The BCS-BEC crossover connects the two seemingly different phenomena of fermion systems: the pair formation and condensation in the weak-coupling regime described by the Bardeen-Cooper-Schrieffer (BCS) theory and the Bose-Einstein condensation (BEC) of tightly bound pairs in the strong-coupling regime. The BCS-BEC crossover has been explored in ultracold atomic systems by using the Feshbach resonance to tune the attractive interaction strength. On the other hand, in superconductors, the realization of the BCS-BEC crossover has been difficult since electrons are typically in the weak-coupling regime.

Excitingly, recent experiments have suggested that the attractive interaction between electrons in FeSe and its family is so strong that the system may be in the BCS-BEC crossover regime [1]. Thus, these materials can open up an opportunity to study novel physical properties of electron systems in the BCS-BEC crossover regime [2]. In particular, it has been observed that the diamagnetic response due to the superconducting fluctuation is much larger than expected from the weak-coupling fluctuation theory [3].

In our study, we theoretically investigate the effects of strong attractive interaction on the superconducting fluctuation properties and the superconducting phase diagram under magnetic fields. Analyzing a simple model, we find an enhancement of the fluctuation-induced diamagnetism in increasing the attractive interaction strength [4]. In addition, increasing the interaction strength enlarges both the vortex liquid region and the preformed pair region in the field-temperature phase diagram [5]. In this talk, I will explain these results and discuss the possible relation to the experiments of FeSe.

[1] S. Kasahara et al., Proc. Nat. Acad. Sci. USA 111, 16309 (2014).
[2] T. Shibauchi, T. Hanaguri, and Y. Matsuda, arXiv:2005.07315.
[3] S. Kasahara et al., Nat. Commun. 7, 12843 (2016).
[4] K. Adachi and R. Ikeda, Phys. Rev. B 96, 184507 (2017).
[5] K. Adachi and R. Ikeda, Phys. Rev. B 99, 064508 (2019).