Temperature dependence and limiting mechanisms of the upper critical field of FeSe thin films

We use magnetoresistance measurements at high magnetic field (μ₀H≤65 T) and low temperature (T≥500 mK) to gain fresh insights into the behavior of the upper critical field Hc2 in superconducting ultrathin FeSe films of varying degrees of disorder, grown by molecular beam epitaxy on SrTiO₃. Measurements of Hc2 across samples with a widely varying superconducting critical temperature (1.2 K ≤Tc≤21 K) generically show similar qualitative temperature dependence. We analyze the temperature dependence of Hc2 in the context of Werthamer-Helfand-Hohenberg (WHH) theory. The analysis yields parameters that indicate a strong Pauli paramagnetic pair-breaking mechanism which is also reflected by pseudoisotropic superconductivity in the limit of zero temperature. In the lower Tc samples, we observe a spin-orbit scattering-driven enhancement of Hc2 above the strongly-coupled Pauli paramagnetic limit. We also observe clear deviations from WHH theory at low temperature, regardless of Tc. We attribute this to the multiband superconductivity of FeSe and possibly to the emergence of a low-temperature, high-field superconducting phase.