Unconventional superconducting gap in underdoped cuprates

A generic theory of the quasi-particle superconducting gap in underdoped cuprates is derived in the strong coupling limit, and found to describe extremely well the experimental "second gap" in \textit{absolute scale}. In drastic contrast to the standard theories of Bogoliubov quasi-particle excitations, the quasi-particle gap is shown to originate from anomalous kinetic process, completely unrelated to the pairing strength. Furthermore, the $k$-dependence of the gap deviates significantly from the pure $d_{x^2-y^2}$-wave of the order parameter. Our study reveals a new paradigm for the nature of superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments toward a more coherent understanding of high-temperature superconductivity.