Le 13ème séminaire des jeunes chercheurs du CPHT aura lieu le 22 septembre 2023 à 15h00 à la Salle de conférence Louis Michel avec deux présentations :

Erik Linnér : "A theoretical description of competing instabilities in strongly correlated fermionic systems" 

Matthieu Vilatte : "Probing the asymptotic structure of gravity"

Adrien Loty : "Does string theory saturate unitarity bounds?"

Le séminaire sera suivi d'un goûter.

Colloquium Friedmann 28-9-2023 17:00 Amphi Faure

Slava Mukhanov (Ludwig-Maximilians-Universität München)
How predictive are cosmological theories ?

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Biséminaire Friedmann CPHT-LLR  29-9-2023 salle de conférences du LLR (aile 5 - centre)

11:00
Oleg Lebedev (University of Helsinki)
Gravity, dark matter and its detection

14:30
Slava Mukhanov (Ludwig-Maximilians-Universität München)
False vacuum decay: thick wall approximation

Affiche Biséminaire

Tuesday, September 19 at 14:00

at CPHT, Conference room Louis Michel

Hareram Swain (IIT Madras)

A semi-holographic approach to strange metallic behaviour

Abstract: "Strange metals" exhibit an anomalous temperature dependence of the low temperature resistivity. This suggests the absence of electron quasiparticles which are the elementary excitations of the Fermi sea. This property is at odds with conventional Fermi liquids, justifying the “strangeness” of this phase. The measurement of the spectral function via ARPES has given us key insights into the nature of elementary constituents in strongly correlated electronic systems that do not admit quasiparticle description, and also demonstrate a rich variety of novel superconducting, metal, and insulating phases. The holographic approach, though allows one to dispense with the notion of quasiparticles, at the same time, this makes it difficult to understand what are the effective microscopic degrees of freedom of the system. In my talk, I will explain how we construct a semi-holographic description for such behaviours. Based on the key insights provided by Faulkner and Polchinski, we propose an effective theory in which the electron of a two-dimensional band hybridizes with a fermionic operator of a critical holographic sector, while also interacting with other bands that preserve quasiparticle characteristics. Besides the scaling dimension $\nu$ of the fermionic operator in the holographic sector, the effective theory has two dimensionless couplings α and γ determining the holographic and Fermi-liquid-type contributions to the self-energy respectively. In the case of DC conductivity that irrespective of the choice of the holographic critical sector, there exists a ratio of the effective couplings for which we obtain linear-in-T resistivity for a wide range of temperatures. This scaling persists to arbitrarily low temperatures when $\nu$ approaches unity in which limit we obtain a marginal Fermi liquid with a specific temperature dependence of the self-energy. Interestingly, we explain the origin of the linear-in-T resistivity and strange metallic behavior as a consequence of the emergence of a universal form of the spectral function which is independent of the model parameters when the ratio of the two couplings takes optimal values determined only by the critical exponent. This universal form fits well with photoemission data of copper oxide samples for under/optimal/over-doping with a fixed exponent over a wide range of temperatures. We further obtain a refined Planckian dissipation scenario in which the scattering time τ=f*h_bar/(k_B*T), with f being O(1) at strong coupling, but O(10) at weak coupling.

Ephraim Bernhardt soutiendra publiquement ses travaux de thèse le 18 septembre 2023 à 10h30 au CPHT dans la salle de conférence Louis Michel.

Titre : Topologie et effets de désordre dans les systèmes de spin quantique ouverts et en interaction et leurs analogues

Directrice de thèse: Karyn Le Hur

Résumé

Jury :
Alberto Rosso, Directeur de recherche au CNRS, LPTMS, Université Paris-Saclay (Rapporteur)
Thomas Schmidt, Professeur, Université du Luxembourg (Rapporteur)
Cristiano Ciuti, Professeur, MPQ, Université Paris-Cité (Examinateur)
Michel Ferrero, Chargé de recherche au CNRS, CPHT, École Polytechnique (Examinateur)
Loic Henriet, Chief Technology Officer, PASQAL (Examinateur)
Peter P. Orth, Professeur, Universität des Saarlandes (Examinateur)
Matteo Rizzi, Professeur, Universität zu Köln, (Examinateur)
Karyn Le Hur, Directrice de recherche au CNRS, CPHT, École Polytechnique (Directrice de thèse)
Cyril Elouard, Junior Professor, LPCT, Université de Lorraine (Invité)

La soutenance sera suivie du pot de thèse dans la salle Jean Lascoux au CPHT.

Après le pot de thèse, une conférence sera organisée sur le thème "Quantum many-body physics - new directions and modern applications"

Chair: Karyn Le Hur, thesis supervisor, CNRS, CPHT, École Polytechnique

- 14h15 - 14h45 Matteo Rizzi (Universität zu Köln): Phase Diagram Detection via Gaussian Fitting of Number Probability Distribution

- 14h45 - 15h15 Thomas Schmidt (Université du Luxembourg) Topology and semiclassical transport in interacting electron system

-15h15 - 15h45 Peter P. Orth (Universität des Saarlandes) Nonlinear interrogation of quantum materials: why higher order response tells you more

- 15h45 - 16h15 Cyril Elouard (LPCT, Université de Lorraine) Extending the laws of thermodynamics for arbitrary autonomous quantum systems

