Strain-induced structural change and diffuse scattering in HgBa₂CuO₄₊δ

We study the structural change and diffuse scattering induced by a-axis compressive strain in HgBa2 CuO4.09 at Tc = 78 K, using synchrotron X-ray diffraction. The high-temperature superconductor exhibits neg- ative Poisson ratio below 0.6% strain. Although the c-axis lattice parameter at 1% strain is 0.1% larger than its zero-strain value, the Cu-O bond length is 1.2% shorter than the zero-strain value. Strain-induced diffuse scattering appears at the two sides of Bragg peaks along the strain direction. Such diffuse scattering has a wavenumber of (0.5,0,0), corresponding to a modulation with a period of twice the a-axis lattice parameters. The diffuse-scattering features saturate at 0.2% strain and exhibit little change on cooling below Tc.