The BCS-BEC crossover from strongly overlapping Cooper pairs to non-overlapping composite bosons in the strong coupling limit has been a long-standing issue of interacting many-body fermion systems. It has been extremely challenging to realize such a crossover state in electronic systems over the past decade [1, 2]. Recently, FeSe semimetal [3, 4] emerged as a high-T_c superconductor located in the BCS-BEC crossover regime, owing to its very small Fermi energies [5, 6]. In FeSe, however, an ordinary BCS-like heat-capacity jump is observed at T_c [7–9], posing a fundamental question on the characteristics of the BCS-BEC crossover. To study this issue, we focus on iron chalcogenide FeSe_1−xS_x, where the nematic phase transition at 90 K in FeSe can be tuned by isovalent sulfur(S) substitution. When the nematic order is suppressed at x > 0.17, strong deviation from mean-field BCS transition is found in the thermodynamic quantities with giant superconducting fluctuations extending far above T_c. However, no pseudogap formation is observed in the tunneling spectra. These results illuminate highly unusual features of the BCS-BEC crossover in superconductors with multiband electronic structure and competing electronic instabilities.

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Keywords: iron chalcogenide, BCS-BEC crossover, specific heat, electron nematicity