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

• Published in: Nature (London) Vol. 456; no. 7218; pp. 77 - 80
• Main Authors: Yang, H.-B., Rameau, J. D., Johnson, P. D., Valla, T., Tsvelik, A., Gu, G. D.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 06.11.2008; Nature Publishing Group
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electronic structure; Exact sciences and technology; Humanities and Social Sciences; letter; multidisciplinary; Physics; Properties of type I and type II superconductors; Science; Science (multidisciplinary); Superconductivity; Energy gap; Cooper pairs; Fermi level; Mott insulating; Doping; High-Tc superconductors; Angle resolved photoemission spectroscopy; Bismuth Calcium Copper Strontium Oxides Mixed; Particle-hole model; Pairing
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature07400

High-temperature superconductivity: primed for the pseudogap In conventional superconductors, the superconductivity is generated by pairs of electrons, called Cooper pairs. The energy needed to break these pairs is observable spectroscopically as an 'energy gap'. The high-transition temperature copper oxides have an additional 'pseudogap', detected at temperatures well above those associated with superconductivity. The origin of this pseudogap may provide clues as to the mechanism of high-temperature superconductivity. It has been unclear whether this represents preformed pairs of electrons that have not achieved the coherence necessary for superconductivity. Yang et al . now report a photoemission study of the underdoped copper oxide Bi 2 Sr 2 CaCu 2 O 8+ δ that reveals preformed pairs in the pseudogap, although the pairing occurs only in the anti-nodal region. 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 superconductivity 1 . 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 Bi 2 Sr 2 CaCu 2 O 8+ 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.