Magnetic excitations in the high-temperature superconductors HgBa$_{2}$CuO$_{4+\delta }$ and Nd$_{1.845}$Ce$_{0.155}$CuO$_{4}$

We report inelastic neutron scattering results for the magnetic excitations in hole-doped HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) and electron-doped Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ (NCCO). The magnetic resonance mode has been observed previously in the superconducting state of several hole-doped systems. Recently, this mode has also been claimed to be present in the electron-doped compounds. We found in underdoped Hg1201 (T$_{c}\sim $85 K) the resonance-like feature appearing at rather high energy of 57(2)meV. Surprisingly, the dynamic susceptibility enhancement appears below the pseudogap temperature T$^{\ast }$ and shows no anomaly at T$_{c}$. Unlike recent reports for optimally-doped NCCO and (Pr,La,Ce)$_{2}$CuO$_{4+\delta }$, we found no evidence for a resonance mode in NCCO (x=0.155) in the 7-12 meV range. Instead, we identify two lower-energy features. One is associated with spectral weight redistribution below T$_{c}$ due to the electronic gap 2$\Delta $, the other already present in normal state is likely associated with the significant spin correlations in the electron-doped cuprates.