Quantum Materials With and Without Quasiparticles

Coordinators: Lara Benfatto, Andrey Chubukov, Sean Hartnoll, and Dmitrii Maslov

Materials not conforming to the Fermi-liquid paradigm continue to be one of the central subjects in condensed matter physics. The vitality of the field is fueled by a stream of new theoretical and experimental results that continue to change our view of quantum matter. However, while the existing experimental evidence for various deviations from Fermi-liquid behavior is extensive and compelling, there is a lack of consensus on even the most basic question: ``Which non-Fermi liquid observations can be understood by cleverly applying textbook formulae and which observations require a more radical departure from the quasiparticle framework?” This uncertainty has fueled ongoing debates surrounding topics including the proper description of electron transport beyond the quasiparticle paradigm, the status of a putative Planckian limit on scattering, and the nature of Cooper pairing without quasiparticles.

Discussion of these topics will be at the heart of this program. Even if a metal does not have well-defined quasiparticles, it is still expected to harbor collective modes, whose existence is protected by conservation laws. Nevertheless, as recent measurements on cuprates and related materials have emphasized, such modes may bear little resemblance with their counterparts in conventional metals. The program will discuss recent developments in the observations and theoretical analysis of collective excitations in unconventional correlated electron systems. This discussion will furthermore connect to ongoing developments in quantum Monte Carlo simulations, which have led to reliable numerical results that can be compared directly with analytical results of effective low-energy theories.