Published by the American Physical Society through the American Institute of Physics
Physical Review B, Condensed Matter and Materials Physics vol:88 issue:17
We investigate the temperature dependence of the lower critical field Hc1(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 Hc1(T )
was deduced by taking into account demagnetization factors. A pronounced change in the Hc1(T) curvature is
observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth
λab(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 1 = 0.41 ± 0.1 meV and 2 = 3.33 ± 0.25 meV or a single anisotropic
s-wave order parameter, the temperature dependence of the lower critical field Hc1(T ) can be well described.
These observations clearly show that the superconducting energy gap in FeSe is nodeless.