Impact of Disorder on the Superconducting Properties and BCS-BEC Crossover in FeSe Single Crystals

We investigate the crystal structure, transport properties and specific heat in five selected FeSe single crystals containing different amounts of disorder. Transport measurements show that disorder significantly suppresses superconducting transition temperature, $T_{\mathrm{c}}$, and upper critical field, $H_{\mathrm{c}2}$. Specific heat results confirm a robust multi-gap nature, a larger isotropic gap (${\mathrm{\Delta }}_{\mathrm{s}}$) and a smaller anisotropic gap (${\mathrm{\Delta }}_{\mathrm{e}\mathrm{s}}$). The smaller gap ${\mathrm{\Delta }}_{\mathrm{e}\mathrm{s}}$ becomes more isotropic with increasing disorder. Additionally, FeSe is regarded as a superconductor in the crossover regime from Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) because of its comparable $\mathrm{\Delta }$ and Fermi energy $E_{\mathrm{F}}$. By introducing disorder, the BCS-BEC crossover in FeSe can be tuned closer to BCS limit, reducing $\mathrm{\Delta }\mathrm{/}{E}_{\mathrm{F}}$ from 1.3 to 0.4.