New Insights Into the Physics of the High-Tc Cuprates from neutron, X-ray and Transport Measurements of HgBa2Cu04
Upon introducing charge carriers into the CuO2 sheets of the lamellar cuprates the ground state evolves from a Mott insulator into a superconductor, and eventually into a seemingly conventional metal (Fermi liquid). Much has remained elusive about the nature of this evolution, and about the peculiar metallic state at intermediate carrier concentrations where the superconducting transition temperature is maximized. Following our success in growing sizable crystals of the structurally simple compound HgBa2CuO4+ , we used polarized neutron diffraction to demonstrate the universal existence of a novel type of magnetic order in superconducting samples .
Unlike antiferromagnetism, this order in the so-called pseudogap phase does not break the lattice translational symmetry. Our inelastic neutron scattering measurements confirmed the existence of the well-known magnetic resonance at the antiferromagnetic point  and led to the insight that the resonance energy is universally related to the superconducting gap in several families of unconventional superconductors . These experiments also resulted in the discovery of several excitations branches that appear to be fundamental collective modes associated with the novel magnetic order .
This magnetism is consistent with the notion that the phase diagram is controlled by an underlying quantum critical point. I will furthermore discuss hard X-ray experiments that reveal the formation of oxygen chains in the interstitial HgOδ layers near optimal doping , as well as recent charge transport measurements that reveal Fermi-liquid-like behavior in the pseudogap phase . (This work was supported by DOE-BES.)
 X. Zhao et al., Adv. Mat. 18, 3243 (2006); N. Barisic et al., Phys. Rev. B 78, 054518 (2008).
 Y. Li et al., Nature 455, 372 (2008); Phys. Rev B 84, 224508 (2012).
 G. Yu et al., Phys. Rev. B 81, 064518 (2010).
 G. Yu et al., Nature Physics 5, 873 (2009).
 Y. Li et al., Nature 468, 283 (2010); Nature Physics 8, 404 (2012).
 G. Chabot-Couture et al., in preparation.
 N. Barišić et al., arXiv:1207.1504.