Session J06: Correlations and Dynamics of Ultracold Fermions

Probing dynamical properties of Fermi-Hubbard systems with a quantum gas microscope

The normal state of high-temperature superconductors exhibits anomalous transport and spectral properties that are poorly understood. Cold atoms in optical lattices have been used to realize the celebrated Fermi-Hubbard model, widely believed to capture the essential physics of these materials. The recent development of fermionic quantum gas microscopes has enabled studying Hubbard systems with single-site resolution. Most studies have focused on probing equal-time spin and density correlations. In this talk, I will report on using a microscope to probe response functions associated with unequal-time correlations relevant for understanding the pseudogap and strange metal regimes of Fermi-Hubbard systems. First, I will describe the development of a technique to measure microscopic diffusion, and hence resistivity, in doped Mott insulators. We have found that this resistivity exhibits a linear dependence on temperature and violates the Mott-Ioffe-Regel limit, two signatures of strange metallic behavior. Next, I will report on the development of angle-resolved photoemission spectroscopy (ARPES) for Hubbard systems and its application to studying pseudogap physics in an attractive Hubbard system, setting the stage for future studies of the pseudogap regime in repulsive Hubbard systems.   

Ultracold atom quantum simulations: Exploring low temperature Fermi-Hubbard phases

Ultracold-atom model-systems offer a unique way to investigate many-body quantum physics in uncharted regimes. Quantum gas microscopy enables us to “zoom in” on a single particle level. We can explore many-body quantum physics in regimes that are not computationally accessible. In my talk I will present the realization of an anti-ferromagnetic phase of Fermions in an optical lattice, and results on probing string pattern in the doped Fermi-Hubbard model.   

Quantum transport in strongly interacting Fermi gases

Transport is the defining property of states of matter, but often the most difficult to understand. Strongly interacting Fermi gases are especially challenging, despite their ubiquitous presence across many fields of physics.\\ \\ Experiments on ultracold fermionic atoms allow the direct measurement of transport properties in ideal model systems where the hamiltonian is precisely known while transport properties are difficult to calculate theoretically.\\ \\ In this talk I will present transport measurements on two strongly interacting Fermi systems, the unitary Fermi gas and the Fermi-Hubbard gas, both realized in uniform box potentials. In the unitary gas, we excite first and, for the superfluid, also second sound waves and demonstrate a quantum limited sound diffusivity given by Planck’s constant divided by the particle mass. Second sound waves are directly imaged via local thermometry, making use of the temperature dependence of radiofrequency spectroscopy. For the Fermi-Hubbard gas, we measure spin diffusion and spin conductivity in the Mott insulator at half filling. For strong interactions, spin diffusion is driven by super-exchange and doublon-hole-assisted tunneling, and strongly violates the quantum limit of charge diffusion. This work sheds light on the complex interplay between spin and charge transport in the Hubbard model.\\ \\ Our experiments provide benchmarks for the highly challenging theoretical calculations of these transport coefficients.   

From spin-charge separation to magnetic polarons in Hubbard systems

The Hubbard model offers an intriguing playground to explore strongly correlated many-body systems. Much of its complexity arises from the interplay of spin and charge degrees of freedom. Here we report on the experimental study of one- and two-dimensional synthetic Hubbard systems implemented on the optical lattice platform. We discuss our recent observations of spin-charge separation in one dimension and the imaging of magnetic polarons in two dimensions. Due to our spin and charge resolved imaging technique, our measurements are largely independent of presumed models. Future extensions of these experiments may allow one to study the interaction of polarons as a precursor to collective many body physics in the Hubbard model.