Breakpoint in the evolution of the gap through the cuprate phase diagram

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 breakpoint close to optimal doping, below which the antinodal gap is gradually disconnected from superconductivity. The nature of both the superconducting and normal states is distinctly different on each side of this breakpoint.