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

Lee, W. S.
Vishik, I. M.
Tanaka, K.
Lu, D. H.
Sasagawa, T.
Nagaosa, N.
Devereaux, T. P.
Hussain, Z.
Shen, Z.-X.

¹Department of Physics, Applied Physics, and Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford, California 94305, USA. ²Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, California 94720, USA. ³Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan. ⁴Department of Physics, University of Waterloo, Ontario N2L 3G1, Canada.

doi:10.1038/nature06219

Received 28 May; accepted 4 September 2007.

Supplementary Information is linked to the online version of the paper at www.nature.com/nature.

Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to Z.X.S. ([email protected]) or W.S.L. ([email protected]).

Nature 450(7166):p 81-84, November 1, 2007. | DOI: 10.1038/nature06219

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_csuperconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well aboveT_c(refs). Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap aboveT_cis one of the central questions in high-T_cresearch. 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 Bi₂Sr₂CaCu₂O_{8 + δ}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 atT_cand 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.