Resonant Spin Excitation in an Overdoped High Temperature Superconductor
  An inelastic neutron scattering study of overdoped Bi_2Sr_2CaCu_2O_{8+\delta} $ (T_c = 83 K) has revealed a resonant spin excitation in the superconducting state. The mode energy is E_res=38 meV, significantly lower than in optimally doped Bi_2Sr_2CaCu_2O_{8+\delta} (T_c = 91 K, E_ res =43 meV). This observation, which indicates a constant ratio E_res /k_B T_c \sim 5.4, helps resolve a long-standing controversy about the origin of the resonant spin excitation in high-temperature superconductors. 
H. He, Y. Sidis, P. Bourges, G.D. Gu, A.Ivanov, N. Koshizuka, B.   Liang, C.T. Lin, L. P. Regnault, E. Schoenherr, and B. Keimer