Abrupt Onset of Second Energy Gap at 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. Quite differently, in underdoped high-T C superconducting cuprates, a pseudogap, whose relation to the superconducting gap remains a mystery, develops well above T C 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. While some experimental evidence suggests they 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