Enhanced paraconductivity-like fluctuations in the radiofrequency spectra of ultracold Fermi atoms

In gases of ultracold Fermi atoms, the crossover from Bardeen–Cooper–Schrieffer superconductivity to Bose– Einstein condensation can be realized by continuously varying the attraction between fermions of different species. In this context, radiofrequency spectroscopy provides a microscopic probe to infer the nature of fermionic pairing. In the regime of strong interaction, the pairing affects a wide temperature range, which includes the critical temperature Tc, in analogy to the pseudogap physics for high-temperature superconductors. Here, we establish a direct connection between the theory of radiofrequency spectra for ultracold fermions above Tc and the theory of paraconductivity fluctuations in superconductors. Our calculations compare favourably to available experimental radiofrequency spectra, and demonstrate that the role of fluctuations for ultracold fermions is considerably enhanced with respect to superconductors. In addition, we illustrate how to extract from the spectra an energy scale associated with pairing and relate it to a universal quantity recently introduced for Fermi gases.