The Condition for Universality at Resonance and Direct Measurement of Pair Wavefunctions Using rf Spectroscopy

We show that when the Fermi energy of a Fermi gas is much smaller than the intrinsic energy width of a Fashbach resonance, the system behaves like a Fermi gas interacting with contact potential. This in turn implies universality at resonance, and large fermionic pairs in the strongly interacting regime. The recent experiments of JILA (PRL. 92, 040403 (2004)) and MIT (PRL. 92, 120403 (2004)) turn out to be deep inside this universal regime, which explains the perfect fit of these experiments by the BEC-BCS crossover theory with contact potential (cond-mat/0404517). We also show that rf spectrocopy can be used to map out the pair wavefunction directly.