Observation of a Strongly Interacting Degenerate Fermi Gas of Atoms

Abstract

We report on the observation of a highly-degenerate, strongly-interacting Fermi gas of atoms. Fermionic 6 Li atoms in an optical trap are evaporatively cooled to degeneracy using a magnetic field to induce strong, resonant interactions. Upon abruptly releasing the cloud from the trap, the gas is observed to expand rapidly in the transverse direction while remaining nearly stationary in the axial. We interpret the expansion dynamics in terms of collisionless superfluid and collisional hydrodynamics. For the data taken at the longest evaporation times, we find that collisional hydrodynamics does not provide a satisfactory explanation, while superfluidity is plausible.As the fundamental constituents of matter are interacting fermions, the experimental study of stronglyinteracting, degenerate Fermi gases will impact theories in fields from particle physics to materials science. Although the interactions between fermions are understood when they are weak (e.g., quantum electrodynamics), the treatment of very strong interactions requires the development of new theoretical approaches. To test these new approaches, there is a need for experimental systems with widely tunable interaction strengths, densities, and temperatures. Ultracold atomic Fermi gases have exactly these properties, and thus enable tests of calculational techniques for fundamental systems ranging from quarks in nuclear matter to electrons in high temperature superconductors [1,2, 3,4,5,6,7,8,9,10,11]. For this reason, a number of groups are developing methods for creating and exploring ultracold atomic Fermi gases [12,13,14,15,16,17]. We report on the study of a strongly-interacting, degenerate Fermi gas. In contrast to the isotropic expansion previously observed for a noninteracting degenerate Fermi gas [12], we observe anisotropic expansion when the gas is released from an optical trap.An exciting feature of strongly-interacting atomic Fermi gases is the possibility of high-temperature superfluids that are analogs of very high temperature superconductors [8,9,10,11]. Our experiments produce the conditions predicted for this type of superfluid transition. Further, the anisotropic expansion we observe has been suggested as a signature of the onset of superfluidity in a Fermi gas [18]. We interpret the observed anisotropic expansion in terms of both collisionless superfluid hydrodynamics [18] and a new form of collisional hydrodynamics.Strong, magnetically-tunable interactions are achieved in our experiments by employing a Fermi gas comprising a 50-50 mixture of the two lowest hyperfine states of 6 Li, i.e., the |F = 1/2, M = ±1/2 states in the low-magnetic-field basis. This mixture has a predicted broad Feshbach resonance near an applied magnetic field of 860 G [19,20], where the energy of a bound 6 Li-6 Li molecular state is tuned into coincidence with the total energy of the colliding atoms. This enables the interaction strength to be widely varied [19,20,21,22]. It has also been suggested that interactions between fermions can be modified by immers...

Many body methods and effective field theory

“…For free fermions ξ = 1, but for strongly correlated fermions the theory contains no obvious expansion parameter and the determination of ξ is a difficult non-perturbative problem. Recent interest in this problem has been fueled by experimental advances in creating cold, dilute gases of fermionic atoms tuned to be near a Feshbach resonance [15]. These experiments are beginning to yield results for the equation of state of non-relativistic fermions in the limit |k F a| → ∞.…”

Lattice Monte Carlo calculations for unitary fermions in a harmonic trap

“…Thus the s-wave phase shift satisfies δ(k) = π/2 for all k and the field theory describing the many-body system is at a conformal fixed point 1 ; in 1998 it was suggested that unitary fermions could serve as the starting point for an effective field theory expansion for nuclear physics [2,3]. Since then the unitary fermion gas has been created and studied experimentally by trapping atoms tuned to a Feshbach resonance by means of an applied magnetic field, exhibiting collective effects interpolating between the well understood phenomena of BCS pairing and Bose-Einstein condensation [4][5][6][7][8][9][10][11][12][13]. The nonperturbative nature of the strongly coupled interaction between unitary fermions poses a nontrivial challenge for theory, and numerical simulation has played an essential role in making progress.…”

Cluster formation and the virial equation of state of low-density nuclear matter

“…In a separate paper [6], we have calculated the equation of state of low-density neutron matter using the virial expansion, in order to assess quantitatively how close the system is to universal behavior at finite temperature. The equation of state of resonant and dilute Fermi gases has also been studied in laboratory experiments with trapped atoms [7,8,9,10], and Ho et al have used the virial expansion to describe Fermi gases at high temperatures in the vicinity of Feshbach resonances [11,12].…”