Are strange metals hydrodynamic or quantum critical? – Strange

Since their discovery in cuprate high temperature superconductors (CHTCs) and then in other quantum materials, strange metals continue to defy our understanding, placing them among the outstanding puzzles of contemporary physics. A close interplay between theory and experiment is key to unraveling their nature, yet defining the essence of their "strangeness" remains a complex challenge. Is it a consequence of material-specific properties that obscure the straightforward application of Fermi liquid theory or is it rooted in strong interactions and the absence of long-lived quasiparticles?

The aim of this collaborative project is to elucidate the nature of their strangeness by exploiting their unconventional dynamics near and far from equilibrium under the light of their collective excitations in the complex frequency plane. This novel perspective will allow to categorize strange metals along the continuum from hydrodynamic to quantum critical regimes. In hydrodynamic metals, slow momentum relaxation causes a hierarchy of scales between momentum and other collective excitations, leading to a coherent Drude peak in charge transport. In contrast, temperature is the only relevant scale at low energies in quantum critical metals, which exhibit frequency over temperature scaling of physical observables and incoherent conductivities.
On the theory front, we will employ a rich arsenal of modern theoretical approaches such as Sachdev-Ye-Kitaev-based models, gauge/gravity duality, Boltzmann transport theory, and hydrodynamics. To fuel and challenge theoretical development, we will probe the disorder dependence of the properties of strange metals (particularly CHTCs) using the electron irradiator located at École Polytechnique and measure their thermoelectric properties. The project will run for 36 months, and involves hiring two PhD candidates supervised jointly by the PI’s and one postdoctoral fellow, with regular secondment periods and visits in France and Germany.