The fruit fly larva is a maggot which looks like a dull white cylinder. Within a few days, and without any changes in its genome sequence, it metamorphoses. It gets its sophisticated adult fly shape with wings, legs, antennas, and compound eyes. How do cells migrate, deform, and rearrange to shape a tissue ? To approach step by step the dynamics of this morphogenesis, we will journey from developmental biology to mechanics, from discrete description of cellular material to continuum mechanics quantification, and from experiments to modeling. We will investigate flows within geometries specifically designed to discriminate between models.

Immune cells, particularly lymphocytes, must move, interface with other cell types, extract information from these interactions, and, if necessary, respond rapidly through endocytosis, status changes, proliferation, or cell killing. These critical biological processes rely on cytoskeletal rearrangement, mechanical sensing, and force production. In our research, we use microfluidics and micro-fabricated tools to investigate the forces at cell-cell interfaces and the cellular rearrangements triggered by antigen recognition. Recently, we introduced functionalized oil droplets as a novel antigen-presenting tool in the context of B cells, uncovering an unexpected role of microtubules in limiting F-actin polymerization, which facilitates the formation and maintenance of a distinct immune synapse. Additionally, I will present new applications of these methods, focusing on lymphocyte interactions with the microenvironment of lymph nodes and examining how mechanical factors influence their immune functions.

Propriétés électroniques et magnétiques de graphène Kagomé Alain Rochefort Polytechnique Montréal, Département de génie physique, Montréal, Canada Résumé La formation de polymères Kagomé avec des propriétés qui s’apparentent à celles du graphène a récemment généré de nombreux travaux sur ce type de matériau 2D aux propriétés parfois exotiques telles que des phases topologiques, des structures de bandes plates ainsi que des phases magnétiques liées à la structure des monomères. Dans cette présentation, je montrerai différents exemples de matériaux pouvant être produits par une approche de synthèse de surface (OSS) qui possèdent des propriétés électroniques et magnétiques prometteuses et pour lesquelles nous proposons également différentes approches pour moduler ces propriétés. Je montrerai également comment la symétrie des unités de base du polymère est au centre du contrôle de ces propriétés.

There has been a growing realization that the properties of a material can be modified just by placing it in an optical cavity. The quantum vacuum fields surrounding the material inside the cavity can cause nonintuitive modifications of electronic states through ultrastrong vacuum–matter coupling, producing a vacuum-dressed material with novel properties. Existing theoretical predictions include cavity-enhanced, cavity-induced, and cavity-mediated enhancement of electron–phonon coupling and superconductivity, electron pairing, anomalous Hall effect, ferroelectric phase transitions, quantum spin liquids, and photon condensation. Achieving the so-called ultrastrong coupling (USC) regime is a prerequisite for observing these effects, which arise when the interaction energy becomes a significant fraction of the bare photonic mode and matter excitation frequencies. Most intriguingly, when a material is ultrastrongly coupled with cavity-enhanced vacuum electromagnetic fields, its ground state will contain virtual photons. This nonperturbative virtual driving without external fields can lead to phase transitions in thermal equilibrium. This talk will describe our recent studies of USC phenomena in various solid-state cavity quantum electrodynamics systems in search of such vacuum-induced phases of matter. We utilize the phenomenon of Dicke cooperativity, i.e., many-body enhancement of light–matter interaction, to explore quantum-optical strategies for creating, controlling, and utilizing novel phases in condensed matter enabled by the quantum vacuum.

How well do we understand string theory? As one indicator can be thought of the degree of our understanding of its spectrum. Yet, other than comprising infinitely many physical states of arbitrarily high spin and mass, what does the string spectrum look like? Traditional methodologies can yield its state content on a level-by-level basis, a straightforward procedure which however becomes cumbersome as the level increases. In this seminar, we will discuss a new, covariant and efficient technology with which entire physical trajectories can be excavated. It is based on the observation that the Virasoro constraints in fact encode the generators of a bigger algebra, that is a symplectic algebra, which commutes with the spacetime Lorentz algebra. This enables constructing trajectories deeper inside the spectrum as “clones” of simpler ones, upon suitably dressing the latter, depending on the “depth” of the trajectory we aim to reach.

