Magnetoresistance of Sr_1-xK_xBiO₃: A second-family of bismuth-oxide-based superconductors

The superconducting Sr_1-xK_xBiO₃ samples with x = 0.45 - 0.6 were synthesized by the high-pressure-high-temperature technique in a belt type apparatus(2 GPa, 700°C, 1 h, Pt capsules) with stoichiometric mixtures of Sr₂Bi₂O₅, Bi₂O₃, and KO₂ as described earlier.¹ The X-ray diffraction results appear as a single perovskite-like phase. The superconductivity occurs at T_c ∼ 12 K in the A. C. susceptibility measurement. The onset (zero resistivity) temperature of superconductivity in resistivity measurement of the investigated sample was T_Conset = 12.5 K (T_Czero = 10.2 K). The transition temperature region was a little bit broad and a shoulder was present about 11.3 K indicating probably the existence of crystallites of different K content. The particularly interesting point is that the resistance begins to reappear at T < 6 K at zero magnetic field. As the external magnetic field is applied, the reentrant resistance disappears and superconductivity is recovered until the applied magnetic field becomes higher than 0.65 Tesla. The superconductivity for T < 6 K is destroyed for the higher magnetic field. The T_Conset decreases as the magnetic field increases like in the BCS type superconductors. The transition region becomes broader under the magnetic field, which indicate a kind of vortex transition as in the case of high T_c cuprates.