Ultracold Atomic Fermions in Condensed Matter Physics - BCS/BEC Crossover in 6Li Randall G. Hulet Physics Department Rice University   Recent progress in cooling atomic Fermi gasses has highlighted their value for realizing some of the paradigm models of condensed matter physics. Unlike most condensed matter systems, the parameters of ultracold atomic gases, including their density, temperature, and interaction strength and sign, can all be controlled with high precision. Furthermore, periodic confining potentials, known as optical lattices, can be formed from pairs of interfering laser beams in one, two, or three dimensions. Therefore, it should be possible to simulate the Hubbard model of high-Tc in two dimensions or the one-dimensional geometry of a Luttinger liquid. In this talk, I will discuss the methods of cooling atomic Fermi gases to quantum degeneracy and the realization of a strongly interacting Fermi gas using 6Li atoms. By employing a magnetically-tunable collisional resonance known as a Feshbach resonance, the atoms are bound into weakly bound molecules/pairs. We have created a Bose-Einstein condensate of these pairs and used the condensate as a starting point to explore the BEC/BCS crossover that occurs at the Feshbach resonance. The molecular content of the many-body state is measured using optical molecular spectroscopy. We find that the pairs are much more molecular in nature than expected by theory, an observation that is seemingly unexplained by two-body physics. We expect that the large molecular content has important implications for the interpretation of all atomic fermion experiments performed within a Feshbach resonance.