UC Berkeley

We report a comprehensive study of the spin ladder compound BaFe2S2.5Se0.5 using neutron diffraction, inelastic neutron scattering, high pressure synchrotron diffraction, and high pressure transport techniques. We find that BaFe2S2.5Se0.5 possesses the same Cmcm structure and stripe antiferromagnetic order as does BaFe2S3, but with a reduced Néel temperature of TN=98 K compared to 120 K for the undoped system, and a slightly increased ordered moment of 1.40μB per iron. The low-energy spin excitations in BaFe2S2.5Se0.5 are likewise similar to those observed in BaFe2S3. However, unlike the reports of superconductivity in BaFe2S3 below Tc∼14 K under pressures of 10 GPa or more, we observe no superconductivity in BaFe2S2.5Se0.5 at any pressure up to 19.7 GPa. In contrast, the resistivity exhibits an upturn at low temperature under pressure. Furthermore, we show that additional high-quality samples of BaFe2S3 synthesized for this study likewise fail to become superconducting under pressure, instead displaying a similar upturn in resistivity at low temperature. These results demonstrate that microscopic, sample-specific details play an important role in determining the ultimate electronic ground state in this spin ladder system. We suggest that the upturn in resistivity at low temperature in both BaFe2S3 and BaFe2S2.5Se0.5 may result from Anderson localization induced by S vacancies and random Se substitutions, enhanced by the quasi-one-dimensional ladder structure.