Laboratoire de Physique Théorique de la Matière Condensée

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Séminaires du LPTMC

Cette page contient les annonces des séminaires à venir, ainsi que les archives des séminaires.

Pour accéder aux archives, saisir une date de début (sous la forme JJ.MM.AAAA) et de fin dans les champs ci-dessous et éventuellement le nom d'un orateur ou un mot-clé dans le champ de recherche en dessous et cliquer sur 'Valider'.

25.12.2024 - 21.2.2025
  • Thibault Bonnemain (PhLAM, Lille) & Simon Metayer (Shanghai)

    Date 18.02.2025 10:45 - 11:45
    Séminaires

    Thibault Bonnemain : Generalised Hydrodynamics of the KdV Soliton Gas

    Generalised Hydrodynamics (GHD) is a rather recent theory that provides a framework for studying a wide family of many-body integrable and nearly integrable systems out of equilibrium. I will introduce the notion of soliton gas, associated with the Korteweg-de Vries (KdV) equation among others, and use it as a paradigmatic example for the GHD of integrable, classical field theories. In particular, by way of a heuristic argument based on the analogy between solitons and classical particles, I will construct the thermodynamics of the KdV soliton gas (free energy, entropy, static covariance...), as well as the Euler-scale hydrodynamic equations describing the evolution of weakly inhomogeneous gases. The results thus produced agree with numerical simulations; moreover they are consistent with and supplement the more rigorous, albeit less transparent, construction pioneered by Gennady El, based on the so-called thermodynamic spectral limit of finite-gap solutions

    Simon Metayer : Precision Field Theory Applied to Statistical Physics Systems

    I present an overview of my recent results in higher-order analytical computations of renormalization group observables in interacting field theories relevant to condensed matter many-body systems. Extending beyond leading-order approximations is often challenging but crucial. It enhances theoretical precision, provides benchmarks for less controlled methods, and, more interestingly, can unveil new physical phenomena. I will illustrate these points through concrete examples from my recent works. For clean polymerized membranes, cutting-edge four-loop calculation yield unprecedented precision for the anomalous elasticity exponent, thanks to surprisingly well-behaved epsilon expansions. These results benchmark precisely NPRG and SCSA methods and align closely with simulations and experiments on various membranes. Generalizing to the quenched disordered case, three-loop corrections reveal a previously unseen disorder-induced wrinkling transition towards a glassy phase fixed point, observed in partial polymerization experiments. In three-dimensional QED, higher-order large-N corrections refine the optical conductivity of graphene, while solving self-consistently the Schwinger-Dyson equations truncated beyond leading order sheds new light on fermion mass generation and interaction-driven metal-insulator transitions in graphene-like systems. In a minimal supersymmetric extension, higher-order large-N estimates predict the optical properties of "supergraphene" and uncover the previously unseen impact of supersymmetry in preventing dynamical symmetry breaking, making supergraphene a permanent conductor. These results demonstrate how precision calculations in field theory still remain a powerful framework for understanding and unveiling new emergent phenomena in complex statistical and quantum physics systems. 

  • Nicolas Regnault (LPENS, Paris)

    13.02.2025 14:00 - 15:00
    Séminaires TQM

    Nicolas Regnault (Flatiron institute, NY et LPENS Paris)

    Engineering quantum phases of matter through moire materials: The case of Fractional Chern insulators

    Progress in stacking two dimensional materials, such as graphene or transition metal dichalcogenides (TMDs), has paved the way to engineer new structures relying on moire patterns. These patterns induced for example by slightly twisting one layer compared to the other, could lead to strongly correlated quantum phases such as superconductivity or the quantum anomalous Hall effects. In the realm of condensed matter physics, the fractional quantum Hall effect stands as a singular experimental manifestation of topological order, characterized by the presence of anyons—quasiparticles that bear fractional charge and exhibit exchange statistics diverging from conventional fermions and bosons. This phenomenon, observed over four decades ago, was still missing the direct observation of similar topological orders arising purely from band structure—without the application of strong magnetic fields. In 2023 within the span of a few months, several pioneering experiments have illuminated this once theoretical domain. Studies on twisted homobilayer MoTe2 and pentalayer rhombohedral graphene placed on hBN have finally unveiled the existence of fractional Chern insulators (FCIs), the zero-magnetic field analog of fractional quantum Hall states.

