Weak-couplingd-wave BCS superconductivity and unpaired electrons in overdopedLa2−xSrxCuO4single crystals
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Abstract
The low-temperature specific heat (SH) of overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ single crystals $(0.178\ensuremath{\leqslant}x\ensuremath{\leqslant}0.290)$ has been measured. For the superconducting samples $(0.178\ensuremath{\leqslant}x\ensuremath{\leqslant}0.238)$, the derived gap values (without any adjusting parameters) approach closely onto the theoretical prediction ${\ensuremath{\Delta}}_{0}=2.14{k}_{B}{T}_{c}$ for the weak-coupling $d$-wave BCS superconductivity. In addition, the residual term $\ensuremath{\gamma}(0)$ of SH at $H=0$ increases with $x$ dramatically when beyond $x\ensuremath{\sim}0.22$, 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 $\ensuremath{\gamma}(0)$ cannot be simply attributed to the pair breaking induced by the impurity scattering, instead the phase separation is possible.
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