| Utilizing the newly-developed, high-resolution experimental technique of low-energy angle-resolved photoemission spectroscopy (le-ARPES), the electronic structure of high temperature superconductors was probed. In an ARPES spectrum, a direct picture of the electron energy vs. momentum dispersion relation is obtained. By substituting 18O for 16O, it was hoped that the effect of the lattice ions on these dispersions could be uncovered and light could be shed on the nature of the electron-phonon coupling in these materials. Contrary to an earlier report, no isotope effect was found at deep binding energies. Through careful control of several experimental parameters, it was determined that the large "unusual" isotope effect seen earlier could likely be explained by sample misalignment. However, with the higher resolution afforded by le-ARPES, it was determined that the well-known nodal (the (0,0)-(pi,pi) direction of the Brillouin Zone) "kink" in ARPES at ∼70 meV binding energy shifted by an amount of ∼3 meV with oxygen isotope substitution. This result is consistent with the idea that the nodal kink is caused by electron-phonon coupling to a phonon of energy close to ∼70 meV. We argue that the primary candidate meeting this criterion is the "half-breathing" oxygen vibrational mode and present results from various other experimental probes to support this view. |
