Atomic Physics Seminar: Giulia Del Pace, University of Florence/LENS, Italy, “Extracting the order parameter of fermionic superfluids from Josephson supercurrent”

The dc Josephson effect provides a formidable phase-sensitive tool for investigating superfluid order parameters, whose experimental determination has been so far indirect and inconclusive.
In this talk, I will present our recent results in probing condensation of strongly correlated fermionic superfluids, by injecting controlled Josephson currents in an ultracold atom two-reservoirs system weakly-coupled through a tunable tunneling barrier. In the absence of any applied chemical potential difference, we directly measure supercurrents, that depend sinusoidally on the relative phase in agreement with Josephson’s original prediction [1]. For different junction parameters and interaction strengths along the BCS-BEC crossover, we extract the Josephson critical current which, for fixed barrier height, exhibits a non-monotonic trend peaked around unitarity. We attribute this behavior to the competition of an increasing pair condensate amplitude and a decreasing chemical potential when moving from the BCS towards the BEC regime, finding quantitative agreement with a simple analytic model throughout the crossover region [2]. Our work demonstrates the effectiveness of coherent Josephson transport for pinning down superfluid order parameters, with great promise towards applications in diverse atomic systems, even with strong correlations.
[1] B. D. Josephson, Phys. Lett. 1, 251 (1962).
[2] M. Zaccanti and W. Zwerger, arXiv:1907.08052 (2019)
Host: Nir Navon (nir.navon@yale.edu)