Magnetoresistance of Sr1-xKxBiO3: a second-family of bismuth-oxide-based superconductors

The superconducting Sr1-x KxBiO3 samples with x = 0.45 - 0.6 were synthesiz ed by the high-pressure-high-temperature technique in a belt type apparatus (2 GPa, 700 degrees C, 1 h, Pt capsules) with stoichiometric mixtures of S r2Bi2O5, Bi2O3 and KO2 as described earlier.(1) The X-ray diffraction resul ts appear as a single perovskite-like phase. The superconductivity occurs a t T-c similar to 12 K in the A. C. susceptibility measurement. The onset (z ero resistivity) temperature of superconductivity in resistivity measuremen t 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 pre sent about 11.3 K indicating probably the existence of crystallites of diff erent K content. The particularly interesting point is that the resistance begins to reappear at T < 6 K at zero magnetic field. As the external magne tic field is applied, the reentrant resistance disappears and superconducti vity is recovered until the applied magnetic field becomes higher than 0. 6 5 Tesla. The superconductivity for T < 6 K is destroyed Sor the higher magn etic field. The T-conset decreases as the magnetic field increases like in the BCS type superconductors. The transition region becomes broader under t he magnetic field, which indicate a kind of vortex transition as in the cas e of high T-c cuprates.