Francesco Barantani

Abstract Symmetry provides a unifying framework for understanding the ordering and dynamics of matter, and imposes constraints on the interactions between distinct collective excitations. In crystalline solids, these constraints are expressed through selection rules that prohibit harmonic coupling between zone-centre modes that transform under inequivalent irreducible representations.

Abstract The study of collective behaviour driven by spontaneous symmetry breaking and topology is crucial for understanding phase transitions in quantum matter. The two-dimensional (2D) XY model, describing spins with continuous in-plane rotational symmetry, hosts the topological Berezinskii–Kosterlitz–Thouless (BKT) transition, where vortex–antivortex binding induces quasi-long-range order.

Abstract Scattering between charges and collective modes in materials governs phenomena such as electrical resistance, energy dissipation, and phase switching. Studying such scattering requires simultaneous access to ultrafast and momentum-resolved dynamics of single-particle and collective excitations, which remains an experimental challenge.