Softening of Cu-O bond stretching phonons in tetragonal HgBa ₂CuO_4+δ

The phonon dispersion of Hg1201 was studied using inelastic x-ray scattering. The crystallinity was examined using x-ray diffraction, and four spherically curved analyzer crystals were used on the 10 m horizontal arm to analyze the scattered radiation. Magnetic susceptibility measurement showed a sharp superconductivity transition at 94 K, indicating that the sample was nearly optimally doped. The results show that the shell model described well the dispersion of the low-energy modes, while the Cu-O bond stretching mode at high energy showed strong softening towards the zone boundary, deviating strongly from the model.