infrared stable real fixed points in short range and a line of infrared stable real fixed points in long range. Surprisingly, the only fixed point in rank 5 is the Gaussian one. For the rank 3 model, in the short-range case, we still find two infrared stable fixed points at next-to-leading order. However, in the long-range case, the corrections to the fixed points are non-perturbative and hence unreliable: we found no precursor of the large N fixed point.  The last part of the thesis investigates the class of model exhibiting a melonic large N limit. Indeed, this limit was lacking for models with ordinary tensor representations of O(N) and Sp(N), such as symmetric traceless or antisymmetric ones. Recently, it was proven that models with tensors in an irreducible representation of O(N) or Sp(N) in rank 3 indeed admit a large N limit. This proof is here extended in rank 5. This generalization relies on recursive bounds derived from a detailed combinatorial analysis of Feynman graphs involved in the perturbative expansion of our model.

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