Séminaires du LPTMC
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[Séminaire TQM] Pascal Simon
14.11.2024 14:00  15:00Séminaires TQMHund’s assisted multichannel quantum phase transition in an iron superconductor
Understanding the interplay between individual magnetic impurities and superconductivity is crucial for bottomup construction of novel phases of matter, as well as to exploit the local response as a probing tool. For decades, the description by Yu, Shiba and Rusinov (YSR) of single spins in a superconductor and its extension to include quantum effects has proven highly successful: the pairbreaking potential of the spin generates subgap bound states. I will first show how atomicallyresolved shot noise can be used to reveal the coherent and incoherent dynamics of such subgap bound states [1].
By tuning the energy of the subgap states through zero, the impurity screening by the superconductor makes the ground state gain or lose an electron, signalling a parity breaking quantum phase transition. I will present a set of scanning tunneling microscopy (STM) measurements that explicitly invalidate the classical YSR paradigm, and propose an interpretation in terms of a multiorbital Anderson impurity model [2]. In particular, I show that in multiorbital impurities, electronic correlations can conversely lead to a quantum phase transition where the impurity mean occupation changes dramatically, without significant effect of the screening by the superconductor. This finding implies that the YSR treatment is not always valid, and that intraatomic interactions, particularly Hund’s coupling that favours highspin configurations, are an essential ingredient for understanding the subgap states.
[1] U. Thupakula, V. Perrin, A. PalacioMorales, L. Cario, M. Aprili, P. Simon, F. Massee, Phys. Rev. Lett. 128, 247001 (2022)[2] M. Uldemolins, A. Mesaros, G. D. Gu, A. PalacioMorales, M. Aprili, P. Simon, and F. Massee, “Interactiondriven quantum phase transition of a single magnetic impurity in Fe(Se,Te)” 2023, arXiv:2310.06030. 
JeanNoël Fuchs (LPTMC)
12.11.2024 10:45  11:45SéminairesPanorama of topological phases of matter
In this talk, I will try to give a pedagogical overview of phases of matter that are classified using ideas from topology rather than symmetry. I will insist on the difference between two families of such phases of matter: topological insulators and topological order.On the one hand, topological insulators can be seen as a refinement of band insulators, in which bands carry topological invariants (for example, Chern numbers) such that several classes of insulators can be distinguished. This is a generalization of ideas of D. Thouless and coworkers concerning the integer quantum Hall effect. Interactions do not play a major role in this context.On the other hand, topological order is a notion proposed by X.G. Wen that takes its origin in the fractional quantum Hall effect, which crucially depends on interactions between electrons. A topologicallyordered system is characterized by a robust groundstate degeneracy that depends on the genus of the surface on which the system is placed (whether a sphere or a torus, for example) and it features fractionalized quasiparticles known as anyons. Important aspects are longrange quantum entanglement and the nonlocal nature of the order. 
Gwendal Fève (LPENS, Sorbonne)
05.11.2024 10:45  11:45SéminairesAnyon braiding in mesoscopic colliders
In threedimensional space, elementary particles are divided between fermions and bosons according to the properties of symmetry of the wave function describing the state of the system when two particles are exchanged. The situation is different in twodimensional systems which can host exotic quasiparticles, called anyons, which obey intermediate quantum statistics characterized by an exchange phase varying between and [1,2]. As a consequence, contrary to fermions and bosons, anyons keep a robust memory of braiding operations, which consist in moving one anyon around another one.In particular anyons have been predicted to the be the elementary excitations of the fractional quantum Hall regime, obtained by applying a strong magnetic field perpendicular to a twodimensional conductor. I will discuss recent experiments realized in fractional quantum Hall conductors to demonstrate the fractional statistics of anyons [3, 56], focusing on the anyon collider geometry [7], where anyon braiding can be revealed by studying the partitioning of dilute anyon beams by a beam splitter.References:[1] B. I. Halperin, Phys. Rev. Lett. 52, 1583–1586 (1984).[2] D. Arovas, J. R. Schrieffer, F. Wilczek, Phys. Rev. Lett. 53, 722–723 (1984).[3] H. Bartolomei et al., Science 368, 173 (2020).[4] J. Nakamura S. Liang, G. C. Gardner, and M. J. Manfra, Nature Physics 16, 931 (2020).[5] M. Ruelle et al., Phys. Rev. X 13, 011031 (2023).[6] M. Ruelle et al., arXiv:2409.08685 (2024)[7] B. Rosenow, I. P. Levkivskyi, B. I. Halperin, Phys. Rev. Lett. 116, 156802 (2016). 
Kirone Mallick (IPhT, Saclay)
08.10.2024 10:45  11:45SéminairesMacroscopic ﬂuctuations out of equilibrium
A system, subject to continuous exchanges of matter, energy or information with its surroundings, may reach a nonequilibrium steady state in which various currents break timereversal invariance and continuously generate entropy. Such a state can not be accounted for by the Principles of Thermodynamics or the GibbsBoltzmann laws of statistical physics. Besides, linear response theory and the OnsagerMachlup functional provide useful descriptions of large scale fluctuations in such driven systems only at first order, in the vicinity of equilibrium.
In the last two decades, important advances in our understanding of processes far from equilibrium have been achieved, for which rare events, large deviations and ﬂuctuations relations provide a uniﬁed framework. The emergence of universal features can be studied thanks to a variational principle, proposed by G. JonaLasinio and his collaborators, known as the Macroscopic Fluctuation Theory (MFT). In this theory, optimal ﬂuctuations far from equilibrium are determined at a coarsegrained scale by two coupled nonlinear hydrodynamic equations. The objective of this talk is to present these concepts and to illustrate them with some exact solutions of the MFT equations.

Andrea Tononi (ICFO, Barcelona, Spain)
01.10.2024 12:45  13:45SéminairesTemporal Bell inequalities in a manybody system
We formulate a temporal ClauserHorne inequality by considering two parties choosing two observables to measure at different consecutive times. For two entangled antipodal spins joined by a spin chain, we show that the inequality is violated during a small finite time interval between the measurements. This fact contrasts with the time evolution in vacuum, which is describable in terms of a hiddenvariable theory. Our result demonstrates that the finite velocity for quantum information spreading in the chain prevents signaling and therefore the immediate vanishing of quantumness.

[Séminaire exceptionnel] Sofyan Iblisdir
30.09.2024 11:00  12:00SéminairesCollective Monte Carlo updates through tensor network renormalization
We introduce a MetropolisHastings Markov chain for Boltzmann distributions of classical spin systems. It relies on approximate tensor network contractions to propose correlated collective updates at each step of the evolution. We present benchmarks for a variety of instances of the twodimensional Ising model, including ferromagnetic, antiferromagnetic, (fully) frustrated and EdwardsAnderson spin glass cases. With modest computational effort, our Markov chain achieves sizeable acceptance rates, even in the vicinity of critical points. It compares well with other Monte Carlo schemes such as the Metropolis or Wolff algorithm: equilibration times appear to be reduced by a factor that varies between 40 and 2000, depending on the model and the observable being monitored. The scheme can be adapted to three dimensions, matrix models, or a confined gas of hard spheres.