I will discuss some interesting results on instanton partition functions of Vafa Witten theory on CP^2 with gauge group SU(N). These results can be related to counting of sheaves on CP^2. In 2301.06711 paper we observed the growth of certain mock cusp forms associated with these mock modular forms. Interestingly the growth of these coefficients remain higher than those expected from Deligne bound. In our ongoing work we show that the holomorphic anomaly in theories are associated with these partition functions corresponding to gauge groups of lower ranks.

Finding the right degrees of freedom is a key step in solving strongly interacting field theories. Considerable progress in this direction was made in the last decades for higher dimensional supersymmetric gauge theories, guided by dualities with string theory. Two complementary approaches have been particularly useful in the exploration of the quantum structure of these theories: one is supersymmetric localisation and the other is integrability. Very recently it was noticed that common structures appear in both approaches when they are applied to the N=2 super-conformal theory in four dimensions which is obtained as a Z_K orbifold of the N=4 super Yang-Mills (SYM) theory. This hints at a deeper structure underlying both localisation and integrability. A special class of three-point functions in the N=2 SYM theory were computed first by localisation, in terms of a Fredholm determinant of a Bessel operator generalising the Tracy-Widom distribution. The talk, based on a recently completed work in collaboration with Gwenaël Ferrando, Shota Komatsu and Gabriel Lefundes, will explain how this result can be derived from the integrability approach to correlation functions, which was developed initially for the N=4 SYM theory and where the Fredholm determinant appears naturally.

In Ikushima group, we are working on the ultimate control of electrons and light using advanced semiconductor technology. One of our targets is terahertz (THz) light, which lies in the band between radio waves and optical waves. We conduct research on fundamental physics utilizing semiconductor quantum structures, including single THz photon detection, THz amplification, and electron-phonon strong coupling (resonant polaron). Furthermore, we are exploring novel mechanisms that could bring transformative advancements not only in electronics and photonics but also in the field of information science, such as terahertz photonic circuits and optically programmable CMOS. In this seminar, I would like to focus on my work “detection of Landau emission in graphene” [1]. We provide experimental evidence for the occurrence of inter-LL radiative transitions in an electrically biased graphene Hall bar, where the wavelength of emission can be tuned by varying the applied magnetic field. A quantum-well based charge sensitive infrared phototransistor (CSIP) is used for detecting weak THz emission from graphene (Fig. 1(a)). THz emission is observed at around 5 T when the Hall voltage exceeds the corresponding LL energy spacing ΔELL01/e between the zero-energy (N = 0) and first excited (N = +1 or N = −1) LLs. We also investigated the emission spectra through measurements of the QW spectrum of the CSIP (Fig. 1(b)). The emission spectra are well explained by the N = +1 → 0 (or N = −1 → 0) inter-LL radiative transition in monolayer graphene. The linewidth of the emission spectra is estimated to be on the order of LF meV, even though no explicit LL splitting is observed in the magnetotransport at 5 T. I would like to discuss the possibilities and challenges of amplifying THz waves also. [1] F. Inamura et al., APL Photonics 9, 116LF1 (2024)

2-dimensional stacks are versatile platforms that host many correlated phases. Some of these exotic phases not only rely on electronic correlations but also the non-trivial topology of the bands of these systems. This is the case in twisted bilayers of graphene (tBLG), for the orbital magnet state for example [1] ; or in twisted MoTe2, in which topology expresses with the apparition of the fractional quantum anomalous Hall effect [2]. I will discuss how it is possible to access this non-trivial topology with scanning tunneling microscopy, using the atomic-scale local density of states (LDOS) of 2D materials as a topological observable. I will present two different approaches to tackle this : exploiting the energy dependence of the LDOS in tMoTe2, or using defects to create interferograms in tBLG. [1] Lu. X et al. Nature 574 (2019) 653 [2] H. Parks et al, Nature 622, 2023