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# 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 (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+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 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.

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## References

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## Acknowledgements

We thank R. Moore for experimental assistance, and D. J. Scalapino, S. Kivelson and T. K. Lee for helpful discussions. This work is supported by the DOE Office of Basic Energy Science, Division of Materials Science and Engineering, and the National Science Foundation. ARPES experiments were performed at the Stanford Synchrotron Radiation Laboratory (SSRL), which is operated by the Department of Energy Office of Basic Energy Science.

## Author information

Authors

### Corresponding authors

Correspondence to W. S. Lee or Z.-X. Shen.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

This file contains Supplementary Methods, Supplementary Figures S1-S3 with Legends and Supplementary Discussion. (PDF 333 kb)

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Lee, W., Vishik, I., Tanaka, K. et al. Abrupt onset of a second energy gap at the superconducting transition of underdoped Bi2212. Nature 450, 81–84 (2007). https://doi.org/10.1038/nature06219

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• DOI: https://doi.org/10.1038/nature06219

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