Is Ba₃In₂O₆a high-T_c superconductor?

Immediately following its first synthesis in the 1980’s Ba₃In₂O₆ was, due to its structural similarity to La_(2-x)Sr_xCaCu₂O₆ and La₂CaCu₂O_(6+δ), hypothesized to be a high-T_c superconductor.¹ Ba₃In₂O₆ being highly hygroscopic, however, inhibited any characterization of its transport properties. In the following years the material was all but forgotten until recently machine learning predicted its T_c to be 45.9 K.² To answer the question whether Ba₃In₂O₆ is a high T_c superconductor, Ba₃In₂O₆ films were grown by molecular-beam epitaxy and capped by amorphous SiO₂.³ The indium species were supplied by a newly developed suboxide source emanating a beam of In₂O.^3,4 In the course of successfully growing epitaxial Ba₃In₂O₆ films, the previously unknown member of the same Ruddlesden-Popper series, Ba₄In₂O₇, was also epitaxially grown.³ Despite the high quality of our films, which were probed by x-ray diffraction and scanning tunneling microscopy, Ba₃In₂O₆ turns out to be insulating. Optical bandgap measurements of the epitaxial films by ellipsometry reveal a higher-than-expected bandgap (3.0 eV in-plane and 2.8 eV out-of-plane). We present different doping strategies and utilize first-principles calculations to show why all of them have been unsuccessful so far.

References

1 K. Mader and H. Müller-Buschbaum, Zur Kristallstruktur von Ba₃In₂O₆, Zeitschrift für anorganische und allgemeine Chemie 559, 89-94 (1988).

2 L. Liu, P. Kang, Y. Zhu, L. Liu, and H. Guo, Material informatics for layered high- T_C superconductors," APL Materials 8, 061104 (2020).

3 F.V.E. Hensling, M.A. Smeaton, V. Show, K. Azizie, M. Barone, L.F. Kourkoutis, D.G. Schlom, Epitaxial growth of the first two members of the Ba_n+1In_nO_2,5n+1 Ruddlesden-Popper homologous series, JVST A: Journal of Vacuum Science and Technology, (in press, 2022).

4 D.G. Schlom, P. Vogt, F.V.E. Hensling, K. Azizie, Z.-K. Liu, B.J. Bocklund, S. Shun-Li, Suboxide Molecular Beam Epitaxy and Related Structures, U.S. Patent Application No. 0122843 (published on April 21, 2022).