16h15 - 16h30 Café et en-cas

- 16h30 - 17h00 Cristiano Ciuti (MPQ, Université Paris-Cité) New quantum many-body effects in high-impedance multi-mode circuit QED

- 17h00 - 17h30 Loic Henriet (Pasqal) tba

- 17h30 - 18h00 Alberto Rosso (CNRS, LPTMS, Université Paris-Saclay) tba

- 18h00 - 18h30 Michel Ferrero (CNRS, CPHT, École Polytechnique) tba

Tuesday, September 12 at 14:00

at CPHT, Conference room Louis Michel

Naoya Iwahara (Chiba University, Japan)

Dynamic Jahn-Teller effect in cubic spin-orbit Mott insulators

Abstract: The synergistic interplay of different interactions in materials leads to the emergence of novel quantum phenomena. Spin-orbit and vibronic couplings usually counteract each other; however, in cubic 4d/5d double perovskites they can coexist and give rise to spin-orbit-lattice entanglement (dynamic Jahn-Teller effect) on the metal sites. The correlation of these entangled states induced by intersite interactions has not been assessed so far. In the seminar, I will first show that the dynamic Jahn-Teller effect develops in iridium compounds based on the analysis of the resonant inelastic x-ray scattering spectra [1]. Then, I will talk about the ordering of the entangled states in a series of cubic 5d1 double perovskites [2]. The magnetically ordered states in these systems coexist with a vibronic order characterized by the ordering of vibronic quadrupole moments on sites. This treatment allows for the rationalization of a number of unexplained features of experimentally investigated phases.

[1] N. Iwahara and W. Furukawa, Phys. Rev. B 108, 075136 (2023).
[2] N. Iwahara and L. F. Chibotaru, Phys. Rev. B 107, L220404 (2023).

Chargée de recherche du CNRS au Centre de physique théorique de l’École polytechnique, Olesia DMYTRUK a été distinguée par une bourse ERC Starting Grant pour son projet intitulé Quantum light-controlled topological phases of matter ("Q-Light-Topo")

Le contrôle des propriétés des matériaux par la lumière est une nouvelle direction de recherche de la physique de la matière condensée. Les matériaux topologiques sont très importants dans cette direction en raison de leur robustesse et de leur application possible dans les technologies quantiques. Le projet théorique « Q-Light-Topo » se concentrera sur l'utilisation de la lumière quantique pour sonder, contrôler et créer des phases topologiques de la matière dans des matériaux à l'état solide couplés avec des photons de cavité. Ce projet explorera comment contrôler les transitions de phase topologiques dans divers matériaux fortement couplés à la lumière, et étudiera la caractérisation topologique des excitations polaritoniques hybrides lumière-matière apparaissant dans de tels systèmes. L'objectif principal du projet « Q-Light-Topo » est de proposer un protocole d'ingénierie d'états topologiques dans des matériaux électroniques initialement triviaux avec de la lumière quantique. Concevoir de nouveaux systèmes topologiques contribuera à faire avancer le domaine des technologies quantiques.

Renaud Garioud will publicly defend his thesis work on Tuesday 4th July. The defense will be held at 3p.m. in room 2 of Collège de France (Collège de France, 11 Pl. Marcelin Berthelot, 75231 Paris)

Title: When perturbation theory becomes non-perturbative : application to strongly correlated systems

Advisor: Michel Ferrero

Abstract : Strongly correlated materials reveal remarkable physical phenomena at low temperatures. Depending on external parameters, they exhibit extremely different electronic phases, ranging from insulating magnetic orders to strong superconductivity with infinite electrical conductivity. The richness of these physical phenomena takes its roots in the strong interactions that impact heavily the behaviour of electrons. To accurately describe these properties, one must solve the quantum many-body problem of interacting particles. In this thesis, we focus on the development of new algorithms to address strongly interacting fermionic systems.

By considering electronic interactions as a perturbation to the non-interacting system, we focus on computing efficiently, and up to high orders, the perturbation series, which can be expressed as sums of Feynman diagrams. We present the CDet (Connected Determinants) state-of-the-art algorithm which allows us to reach high perturbation orders. We overcome one of the main limitations of perturbation theory by introducing a novel chemical potential shift that breaks a symmetry. We show that this approach allows us to describe perturbatively the physics of ordered phases in the thermodynamic limit. We apply this new algorithm to the cubic half-filled Hubbard model and provide a quantitative description of the Néel order both near the phase transition and at low temperature up to the high coupling regime. This study enables us to detail the limitations to our method and to present the numerical tools that ensure an efficient implementation of the CDet algorithm and an accurate resummation of the resulting perturbative series. The attractive counterpart of this model shows a superconducting phase that can also be described by adapting our symmetry-breaking approach.

We are happy to announce the seminar entitled

"Control of Auto-Resonant Beat-Wave Excitation of Plasma Waves."
given by our former group member
Dr. Mufei Luo
(Chalmers University of Technology, Göteborg, Sweden)

Thursday 29th of June 2023 , 15 h
Salle Louis Michel of CPHT, building 06

La soutenance d’habilitation à diriger des recherches de Michel Ferrero sur le sujet : « Diagrammatic Monte Carlo: recent developments and applications to the Hubbard model » aura lieu le 7 juin 2023 à 14h00 au Collège de France (Salle 2) à Paris.