Broad-spectrum radiofrequency spectroscopy of a unitary Fermi gas

We explore how the radiofrequency (rf) transfer spectrum of a degenerate Fermi gas depends on the correlation strength between atoms. We find a dimer resonance that had not previously been characterized and demonstrate that it can be used to measure the s-wave contact parameter. Using potassium-40 to create a degenerate Fermi gas, we tune the scattering length such that the system is at unitarity. We then measure a broadband rf spectrum, spanning 5 MHz in frequency, far larger than the typical 20 kHz Fermi energy. Salient features in this spectrum include the dimer feature at deeply negative detuning and the high-frequency tail with a distinct ω^-3/2 power-law dependence. The average frequency of the overall spectrum is the clock shift, which is negative, despite the broad positive-frequency tail. The strength of the dimer feature is compared to predictions from coupled-channel calculations and from an analytic square-well method. Overall, our results give a deeper understanding of the clock shift and provide a new technique to measure the contact parameter.