We perform a theoretical study into how dipole-dipole interactions modify the properties of superfluid vortices within the context of a two-dimensional atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts like a giant antidipole, changing the density profile and generating an effective dipolar potential centred at the vortex core whose most slowly decaying terms go as 1/ρ(2) and ln(ρ)/ρ(3). These effects modify the vortex-vortex interaction which, in particular, becomes anisotropic for dipoles polarized in the plane. Striking modifications to vortex-vortex dynamics are demonstrated, i.e., anisotropic corotation dynamics and the suppression of vortex annihilation.

We theoretically investigate how quasi-particle properties of an attractive Fermi polaron are affected by nonzero temperature and finite impurity concentration. By applying both non-selfconsistent and self-consistent many-body T -matrix theories, we calculate the polaron energy (including decay rate), effective mass, and residue, as functions of temperature and impurity concentration. The temperature and concentration dependences are weak on the BCS side with a negative impurity-medium scattering length. Toward the strong attraction regime across the unitary limit, we find sizable dependences. In particular, with increasing temperature the effective mass quickly approaches the bare mass and the residue is significantly enhanced. At the temperature T ∼ 0.1TF , where TF is the Fermi temperature of the background Fermi sea, the residual polaron-polaron interaction seems to become attractive. This leads to a notable down-shift in the polaron energy. We show that, by taking into account the temperature and impurity concentration effects, the measured polaron energy in the first Fermi polaron experiment [A. Schirotzek et al., Phys. Rev. Lett. 102, 230402 (2009)] can be better theoretically explained.

Ultracold Fermi gases subject to tight transverse confinement offer a highly controllable setting to study the two-dimensional (2D) BCS to Berezinskii-Kosterlitz-Thouless superfluid crossover. Achieving the 2D regime requires confining particles to their transverse ground state which presents challenges in interacting systems. Here, we establish the conditions for an interacting Fermi gas to behave kinematically 2D. Transverse excitations are detected by measuring the transverse expansion rate which displays a sudden increase when the atom number exceeds a critical value N2D signifying a density driven departure from 2D kinematics. For weak interactions N2D is set by the aspect ratio of the trap. Close to a Feshbach resonance, however, the stronger interactions reduce N2D and excitations appear at lower density.PACS numbers: 03.75. Ss, 03.75.Hh, 05.30.Fk, 67.85.Lm Fermions confined to two-dimensional (2D) planes represent an important paradigm in many-body physics in settings ranging from thin films of superfluid helium-3 [1, 2] to the superconducting planes in high-T c cuprates [3]. Ultracold atomic gases confined in oblate potentials allow access to the 2D regime [4][5][6][7][8][9][10][11][12][13][14][15] where interactions between particles can be controlled using a Feshbach resonance [16]. In 2D Fermi gases, one can realize the BCS to Berezinskii-Kosterlitz-Thouless (BKT) superfluid crossover [17][18][19][20][21][22][23][24][25] by tuning the attractive interaction between particles in different spin states. Of particular interest is the enhanced pairing due to the transverse confinement [26][27][28][29][30] and the consequences this has for the phase diagram of the crossover [15,[31][32][33].Theoretical studies of the BCS-BKT crossover generally assume only two spatial dimensions, however, all atomic gases exist in 3D environments. Lower dimensional behaviour can be realized by freezing out dynamics along one or more directions. For atoms in a harmonic potential, with frequencies ω x , ω y and ω z , the 2D regime is achieved when the transverse (z) confinement is strong enough that occupation of transverse excited states is energetically forbidden. When a gas is frozen in the transverse ground state, dynamics in the x-y plane become decoupled from z and the gas is kinematically 2D. In an ideal gas this requires the thermal energy and chemical potential be much smaller than the transverse confinement energy k B T, µ ω z , where k B is Boltzmann's constant, T is the temperature and µ the chemical potential. When interactions are present, however, these can provide another means for generating transverse excitations which go beyond purely 2D models.In this Rapid Communication, we examine the criteria for an interacting Fermi gas to behave kinematically 2D.By measuring the transverse cloud width after time of flight we observe a rapid growth in the expansion rate when transverse excitations are present. Both the trap geometry and interaction strength are seen to limit the parameter space where interacting sys...