Laser-based angle-resolved photoemission measurements with super-high resolution have been carried out on an optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. New high energy features at $\sim$115 meV and $\sim$150 meV, besides the prominent $\sim$70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which can not be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.
Categories: cond-mat.supr-con cond-mat.str-el