Softening of Cu-O bond stretching phonons in tetragonal HgBa2CuO4+delta

Phonons in nearly optimally doped HgBa(2)CuO(4+delta) were studied by inelastic x-ray scattering. The dispersion of the low-energy modes is well described by a shell model, while the Cu-O bond stretching mode at high energy shows strong softening towards the zone boundary, which deviates strongly from the model. This seems to be common in the hole-doped high-T(c) superconducting cuprates, and, based on this work, not related to a lattice distortion specific to each material.     

Charge-transfer excitations in the model superconductor HgBa(2)CuO(4+delta)

We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa(2)CuO(4+delta) at optimal doping (T(c)=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.     

Charge states of atoms in HgBa2CuO4 and HgBa2CaCu2O6 lattices

Mössbauer emission spectroscopy on the ⁶⁷Cu(⁶⁷Zn) isotope has been used to determine the parameters of the electric-field gradient tensor at the copper sites of the HgBa₂Ca _n?1Cu_nO_2n+2 lattices (n=1,2), as well as to calculate these parameters in the point-charge approximation. An analysis of the results, combined with available NQR literature data for the ⁶³Cu isotope, has shown that the best fit of calculated to experimental data can be reached by assuming that the holes due to the presence of defects in the material localize primarily in the sublattice of the oxygen lying in the copper plane.