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

We use magnetoresistance measurements at high magnetic field (${\mu }_{0}H\le 65$ T) and low temperature ($T\ge 500$ mK) to gain fresh insights into the behavior of the upper critical field $H_{c2}$ in superconducting ultrathin FeSe films of varying degrees of disorder, grown by molecular beam epitaxy on ${\mathrm{SrTiO}}_3$. Measurements of $H_{c2}$ across samples with a widely varying superconducting critical temperature (1.2 K $\le T_c\le 21$ K) generically show similar qualitative temperature dependence. We analyze the temperature dependence of $H_{c2}$ 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 $T_c$ samples, we observe a spin-orbit scattering-driven enhancement of $H_{c2}$ above the strongly-coupled Pauli paramagnetic limit. We also observe clear deviations from WHH theory at low temperature, regardless of $T_c$. We attribute this to the multiband superconductivity of FeSe and possibly to the emergence of a low-temperature, high-field superconducting phase.