The authors report that key signs of “Mottness” can appear at finite temperature before a full single-electron gap forms. Using exact numerical simulations of the repulsive Fermi Hubbard model, they find that charge fluctuations become strongly suppressed while the single-particle spectrum remains gapless. In other words, measures that probe pairs of particles show insulating-like behavior earlier than measures that probe individual electrons.

To reach this conclusion the researchers simulated the Hubbard model above the temperature where spins order. They used determinant quantum Monte Carlo, a computational method that directly samples the quantum model and is described in the paper as numerically exact for the conditions studied. The study focuses on how observables change as the system crosses from a normal metal into an ‘‘anomalous metallic’’ regime that precedes full Mott insulating behavior.

A main result is that the finite-temperature crossover shows a pronounced suppression of charge fluctuations even though single-particle spectra remain gapless. That means two-particle responses (for example, how the system reacts to changes in charge) reveal Mottness sooner than single-particle probes (which look at individual electron excitations). The authors call this an anomalous metallic regime because the system is metallic by single-particle measures but already shows strong signatures of interaction-driven localization in two-particle quantities.

They also find that a gap in the density of states—the number of electron states available at each energy—forms through a momentum-resolved reshuffling of spectral weight across the Brillouin zone (momentum space). This gap does not appear first as a gap at particular momenta. Instead, the system redistributes where spectral weight sits across momentum space, and that redistribution drives the gap in the total density of states.