DOE OSTI2018
We present the discovery of superconductivity in the two-leg ladder compound BaFe 2 S 3 has established the 123-type iron chalcogenides as a novel and interesting subgroup of the iron-based superconductor family. However, in this 123 series, BaFe 2 Se 3 is an exceptional member, with a magnetic order and crystalline structure different from all others. Recently, an exciting experiment reported the emergence of superconductivity in BaFe 2 Se 3 at high pressure [J. Ying et al., Phys. Rev. B 95, 241109(R) (2017)]. In this paper, we report a first-principles study of BaFe 2 Se 3 . Our analysis unveils a variety of qualitative differences between BaFe 2 S 3 and BaFe 2 Se 3 , including in the latter an unexpected chain of transitions with increasing pressure. First, by gradually reducing the tilting angle of iron ladders, the crystalline structure smoothly transforms from Pnma to Cmcm at ~6 GPa. Second, the system becomes metallic at 10.4 GPa. Third, its unique ambient-pressure Block antiferromagnetic ground state is replaced by the more common stripe (so-called CX-type) antiferromagnetic order at ~12 GPa, the same magnetic state as the 123-S ladder. This transition is found at a pressure very similar to the experimental superconducting transition. Finally, all magnetic moments vanish at 30 GPa. This reported theoretical diagram of the complete phase evolution is important because of the technical challenges to capture many physical properties in high-pressure experiments. The information obtained in our calculations suggests different characteristics for superconductivity in BaFe 2 Se 3 and BaFe 2 S 3 : in 123-S pairing occurs when magnetic moments vanish, while in 123-Se the transition region from Block- to CX-type magnetism appears to catalyze superconductivity. Lastly, an additional superconducting dome above ~30 GPa is expected to occur.