Ongoing investigations of ultracold fermionic gases deal with the properties of imbalanced mixtures of fermions in different spin states, where one can explore new regimes of superfluidity and pairing beyond the conventional Cooper pairs. Recent work has focused on the behavior of an impurity immersed in a Fermi sea, which appears to be a key issue for understanding the physics of Fermi superfluids and for solving a continuing controversy about experiments performed in 2006 at MIT and Rice University on the phase separation of spin mixtures in trapped fermions.
One way to create an impurity is to add a spin-up atom to a gas of spin-down atoms of the same atomic species. When the interaction between particles of opposite spins is weakly attractive, the impurity may behave as a Fermi polaron, that is, a quasiparticle enveloped or “dressed” by particle-hole excitations of the background Fermi sea. Writing in Physical Review A, Christophe Mora and Frédéric Chevy at Ecole Normale Supérieure and CNRS in Paris instead explore the case where the interaction is such that the impurity pairs up with an atom of the Fermi sea to form a deeply bound bosonic dimer. They propose an instructive model for describing the ground-state properties of this dimer in the Fermi sea, based on both a perturbative expansion and a variational method. Their analytic results explain, in a rather intuitive way, how to dress the dimer in order to account for all relevant many-body correlations and to what extent the dimer is a composite object instead of a pointlike particle. Their results nicely complement Monte-Carlo simulations and shed light on intriguing properties of a new type of fermionic pairing. – Franco Dalfovo