The behavior of the doped Hubbard model at low temperatures is a central problem in modern condensed matter physics, with relevance to correlated materials such as cuprate superconductors. Despite extensive computational studies, many open questions remain on its low-temperature phase diagram, motivating its study through quantum simulation with ultracold fermionic atoms in optical lattices. Here, leveraging a recent several-fold reduction in experimental temperatures, we report the first direct experimental observation of the pseudogap metal in the Hubbard model. These measurements are enabled by a novel, efficient spectroscopic technique with which we resolve the opening of a partial gap, which we further correlate with a thermodynamic anomaly in the equation of state that emerges at low temperatures. Our results partially characterize the pseudogap regime and hint at a link between the pseudogap and charge order, which can be probed in future work. Furthermore, these results demonstrate the utility of quantum simulation in addressing frontier problems in correlated electron physics.
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