The Shrinking Fermi Liquid Scenario for Cuprates Under the Scrutiny of Optical Conductivity Measurements

In a recent paper [B. Michon et al., Nat. Commun. (2023) 14:3033] optical conductivity experiments in cuprate superconductors were shown to display scaling properties consistent with the Marginal Fermi Liquid theory. Here we argue, that the temperature regime studied in these experiments does not allow to distinguish between Marginal Fermi Liquid and a Shrinking Fermi Liquid. In the latter scenario, which we recently proposed and which applies near a quantum critical point, dynamical fluctuations of the order parameter with short correlation length mediate a nearly isotropic scattering among the quasiparticles over the entire Fermi surface leading to strange-metal behavior. If the damping of these nearly local fluctuations increases by decreasing the temperature, the Fermi liquid regime shrinks and the strange-metal behavior is extended to the lowest temperatures. This Shrinking Fermi liquid scenario has many similarities and some differences with respect to the Marginal Fermi liquid theory. In particular, we show that the approximate scaling properties of the optical conductivity in some high-frequency regime predicted by the Shrinking Fermi liquid scenario account to a very good extend for the experimental data.