Pairing effects in the normal phase of a two-dimensional Fermi gas

In recent experiments [1, 2], a pairing gap was detected in a two-dimensional (2D) Fermi gas with attractive interaction at temperatures where superfluidity does not occur. A relevant question is whether this gap is a pseudogap phenomenon or is due to a molecular state, which is always present in 2D. In this talk I will discuss how the boundary between the pseudogap and molecular regimes can be set, and compare the theoretical results obtained by using a t-matrix approach [3] with the above experimental data for a 2D Fermi gas. I will also show that pseudogap phenomena occurring in 2D and 3D can be related through a variable spanning the BCS-BEC crossover in a universal way. [1] M. Feld, B. Fr¨ohlich, E. Vogt, M. Koschorreck, and M. K¨ohl, Nature 480, 75 (2011). [2] P. A. Murthy, M. Neidig, R. Klemt, L. Bayha, I. Boettcher, T. Enss, M. Holten, G. Z¨urn, P. M. Preiss, and S. Jochim, arXiv:1705.10577 (2017). [3] F. Marsiglio, P. Pieri, A. Perali, F. Palestini, and G. C. Strinati, Phys. Rev. B 91, 054509 (2015)