In materials with strong electron-electron interactions, the common single-particle picture used to describe the material’s properties is insufficient to explain all observations. These strongly-correlated systems are characterized by a wide range of exotic phases, ranging from superconducting to uncommon insulating phases, which are interesting both from the standpoint of fundamental physics and for their possible applications. Despite decades of research, questions persist on how these materials behave, and which degrees of freedom, including charge, lattice, spin, and orbital, contribute or compete in each phase. A better understanding of the physics of these systems could allow for their design and control, and therefore remains a topic of great interest. This is particularly true as heterostructures of 2D materials have, in recent years, raised the possibility of obtaining a wide range of strongly correlated phases ‘on demand,’ thanks to the unprecedented level of material engineering they offer.

Spectroscopic measurements can be a useful tool to investigate these systems, as measurements can be made on many different energy scales, probing phonons, interband transitions, and core-level spectroscopy. Thus, the response of quasi-particles and collective modes can be studied while other parameters, like temperature, doping, or pressure, are varied. Steady-state measurements often offer a simplicity that allows for mapping states in thermal equilibrium across these different axes. Yet these measurements have their limits, and disentangling the role of contributing interactions can be challenging if they fall on the same energy scale. By perturbing the system and tracking the timescale of its response, the contributions of electron-electron, electron-phonon, or phonon-phonon interactions can be separated. Out-of-equilibrium experiments such as these can also be useful to observe extremely early-time dynamics, or to reach exotic metastable states, only accessible through perturbations of the potential landscape.

We will aim to cover three themes in this seminar. The first theme is steady-state spectroscopy of strongly-correlated systems, the second on out-of-equilibrium techniques, and the third on theoretical studies which develop optical observables or discuss the interpretation of spectroscopic data. One topic for exploration is how dimensionality contributes to strongly correlated systems, stabilising phases to higher temperatures and therefore leading to new regimes of interaction. Quasi 2D and 1D systems have long been a focus for study in strongly-correlated systems, and now ‘real’ 2D systems have emerged, with highly tuneable properties due to strain, dielectric engineering, lattice structure, and stacking. How can both the 2D limit shed light on the quasi-2D bulk, and vice versa? What new exotic phenomena emerge on the atomically-thin limit?  Through these themes, we hope to explore new avenues of research, juxtaposing how each of the three can bring new insights to the others.