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 Ba1-xKxBiO3 Single Crystals" JETP Letters, 83, 405 (2006)

This is well known that the electron concentration in superconductor KxBa1-xBiO3 diminishes together with the critical temperature Tc when the potassium doping x increases above x=0.35. Today the optimally doped crystals (x=0.35-0.4, Tc=31 K) are well studied and are known to demonstrate some unusual effects which are similar to those observed in high-Tc superconductors. We mean, in particular, the positive curvature of the temperature dependence of the upper critical field Bc2(T) and the linear dependence of the field penetration length at T< Tc/3. We have shown that in the course of doping level x increase (and Tc 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 Bc2(T) for the samples Ba1-xKxBiO3 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 Bc2(T) curves of the crystals with Tc < 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<Tc/3 for this group of crystals in full agreement with BCS theory.
Thus, it is stated that when potassium doping x increases in Ba1-xKxBiO3 compound, the unknown earlier transition accompanied by the radical change of the superconducting properties takes place. The result is of high interest because inBa1-xKxBiO3 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 Ba1-xKxBiO3 by extended saddle point model (dashed line Bc2(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.