Magnetic resonance in the model high-temperature superconductor   HgBa$_2$CuO$_{4+\delta}$
  We present an inelastic neutron scattering study of the structurally simple single-layer compound HgBa$_2$CuO$_{4+\delta}$ close to optimal doping ($T_c \approx 96$ K). A well-defined antiferromagnetic resonance with energy $\omega_r = 56$ meV ($\approx 6.8 k_BT_c$) is observed below the superconducting transition temperature $T_c$. The resonance mode is energy-resolution limited and exhibits an intrinsic momentum width of about $0.2 \mathrm{\mathring{A}^{-1}}$, consistent with prior work on several other cuprates. However, the unusually large value of the mode energy implies a non-universal relationship between $\omega_r$ and $T_c$ across different families of cuprates. 
G. Yu, Y. Li, E. M. Motoyama, X. Zhao, N. Barisic, Y. Cho, P. Bourges,   K. Hradil, R. A. Mole, M. Greven