title: Abrupt onset of a second energy gap at the superconducting transition of underdoped Bi2212.
abstract: 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. Although some experimental evidence suggests that the two gaps are distinct, 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.
authors.affiliation: Department of Physics, Applied Physics, and Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford, California 94305, USA. leews@stanford.edu
authors.forename: W S
authors.initials: WS
authors.lastname: Lee
authors.forename: I M
authors.initials: IM
authors.lastname: Vishik
authors.forename: K
authors.initials: K
authors.lastname: Tanaka
authors.forename: D H
authors.initials: DH
authors.lastname: Lu
authors.forename: T
authors.initials: T
authors.lastname: Sasagawa
authors.forename: N
authors.initials: N
authors.lastname: Nagaosa
authors.forename: T P
authors.initials: TP
authors.lastname: Devereaux
authors.forename: Z
authors.initials: Z
authors.lastname: Hussain
authors.forename: Z-X
authors.initials: ZX
authors.lastname: Shen