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The presence of phonon signatures in the electron tunneling spectra of classical super-conductors and their quantitative explanation by the Eliashberg equations stand among the most convincing validations of the BCS phonon-mediated pairing theory. For high-Tc superconductors, the pairing mechanism still remains as an intriguing mystery. Several cuprate superconductors present a spectroscopic feature, the so-called dip-hump, that resembles the phonon signatures of classical superconductors. There is still no consensus on the origin of the dip-hump nor on its connection to high-Tc superconductivity.


In this talk we will present a scanning tunneling microscopy (STM) study on two and three-layer Bi-based cuprates. We observe that the gap magnitude, a direct measure of the pairing strength, is periodically modulated on a length scale of about 5 crystal unit-cells. This variation mimics the periodic modulation of atomic positions naturally present in Bi-based cuprates. By fitting the STM data with a strong-coupling model we demonstrate that the dip feature is a fingerprint of a collective excitation. This allows us to image the collective mode energy (CME) at the atomic scale and to reveal that it is modulated with the same period as the local gap variation. The CME and the gap are locally anti-correlated. These findings support that the collective mode probed in our study is related to superconductivity, and is most likely the anti-ferromagnetic spin resonance detected by neutron scattering. Our results, in particular the CME value of 30–40 meV, are in agreement with the spin-fluctuation-mediated pairing scenario in which the spin resonance in high-Tc’s is a consequence of pairing.


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