Find the upcoming seminars below.
TQM seminar #33: Dec 12 2024, 2pm (LPTMC seminar room, towers 13-12, 5th floor, room 523)
Vincent Renard (CEA Grenoble)
Twist and strain in graphene bilayers
The study of moiré engineering started with the advent of van der Waals heterostructures, in which stacking 2D layers with different lattice constants leads to a moiré pattern controlling their electronic properties. The field entered a new era when it was found that adjusting the twist between two graphene layers led to strongly-correlated-electron physics [1] and topological effects associated with atomic relaxation [2]. A twist is now routinely used to adjust the properties of 2D materials. I will first discuss the effect of heterostrain where a layer is strained with respect to the other and its strong impact on electronic properties of twisted bilayer graphene near the magic angle [3.4]. I will then discuss a new type of moiré superlattice in bilayer graphene when one layer is biaxially strained with respect to the other—so-called biaxial heterostrain. Scanning tunneling microscopy measurements uncover spiraling electronic states associated with a novel symmetry-breaking atomic reconstruction at small biaxial heterostrain [5]. Atomistic calculations using experimental parameters as inputs reveal that a giant atomic swirl forms around regions of aligned stacking to reduce the mechanical energy of the bilayer. Tight-binding calculations performed on the relaxed structure show that the observed electronic states decorate spiraling domain wall solitons as required by topology. This study establishes biaxial heterostrain as an important parameter to be harnessed for the next step of moiré engineering in van der Waals multilayers.
[1] Y.Cao et al. Nature 556 (2018) 43–50
[2] S. Huang et al. Phys. Rev. Lett 121 (2018) 077702
[3] L. Huder et al. Phys. Rev. Lett. 120 (2018) 156405 [3] F. Mesple et al. Phys. Rev. Lett. 127 (2021) 126405
[5] F. Mesple et al. Adv. Mater 35 (2023) 2306312
TQM seminar #34 : January 16th 2025, 2pm (LPTMC seminar room, towers 13-12, 5th floor, room 523)
Benjamin Wieder (IPhT Saclay)
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.
TQM seminar #35 : Feb 6 2025, 2pm (LPTMC seminar room, towers 13-12, 5th floor, room 523)
Luca de Medici (LPEM, ESPCI)
TBA
TQM seminar #36 : Feb 13 2025, 2pm (LPTMC seminar room, towers 13-12, 5th floor, room 523)
Nicolas Regnault (LPENS, Paris)
TBA
TBA
TQM seminar #37 : March 27 2025, 2pm (LPTMC seminar room, towers 13-12, 5th floor, room 523)
Tarik Yefsah (LKB Paris)
TBA
TBA
TQM seminar #34 : January 16th 2025, 2pm (LPTMC seminar room, towers 13-12, 5th floor, room 523)
Benjamin Wieder (IPhT Saclay)
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.
