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.

Flocks of birds, bacteria colonies, sheep herds, crowds of people ... are just some common examples of large groups of living entities behaving in a remarkable collective fashion. From a physicists viewpoint, these are systems composed of many interacting units, each somehow able to convert energy from their environment to stay 'alive' and perform some task. Such dynamics lifts the constraints imposed by equilibrium, opening many challenges and eventually the possibility for the rich phenomena displayed in the natural world. However, we lack a general theoretical framework to describe it. Much understanding has been gained from the investigation of model systems (both theoretical and experimental), usually dealing with 'active units' consisting in self-propelled particles with simple physical interactions, such as excluded volume. Interestingly, the mere competition between self-propulsion and crowding, is already enough to trigger a novel non-equilibrium phase transition. I will review our current understanding of this phenomenon, and I will then delve into a complementary perspective on active systems, as made of entities that interact in a non-reciprocal way. This will lead us to consider 'active' extensions of paradigmatic models in statistical mechanics, such as the Ising model, to decipher what such new ingredient brings into the classical picture of phase transitions.

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.

The steady-state theory, which held that the density of the universe remains constant due to the continuous creation of matter during expansion, died during the 1960s. This death is commonly attributed to the discovery of the Cosmic Microwave Background (CMB) in 1965, which was interpreted as evidence that the universe had evolved from a superdense state—contradicting the core assumption of the steady-state model. Historian Stephen Brush argued that the theory's supporters, who embraced a Popperian view of science, abandoned it in light of the CMB's apparent falsification. However, this narrative has been challenged by other historians, notably Helge Kragh, who downplay both Popper’s influence and the CMB's role in the theory’s downfall. This talk reopens the cold case of the steady-state theory's death. Did the discovery of the CMB really shoot the fatal blow? Was Popper an (unwilling) accomplice? Is the real murderer still unidentified? Drawing on new testimonies from both theorists and observers, I propose an alternative explanation: it was not the CMB, nor Popper, that "killed" the steady-state model, but its declining fecundity during the 1960s. Using new tools of history and philosophy of science, I argue that while Popper played a significant role in the theory’s development during the 1950s, he had little influence on its fate in the 1960s. Likewise, the CMB was not seen as a straightforward falsification of the steady-state theory, nor as direct confirmation of its rival, the big bang model. Rather, it was perceived as a fertile phenomenon that opened new avenues for connecting nuclear and particle physics, general relativity, and radio astronomy.

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