Upper critical fields in Ba2Ti2Fe2As4 O single crystals: Evidence for dominant Pauli paramagnetic effect

Abstract

We report on magneto-optical imaging and the temperature dependency of the upper critical fields Hc2c(T) parallel to the c axis and Hc2ab(T) parallel to the ab plane in Ba2Ti2Fe2As4O single crystals. These data were inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 60 T. Hc2 values are found to be 52 and 50 T for Hab and Hc, respectively. These values are 1.2-1.35 times larger than the weak-coupling Pauli paramagnetic limit (Hp∼1.84Tc), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. Our observations of strong bending in the Hc2ab(T) curves and a nearly isotropic maximum upper critical field Hc2ab(0)≈Hc2c(0) support the presence of a strong Pauli paramagnetic effect. We show that the Werthamer-Helfand-Hohenberg (WHH) formula that includes the spin-orbit scattering can effectively describe the Hc2ab(T) curve, whereas Hc2 deviates from the conventional WHH theoretical model without considering the spin paramagnetic effect for the Hc and Hab directions. For Hc, a two-band model is required to fully reproduce the behavior of Hc2, while for Hab the spin paramagnetic effect is responsible for the behavior of Hc2. The anisotropy of Hc2 is close to 3 near Tc and decreases rapidly at lower temperatures.