Emergence of preformed Cooper pairs from the doped Mott insulating state in Bi2Sr2CaCu2O8+δ

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Abstract

Superconductors are characterized by an energy gap that represents the energy needed to break the pairs of electrons (Cooper pairs) apart. At temperatures considerably above those associated with superconductivity, the high-transition-temperature copper oxides have an additional ‘pseudogap’. It has been unclear whether this represents preformed pairs of electrons that have not achieved the coherence necessary for superconductivity, or whether it reflects some alternative ground state that competes with superconductivity1. Paired electrons should display particle–hole symmetry with respect to the Fermi level (the energy of the highest occupied level in the electronic system), but competing states2,3,4 need not show such symmetry. Here we report a photoemission study of the underdoped copper oxide Bi2Sr2CaCu2O8+δ that shows the opening of a symmetric gap only in the anti-nodal region, contrary to the expectation that pairing would take place in the nodal region. It is therefore evident that the pseudogap does reflect the formation of preformed pairs of electrons and that the pairing occurs only in well-defined directions of the underlying lattice.

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Figure 1: Analysis of spectra from the optimally doped material.
Figure 2: Spectra from the optimally doped and underdoped material in the superconducting state.
Figure 3: Spectra from the optimally doped and underdoped material in the normal state.
Figure 4: Analysis of spectra recorded along the anti-nodal direction.

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Acknowledgements

We thank S. Chakravarty, A. Chubukov, P. Lee, M. Norman, M. Rice, D. Scalapino and J. Tranquada for discussions. The assistance of J. Wen and Z. Xu with the preparation of underdoped crystals is also acknowledged. This work was supported by the US Department of Energy.

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Correspondence to P. D. Johnson.

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This file contains Supplementary Notes incorporating Supplementary Figures SI1and SI2 with Legends and Supplementary References. (PDF 696 kb)

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Yang, H., Rameau, J., Johnson, P. et al. Emergence of preformed Cooper pairs from the doped Mott insulating state in Bi2Sr2CaCu2O8+δ. Nature 456, 77–80 (2008) doi:10.1038/nature07400

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