Abstract
We report on magneto-optical imaging and the temperature dependency of the upper critical fields H-c2(c)(T) parallel to the c axis and H-c2(ab)(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. H-c2 values are found to be 52 and 50 T for H parallel to ab and H parallel to c, respectively. These values are 1.2-1.35 times larger than the weak-coupling Pauli paramagnetic limit (H-p similar to 1.84T(c)), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. Our observations of strong bending in the H-c2(ab)(T) curves and a nearly isotropic maximum upper critical field H-c2(ab)(0)H-c2(c)(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 H-c2(ab) (T) curve, whereas H-c2 deviates from the conventional WHH theoretical model without considering the spin paramagnetic effect for the H parallel to c and H parallel to ab directions. For H parallel to c, a two-band model is required to fully reproduce the behavior of H-c2, while for H parallel to ab the spin paramagnetic effect is responsible for the behavior of H-c2. The anisotropy of H-c2 is close to 3 near T-c and decreases rapidly at lower temperatures.