Although more than twenty years have passed since the discovery of high temperature cuprate superconductivity, the identification of the superconducting order parameter is still under debate. Here, we show that the nodal gap component is the best can
didate for the superconducting order parameter. It scales with the critical temperature $T_c$ over a wide doping range and displays a significant temperature dependence below $T_c$ in both the underdoped and the overdoped regimes of the phase diagram. In contrast, the antinodal gap component does not scale with $T_c$ in the underdoped side and appears to be controlled by the pseudogap amplitude. Our experiments establish the existence of two distinct gaps in the underdoped cuprates.
The actual physical origin of the gap at the antinodes, and a clear identification of the superconducting gap are fundamental open issues in the physics of high-$T_c$ superconductors. Here, we present a systematic electronic Raman scattering study of
a mercury-based single layer cuprate, as a function of both doping level and temperature. On the deeply overdoped side, we show that the antinodal gap is a true superconducting gap. In contrast, on the underdoped side, our results reveal the existence of a break point close to optimal doping below which the antinodal gap is gradually disconnected from superconductivity. The nature of both the superconducting and normal state is distinctly different on each side of this breakpoint.
