Brendan C. Mulkerin

Comparison of strong-coupling theories for a two-dimensional Fermi gas

Understanding the formation of Cooper pairs and the resulting thermodynamic properties of a low-dimensional Fermi gas is an important area of research, elucidating our understanding of high temperature superconductors. In lower dimensions quantum fluctuations are expected to play an increasingly important role and the reliability of strong-coupling theories becomes questionable. Here, we present a comparison of recent thermodynamic measurements and theoretical predictions from different many-body

Criteria for two-dimensional kinematics in an interacting Fermi gas

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Superfluid density and critical velocity near the Berezinskii-Kosterlitz-Thouless transition in a two-dimensional strongly interacting Fermi gas

-matrix theories for a two-dimensional strongly interacting Fermi gas in the normal state. We find that the fully self-consistent

Breathing mode frequency of a strongly interacting Fermi gas across the 2D-3D dimensional crossover

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Larkin-Ovchinnikov superfluidity in a two-dimensional imbalanced atomic Fermi gas

-matrix theory provides the best description of the experimental data over a wide range of temperatures and interatomic interactions. Our comparison reveals the crucial role played by the interactions between Cooper pairs and suggests that the future development of a quantitative strong-coupling theory for two-dimensional Fermi superfluids must explicitly take into account the diagrams that are responsible for pair-pair interactions.

Dimensional crossover in a strongly interacting ultracold atomic Fermi gas

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 display 2D kinematics. Transverse excitations are detected by measuring the transverse expansion rate which displays a sudden increase when the atom number exceeds a critical value N-2D signifying a density driven departure from 2D kinematics. For weak interactions N-2D is set by the aspect ratio of the trap. Close to a Feshbach resonance, however, the stronger interactions reduce N-2D and excitations appear at lower density.

Beyond Gaussian pair fluctuation theory for strongly interacting Fermi gases

We theoretically investigate superfluidity in a strongly interacting Fermi gas confined to two dimensions at finite temperature. Using a Gaussian pair fluctuation theory in the superfluid phase, we calculate the superfluid density and determine the critical temperature and chemical potential at the Berezinskii-Kosterlitz-Thouless transition. We propose that the transition can be unambiguously demonstrated in cold-atom experiments by stirring the superfluid Fermi gas using a red detuned laser beam, to identify the characteristic jump in the local Landau critical velocity at the superfluidnormal interface, as the laser beam moves across the cloud.

Equation of state and contact of a strongly interacting Bose gas in the normal state

We address the interplay between dimension and quantum anomaly on the breathing mode frequency of a strongly interacting Fermi gas harmonically trapped at zero temperature. Using a beyond mean-field, Gaussian pair fluctuation theory, we employ periodic boundary conditions to simulate the dimensionality of the system and impose a local density approximation, with two different schemes, to model different trapping potentials in the tightly-confined axial direction. By using a sum-rule approach, we compute the breathing mode frequency associated with a small variation of the trapping frequency along the weakly-confined transverse direction, and describe its behavior as functions of the dimensionality, from two- to three-dimensions, and of the interaction strength. We compare our predictions with previous calculations on the two-dimensional breathing mode anomaly and discuss their possible observation in ultracold Fermi gases of

Quantum anomaly in a quasi-two-dimensional strongly interacting Fermi gas

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Breakdown of the Fermi polaron description near Fermi degeneracy at unitarity.

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