Novel magnetic excitations in a model cuprate high-$T_{c}$ superconductor*

Magnetic fluctuations might be essential to the mechanism of high-temperature superconductivity in the cuprates. For a long time, such fluctuations have been theoretically regarded as arising from the antiferromagnetic correlations within the copper-oxygen layers, and experimental studies of magnetic excitation spectrum have mainly been carried out near the corresponding wave vector (1/2,~1/2). Following neutron diffraction experiments which demonstrated the universal existence of a ``$q$~=~0 antiferromagnetic order'' in the pseudogap phase of three different cuprates [1-3], our recent inelastic neutron scattering experiments on the model compound HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) revealed the existence of unusual magnetic excitations that weakly disperse throughout the entire Brillouin zone [4,5]. Like the $q$~=~0 antiferromagnetic order, the new excitations are observed in the pseudogap phase and therefore appear to be associated with the order. The excitations possess very large spectral weights at well-defined characteristic energies that are comparable to the resonance energy [6] and to those of electron-boson-coupling features observed in a wide range of cuprates, highlighting their possible influence on the electronic structure. These findings demonstrate that the pseudogap state is a distinct phase of matter rather than a mere crossover. They furthermore cast doubt on the presumed predominant importance of the wave vector (1/2,~1/2) in the magnetic excitation spectrum, and have the profound implication that a single-band description of the cuprates is insufficient. \\[4pt] [1] B. Fauque \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{96}, 197001 (2006). \\[0pt] [2] Yuan Li \textit{et al.}, \textit{Nature} \textbf{455}, 372 (2008). \\[0pt] [3] V. Baledent \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{105}, 027004 (2010). \\[0pt] [4] Yuan Li \textit{et al.}, \textit{Nature} \textbf{468}, 283 (2010). \\[0pt] [5] Yuan Li \textit{et al.}, unpublished. \\[0pt] [6] G. Yu \textit{et al.}, \textit{Phys. Rev. B} \textbf{81}, 064518 (2010).