    The journey to this point, preceded by over a decade of theoretical frameworks and predictions surrounding FCIs, yet the experimental revelations have proved to be richer and more surprising than expected. In this talk, we will present how the combination of ab-initio and quantum many-body calculations can help us capture the different features observed in experiments. We will discuss the potential future for this exciting booming field, including the possible observation of fractional topological insulators, a yet-never observed topological ordered phase preserving the time reversal symmetry.

  • Matias Gonzalez (Berlin) & Ruben Zakine (LadHyX)

    11.02.2025 10:45 - 11:45
    Séminaires

    Matias Gonzalez : Spiral spin liquids and surrounding phases in the square lattice XY model

    Spiral spin liquids possess a subextensively degenerate ground-state manifold, represented by a continuum of energy minima in reciprocal space. Since a small change of the spiral state wave vector requires a global change of the spin configuration in real space, it is a priori unclear how such systems can fluctuate within the degenerate ground-state manifold. Only recently it was proposed that momentum vortices are responsible for the liquidity of the spiral phase and that these systems are closely related to an emergent rank-2 U(1) gauge theory. As a consequence of this gauge structure, fourfold pinch-point singularities were found in a generalized spin correlator. In this paper, we use classical Monte Carlo and molecular dynamics calculations to embed the previously studied spiral spin liquid into a broader phase diagram of the square lattice XY model. We find a multitude of unusual phases and phase transitions surrounding the spiral spin liquid such as an effective four-state Potts transition into a collinear double-striped phase resulting from the spontaneous breaking of two coupled Z2 symmetries. Since this phase is stabilized by entropic effects selecting the momenta away from the spiral manifold, it undergoes a second phase transition at low temperatures into a nematic spiral phase which only breaks one Z2 symmetry. We also observe a region of parameters where the phase transition into the spiral spin liquid does not break any symmetries and where the critical exponents do not match those of standard universality classes. We study the importance of momentum vortices in driving this phase transition and discuss the possibility of a Kosterlitz-Thouless transition of momentum vortices. Finally, we explore the regime where the rank-2 U(1) gauge theory is valid by investigating the fourfold pinch-point singularities across the phase diagram.
     
    References:
    - H. Yan and J. Reuther, Phys. Rev. Research 4, 023175 (2022)
    - M. G. Gonzalez, A. Fancelli, H. Yan, and J. Reuther, Phys. Rev. B 110, 085106 (2024)

    Ruben Zakine : Patterns robust to Disorder in spatially-interacting Generalized Lotka-Volterra Ecosystems

    How do interactions between species influence their spatial distribution in an ecosystem? In this seminar, I will introduce a spatially-extended ecosystem of N interacting species (N large), where species can diffuse and interactions are nonlocal. I will compute the criterion for the loss of stability of the spatially homogeneous ecosystem, and I will show that the stability of the uniform state crucially depends on the most abundant species, and on the interplay between space exploration during one species generation and the interaction range. Focusing on the spectrum of the interaction matrix weighted by the species abundances, we identify a second order transition (of BBP type) that translates into a transition in the final patterns of the species repartition. Assuming that the disorder is small, we exhibit an explicit solution of the dynamical mean-field equation for the species density, obtained as the fixed point of a nonlocal Fisher-Kolmogorov-Petrovski-Piskounov equation. This work paves the way for future combined approaches at the frontier of active matter and disordered systems, with the hope of better understanding complex ecosystems like bacterial communities.

