Strain-induced structural change and nearly-commensurate diffuse scattering in the model high-temperature superconductor HgBa 2 CuO 4 + δ

We investigate the strain response of underdoped HgBa 2 CuO 4 + δ (Hg1201), by synchrotron X-ray diffraction and corresponding simulations of thermal diffuse scattering. The compression in the crystallographic a direction leads to relatively small expansion in the b and c directions, with Poisson ratios ν ba = 0.16 and ν ca = 0.11 , respectively. However, the Cu-O distance in the c direction exhibits a notable 0.9% increase at 1.1% a -axis compression. We further find strain-induced diffuse scattering that corresponds to a new type of two-dimensional charge correlation. Interestingly, this signal is insensitive to the onset of superconductivity and instead corresponds to a short-range, nearly commensurate modulation with a wave vector close to (0.5,0,0) and a correlation length of approximately 4 unit cells. It closely resembles the charge order theoretically predicted in the phase diagram of the spin-liquid model with resonating valence bonds on a square lattice.