Strange metal transport from coupling to fluctuating spins: numerical results and analytical insights

Metals hosting strong electronic interactions, including high-temperature superconductors, conduct electrical current in ways that do not conform to normal Fermi liquid theory. To pinpoint the microscopic origin of this ”strange metal” behavior, here we reexamine the d.c. and frequency-dependent conductivity of the two-dimensional t-J model taking advantage of recent improvements made on the finite temperature Lanczos method, enabling numerically exact calculations at unprecedentedly low temperatures and high spectral resolution. We find that strange metallicity is pervasive in the temperature-doping phase diagram wherever anti-ferromagnetic correlations are suppressed, being instead driven by paramagnetic spin fluctuations and unrelated to quantum criticality. Moreover, we identify an ubiquitous "stretched Drude" optical absorption shape that can be directly associated with the Planckian carriers responsible for the strange metal resistivities. These findings rationalize the anomalous optical properties commonly seen in experiments on strongly correlated systems, and highlight striking similarities with the universal relaxation of glasses and dielectrics.

Next, I will introduce an analytical theory of transport in quantum materials that generalizes Drude transport to those cases where the carriers' de Broglie wavelength is comparable to the semi-classical mean-free-path, explicitly restoring quantum uncertainty. In bad metals, where wave-like coherence is lost at each hop between neighboring atoms or molecules, the theory naturally leads to strange T-linear resistivities with apparently Planckian scattering rates as well as both the stretched Drude and displaced Drude peaks that are commonly observed in optical absorption experiments.