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- Title
How Cooper pairs vanish approaching the Mott insulator in Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+δ</sub>.
- Authors
Kohsaka, Y.; Taylor, C.; Wahl, P.; Schmidt, A.; Jhinhwan Lee; Fujita, K.; Alldredge, J. W.; McElroy, K.; Lee, Jinho; Eisaki, H.; Uchida, S.; Lee, D.-H.; Davis, J. C.
- Abstract
The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the mysterious ‘pseudogap’ excitations are found, whereas, at lower energies, Bogoliubov quasi-particles—the excitations resulting from the breaking of Cooper pairs—should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi2Sr2CaCu2O8+δ in r-space and k-space simultaneously. We find that although the low-energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy-independent way. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.
- Subjects
ANTIFERROMAGNETISM; COPPER oxide; ELECTRONS; LOW energy electron diffraction; COPPER; DENSITY; EXCITON theory; SEMICONDUCTOR doping; SYMMETRY (Physics)
- Publication
Nature, 2008, Vol 454, Issue 7208, p1072
- ISSN
0028-0836
- Publication type
Article
- DOI
10.1038/nature07243