Weak-coupling d-wave BCS superconductivity and unpaired electrons in overdoped La2-xSrxCuO4 single crystals

The low-temperature specific heat (SH) of overdoped La2-xSrxCuO4 single crystals (0.178 <= x <= 0.290) has been measured. For the superconducting samples (0.178 <= x <= 0.238), the derived gap values (without any adjusting parameters) approach closely onto the theoretical prediction Delta(0)=2.14k(B)T(c) for the weak-coupling d-wave BCS superconductivity. In addition, the residual term gamma(0) of SH at H=0 increases with x dramatically when beyond x similar to 0.22, and finally evolves into the value of a complete normal metallic state at higher doping levels, indicating a growing amount of unpaired electrons. We argue that this large gamma(0) cannot be simply attributed to the pair breaking induced by the impurity scattering, instead the phase separation is possible.