Upper critical fields in Ba2Ti2Fe2As4O single crystals: Evidence for dominant Pauli paramagnetic effect

Abstract

We report on magneto-optical imaging and the temperature dependency of the upper critical fields H c c2 (T ) parallel to the c axis and H ab c2 (T ) parallel to the ab plane in Ba 2 Ti 2 Fe 2 As 4 O 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 ab and H c, respectively. These values are 1.2-1.35 times larger than the weak-coupling Pauli paramagnetic limit (H p ∼ 1.84T c ), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. Our observations of strong bending in the H ab c2 (T ) curves and a nearly isotropic maximum upper critical field H ab c2 (0) ≈ H c c2 (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 ab c2 (T ) curve, whereas H c2 deviates from the conventional WHH theoretical model without considering the spin paramagnetic effect for the H c and H ab directions. For H c, a two-band model is required to fully reproduce the behavior of H c2 , while for H 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.

Comparative study of superconducting and normal-state anisotropy in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ superconductors with controlled amounts of interstitial excess Fe

“…The upturn in H c c2 may be due to the contribution from another band. Similar upturn behavior has also been observed in S-doped FeSe [60] and Ba 2 Ti 2 Fe 2 As 4 O [61]. We want to point out that such upturn behavior only occurs iv at low temperatures, which will not affect the value of anisotropy at high temperatures close to T c .…”