  Antiferromagnetic correlations have been argued to be the cause of the d-wave
superconductivity and the pseudogap phenomena exhibited by the cuprates.
Although the antiferromagnetic response in the pseudogap state has been
reported for a number of compounds, there exists no information for
structurally simple HgBa$_2$CuO$_{4+\delta}$. Here we report neutron scattering
results for HgBa$_2$CuO$_{4+\delta}$ (superconducting transition temperature
T$_c$ $\sim$ 71 K, pseudogap temperature T* $\sim$ 305 K) that demonstrate the
absence of the two most prominent features of the magnetic excitation spectrum
of the cuprates: the X-shaped 'hourglass' response and the resonance mode in
the superconducting state. Instead, the response is Y-shaped, gapped, and
significantly enhanced below T*, and hence a prominent signature of the
pseudogap state.
