Observation of Superconductivity Up to 8.7 K in Reduced Potassium Tantalate

The observation of superconductivity with a transition temperature (T_c) up to 8.7 K in KTaO₃ single crystals annealed with CaH₂ at 900–1000 °C is reported. The superconductivity is confirmed by both resistance and magnetization measurements and is 3D in nature. Characterizations of X-ray photoelectron spectroscopy, X-ray diffraction, and scanning transmission electron microscopy reveal that it locates in a 1-µm-order-thick polycrystalline surface layer that shows a rock-salt type structure, with a lattice constant of 0.454 nm, and can be chemically identified as K_xTaO_y (0.04 ≤ x ≤ 0.08, 1.24 ≤ y ≤ 1.35), depending on annealing conditions. Within the experimental ranges, the T_c is peaked at x ≈0.05, and increases with decreasing y, and the highest T_c is observed in K_0.05TaO_1.24. The T_c observed here is much higher than that of KTaO₃, Ta, and pure TaO, and is also one of the highest among of all the known oxide superconductors with the same rock-salt structure. The rather high T_c and its close connection with KTaO₃ and Ta, both of which are promising materials for quantum computing, make K_xTaO_y potentially interesting as a building block in constructing future superconducting quantum devices.