Fine Details of the Nodal Electronic Excitations in   Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$
  Very high energy resolution photoemission experiments on high quality samples of optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ show new features in the low-energy electronic excitations. A marked change in the binding energy and temperature dependence of the near-nodal scattering rates is observed near the superconducting transition temperature, $T_C$. The temperature slope of the scattering rate measured at low energy shows a discontinuity at ~$T_C$. In the superconducting state, coherent excitations are found with the scattering rates showing a cubic dependence on frequency and temperature. The superconducting gap has a d-wave magnitude with negligible contribution from higher harmonics. Further, the bi-layer splitting has been found to be finite at the nodal point. 
T. Valla, T. E. Kidd, J. D. Rameau, H.-J. Noh, G. D. Gu, P. D.   Johnson, H.-B. Yang and H. Ding