Del Pace, Giulia

Date:   Wednesday, December 2, 2020
Time:   10:00
Place:   scheduled Zoom meeting
Host:    Jean-Philippe Brantut

Tunneling transport of strongly interacting Fermi gases

Giulia Del Pace
European Laboratory for Nonlinear Spectroscopy (LENS) and Department of Physics and Astronomy, University of Florence, Italy

Tunneling transport measurements provide a powerful tool for unveiling both the coherence properties of a many-body system, through the celebrated Josephson effect, and the role of excitations in its conduction dynamics.
In this talk, I will present our recent results in probing condensation of strongly interacting fermionic superfluids, by injecting controlled currents in an ultracold atom two-reservoirs system weaklycoupled through a tunable tunneling barrier [1]. In the absence of any applied chemical potential difference, we directly measure Josephson supercurrents, that depend sinusoidally on the relative phase, in agreement with Josephson's original prediction [2]. By comparing the measured Josephson critical current throughout the BEC-BCS crossover with an analytic model [3], we extract the order parameter of fermionic superfluids, namely their condensed fraction, the quantitative determination of which has been so far indirect and somewhat inconclusive.
In a more recent work, we characterize the operation of a resonantly interacting atomic junction across the superfluid transition [4]. The condensate depletion causes Josephson supercurrents to vanish when approaching the critical temperature, providing a striking signature of the superfluid transition. Remarkably, we observe the condensate to feed not only the supercurrents, but also the resistive ones. In stark contrast with superconducting junctions, we detect indeed a large anomalous normal conductance at low temperature, arising from the coherent coupling between the condensate and phononic Bogoliubov-Anderson excitations. Furthermore, we measure a large conductance even above the critical temperature, which we ascribe to the hydrodynamic behavior of unpaired fermions at unitarity.
Our work highlights the key role of transport measurements to disclose the intrinsic nature of
quantum materials and demonstrates Josephson effect as an effective probe for any condensed
state.

[1] W. J. Kwon et al., Science 369, 6499 (2020).
[2] B. D. Josephson, Phys. Lett. 1, 251 (1962).
[3] M. Zaccanti and W. Zwerger, Phys. Rev. A 100, 063601 (2019).
[4] G. Del Pace, et al., arXiv:2010.00582 (2020).