G. E. Tsydynzhapov, A. F. Shevchun, M. R. Trunin, V. N. Zverev, D. V. Shovkun, N. V. Barkovskiy, and L. A. Klinkova "Observation of a Transition from BCS to HTSC-like Superconductivity in Ba_1-xK_xBiO₃ Single Crystals" JETP Letters, 83, 405 (2006)

This is well known that the electron concentration in superconductor K_xBa_1-xBiO₃ diminishes together with the critical temperature T_c when the potassium doping x increases above x=0.35. Today the optimally doped crystals (x=0.35-0.4, T_c=31 K) are well studied and are known to demonstrate some unusual effects which are similar to those observed in high-T_c superconductors. We mean, in particular, the positive curvature of the temperature dependence of the upper critical field B_c2(T) and the linear dependence of the field penetration length at T< T_c/3. We have shown that in the course of doping level x increase (and T_c decrease), the sharp transition from this kind of unusual behavior to the usual superconductivity well described by BCS theory takes place. The set of curves B_c2(T) for the samples Ba_1-xK_xBiO₃ with different critical temperatures is shown in the left Figure. The strike separation of the curves on two groups with Tc lower then 15 K and above 20 K is clearly seen. One can see from the insert that B_c2(T) curves of the crystals with T_c < 15 K are well described by the usual Werthamer-Helfand-Hohenberg formula. The fact that this group of samples are classical BCS-superconductors is confirmed by our microwave impedance studies (see the right Figure). Both the field penetration length and the residual losses saturate exponentially at T<T_c/3 for this group of crystals in full agreement with BCS theory.
Thus, it is stated that when potassium doping x increases in Ba_1-xK_xBiO₃ compound, the unknown earlier transition accompanied by the radical change of the superconducting properties takes place. The result is of high interest because inBa_1-xK_xBiO₃ in this doping level region there are no any structural phase transitions, which could lead to the change of the electron properties. So, the nature of high-temperature superconducting state in our crystals may be determined by the same fundamental interaction as that for BCS. Our analysis points out on the opportunity to express the high-temperature superconducting state in Ba_1-xK_xBiO₃ by extended saddle point model (dashed line B_c2(T) in the left Figure) proposed by Abrikosov [Abrikosov, Int. J. of Modern Phys, 13, 3405 (1999)]. This state transforms to the low-temperature one due to the decrease of the carrier concentration and the change of the compound structure in the microscopic scale.