Magnetic signatures of the pseudogap phase and doping dependence of the antiferromagnetic resonance in HgBa$_{2}$CuO$_{4+\delta }$

We present our latest experimental results for the pseudogap phase of the model high-Tc superconductor HgBa$_{2}$CuO$_{4+\delta }$. Refinement of our polarized neutron diffraction experiments [Y. Li \textit{et al.}, Nature \textbf{455}, 372 (2008)] using sizable single-crystals of this structurally simple compound consistently show a novel magnetic order below the pseudogap temperature. Furthermore, DC magnetic susceptibility measurements on the highest-quality crystals exhibit a response with highly anisotropic temperature- and doping-dependence, in which the pseudogap anomaly is only visible when the magnetic field is applied along the $c$-axis. These findings strongly suggest that the new magnetic order competes with the superconductivity. Building on our initial work for optimally-doped HgBa$_{2}$CuO$_{4+\delta }$ [G. Yu \textit{et al., }arXiv: 0810:5759], we also briefly discuss preliminary results for the doping dependence of the antiferromagnetic resonance.