Abstract
We investigate the temperature dependence of the lower critical field ${H}_{c1}(T)$ of a high-quality FeSe single crystal under static magnetic fields $H$ parallel to the $c$ axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding ${H}_{c1}(T)$ was deduced by taking into account demagnetization factors. A pronounced change in the ${H}_{c1}$(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth ${\ensuremath{\lambda}}_{ab}(T)$ calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean $s$-wave superconductor. Using either a two-band model with $s$-wave-like gaps of magnitudes ${\ensuremath{\Delta}}_{1}=0.41\ifmmode\pm\else\textpm\fi{}0.1$ meV and ${\ensuremath{\Delta}}_{2}=3.33\ifmmode\pm\else\textpm\fi{}0.25$ meV or a single anisotropic $s$-wave order parameter, the temperature dependence of the lower critical field ${H}_{c1}(T)$ can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.
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