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 density1,2. Such a state has been created in ultracold 6Li gas3 but never observed directly in any superconductor. It is now widely hypothesized that the pseudogap phase4 of the copper oxide superconductors contains such a ‘pair density wave’ state5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21. Here we report the use of nanometre-resolution scanned Josephson tunnelling microscopy22,23,24 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 atoms25 and at the Bi2Sr2CaCu2O8+x crystal supermodulation26. 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 theory5,13,14 when a charge density wave5,27 with d-symmetry form factor28,29,30 and wavevector QC = QP coexists with a d-symmetry superconductor; it is also predicted by several contemporary microscopic theories for the pseudogap phase18,19,20,21.
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We acknowledge and thank D. Agterberg, A. V. Balatsky, D. Chowdhury, A. Chubukov, E. Fradkin, R. Hulet, S. A. Kivelson, P. A. Lee, M. Norman, J. W. Orenstein, C. Pepin, S. Sachdev, J. Tranquada and Y. Wang for discussions and advice. The development and operation of HTS SJTM technology and M.H.H. and A.K. were funded by the Moore Foundation’s EPiQS Initiative through grant number GBMF4544. S.D.E. acknowledges studentship funding from the EPSRC under grant number EP/G03673X/1. J.C.S.D. and A.P.M. acknowledge research support from the EPSRC through the grant programme ‘Topological Protection and Non-Equilibrium States in Correlated Electron Systems’. S.U. and H.E. acknowledge support from a Grant-in-Aid for Scientific Research from the Ministry of Science and Education (Japan). S.H.J. and J.L. acknowledge support from the Institute for Basic Science, Korea under grant number IBS-R009-D1. J.C.S.D. and K.F. acknowledge salary support from the US Department of Energy, Office of Basic Energy Sciences, under contract number DEAC02-98CH10886. E.-A.K. acknowledges support from the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under award DE-SC0010313.
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Nature Communications (2017)