1. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA

Correspondence to: P. D. Johnson¹ Correspondence and requests for materials should be addressed to P.D.J. (Email: pdj@bnl.gov).

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 states^{2, 3, 4} need not show such symmetry. Here we report a photoemission study of the underdoped copper oxide Bi₂Sr₂CaCu₂O₈₊ 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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