We show that electromagnetically-induced isotropic dipole–dipole interactions in a spin-polarized non-degenerate fermionic gas can cause an extremely exothermic phase transition, analogous to the isothermal collapse in gravitationally interacting star clusters. This collapse may result in fragmentation of the gas into a hot "halo" and a highly degenerate "core". Possible realization is envisaged in microwave-illuminated fermionic molecular gases at microkelvin temperatures.
We show that electromagnetically-induced isotropic dipole–dipole interactions in a spin-polarized non-degenerate fermionic gas can cause an extremely exothermic phase transition, analogous to the isothermal collapse in gravitationally interacting star clusters. This collapse may result in fragmentation of the gas into a hot "halo" and a highly degenerate "core". Possible realization is envisaged in microwave-illuminated fermionic molecular gases at microkelvin temperatures.
We study the two mechanisms of the interplay of long- and short-range interactions in different geometries of ultracold fermionic atomic or molecular gases. We show that in the range of validity of the one-dimensional (1D) approximation, both mechanisms yield similar superconductivity. We show that electromagnetically induced isotropic dipole–dipole interactions in a spin-polarized non-degenerate fermionic gas can cause an extremely exothermic phase transition, analogous to the isothermal collapse in gravitationally interacting star clusters. This collapse may result in fragmentation of the gas into a hot ‘halo’ and a highly degenerate ‘core’. Possible realization is envisaged in microwave-illuminated fermionic molecular gases at microkelvin temperatures.
We theoretically investigate the thermodynamic properties of a quasi-one-dimensional single-component dipolar Fermi gas at finite temperatures. A self-bound fermionic droplet can be achieved by exchange correlations with long-range dipole-dipole interactions under quasi-one-dimensional confinement, where the interaction can be tuned by tilting the dipoles along the system coordinate. Using the Hartree-Fock approximation, we show how the liquid-gas phase transition occurs in this system and elucidate the finite-temperature phase structure consisting of a gas phase, liquid phase, gas-liquid coexistence phase, and spinodal phase. We also discuss its similarity to the liquid-gas phase transition in nuclear matter through a comparison with phenomenological models. By examining the experimental conditions for realizing self-bound fermionic droplets, we find that microwave-shielded fermionic polar molecules are promising candidates. Our results will be useful for an interdisciplinary understanding of self-bound fermionic matter as well as an analog quantum simulation of nuclear systems.
Abstract Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter 1–9 , implement quantum information schemes 10–12 and test the fundamental symmetries of nature 13 . Realizing their full potential requires cooling interacting molecular gases deeply into the quantum-degenerate regime. However, the intrinsically unstable collisions between molecules at short range have so far prevented direct cooling through elastic collisions to quantum degeneracy in three dimensions. Here we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium–potassium molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such cold and dense samples of polar molecules open the path to the exploration of many-body phenomena with strong dipolar interactions.
Ultracold polar molecular gases promise new directions and exciting applications in collisions and chemical reactions at ultralow energies, precision measurements, novel quantum phase transitions, and quantum information science. Here we briefly discuss key experimental requirements for observing strong dipole-dipole interactions in an ultracold dipolar gas of molecules. We then survey current experimental progress in the field with a focus on our recent work creating a near quantum degenerate gas of KRb polar molecules [Ni et al., Science, 2008, 322, 231].
Ultracold dilute atomic gases are highly controllable systems whose properties are fundamentally determined by the interatomic interactions.In typical experiments on ultra cold gases particles interact dominantly via a short-range isotropic potential.However, during the last years a new generation of experiments is starting to explore systems, where an additional interaction, the long-range and anisotropic dipole-dipole interaction plays a significant or possibly dominant role.The effects induced by the anisotropy of the dipole-dipole interaction are crucially enhanced if the dipolar gas is confined in a deep optical lattice.In this thesis we study novel phenomena in such systems of dipolar gases in deep optical lattices.The nonlocal nonlinearity introduced by the dipole-dipole interaction plays a crucial role in the physics of dipolar Bose-Einstein condensates.In particular, it may distort significantly the stability of straight vortex lines.Remarkably, in the presence of a periodic potential along the vortex line, the spectrum of transverse modes shows a roton-like minimum, which eventually destabilizes the straight vortex when the BEC as a whole is still stable, opening the possibility for new scenarios for vortex line configurations in dipolar gases.We have analyzed this instability, and showed that it leads to a second-order-like phase transition from a straight vortex line into novel helical or snake-like configurations, depending on the dipole orientation.Strong 1D lattices usually lead to unconnected two-dimensional gases.The long-range character of the dipole-dipole interactions leads to a novel scenario where non-overlapping gases at different sites may interact significantly.We show that the excitations of non-overlapping condensates in 1D optical lattices acquire a band-like character, being collectively shared by different sites.In particular, the hybridization of the modes significantly enhances the rotonization of the excitations, and may induce roton-instability.We discuss the observability of this effect in on-going experiments.Fermionic polar molecules in deep 1D optical lattices may form selfassembled filaments when the electric dipoles are oriented along the lattice axis.These composites are bosons or fermions depending on the number of molecules per chain, leading to a peculiar and complex Bose-Fermi mixture, vi which we discuss in detail for the simplest case of a three-well potential.We show that the interplay between filament binding energy, transverse filament modes, and trimer Fermi energy leads to a rich variety of possible scenarios ranging from a degenerate Fermi gas of trimers to a binary mixture of two different types of bosonic dimers.We study the intriguing zero temperature and finite temperature physics of these composites for the particular case of an ideal filament gas loaded in 1D sites, and discuss possible methods to probe these chain mixtures.
Long-range anisotropic dipole-dipole interactions between ultracold polar molecules are predicted to drive exotic quantum phases, yet direct many-body signatures of these interactions in degenerate Fermi gases have remained elusive. Here, we report the observation of an interaction-induced controlled deformation of the Fermi surface, providing a clear many-body signature in a deeply degenerate Fermi gas of 23Na40K molecules. Using double microwave (MW) shielding, we prepare 8×103 molecules at 0.23(1) times the Fermi temperature, achieving a three-fold suppression of inelastic losses compared to single MW shielding while preserving strong elastic dipolar scattering. We observe Fermi surface deformations of up to 7%, more than two times larger than those observed in magnetic atoms, despite operating at two orders of magnitude lower densities. Crucially, we demonstrate continuous tuning of the interaction potential from axial U(1) to biaxial C2 symmetry, directly imprinting this geometry onto the Fermi surface. We find excellent agreement between our experimental results and parameter-free Hartree-Fock theory. These results establish MW-shielded polar molecules as a highly tunable platform for exploring strongly correlated dipolar Fermi matter and offer a promising path towards topological superfluidity.