Evidence of Electron Coupling to High Energy Excitations in a High Temperature Superconductor

We have carried out super-high resolution angle-resolved photoemission measurements on temperature evolution of the electron dynamics along the (0,0)-($\pi$,$\pi$) nodal direction in an optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. The nodal photoemission spectra exhibit dramatic sharpening with decreasing temperature, with an obvious change in the scattering rate across T$_c$. New high energy features are found to develop at $\sim$115meV and $\sim$150meV, besides the prominent $\sim$70 meV one, in the nodal electron self-energy in the superconducting state. These observations provide evidence that, in addition to coupling with low energy excitations like phonons or magnetic resonance mode, there are high-energy excitations involved in the electron coupling.