Abrupt onset of a second energy gap at the superconducting transition of underdoped Bi2212

The superconducting gap-an energy scale tied to the superconducting phenomena-opens on the Fermi surface at the superconducting transition temperature (T-c) in conventional BCS superconductors. In underdoped high-T-c superconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well above T-c (refs 1, 2). Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above T-c is one of the central questions in high-T-c research(3-8). Although some experimental evidence suggests that the two gaps are distinct(9-18), this issue is still under intense debate. A crucial piece of evidence to firmly establish this two-gap picture is still missing: a direct and unambiguous observation of a single-particle gap tied to the superconducting transition as function of temperature. Here we report the discovery of such an energy gap in underdoped Bi2Sr2CaCu2O8+delta in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap that opens at T-c and has a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasi-particles, a classical signature of superconductivity. This is in sharp contrast to the pseudogap near the Cu-O bond direction (antinodal region) measured in earlier experiments(19-21).