Magnetic excitations in the high-T$_{\mathrm{c}}$ superconductor HgBa$_2$CuO$_{4+d}$ at low doping

We report on the observation of magnetic excitations in the very underdoped regime of the high-T$_{\mathrm{c}}$ superconductor HgBa$_{2}$CuO$_{4+d}$ (Hg1201). Our previous inelastic neutron scattering measurements of optimally doped (T$_{\mathrm{c}}$ $\approx $ 95 K) and moderately underdoped (T$_{\mathrm{c}} \approx $ 65 K) samples revealed two novel, weakly-dispersive magnetic excitation branches below the pseudogap temperature T* [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012)]. These excitations are associated with the translational symmetry preserving magnetic order previously established to be a universal property of the pseudogap phase [B. Fauqu\'{e} et al., Phys. Rev. Lett. 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008); Y. Li et al. Phys. Rev. B 84, 224508 (2011)]. In YBa$_{2}$Cu$_{3}$O$_{6+d}$, the strength of this order was found to decrease in very underdoped samples [V. Bal\'{e}dent et al. Phys. Rev. B 83, 104504 (2011)]. Indeed, we find no evidence of pseudogap excitations in very underdoped Hg1201 (T$_{\mathrm{c}} =$ 45 K), and instead we observe strong antiferromagnetic fluctuations over a large energy range (10 -150 meV).