Discovery of Superconductivity in (Ba,K)SbO$_{3}$

TL;DR

This paper reports the discovery of superconductivity in (Ba,K)SbO₃, showing that strong metal-oxygen covalency, rather than charge transfer energy sign, is key to high-temperature superconductivity in main-group perovskite oxides.

Contribution

It introduces (Ba,K)SbO₃ as a new material demonstrating superconductivity and highlights the role of metal-oxygen covalency over charge transfer energy in superconductivity mechanisms.

Findings

Superconductivity observed up to 15 K in (Ba,K)SbO₃.

Enhanced charge density wave gap compared to BaBiO₃.

Superconductivity emerges with potassium doping up to 65%.

Abstract

Superconducting bismuthates (Ba,K)BiO$_{3}$ (BKBO) constitute an interesting class of superconductors in that superconductivity with a remarkably high $T_\mathrm{c}$ of 30 K arises in proximity to charge density wave (CDW) order. Prior understanding on the driving mechanism of the CDW and superconductivity emphasizes the role of either bismuth (negative $U$ model) or oxygen ions (ligand hole model). While holes in BKBO presumably reside on oxygen owing to their negative charge transfer energy, so far there has been no other comparative material studied. Here, we introduce (Ba,K)SbO$_{3}$ (BKSO) in which the Sb 5$s$ orbital energy is higher than that of the Bi 6$s$ orbitals enabling tuning of the charge transfer energy from negative to slightly positive. The parent compound BaSbO$_{3-\delta}$ shows a larger CDW gap compared to the undoped bismuthate BaBiO$_{3}$. As the CDW order is suppressed via potassium substitution up to 65 %, superconductivity emerges, rising up to $T_\mathrm{c}$ = 15 K. This value is lower than the maximum $T_\mathrm{c}$ of BKBO, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced CDW gap and superconductivity in BKSO indicates that the sign of the charge transfer energy may not be crucial, but instead strong metal-oxygen covalency plays the essential role in constituting a CDW and high-$T_\mathrm{c}$ superconductivity in the main-group perovskite oxides.