title: Detection of a Cooper-pair density wave in Bi2Sr2CaCu2O8+x.
abstract: The quantum condensate of Cooper pairs forming a superconductor was originally conceived as being translationally invariant. In theory, however, pairs can exist with finite momentum Q, thus generating a state with a spatially modulated Cooper-pair density. Such a state has been created in ultracold (6)Li gas but never observed directly in any superconductor. It is now widely hypothesized that the pseudogap phase of the copper oxide superconductors contains such a 'pair density wave' state. Here we report the use of nanometre-resolution scanned Josephson tunnelling microscopy to image Cooper pair tunnelling from a d-wave superconducting microscope tip to the condensate of the superconductor Bi2Sr2CaCu2O8+x. We demonstrate condensate visualization capabilities directly by using the Cooper-pair density variations surrounding zinc impurity atoms and at the Bi2Sr2CaCu2O8+x crystal supermodulation. Then, by using Fourier analysis of scanned Josephson tunnelling images, we discover the direct signature of a Cooper-pair density modulation at wavevectors QP ≈ (0.25, 0)2π/a0 and (0, 0.25)2π/a0 in Bi2Sr2CaCu2O8+x. The amplitude of these modulations is about five per cent of the background condensate density and their form factor exhibits primarily s or s' symmetry. This phenomenology is consistent with Ginzburg-Landau theory when a charge density wave with d-symmetry form factor and wavevector QC = QP coexists with a d-symmetry superconductor; it is also predicted by several contemporary microscopic theories for the pseudogap phase.
authors.affiliation: Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Hamidian
authors.affiliation: Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA.|School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, UK.
authors.forename: S D
authors.initials: SD
authors.lastname: Edkins
authors.affiliation: Institute of Applied Physics, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, South Korea.|Center for Correlated Electron Systems, Institute of Basic Science, Seoul 151-742, South Korea.
authors.forename: Sang Hyun
authors.initials: SH
authors.lastname: Joo
authors.affiliation: Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA.
authors.forename: A
authors.initials: A
authors.lastname: Kostin
authors.affiliation: Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Eisaki
authors.affiliation: Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan.|Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0011, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Uchida
authors.affiliation: Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA.|Department of Physics, Binghamton University, Binghamton, New York 13902-6000, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Lawler
authors.affiliation: Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA.
authors.forename: E-A
authors.initials: EA
authors.lastname: Kim
authors.affiliation: School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, UK.|Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany.
authors.forename: A P
authors.initials: AP
authors.lastname: Mackenzie
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: K
authors.initials: K
authors.lastname: Fujita
authors.affiliation: Institute of Applied Physics, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, South Korea.|Center for Correlated Electron Systems, Institute of Basic Science, Seoul 151-742, South Korea.
authors.forename: Jinho
authors.initials: J
authors.lastname: Lee
authors.affiliation: Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA.|School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, UK.|Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.|Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA.
authors.forename: J C Séamus
authors.initials: JC
authors.lastname: Davis