How Cooper pairs vanish approaching the Mott insulator in Bi2Sr2CaCu2O8+δ

Y. Kohsaka, C. Taylor, P. Wahl, A. Schmidt, Jhinhwan Lee, K. Fujita, J. W. Alldredge, K. McElroy, Jinho Lee, H. Eisaki, S. Uchida, D.-H. Lee and J. C. Davis ()
Additional contact information
Y. Kohsaka: LASSP, Cornell University, Ithaca, New York 14853, USA
C. Taylor: LASSP, Cornell University, Ithaca, New York 14853, USA
P. Wahl: LASSP, Cornell University, Ithaca, New York 14853, USA
A. Schmidt: LASSP, Cornell University, Ithaca, New York 14853, USA
Jhinhwan Lee: LASSP, Cornell University, Ithaca, New York 14853, USA
K. Fujita: LASSP, Cornell University, Ithaca, New York 14853, USA
J. W. Alldredge: LASSP, Cornell University, Ithaca, New York 14853, USA
K. McElroy: University of Colorado, Boulder, Colorado 80309, USA
Jinho Lee: LASSP, Cornell University, Ithaca, New York 14853, USA
H. Eisaki: Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
S. Uchida: University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
D.-H. Lee: University of California, Berkeley, California 94720, USA
J. C. Davis: LASSP, Cornell University, Ithaca, New York 14853, USA

Nature, 2008, vol. 454, issue 7208, 1072-1078

Abstract: 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.

Date: 2008
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DOI: 10.1038/nature07243

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