Strongly Interacting Fermi Gases of Ultracold Atoms

The modern world is run by electrons--they flow through our smart phones, computers, and machines to carry out a myriad of tasks from data storage and calculations to heavy lifting when employed in electromagnets. It is surprising that we do not better understand how electrons work together. This award supports studies of a novel substance, an ultracold gas of strongly interacting atoms, which behaves in many ways like electrons. For example, just like a metal becomes "superconducting" at low temperatures and starts to conduct electricity without resistance, the atomic gas becomes "superfluid" and atoms flow without friction. However, scaled to the density of electrons in metals, superfluidity would occur in the atomic gas already far above room temperature, thanks to the strong interatomic interactions. Just like electrons, but also protons and neutrons, the atoms belong to the class of particles called fermions, which cannot share one and the same state. This requirement makes computations extremely difficult and experiments indispensable to learn about the behavior of fermions. Confined in an artificial "box" of light, the atomic Fermi gas will be a pristine platform to learn about the equation of state of strongly interacting fermions, as they occur in modern materials, for example high-temperature superconductors, but also in neutron stars and nuclear matter. With the help of these and other studies, we might be led to an understanding on how to realize room temperature superconductivity. The project also holds the potential for observing new states of fermionic matter such as a supersolid--a superfluid that is also ordered like a crystal. The research will present a stimulating learning experience for graduate students.

Ultracold Fermi gases of atoms represent a paradigmatic form of fermionic matter, where all details of the interparticle interaction, the external confinement, and the spin composition are precisely known and under the control of the experimenter. This project employs a Fermi gas of Lithium-6 atoms to try to answer long-standing questions about 1) the thermodynamics of two- and three-dimensional systems, 2) the fate of fermionic superfluidity in the presence of spin imbalance and 3) non-equilibrium dynamics in fermionic superfluids. The Fermi gas will be confined in tailored potentials, in particular a homogeneous box potential and a hybrid harmonic-box potential. Creating a homogeneous Fermi gas will take away many of the existing experimental limitations in obtaining accurate thermodynamic information. The box potential allows accessing new phases of fermionic matter that have not been observed before. The Berezinskii-Kosterlitz-Thouless superfluid in two dimensions features algebraic order that would be masked in an inhomogeneous trap. A homogeneous 3D Fermi superfluid in the presence of spin imbalance should spontaneously break translational symmetry by forming a train of solitons, where excess fermions reside in the nodes of the order parameter. This describes the famous Larkin-Ovchinnikov (LO) state, a supersolid phase of matter that has not been conclusively observed despite five decades of research. In the present work, soliton trains will be directly created in the presence of spin imbalance, thereby engineering the LO state.