Exploring attractively interacting fermions in 2D using a quantum gas microscope

Abstract

Recent advances in the field of ultracold quantum gases have played an important role in expanding our understanding of strongly correlated quantum matter. These gases are isolated, clean and fully controllable systems, allowing bottom-up engineering of idealized condensed matter models. Interacting Fermi gases are particularly interesting because of their relevance to understanding systems ranging from high temperature superconductors to neutron stars.

In this thesis, I describe the development of a quantum gas microscope for studying Fermi gases of lithium-6 in two dimensions. With this tool, we can probe 2D systems containing over a thousand fermions and measure the spin or density on each site as well as n-point correlations of these quantities. The design of our microscope introduces several new simplifying features, including a novel Raman cooling scheme for imaging that does not require confining the atoms in the Lamb-Dicke regime in all directions.

I report on two experiments we have performed using this instrument. First I present an exploration of attractive spin-imbalanced gases in two dimensions. We observe in-trap phase separation characterized by the appearance of a spin-balanced core surrounded by a polarized gas. In addition, we observe pair condensation in momentum-space measurements even for large polarizations where phase separation vanishes, indicating the presence of a polarized pair condensate.

In a second experiment, we explore fermions in an optical lattice, described by the Fermi-Hubbard model. Compared to the repulsive model, the attractive model has received less experimental attention despite its rich phase diagram, including a possible FFLO phase in the polarized system. Using the microscope, we directly image charge density wave correlations in our system and use them to put a lower bound on pairing correlations. We also demonstrate that these correlations constitute a sensitive thermometer that might be useful in the development of future cooling schemes.

These initial explorations with our fermion quantum gas microscope set the stage for future work that might shed insights on a wide variety of condensed matter problems, ranging from the microscopic mechanisms for pairing in high-temperature superconductors to Cooper pairing at non-zero momentum in large magnetic fields.