New magnetic neutron scattering results for the high-T$_{c}$ superconductors HgBa$_{2}$CuO$_{4+\delta }$and Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$

We have succeeded in growing sizable single crystals of HgBa$_{2}$CuO$_{4+\delta }$, the single-layer hole-doped compound with the highest superconducting transition temperature [1]. Careful characterization demonstrates the high quality of our crystals [2]. Using polarized neutron diffraction, we find an unusual magnetic order in the pseudogap phase [3]. Together with prior results for YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, this observation constitutes a demonstration of the universal existence of such a state. Our inelastic neutron scattering measurements reveal that the antiferromagnetic resonance occurs at a rather high energy in HgBa$_{2}$CuO$_{4+\delta }$ and, hence, that the resonance energy is not universally related to T$_{c}$ [4]. Finally, our results for the antiferromagnetic excitations in Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ provide new insight into magnetic energy scales and the resonance of the electron-doped side of the phase diagram [5]. \\[4pt] [1] X. Zhao et al., Adv. Mater. \textbf{18}, 3243 (2006). \\[0pt] [2] N. Barisic et al., Phys. Rev. B \textbf{78}, 054518 (2008). \\[0pt] [3] Y. Li et al., Nature \textbf{455}, 372 (2008). \\[0pt] [4] G. Yu et al., arXiv:0810.5759. \\[0pt] [5] G. Yu et al., arXiv:0803.3250.