title: Observation of the Berezinskii-Kosterlitz-Thouless Phase Transition in an Ultracold Fermi Gas.
abstract: We experimentally investigate the first-order correlation function of a trapped Fermi gas in the two-dimensional BEC-BCS crossover. We observe a transition to a low-temperature superfluid phase with algebraically decaying correlations. We show that the spatial coherence of the entire trapped system can be characterized by a single temperature-dependent exponent. We find the exponent at the transition to be constant over a wide range of interaction strengths across the crossover. This suggests that the phase transitions in both the bosonic regime and the strongly interacting crossover regime are of Berezinskii-Kosterlitz-Thouless type and lie within the same universality class. On the bosonic side of the crossover, our data are well described by the quantum Monte Carlo calculations for a Bose gas. In contrast, in the strongly interacting regime, we observe a superfluid phase which is significantly influenced by the fermionic nature of the constituent particles.
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: P A
authors.initials: PA
authors.lastname: Murthy
authors.affiliation: Institut für Theoretische Physik, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Boettcher
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: L
authors.initials: L
authors.lastname: Bayha
authors.affiliation: LPTMC, UMR 7600 of CNRS, Université Pierre et Marie Curie, F-75252 Paris Cedex 05, France.|Université Grenoble Alpes, CNRS, LPMMC, UMR 5493, F-38042 Grenoble, France.
authors.forename: M
authors.initials: M
authors.lastname: Holzmann
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Kedar
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Neidig
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: M G
authors.initials: MG
authors.lastname: Ries
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: A N
authors.initials: AN
authors.lastname: Wenz
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Zürn
authors.affiliation: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Jochim