Static Magnetic Order in Underdoped HgBa$_{2}$CuO$_{4+\delta }$

It is believed by many that understanding the pseudogap phase is essential to understanding the mechanism of high-transition-temperature superconductivity. Here we present the first experimental identification, by polarized neutron diffraction, of an exotic magnetic order in the HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201), which is considered the model high-Tc material with (i) simple tetragonal structure, (ii) large spacing between the CuO$_{2}$ planes, and (iii) the highest Tc among all single-layer compounds [1]. The order parameter develops below a characteristic temperature T$_{mag}$ that nicely agrees with the pseudogap temperature T* determined by DC transport. Our result is highly consistent with the previous work by B. Fauque \textit{et al. }on the bi-layer compound YBCO [2], and can be interpreted within an orbital-current picture that breaks time-reversal symmetry but preserves discrete translational symmetry. The magnitude and the orientation of the magnetic moments of the current loops are experimentally investigated. 1. X. Zhao \textit{et al.}, Adv. Mat. \textbf{18}, 3243 (2006). 2. B. Fauque \textit{et al.}, PRL \textbf{96}, 197001 (2006).