Temperature dependence and limiting mechanisms of the upper critical field of FeSe thin films
We use magnetoresistance measurements at high magnetic field (μ 0 H ≤ 65 T) and low temperature (T ≥ 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 SrTiO 3 . Measurements of H c2 across samples with a widely varying superconducting critical temperature (1.2 K ≤ T c ≤ 21 K) generically show similar qualitative temperature dependence. Here, 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.