Commensurate antiferromagnetic excitations as a signature of the   pseudogap in the tetragonal high-Tc cuprate HgBa$_2$CuO$_{4+\delta}$
  Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa$_2$CuO$_{4+\delta}$. Here we report neutron scattering results for HgBa$_2$CuO$_{4+\delta}$ (superconducting transition temperature T$_c$ $\sim$ 71 K, pseudogap temperature T* $\sim$ 305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped 'hourglass' response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped, and significantly enhanced below T*, and hence a prominent signature of the pseudogap state. 
M. K. Chan, C. J. Dorow, L. Mangin-Thro, Y. Tang, Y. Ge, M. J. Veit,   G. Yu, X. Zhao, A. D. Christianson, J. T. Park, Y. Si...
cond-mat.supr-con cond-mat.str-el