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.     

Temperature dependence of the gap size near the brillouin-zone nodes of HgBa2CuO4+delta superconductors

Although more than 20 years have passed, the identification of the superconducting order parameter in cuprates is still under debate. Here, we show that the gap size near the nodes is a good candidate for the order parameter: it scales with the critical temperature Tc over a wide doping range and displays a significant temperature dependence below Tc in both the underdoped and the overdoped regimes. In contrast, the gap size at the antinodes does not scale with Tc in the underdoped regime and appears to be controlled by the pseudogap which persists below Tc.     

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.     

Correlation between the superconducting transition temperature and anisotropic quasiparticle scattering in Tl2Ba2CuO6+delta

Angle-dependent magnetoresistance measurements are used to determine the isotropic and anisotropic components of the transport scattering rate in overdoped Tl2Ba2CuO6+delta for a range of Tc values between 15 and 35 K. The size of the anisotropic scattering term is found to scale linearly with Tc, establishing a link between the superconducting and normal state physics. A comparison with results from angle resolved photoemission spectroscopy indicates that the transport and quasiparticle lifetimes are distinct.     

Volume dependence of the superconducting transition temperature of quench-condensed be films

The volume dependence of the superconducting transition temperatureT _c of quench-condensed Be films is investigated by bending the substrate of the film. Tensile strain causes an increase, compressive strain a decrease ofT _c . The volume coefficientd lnT _c /d lnV is about 0.8, a value which is small compared with those of other weakcopling non-transition metals. In addition to pure films, Be films stabilized by codeposition of noble metals or Ge are investigated.Paper based in part on a Habilitationsschrift submitted to the Fakultät für Physik, Universität Karlsruhe (TH), FRG     

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.     

Spontaneous strain in high-temperature superconductor La1.85Sr0.15CuO4

The lattice parameters of La_1.85Sr_0.15CuO₄ were measured by an x-ray Rietveld method between 15 K and room temperature. A change in a lattice parameter could be detected at superconducting phase transition. It is pointed out, for the first time, that the change can be attributed to a spontaneous strain in the superconducting phase caused by coupling between a superconducting order parameter and the strain.