Wolfgang Henggeler

A systematic low-temperature neutron diffraction study of the RBa2Cu3Ox (R=yttrium and rare earths; x=6 and 7) compounds

The crystal structures of RBa2Cu3Ox (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analysed concerning systematic trends. Other than for non-superconducting PrBa2Cu3O7 the lattice parameters and most interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant within error limits, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa2Cu3O6 series compared to the RBa2Cu3O7 series can be explained by the increase of Tc from 90 K for YbBa2Cu3O7 to 96 K for NbBa2Cu3O7. The smaller distance Cu(1)-O(1) for the RBa2Cu3O6 series compared to the RBa2Cu3O7 series may be related to the suggested double-well potential of the apical oxygen ion. For some interionic distances of PrBa2Cu3O7 approximately parallel to the h direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 for the Pr ions. This indicates for PrBa2Cu3O7 a highly anisotropic 4f-CuO2 valence band hybridization. An important structural property with respect to the superconductivity is the puckering of the CuO2 planes: the superconductivity is lost when the puckering angle exceeds a critical value of about 167.3.

Magnetic excitations in rare-earth-based high-temperature superconductors

The unique character of inelastic neutron scattering in the study of magnetic excitations of rare-earth-containing high-temperature superconductors is summarized. It is exemplified for single-ion, cluster and collective magnetic excitations which are directly related to the fundamental interactions associated with the rare-earth sublattice. Crystalline-electric-field and cluster excitations are discussed for which provide information on the electrostatic potential and the magnetic pair coupling at the rare-earth site, respectively. The study of collective magnetic excitations in both grain-aligned samples and single crystals yields more detailed information on the exchange coupling. In particular, we show how Ce doping affects the spin-wave dispersion of , i.e., a strong softening of the excitation spectra is observed which explains the huge Sommerfeld coefficients in (x > 0.1).

Neutron crystal-field spectroscopy in underdoped and overdoped copper oxide superconductors

In many cuprate superconductors rare-earth (R) ions can be placed close to the superconducting copper oxide planes; thus, the crystal-field interaction at the R site constitutes an ideal probe of the local symmetry as well as the local charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the crystal-field spectra observed by inelastic neutron scattering separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for the formation of clusters which make the systems inhomogeneous. Moreover, it is found that the intrinsic linewidths of crystal-field transitions vary with temperature, which is essentially a reflection of the density of states associated with the charge carriers at the Fermi energy. Linewidth studies can therefore reveal information about the energy gap. For underdoped systems there is evidence for the formation of a pseudogap atT* >Tc.

Inelastic neutron scattering in cuprate superconductors: Evidence for inhomogeneous materials properties

There is increasing experimental evidence for the coexistence of different types of local regions in doped high-Tc superconductors of “123” and “214” type. They may be classified into local regions of undoped, intermediately doped, and highly doped character. The doping mechanism makes the system inhomogeneous, and the superconducting state is reached by a percolation process. Very detailed information about the volume fractions and the spatial extent of the local regions can be obtained by neutron spectroscopic investigations of the crystal-field interaction at the rare-earth site which is an ideal local probe of the charge distribution in the superconducting copper oxide planes. The inhomogeneous materials properties have to be taken into account in any interpretation of the physical properties of doped high-Tc superconductors. This is exemplified for the static and dynamic copper spin correlations of YBa2Cu3Ox (6≤x≤7). We show that the spin fluctuations observed in the superconducting state can be associated with the undoped and intermediately doped regions of the system, whereas no magnetic scattering occurs for the highly doped regions.

Neutron spectroscopic study of crystalline electric-field in infinite-layer Sr1−xNdxCuO2

Abstract Inelastic neutron scattering has been employed to search for the crystalline-electric-field (CEF) transitions in the infinite-layer (IL) Sr 1−x Nd x O 2 compounds produced by high-pressure technique. For the first time we have been able to observe the complete energy spectrum of the ground state J-multiplet of Nd 3+ in the IL phase and to determine the CEF parameters. Dependence of the CEF splitting on Nd doping is discussed.