Event

The concept of electronic quasiparticles, as introduced by Landau, is one of the cornerstones of the theory of quantum many-body systems. However, a growing number of recent experiments in strongly correlated quantum materials have forced us to confront the existence of quantum matter for which the concept of electronic quasiparticles does not apply. In the first part of this talk, I will consider the physics of an electrical insulator, where recent experiments have suggested the existence of gapless neutral fermionic excitations. In an effort to understand these paradoxical experiments, I will describe a new phase of matter--- the composite exciton Fermi liquid---which is an electrical insulator with a three-dimensional Fermi surface of neutral fermions. In the second part of the talk, inspired by the rich phenomenology of the parent states of numerous high-temperature superconductors, I will describe some recent progress in our understanding of metallic states that do not admit a quasiparticle description but that nonetheless have a sharply-defined Fermi surface. Finally, if time permits, I will present new experimental results on unconventional transport properties of magic-angle twisted bilayer graphene and comment on their possible connections with the rest of my talk.