Direct evidence for multi-gap nodeless superconductivity in FeSe
A combined study of low-temperature specific heat $C(T)$ and scanning tunneling microscopy measurements on single crystalline FeSe reveals the existence of two superconducting gaps without nodes. The $C(T)$ data can be represented by a phenomenological two-band model with an s-wave plus an extended s-wave ($s$ + $es$) type gap structure. The estimated s-wave gap magnitude $\Delta_{s}$ is significantly smaller than that of the extended s-wave gap $\Delta_{es}$. An overall "U"-shaped gap found in the tunneling spectra away as well as on top of twin boundaries is in excellent agreement with the $C(T)$ analysis, confirming nodeless multi-gap superconductivity throughout the FeSe material.