We report on the creation of homogeneous Fermi gases of ultracold atoms in a uniform potential. In the momentum distribution of a spin-polarized gas, we observe the emergence of the Fermi surface and the saturated occupation of one particle per momentum state: the striking consequence of Pauli blocking in momentum space for a degenerate gas. Cooling a spin-balanced Fermi gas at unitarity, we create homogeneous superfluids and observe spatially uniform pair condensates. For thermodynamic measurements, we introduce a hybrid potential that is harmonic in one dimension and uniform in the other two. The spatially resolved compressibility reveals the superfluid transition in a spin-balanced Fermi gas, saturation in a fully polarized Fermi gas, and strong attraction in the polaronic regime of a partially polarized Fermi gas.

We observe a long-lived solitary wave in a superfluid Fermi gas of 6 Li atoms after phase imprinting. Tomographic imaging reveals the excitation to be a solitonic vortex, oriented transverse to the long axis of the cigar-shaped atom cloud. The precessional motion of the vortex is directly observed, and its period is measured as a function of the chemical potential in the BEC-BCS crossover. The long period and the correspondingly large ratio of the inertial to the bare mass of the vortex are in good agreement with estimates based on superfluid hydrodynamics that we derive here using the known equation of state in the BEC-BCS crossover. DOI: 10.1103/PhysRevLett.113.065301 PACS numbers: 67.85.-d, 03.75.Kk, 03.75.Lm, 03.75.Ss Solitary waves that do not spread as they propagate are ubiquitous in nonlinear systems, from classical fluids and fiber optics to superfluids and superconductors. These waves are localized objects with defined energy and mass, and as such they can be described as an effective single particle emerging from a many-body environment. This distinguishes them from larger-scale collective excitations such as shape oscillations of a superfluid, or from perturbative linear excitations such as phonons. Paradigmatic examples of solitary waves in superfluids are planar solitons that separate regions of differing phase, as well as vortex rings or single vortex lines [see Fig. 1(a)]. The direct creation of such localized and highly nonlinear objects "on demand" in ultracold quantum gases allows for an excellent dynamical probe of novel superfluids, such as strongly interacting Fermi gases [1] or spin-orbit coupled Bose-Einstein condensates [2,3].In a recent experiment on fermionic superfluids at MIT [1], long-lived solitary waves were produced that featured a large ratio of inertial to bare (missing) mass of over 200, evidenced by an oscillation period over 15 times longer than the period for a single atom. The observed absorption images suggested the interpretation of the waves as planar solitons, but the longevity as well as the large effective mass ratio were unexpected for this type of defect [4][5][6][7]. Indeed, the nodal plane of a soliton is energetically more costly than the nodal line of a vortex, and planar solitons can decay into lower energy excitations via the snake instability, the undulation of the soliton plane [4]. Several recent works therefore suggested that these solitary waves are vortex rings [8][9][10]. For weakly interacting BoseEinstein condensates, solitons have been created [11,12] and observed to decay into vortex rings [13,14]. The latter further decay into a vortex-antivortex pair that eventually breaks up, leaving behind a single remnant vortex [15][16][17]. The exact process was recently elucidated in a discussion of apparent soliton oscillations observed in weakly interacting BECs [18,19]. In the case of strongly interacting fermionic superfluids, the understanding of such nontrivial dynamics presents a challenging nonequilibrium many-body problem [8,20].In this Letter,...

We study the thermal evolution of a highly spin-imbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radio-frequency spectroscopy gives access to the energy, lifetime, and short-range correlations of Fermi polarons at low temperatures T . In this regime, we observe a characteristic T 2 dependence of the spectral width, corresponding to the quasiparticle decay rate expected for a Fermi liquid. At high T , the spectral width decreases again towards the scattering rate of the classical, unitary Boltzmann gas, ∝ T −1/2 . In the transition region between the quantum degenerate and classical regime, the spectral width attains its maximum, on the scale of the Fermi energy, indicating the breakdown of a quasiparticle description. Density measurements in a harmonic trap directly reveal the majority dressing cloud surrounding the minority spins and yield the compressibility along with the effective mass of Fermi polarons.Landau's Fermi liquid theory provides a quasiparticle description of the low-temperature behavior for a large class of unordered fermionic states of matter, including most normal metals, atomic nuclei, and liquid helium-3 [1]. Strongly interacting Fermi gases with highly imbalanced spin populations have been identified as belonging to the same class [2][3][4][5][6][7][8][9][10][11][12][13][14]. The quasiparticles in spin-imbalanced Fermi gases are Fermi polarons: spin impurities dressed by an excess cloud of majority fermions. The stability of quasiparticles in a Fermi liquid is a consequence of the restricted phase space for collisions due to Pauli blocking. With increasing temperature T , the accessible phase space increases, and the lifetime of quasiparticles shortens, leading to the breakdown of Fermi liquid theory. In this intermediate temperature regime the gas is neither a Fermi liquid nor a classical Boltzmann gas. For strong interactions, this regime is void of well-defined quasiparticles and controlled by the quantum critical point of the unitary, spin-balanced gas at zero chemical potential and temperature [15][16][17].Ultracold Fermi gases offer a unique opportunity to study the crossover from a low-temperature Fermi liquid to a classical Boltzmann gas, due to the large accessible temperature range. In spin-imbalanced Fermi gases, the two inequivalent Fermi surfaces provide additional richness. As the temperature is lowered from the classical regime, the Fermi surface of the majority forms first, giving minority spins the quasiparticle character of polarons. At even lower temperatures, the polarons themselves become quantum degenerate and form a Fermi surface.In this work, we access the entire crossover from degenerate polarons to the classical Boltzmann gas through the quantum critical region. The internal properties of the polaronic quasiparticles are measured via radio-frequency (rf) spectroscopy [10,[18][19][20] on a homogeneous Fermi gas [21,22]. At low temperatures, the...