Nature450, 81-84 (1 November 2007) | doi:10.1038/nature06219; Received 28 May 2007; Accepted 4 September 2007
Abrupt onset of a second energy gap at the superconducting transition of underdoped Bi2212
W. S. Lee1, I. M. Vishik1, K. Tanaka1,2, D. H. Lu1, T. Sasagawa1, N. Nagaosa3, T. P. Devereaux4, Z. Hussain2 & Z.-X. Shen1
Department of Physics, Applied Physics, and Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford, California 94305, USA
Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, California 94720, USA
Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
Department of Physics, University of Waterloo, Ontario N2L 3G1, Canada
Correspondence to: W. S. Lee1Z.-X. Shen1 Correspondence and requests for materials should be addressed to Z.X.S. (Email: zxshen@stanford.edu) or W.S.L. (Email: leews@stanford.edu).
The superconducting gap—an energy scale tied to the superconducting phenomena—opens on the Fermi surface at the superconducting transition temperature (Tc) in conventional BCS superconductors. In underdoped high-Tc superconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well above Tc (refs 1, 2). Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above Tc is one of the central questions in high-Tc research3, 4, 5, 6, 7, 8. Although some experimental evidence suggests that the two gaps are distinct9, 10, 11, 12, 13, 14, 15, 16, 17, 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+ 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 Tc 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 experiments19, 20, 21.
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 Tc 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
