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).

Olesia DMYTRUK, CNRS research fellow with the Center for Theoretical Physics at École Polytechnique, was awarded an ERC Starting Grant.

She receives the ERC support for her project "Quantum light-controlled topological phases of matter".

Controlling properties of materials with light is a novel research direction of condensed matter physics. Topological materials play a particularly important role in this direction due to their robustness and their possible applications in quantum technologies. The aim of the Q-Light-Topo project led by Olesia Dmytruk is to propose a protocol for engineering topological phases of matter with light. To achieve this goal, solid-state systems such as nanowires and two-dimensional materials coupled to cavity photons will be considered. Olesia Dmytruk's project will explore how to control topological phase transitions in various topological materials strongly coupled to light, and also study properties of hybrid polaritonic light-matter excitations arising in such systems.

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

The defense of Michel Ferrero's habilitation on the subject: "Diagrammatic Monte Carlo: recent developments and applications to the Hubbard model" will take place on June 7, 2023 at 2:00 pm at the Collège de France.

Collège de France
Salle 2
11, place Marcelin Berthelot
75231 Paris 

Seconde journée thématique sur le thème de la renormalisation le lundi 12 juin à l'Ecole Polytechnique, Amphi Curie

https://indico.math.cnrs.fr/e/themaRenormalisation12juin2023

Orateurs

Nils Berglund (université d'Orléans, institut Denis Poisson)

Jonas Lampart (Université de Bourgogne, laboratoire UCB)

Slava Rychkov (IHES)

Monday 15/05 at 14:00

Conference room Louis Michel (CPHT)

Marc-Olivier Renou (Inria and CPHT)

Bell theorem and its generalization: From foundations to applications

Abstract: Quantum correlations are obtained when multiple parties perform independent measurements on a shared quantum state. Bell’s seminal theorem proves that certain correlations predicted by quantum theory resist explanations in terms of any Local Hidden Variable theory based on shared randomness. I’ll first review the Bell theorem, its main foundational consequences and some of its applications. Then, I will discuss recent generalization of this theorem in the context of causal quantum networks, in which now multiple parties perform independent measurements on several, independent quantum states.

Tuesday 02/05 at 14:00

Conference room Louis Michel (CPHT)

Matteo Crispino (ESPCI Paris)

Slave-spin mean field for broken-symmetry states: Néel antiferromagnetism and its phase separation in multi-orbital Hubbard models

Abstract: We introduce the generalization of the Slave-Spin Mean-Field method to broken-symmetry phases. Through a variational approach we derive the single-particle energy shift in the mean-field equations which generates the appropriate self-consistent field responsible for the stabilization of the broken symmetry. With this correction the different flavours of the slave-spin mean-field are actually the same method and they give identical results to Kotliar-Ruckenstein slave-bosons and to the Gutzwiller approximation. We apply our formalism to the Néel antiferromagnetic state and study it in multi-orbital models as a function of the number of orbitals and Hund's coupling strength, providing phase diagrams in the interaction-doping plane. We show that the doped antiferromagnet in proximity of half-filling is typically unstable towards insulator-metal and magnetic-non magnetic phase separation. Hund's coupling extends the range of this antiferromagnet, and favors its phase separation.

L’aménagement du patio avance bien. Nous avons déjà notre petit verger (pommiers, poiriers, figuier…), et surtout de la terre qui est préparée pour les plantations, nous avons 5 parcelles prêtes à l’emploi qui n’attendent que nous !

Vendredi 21 avril à 13h15  réunion mensuelle dans le patio

C’est une réunion très importante car nous devons décider :

- Qu’est-ce que nous allons planter ? (tomates ? concombres ? pommes de terres ? herbes aromatiques ? herbes médicinales ? etc)

- Comment nous allons planter sur 5 parcelles ? Parcelles thématiques (ex. : légumes ; herbes …) ? Parcelles par famille botanique (ex : Cucurbitacées …) ? Autre ?

- Quand allons-nous planter ? Nous devons établir un planning de plantation. Les jardiniers nous conseillent vivement de planter très rapidement, c’est-à-dire la semaine prochaine, tant que la terre n’est pas encore très dure.

- Combien nous sommes pour planter ?

Venez nombreux avec vos idées ! Parlez-en autour de vous avec vos collègues !

Le 29 juin 2023 aura lieu à l'ENS-Lyon la sixième  Journée Parité 2023 organisée par Olga Paris-Romaskevich, Frédérique Bienvenüe, Vincent Borrelli, Jean-René Chazottes, Claire Lacour, Bertrand Rémy.

Cette 6ème Journée Parité s'adresse à celles et ceux intéressé·e·s qui veulent réfléchir et échanger sur les manières d'agir pour une communauté mathématique inclusive, apaisée et ouverte. Nous espérons qu'elle vous apportera du soutien, des outils, des idées et de belles rencontres pour encore plus de sens dans cette aventure collective.