Novel Magnetism in the Pseudogap Phase of the Cuprates

Magnetic correlations might cause the superconductivity in the cuprates and are generally believed to be antiferromagnetic. Following our success in growing sizable crystals of the tetragonal compound HgBa$_2$CuO$_{4+\delta}$ [1], we used polarized neutron diffraction to demonstrate that the unusual magnetic order previously observed in YBa$_2$Cu$_3$O$_{6+\delta}$ [2] is a universal property of the pseudogap phase [3]. Subsequent inelastic neutron scattering experiments revealed several accompanying, weakly-dispersive magnetic excitation branches in HgBa$_2$CuO$_{4+\delta}$ [4]. Unlike antiferromagnetism, the novel magnetic order does not break the lattice translational symmetry. Nevertheless, the excitations mix with conventional antiferromagnetic fluctuations. Our results point toward the need for a multi-band description of the cuprates, and they are consistent with the notion that the phase diagram is controlled by an underlying quantum critical point [5]. The neutron scattering results will be discussed together with new dc resistivity data for the pseudogap phase of HgBa$_2$CuO$_{4+\delta}$ [6].\\[4pt] [1] X. Zhao {\it et al.}, Adv. Mat. {\bf 18}, 3243 (2006).\\[0pt] [2] B. Fauque {\it et al.}, Phys. Rev. Lett. {\bf 96}, 197001 (2006).\\[0pt] [3] Y. Li {\it et al.}, Nature {\bf 455}, 372 (2008).\\[0pt] [4] Y. Li {\it et al.}, Nature {\bf 468}, 283 (2010), and unpublished results.\\[0pt] [5] C. Varma, Nature {\bf 468}, 184 (2010).\\[0pt] [6] N. Barisic {\it et al.}, unpublished results.