Coexistence of Two Sharp-Mode Couplings and Their Unusual Momentum   Dependence in the Superconducting State of Bi2Sr2CaCu2O8+d Superconductor   Revealed by Laser-Based Angle-Resolved Photoemission
  Super-high resolution laser-based angle-resolved photoemission measurements have been carried out on Bi2Sr2CaCu2O8+d (Bi2212) superconductors to investigate momentum dependence of electron coupling with collective excitations (modes). Two coexisting energy scales are clearly revealed over a large momentum space for the first time in the superconducting state of an overdoped Bi2212 superconductor. These two energy scales exhibit distinct momentum dependence: one keeps its energy near 78 meV over a large momentum space while the other changes its energy from $\sim$40 meV near the antinodal region to $\sim$70 meV near the nodal region. These observations provide a new picture on momentum evolution of electron-boson coupling in Bi2212 that electrons are coupled with two sharp modes simultaneously over a large momentum space in the superconducting states. Their unusual momentum dependence poses a challenge to our current understanding of electron-mode-coupling and its role for high temperature superconductivity in cuprate superconductors. 
Junfeng He, Wentao Zhang, Jin Mo Bok, Daixiang Mou, Lin Zhao, Yingying   Peng, Shaolong He, Guodong Liu, Xiaoli Dong, Jun Zh...
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el