Séminaires du LPTMC

Salle 523, couloir 12-13, 5è étage

Competing orders in many-electron systems: a renormalization group perspective

The renormalization group is an established approach to study quantum many-body systems, and this applies especially to one of its modern implementations known as the functional renormalization group (FRG). In particular, the FRG constitutes a flexible and unbiased tool for the study of competing orders. In this talk, I will outline recent progress in this direction for correlated electron systems. To this end, I will first discuss the competition between antiferromagnetism, charge density waves and superconductivity in the 2D Hubbard model, thus making a connection with high-temperature superconductors. The special role of bosonization methods will be emphasized along the way. I will also show how the FRG can be combined with dynamical mean-field theory to treat strongly interacting regimes, with a focus on d-wave superconductivity. As a next step, I will increase the complexity of the model by including non-local interactions and discuss unconventional superconductivity in an extended Hubbard model with a connection to moiré materials. Special consideration will also be given to the treatment of retarded interactions with electron-phonon couplings. Finally, I will highlight recent FRG studies of quantum criticality in Dirac materials, with a connection to graphene.

Salle 523, couloir 12-13, 5è étage

Equilibrium phase transition between a fluid and an amorphous solid

When a liquid is cooled, it can form a glass: a mechanically rigid but structurally disordered solid. Experimentally, this transformation occurs when the system falls out of equilibrium and no longer explores all accessible configurations on experimental timescales. A central open question, dating back more than a century, is whether this dynamical arrest reflects an underlying equilibrium phase transition. While theory predicts such a transition in idealized models (with deep connections to spin glass physics), its existence in realistic finite-dimensional systems remains unsettled. I will review this problem and present numerical results for a two-dimensional glass-forming liquid. By combining complementary Monte Carlo techniques, we equilibrate the system down to zero temperature over a range of system sizes and directly measure its equilibrium thermodynamic and structural properties. These results provide evidence for an equilibrium phase transition between a fluid and an amorphous solid. I will conclude by discussing the implications and open questions raised by this finding.

Salle 523, couloir 12-13, 5è étage

Collective dynamics in binary superfluids: From dissipationless flow to dispersive shock waves

Binary superfluids are typically characterized by two distinct internal states, giving rise to a spin mode in addition to the conventional density mode. These systems can be realized, for example, using Bose-Einstein condensation into two hyperfine atomic states, or the propagation of a two-polarization laser in a birefringent nonlinear medium. In this talk, I will mainly present two theoretical investigations into collective phenomena in these systems. First, inspired by a recent experiment at LKB [1], I will discuss the critical speed for dissipationless flow of a two-dimensional binary superfluid of light past an impurity [2]. For a weak impurity, the drag is determined within linear-response theory and aligns with Landau’s criterion. For an impurity of arbitrary strength, the critical speed is obtained from the conditions for strong ellipticity of the stationary flow equations. We identify the emission of linear waves and vortex structures in the density and spin sectors as primary mechanisms for dissipative flow. Second, I will examine a coherently driven binary Bose-Einstein condensate, inspired by an experiment at Institut d’optique [3]. In its ground state, this system can be effectively described by a single coherent field satisfying the cubic-quintic nonlinear Schrodinger equation. In a one-dimensional geometry, we investigate nonlinear periodic solutions arising from steplike initial conditions. Using modulation theory, we analyze contact dispersive shock waves [4], nonlinear structures that are fundamentally absent in the standard, cubic nonlinear Schrodinger framework. To conclude, I will briefly complement these results by looking toward simpler, single-component systems, to highlight a recent experimental result at LKB [5, 6]: the observation that a finite-mass impurity can self-propel against a two-dimensional superfluid flow via vortex-antivortex shedding. Reducing the impurity to its center of mass and using a point-vortex model, I will describe how quantized vortices can serve as momentum-transfer agents, effectively bridging the physics of quantum fluids with the field of active matter.

[1] C. Piekarski, N. Cherroret, T. Aladjidi, and Q. Glorieux, Spin and density modes in a binary fluid of light, Phys. Rev. Lett. 134, 223403 (2025).
[2] P.-E. Larré, C. Michel, and N. Cherroret, Critical speed of a binary superfluid of light (2026).
[3] A. Hammond, L. Lavoine, and T. Bourdel, Tunable three-body interactions in driven two-component Bose-Einstein condensates, Phys. Rev. Lett. 128, 083401 (2022).
[4] T. Congy, P.-E. Larré, and P. Sprenger, Modulation theory for cubic-quintic nonlinear Schrodinger equations (2026).
[5] M. Baker-Rasooli, T. Aladjidi, T. D. Ferreira, A. Bramati, M. Albert, P.-E. Larré, and Q. Glorieux, Swimming against a superfluid flow: Self-propulsion via vortex-antivortex shedding in a quantum fluid of light, arXiv:2512.09028 (2025).
[6] T. Aladjidi, M. Baker-Rasooli, T. D. Ferreira, A. Bramati, M. Albert, P.-E. Larré, and Q. Glorieux, Critical velocity of a flow of superfluid light past a finite-mass impurity of tunable width (2026).

Salle 523, couloir 12-13, 5è étage

Unbiased numerical methods for strongly correlated quantum matter

Unbiased numerical approaches are essential for understanding strongly correlated quantum systems, but are often limited by the quantum Monte Carlo sign problem, particularly in frustrated spin systems and doped fermionic models. In this seminar, I will discuss two different quantum Monte Carlo strategies addressing these regimes.

I will first introduce the stochastic series expansion (SSE), a finite-temperature quantum Monte Carlo method based on a high-temperature expansion, and explain how cluster-based computational bases can reduce or eliminate sign problems in frustrated quantum magnets. As an illustration, I will present results for the spin-1/2 Heisenberg antiferromagnet on the diamond-decorated square lattice, a highly frustrated system of coupled orthogonal dimers.

I will then turn to diagrammatic Monte Carlo (DiagMC), in which thermodynamic observables are computed from perturbative expansions in terms of connected Feynman diagrams directly in the thermodynamic limit. I will introduce a recently developed DiagMC formalism that reorganizes these expansions around generic shifted quadratic reference points, and explain how automatic differentiation provides a systematic way to implement such shifted expansions.