Multiband description of the upper critical field of bulk FeSe

The upper critical field of multiband superconductors can be an essential quantity to unravel the nature of
superconducting pairing and its interplay with the electronic structure. Here we experimentally map out the
complete upper critical field phase diagram of FeSe for different magnetic field orientations at temperatures
down to 0.3 K using both resistivity and torque measurements. The temperature dependence of the upper critical
field reflects that of a multiband superconductor and requires a two-band description in the clean limit with band
coupling parameters favoring interband over intraband interactions. Despite the relatively small Maki parameter
in FeSe of α ∼ 1.6, the multiband description of the upper critical field is consistent with the stabilization of
a Fulde-Ferrell-Larkin-Ovchinnikov state below T /Tc ∼ 0.3. We find that the anomalous behavior of the upper
critical field is linked to a departure from the single-band picture, and FeSe provides a clear example of where
multiband effects and the strong anisotropy of the superconducting gap need to be taken into account.