  • [Séminaire TQM] Luca de Medici (LPEM, ESPCI)

    06.02.2025 14:00 - 15:00
    Séminaires TQM

    Luca de’ Medici (ESPCI, Paris)

    Insight into Hund metals and interplay with Mott physics

    Hund metals are paramagnetic phases in which high-spin local configurations dominate. This paradigm is now a useful guidance to interpret the physics of many transition-metal compounds, like Ruthenates and Iron-based superconductors. I will show how this physics is extremized by moving towards a half-filled Mott insulator, and that it gives rise to charge instabilities and heavy fermionic behavior along the way.

    A. Georges and G. Kotliar, Physics Today 77, 46 (2024)

    M. Chatzieleftheriou et al. Phys. Rev. Lett. 130, 066401 (2023)

    M. Crispino et al. ArXiv:2312.06511 (2023)

  • Nina Javerzat (LIPhy) & Enrico Ventura (Milan)

    04.02.2025 10:45 - 11:45
    Séminaires

    Nina Javerzat : Conformal Invariance of Rigidity Percolation

    Rigidity Percolation (RP) attracted much attention in soft matter, as an elegant framework to understand the non-trivial emergence of solidity, in media that not present any long-range structural order. The solidification of amorphous systems like gels, fiber networks or living tissues can indeed be understood by focusing on locally rigid structures --clusters, that grow and coalesce until one eventually percolates the whole system, ensuring macroscopic mechanical stability. As a statistical model, Rigidity Percolation is defined from the concept of graph rigidity. I will explain that RP possesses a unique non-local character, leading to a rich behaviour that is absent in the usual Connectivity Percolation problem.

    Inspired by the great success of conformal field theory to understand critical phenomena, I have recently examined conformal invariance in 2D Rigidity Percolation. I will present two works where I gave numerical evidence of conformal invariance: i) from properties of the so-called connectivity functions, and ii) from consistence of the boundaries of clusters with Schramm-Loewner Evolution processes. These works reveal furthermore unexpected similarities with Connectivity Percolation, and allow to obtain a new relation between two of the critical exponents of RP.
    A lot remains to be understood about Rigidity Percolation, and I will end with my favourite perspectives.

    Based on Phys. Rev. Lett. 130, 268201 (2023) and Phys. Rev. Lett. 132, 018201(2024)

    Slides (pdf)

    Enrico Ventura : Memorization as Generalization in Physics-inspired Generative Models

    Our daily experience proves that humans are able to acquire and manipulate the hidden structure of the surrounding environment to generate creative ideas and survive. Artificial machines are also able to learn the unknown distribution of a set of data-points and use it to generate new examples. This capability, known under the name of generalization, is usually opposed to learning specific point-wise examples from the training-set. This second ability is called memorization. In this talk I will report some recent results supporting the picture of generalization as a “thermal” version of memorization with respect to a fictitious learning temperature. Both biologically-inspired (i.e spin-glass like neural networks) and artificial learning systems (i.e. diffusion models) will be analyzed under the lens of statistical mechanics.

    Slides (pdf)

  • Léo Mangeolle (TUM, Munich)

    28.01.2025 10:45 - 11:45
    Séminaires

    Thermal Hall conductivity of neutral bosons from the quantum kinetic equation

    Thermal Hall conductivity has recently emerged as an experimentally accessible property of insulating materials. Theoretical understanding thereof has remained a challenge, in particular since the breaking of time-reversal symmetry by neutral particles is nontrivial and can emerge from multiple mechanisms (semiclassical dynamics, skew-scattering, etc). In a first part, I will present a general formulation of inelastic skew-scattering of energy-carrying bosons by other collective excitations. Specializing to phonon-magnon interactions, I will show that a phonon thermal Hall effect from skew-scattering in antiferromagnets is allowed by magnetoelastic and spin-orbit couplings. In a second part, I will focus on the free semiclassical dynamics of neutral bosons, and present a systematic derivation of their kinetic equation, incorporating the topological dynamics of wavepackets in the form of Berry curvatures (generalized to phase space). This makes it possible to treat inhomogeneous systems, including boundaries, textures, etc., in a compact and natural manner.

