Observation of pseudogap and enhanced rotational symmetry breaking in a Fermi-Hubbard quantum simulator

Understanding doped Mott insulators is a central goal in condensed matter physics, with relevance to cuprate superconductors.The doped Hubbard model captures the key physics of such systems. Despite its simplicity, many open questions remain concerning the anomalous metallic states at low temperatures and intermediate doping, which, in cuprates, give rise to high-temperature superconductivity upon cooling. Enabled by the recent advance in cooling techniques, we use a cold-atom quantum simulator to probe the normal state properties of the doped Hubbard model. We observe a crossover from a normal metal to a pseudogapped metal, marked by a peak in the compressibility separating the overdoped and the underdoped regimes. Lattice modulation spectroscopy reveals a loss of low-energy spectral weight in the underdoped regime, indicating pseudogap formation. In addition, with site-resolved spin correlations measurements, we observe enhanced rotational symmetry breaking at intermediate doping, consistent with a state of fluctuating stripes. These results demonstrate the utility of quantum simulation in addressing frontier problems in correlated electron physics.