Some of the most vexing present-day problems in physics center on understanding the many-body properties and phases of strongly interacting fermions. Part of the difficulty arises from the fact that while the behavior of interacting fermions is understood in certain well-defined regions of parameter space, this understanding often breaks down in the correlated strongly interacting regime. Achieving this understanding of strongly interacting fermions would have wide-ranging implications: the systems of interest include high-temperature superconductors, high-density quark matter, and ultracold atomic fermion gases. The latter is the subject of a recent experimental investigation by John Stewart, John Gaebler, Tara Drake, and Debbie Jin at JILA in the US, appearing in Physical Review Letters [1]. Stewart et al. succeeded in confirming the validity of certain nonperturbative theoretical formulas, known as the Tan relations [2], which describe the properties of fermions with short-range interactions. The verification of these relations shows that cold-fermion experiments are particularly suited as a testing laboratory for studying the general problem of strongly interacting fermion systems [3, 4].
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