Preformed pairing in superconducting FeSe in the BCS-BEC cross-over regime

The BCS-BEC cross-over bridges the two important theories of bound particles (Bardeen-Cooper-Schrieffer theory and Bose-Einstein condensation) in a unified picture with the ratio of the attractive interaction to the Fermi energy as a tuning parameter. A key issue is to understand the intermediate regime, where new states of matter may emerge. It has been shown that the Fermi energy of FeSe ($T_c \sim 10$ K) is extremely small, with the result that this system is located at the verge of a BCS-BEC cross-over~[1]. Here we show that resistivity, Hall effect, Seebeck and Nernst coefficients all exhibit anomalies at $T\sim 2T_c$, well above the superconducting transition temperature. Moreover, our highly sensitive torque magnetometry shows a suppression of the Pauli susceptibility in the same regime. These anomalies appear to suggest a reduction of the density of states (pseudogap) caused by the onset of pair formation. Based on these results, a new phase diagram of FeSe above $T_c$ is proposed. \\ ~[1] S. Kasahara {\it et al}., Proc.\,Nat.\,Accad.\,Sci.\,(USA), Early Edition, 10.1073/pnas.1413477111 (November 6, 2014).