  • Michel Fruchart (Gulliver, ESPCI)

    21.01.2025 10:45 - 11:45
    Séminaires

    Pattern formation by turbulent cascades

    Fully developed turbulence is a universal and scale-invariant chaotic state characterized by an energy cascade from large to small scales at which the cascade is eventually arrested by dissipation. Here we show how to harness these seemingly structureless turbulent cascades to generate patterns. Pattern formation entails a process of wavelength selection, which can usually be traced to the linear instability of a homogeneous state. By contrast, the mechanism we propose here is fully nonlinear. It is triggered by the non-dissipative arrest of turbulent cascades: energy piles up at an intermediate scale, which is neither the system size nor the smallest scales at which energy is usually dissipated. Using a combination of theory and large-scale simulations, we show that the tunable wavelength of these cascade-induced patterns can be set by a non-dissipative transport coefficient called odd viscosity, ubiquitous in chiral fluids ranging from bioactive to quantum systems. Odd viscosity, which acts as a scale-dependent Coriolis-like force, leads to a two-dimensionalization of the flow at small scales, in contrast with rotating fluids in which a two-dimensionalization occurs at large scales. Apart from odd viscosity fluids, we discuss how cascade-induced patterns can arise in natural systems, including atmospheric flows, stellar plasma such as the solar wind, or the pulverization and coagulation of objects or droplets in which mass rather than energy cascades.

  • [Séminaire TQM] Ben Wieder (IPhT)

    16.01.2025 14:00 - 15:00
    Séminaires TQM

    Monopole Quantum Numbers and Projective Representations in Stable and Fragile Topological Crystalline Insulators

    Over the past 15 years, a dizzying array of noninteracting topological insulator (TI) and topological crystalline insulator (TCI) phases have been theoretically predicted and identified in real materials. While the TI states are well understood, the TCI states – which comprise the majority of topological materials in nature – exhibit more complicated classification groups and boundary states and carry more ambiguous response signatures. For earlier variants of interacting symmetry-protected topological states (SPTs), both the classification and response were clarified through the many-body quantum numbers of the 0D collective excitations bound to crystal and electromagnetic defects, such as magnetic fluxes and monopoles. In particular, when 0D defects exhibit fractionalized quantum numbers, or more generally projective representations of the local many-body symmetry group, this can indicate the presence of quantized responses in the bulk that are governed by long-wavelength topological field theories that are stable to symmetric interactions. In this talk, I will introduce numerical methods for computing defect quantum numbers in stable and fragile TCI states via the reduced density matrix, revealing a deep connection between defect quantum numbers and the entanglement spectrum. Surprisingly, we find that when crystal symmetries are included in the local symmetry group, defects can appear to transform projectively even in Wannierizable (fragile) insulators, casting doubt on the suitability of magnetic monopoles for characterizing the TCI states present in real 3D materials. Our results represent a crucial step towards describing TCIs beyond tight-binding models and frameworks like “higher-order topology,” and facilitate more direct connections between free-fermion TCIs and interacting SPTs.

  • Nicolas Pavloff (LPTMS)

    14.01.2025 10:45 - 11:45
    Séminaires

    Topological Pathways to Two-Dimensional Quantum Turbulence

    I shall present a combined experimental and theoretical investigation of the formation and decay kinetics of vortices in a two-dimensional turbulent superfluid. Fundamental topological conservation laws require that the formation and annihilation of vortices also involve critical points of the velocity field, namely nodes and saddles. Identifying the simplest bifurcations underlying these processes enables to develop an effective kinetic model that closely aligns with experimental observations, and shows that different mechanisms are responsible for vortex number growth and decay.

    Slides (pdf)