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 HTSClike Superconductivity in Ba_{1x}K_{x}BiO_{3} Single Crystals" JETP Letters, 83, 405 (2006) This is well known that the
electron concentration in superconductor K_{x}Ba_{1x}BiO_{3} diminishes together
with the critical temperature T_{c} when the potassium doping x increases
above x=0.35. Today the optimally doped crystals (x=0.350.4, T_{c}=31 K)
are well studied and are known to demonstrate some unusual effects which
are similar to those observed in highT_{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_{1x}K_{x}BiO_{3} 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 WerthamerHelfandHohenberg
formula. The fact that this group of samples are classical BCSsuperconductors
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.
