Nature of the phase transition of superconducting oxides Bi1.8Pb0.8Sr1.9CuOy studied by specific heat

The electronic specific heat, C-el of Bi1.8Pb0.3Sr1.9CuOy samples in the un derdoped and overdoped regions exhibited the anomalies near the T-c of abou t 5 K. Tn the region of / T-T-c / / T-c below 0.15, the Gaussian fluctuatio n to the mean-field theory of d-wave superconductor described well the T-de pendence of C-el of the overdoped sample, while that of the underdoped samp le was not. The T-dependence of C-el of the underdoped sample was presented by critical fluctuation characteristics of the cusp type that, has been ob served in the superfluid transition of liquid He-4, suggesting that the sup erconducting transition of the underdoped sample is due to Bose condensatio n.