title: Beyond linear response spectroscopy of ultracold fermi gases.
abstract: We study rf spectroscopy of ultracold Fermi gas by going beyond the linear response in the field-matter interaction. Higher order perturbation theory allows virtual processes and energy conservation beyond the single-particle level. We formulate an effective higher order theory, which agrees quantitatively with experiments on the pairing gap and is consistent with the absence of the mean-field shift in the spin-flip experiment.
authors.affiliation: Department of Physics, Nanoscience Center, University of Jyväskylä, P.O. Box 35, FIN-40014, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Kinnunen
authors.forename: P
authors.initials: P
authors.lastname: Törmä