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Evolution of Superconductivity with Sr-Deficiency in Antiperovskite Oxide Sr3−xSnO

Mohamed Oudah, Jan Niklas Hausmann, Shinji Kitao, Atsutoshi Ikeda, Shingo Yonezawa, Makoto Seto, Yoshiteru Maeno
2019 Scientific Reports  
Bulk superconductivity was recently reported in the antiperovskite oxide Sr 3−x SnO, with a possibility of hosting topological superconductivity. We investigated the evolution of superconducting properties such as the transition temperature T c and the size of the diamagnetic signal, as well as normalstate electronic and crystalline properties, with varying the nominal Sr deficiency x 0 . Polycrystalline sr 3−x SnO was obtained up to x 0 = 0:6 with a small amount of SrO impurities. The amount
more » ... ities. The amount of impurities increases for x 0 > 0.6, suggesting phase instability for high deficiency. Mössbauer spectroscopy reveals an unusual Sn 4− ionic state in both stoichiometric and deficient samples. By objectively analyzing superconducting diamagnetism data obtained from a large number of samples, we conclude that the optimal x 0 lies in the range 0.5 < x 0 < 0.6. In all superconducting samples, two superconducting phases appear concurrently that originate from sr 3−x SnO but with varying intensities. These results clarify the Sr deficiency dependence of the normal and superconducting properties of the antiperovskite oxide sr 3−x SnO will ignite future work on this class of materials. Discoveries of superconductivity with high critical temperatures (T c 's) in the layered copper oxides 1 and iron pnictides 2 have opened new research fields not only on their superconductivity but also on neighboring and even wider topics such as strong correlation and multi-orbital effects in d-electron systems. Clarification of the composition dependence of various ordered phases and corresponding electronic properties serves as an important basis towards pioneering such novel fields. Indeed, in both copper oxides and iron pnictides, the establishment of the composition phase diagrams has been playing significant roles 3-5 . Very recently, some of the present authors reported superconductivity in the antiperovskite oxide Sr 3−x SnO 6 , a new class of oxide superconductors. The superconductivity of this oxide emerges by hole doping to the parent compound Sr 3 SnO, which is unique in hosting a negative metal ion Sn 4− and as a consequence in exhibiting three-dimensional (3D) bulk Dirac dispersion in its electronic state 7,8 . However, it was not clear how the superconductivity emerges from the parent 3D Dirac compound as the Sr deficiency x is tuned and whether the negative ionic state is actually realized. In this article, we report the dependence of superconductivity on the nominal Sr deficiency x 0 and reveal that the optimal x 0 is located around x 0 ~ 0.55-0.60. Furthermore, we provide microscopic evidence for the Sn 4− state in both stoichiometric and deficient Sr 3−x SnO. Antiperosvskite oxides A 3 BO (A = Mg, Ca, Sr, Ba, Eu, Yb and B = Si, Ge, Sn, Pb) have the perovskite crystal structure but with O 2− ions occupying the center of the octahedron formed by A 2+ ions. To satisfy the charge-neutrality relation, the B ions take an unusual 4− oxidation state and as a consequence their p orbitals are almost filled 9,10 . This unusual electronic configuration can lead to interesting properties. Indeed, theoretical works on Ca 3 PbO predicted a 3D Dirac dispersion in the electronic band 7,8 , similar to recently-studied Dirac-material candidates Au2Pb 11 , Cd3As2 12,13 and Na 3 Bi 14 . This Dirac dispersion originates from the band inversion of the nearly empty Ca-3d and nearly filled Pb-6p bands near the Γ point, as well as from the avoided hybridization between these bands due to crystal symmetry. The Dirac point is expected to have a small gap of the order of ~10 meV 7 , due to higher-order interactions originating from the spin-orbit coupling. This gapped state was later predicted to be a topological cyrstalline insulator state 15 . By changing the A and B ions, one can control the strength of the spin-orbit coupling and band mixing, and eventually tune the system from the topologically trivial Published: xx xx xxxx opeN www.nature.com/scientificreports/
doi:10.1038/s41598-018-38403-8 fatcat:sanl2zvmhvcmjgpjidpmxq7zke