LPTMC Seminars
The seminars take place in room 523, corridor 12-13, 5th floor.
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Lucien Jezequel (KTH)
12.11.2025 11:00 - 12:00SéminairesSalle 523, couloir 12-13, 5è étage
The "Mode-Shell" correspondence, a unifying concept in topological physics
In quantum or classical wave systems, some properties of wave systems are known to be topologically protected. Due to their increased robustness, such properties have attracted much interest in the past decades.
The most studied case is the existence of unidirectional edge states in the quantum Hall effect and, more generally, the existence of protected states at the edges of topologically insulators. An important result is then the bulk-edge correspondence that links the existence of topological edge states to a topological index defined in the volume of the material.
This is not the only case studied in topological physics and different, yet similar, results have been obtained for topological semimetals, higher order insulators or continuous wave systems. In this talk I will explain how all these results can be understood in a unifying theory using the mode-shell correspondence formalism which relates the existence of isolated topological modes in phase space, to a topological invariant defined in the surface which encloses these modes in phase space. Invariant that reduces to Chern or winding numbers in the semiclassical limit.
Mode-shell correspondence, a unifying phase space theory in topological physics
[1] Part I: Chiral number of zero-modes https://www.scipost.org/10.21468/SciPostPhys.17.2.060
[2] Part II: Higher-dimensional spectral invariants https://arxiv.org/abs/2501.13550 -
Achille Mauri (EPFL)
04.11.2025 10:45 - 11:45SéminairesSalle 523, couloir 12-13, 5è étage
Excitations in triangular-lattice antiferromagnets near the Ising limit
Recent experimental studies have stimulated an extensive interest on the excitation spectrum of spin-1/2 triangular magnets with XXZ magnetic anisotropy. Among the materials which were recently investigated, the cobaltite K2Co(SeO3)2 was demonstrated to realize an XXZ triangular magnet with an extremely strong degree of easy-axis anisotropy, implying that the compound is located very close to the limit of a quantum Ising model with transverse exchange. Motivated by the inelastic neutron scattering studies on this compound, this presentation will report on a theoretical analysis of the excitation spectrum in the "up-up-down" and in the low-field phase of the model, focusing on the Ising limit in which the longitudinal exchange Jzz is much larger than the transverse exchange Jxy. In the “up-up-down” phase, stabilized by a c-axis oriented field, we study the magnon excitations, both in the framework of spin wave theory and within a perturbative expansion in the anisotropy parameter α = Jxy/Jzz ≪ 1. We show that the linear-spin wave (LSW) approximation, although exact at leading order in α, severely underestimates the coefficients of the higher-order corrections in α. It will be shown that this discrepancy explains the deviations between LSW and scattering data observed in the up-up-down phase. The presentation will then discuss the spectrum in the case of zero field, for which the system is characterized by "spin-supersolid" long-range order. We analyze the first-order non-linear spin wave corrections to the linear spin wave theory, and show that the 1/S nonlinearities do not provide a simple framework for explaining the anomalous spectral features observed experimentally in K2Co(SeO3)2.
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Miha Srdinsek (CEA Grenoble)
21.10.2025 10:45 - 11:45SéminairesSalle 523, couloir 12-13, 5è étage
Hybrid between biologically inspired and quantum inspired many-body states
Deep neural networks can represent very different sorts of functions, including complex quantum many-body states. Tensor networks can also represent these states, have more structure and are easier to optimize. However, they can be prohibitively costly computationally in two or higher dimensions. In this seminar I will propose a hybrid network [1] which borrows features from the two different formalisms. I will showcase the ansatz by obtaining the representation of a transverse field quantum Ising model with a long range 1/r^6 antiferromagnetic interaction on a 10×10 square lattice. The model corresponds to the Rydberg (cold) atoms platform proposed for quantum annealing.
[1] Srdinsek, Waintal, arXiv:2506.05050 (2025)