Phonon softening in HgBa2CuO4+δ and MgB2

Phonon dispersion of two superconductors was studied with using inelastic X-ray scattering. In MgB2, which is thought as a Eliashberg type superconductor, the phonon dispersion shows softening of the E2g mode around Γ in the [100] direction, in excellent agreement with ab initio calculations. It supports the idea that this phonon gives main contribution to the superconductivity. In HgBa2CuO4+δ, on the other hand, which is a good candidate for phonon measurement in the high-Tc cuprates, the dispersion shows anomalous softening of the bond stretching mode at q∼0.3 in the [100] direction. The softening does not agree with shell model calculations well, suggesting existence of spatial charge fluctuation with periodicity of q∼0.3.