Fig. 4. Superconductivity and weakened oxygen-hole character in Ba_0.35K_0.65SbO₃.

a, Superconducting transition observed in the resistivity (ρ) of optimally doped antimonate (x = 0.65). T denotes temperature. The superconducting transition temperature T_c, defined by the clear onset of the transition, is ~15 K. b, The superconducting transition of the same sample is observed in zero-field-cooled magnetic susceptibility (χ) measured at μ₀H = 0.001 T (red), in comparison with that of Ba_0.34K_0.66BiO₃ (grey)⁴⁰. H is an applied magnetic field, and μ₀ is the vacuum permeability. The diamagnetic volume fraction is near 100%, indicating bulk superconductivity. Here, T_c is 15 K, defined as a temperature where the volume fraction started increasing by 0.1%. c, The superconducting transition of the same sample observed in the specific heat. ΔC denotes the difference between specific heats (C) under each field and 14 T. T_c is estimated to 15 K from the clear onset of jump, which can be suppressed by applying a field of 1 T. The observed jump is broadened, perhaps indicating sample inhomogeneity from the high-pressure synthesis. d, Oxygen K-edge X-ray absorption spectrum of Ba_0.35K_0.65SbO₃ (red open circles) at 300 K, plotted together with that of Ba_0.4K_0.6BiO₃ (ref. ⁴¹; grey open triangles). The intensity of each spectrum is normalized by that at a high energy ~550 eV above the edge. The arrows indicate the prepeak structure originating from oxygen 2p holes in the spσ* band. The suppression of the prepeak intensity in the antimonate indicates the decrease of oxygen holes compared to the bismuthate.