Condensed Matter and Biological Physics Seminars

Superconductivity and Fermi superfluidity have been subjects of increasing interest since their discovery. Recently, the nature of Cooper pairs in the BCS-BEC crossover has regained attention due to the observation of a large fraction of preformed fermion pairs on the BCS side of the Feshbach resonance in ultracold atomic Fermi gases. It is expected, moreover, that an imbalance in the population of the two-component Fermi gas could lead to a rich and complex phase diagram along the BCS-to-BEC crossover. A density asymmetry, in general, makes pairing less favorable and questions about the nature of the resulting competing phases arise. I will start discussing the Cooper-pair concept at the very foundational level by using the exact ground state wavefunction of the "reduced" BCS Hamiltonian and relate to recent experiments in cold Fermi gases. Then, I will introduce a new integrable model for asymmetric superfluid systems able to describe different homogeneous and inhomogeneous competing phases such as, breached superconductivity, deformed Fermi superfluidity, and the elusive Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state. For certain values of the filling and the interaction strength, the model exhibits a new stable exotic pairing phase which combines an inhomogeneous state with an interior gap to pair-excitations. It is proven that this phase is the exact ground state in the strong coupling limit.