P. Allenspach

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.

Crystal field in RBa2Cu3O7 (R rare earth) determined by neutron spectroscopy

Abstract The crystal-field interaction of the high- T c superconductors RBa 2 Cu 3 O 7−δ (R  Nd, Dy, Ho) was determined by neutron spectroscopy. Here we summarize our results obtained for NdBa 2 Cu 3 O 7−δ . The observed energy spectra are strongly influenced by the oxygen vacancy concentration δ, and the quasielastic line width exhibits a sudden drop at T c .

The crystalline electric field as a direct probe for the electron doping process in Nd2-χCeχCuO4

Abstract The neutron spectroscopic data available for the cuprates Nd 2-χ Ce χ CuO 4 are analyzed towards establishing the crystalline electric field (CEF) interaction around the Nd 3+ sites, which are situated close to the CuO 2 planes and thus constitute a local probe of the charge distribution in these planes. Through a comparison of the CEF parameters in the insulating (χ=0) and superconducting (χ=0.15) state we find direct evidence for the addition of electrons to the CuO 2 planes by doping Nd 2 CuO 4 with Ce 4+ . While this compound is widely believed to be favourable for electron doping onto copper, our results indicate that the oxygen sites are also involved in the electron doping mechanism.

Oxygen-vacancy induced changes of the crystal-field interaction in ErBa2Cu3Ox(6.1⩽x⩽7.0) determined by inelastic neutron scattering

Abstract Inelastic neutron scattering has been employed to study the crystalline electric field (CEF) interaction in the high-T c superconductor ErBa 2 Cu 3 O x (6.1 ⩽ x ⩽ 7.0). The second-order CEF parameters are found to exhibit abrupt changes vs. x , which correlates with the observed two-plateau structure of T c . Our observations are interpreted in terms of charge redistributions in the superconducting CuO 2 planes, consistent with the recently proposed concept of charge transfer from the chains to the planes.

Neutron spectroscopy of the crystalline electric field in high-Tc YbBa2Cu3O7

Abstract Inelastic neutron scattering has been employed to study the crystalline electric field (CEF) interaction in the high-T c superconductor YbBa 2 Cu 3 O 7 . The observed energy spectra exhibit three well defined CEF transitions at 88.5, 92.8 and 98.5 meV which completely determine the CEF splitting of the ground-state J-multiplet. A set of CEF parameters is derived which excellently reproduce the neutron spectroscopic data as well as the thermodynamic magnetic properties.

Crystal electric field splitting of R3+-ions in RNi2B2C (R = rare earth)

Abstract The parameters of the crystal electric field (CEF) hamiltonian of the R 3+ -ions in RNi 2 B 2 C (R  Ho, Er, Tm) have been determined from neutron CEF-spectroscopy. These parameters were extrapolated to other lanthanides. The eigen-energies, eigen-states, magnetic single-ion susceptibilities, and magnetic moments in the ordered state were calculated.

Neutron spectroscopy in RBa2Cu3Ox (R=rare earth; 6≤x≤7) and R2-xCexCuO4-δ (0≤x≤0.2) compounds: charge transfer, cluster formation, percolative superconductivity, charge fluctuations

Abstract Neutron spectroscopic experiments performed for the high-T c superconducting hole-doped RBa 2 Cu 3 O x (R=rare earth; 6≤x≤7) and electron-doped R 2-x Ce x CuO 4-δ (0≤x≤0.2) compounds are discussed. In these systems the R ions are situated close to the superconducting CuO 2 planes, thus the crystal-field interaction at the R site constitutes an ideal local probe of the charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the observed crystal-field spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation, i.e., the superconductivity is an inhomogeneous property in the persovskite-type compounds. From a line-width analysis of the observed crystal-field transitions we derive the evolution of the fractal sizes of the clusters versus doping. At high doping the neutron spectroscopic data reveal anomalies which are interpreted in terms of copper-oxyde charge fluctuations.

Effect of light Sr doping on the spin-state transition in LaCoO3

We present an inelastic neutron scattering study of the low energy crystal-field excitations in the lightly doped cobalt perovskite La 0.99 8Sr 0.002 CoO 3 . In contrast to the parent compound LaCoO 3 an inelastic peak at energy transfer ∼0.75 meV was found at temperatures below 30 K. This excitation apparently corresponds to a transition between a ground state orbital singlet and a higher excited orbital doublet, originating from a high-spin triplet split by a small trigonal crystal field. Another inelastic peak at an energy transfer ∼0.6 meV was found at intermediate temperatures starting from T>30K. This confirms the presence of a thermally induced spin-state transition from the low-spin Co 3+ to a magnetic high-spin state in the non-disturbed LaCoO 3 matrix. We suggest that hole doping of LaCoO 3 leads to the creation of a magnetic polaron and hence to the low-to-high spin state transition on the relevant Co sites.

Crystal structures and long-range antiferromagnetic ordering in REBa2Cu3O7-δ (RE=Yb, Nd)

Abstract Neutron diffraction experiments prove three-dimensional long-range antiferromagnetic ordering due to trivalent rare earth ions in NdBa2Cu3O7-δ corresponding to k=[ 1 2 , 1 2 , 0] for δ=0.38, in addition to k = [ 1 2 , 1 2 , 1 2 ] observed for δ = 0.14. In YbBa2Cu3O7 three-dimensional antiferromagnetic ordering corresponding to k=[0, 1 2 , 1 2 ] due to magnetic moments μYb3+ = 1.4μB was established below 400 mK, in agreement with previous Mossbauer experiments. Moreover the crystal structures were refined.

Introducing the Time-of-Flight Backscattering Instrument MARS at SINQ

Current topics engaging the attention of the scientific community place an ever-increasing demand on accessing information in the microwave and far-infrared energy regions of the electromagnetic spectrum. This is manifested not only in the heavy demand for neutron spectrometers, but also in the multitude of high-field EPR facilities springing up across Europe and elsewhere. Conventional neutron backscattering instruments such as IN10 at the ILL and HFBS at NIST proffer sub-μeV resolution but only for energy transfers <ca. 30 μeV. Transitions in the inelastic regime may be readily observed with time-of-flight or three-axis instruments but with a resolution that often cannot compete with optical or magnetic resonance techniques, the inherent advantages of the neutron scattering technique notwithstanding.