Strongly-correlated fermionic systems

The goal of this experiment is the study of strongly-correlated ultracold systems composed by fermionic <sup>6</sup>Li atoms. In particular, we aim at exploring the physics of the BEC-BCS crossover by tuning the scattering length a between the fermions via large magnetic Feshbach resonances that exist for lithium atoms. Thanks to this unique property, we can characterize the transition from a Bose-Einstein condensate (BEC) of Li<sub>2</sub> bosonic molecules (where a>0) to a fermionic superfluid at the centre of the resonance where a=±∞, as shown in Fig. 1. In this regime, the system can be effectively described in the picture of “generalized” Cooper pairs, mimicking the physics of high-Tc superconductors. In fact, the critical temperature T<sub>c</sub> to achieve superfluidity is of the order of the Fermi temperature T<sub>F</sub> (T<sub>c</sub>/T<sub>F</sub>~0.16), making these atomic systems the fermionic superfluids with the highest critical temperature. The correlation length of the fermionic pairs at the crossover is of the order of the inter-particle distance (~0.5 micron), i.e. much smaller than the size of standard BCS Cooper pairs, whose size/correlation length is instead typically as large as the size of the whole gas.In our system, we will study both three dimensional and two-dimensional systems, accessing, in the latter case, the physics of the Berezinskii-Kosterlitz-Thouless transition, a characteristic topological transition to superfluidity occurring in 2D systems.
Our experimental set-up is provided with large optical access to have the possibility of imprinting arbitrary optical potentials to manipulate the atomic sample at will. This opens the possibility of studying different geometrical realizations such as repulsive thin barrier, or speckle patterns.
We are indeed presently investigating the dynamics of ultracold fermions through a thin barrier, generated by focusing an elliptical laser beam on the sample (see Fig.2). In this way we realize the analogous of a Josephson junction of condensed matter systems. In particular, we are studying the coherent oscillation of pairs between the two wells driven by an initial unbalance between the populations of the two wells. Since we can produce both bosonic (BEC of molecules) and fermionic (at the crossover) superfluids, we have the unique possibility of studying Josephson dynamics in all the range of interactions, testing in this the current and still debated theories of strongly-correlated systems.
In the next future, we want to study the effects of a disordered potential introduced on the superfluids via speckle pattern. The competition between superfluidity and disorder is one of the most intriguing but still not fully understood topics in modern physics, influencing the observed behaviour of superfluids and superconductors.

Research & Technical staff:
Roati GiacomoBurchianti AlessiaZaccanti Matteo

Associated Researchers:
Fort ChiaraScazza Francesco