In this talk, I will address the following question: what properties of metals are *universal* in the sense that they hold not just in particular models or theories (e.g. Landau's Fermi liquid theory), but in *any* (clean) metal, even strongly coupled non-Fermi liquids? I will argue that progress can be made on this question by elevating compressibility - that is, the property that the emergent low-temperature behavior is compatible with a continuous range of electron densities - as the defining feature of a metal. I will argue that compressibility is sufficient to deduce various properties, such as the existence of a Fermi surface that obeys Luttinger's theorem. Furthermore I will apply hydrodynamics to the associated emergent conserved quantities to derive the equation of motion for the low-frequency, long-wavelength collective dynamics of a metal and show that it reduces to the same kinetic equation that in Fermi liquid theory governs the dynamics of the quasiparticles (leading me to coin the term "quasi-quasiparticles"). I will explain how non-hydrodynamic modes can nevertheless restore some non-Fermi-liquid-like behavior in the dynamics. The results of this talk illustrate the power of the concept of emergent symmetries as a tool to understand the physics of strongly coupled systems.

# Universal properties of metals: Fermi surfaces, quasi-quasiparticles and dynamics

Host: Hae-Young Kee