U. Staub

A systematic neutron diffraction study of RBa2Cu3O7 (R=yttrium and rare earths) high-T c superconductors

The 10K crystal structures of high-Tc RBa2Cu3O7 (R=Y and rare earths) ceramic materials investigated by neutron diffraction were consistently analysed concerning systematic trends. Several 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, and the plane copper Cu(2)−O(1) distances are increasing across the rare-earth series. For R=Pr the 4f−CuO2 valence band hybridization effects are found to be highly anisotropic, since exclusively the interionic distances involving the b-direction (i.e, the chain direction) indicate that the Pr ions have an intermediate valence of about+3.4.

Combined Electronic-Nuclear Magnetic Ordering of the Ho3+ Ions and Magnetic Stacking Faults in the High-Tc Superconductor HoBa2Cu3O7

Highly unusual magnetic ordering stabilised by nuclear 165Ho polarisation is found to exist below TN = 190 mK in HoBa2Cu3O7 by means of low-temperature neutron scattering experiments. The three-dimensional antiferromagnetic ordering corresponding to K = [0,1/2,1/2] is imperfect, due to finite correlation length ξc = 29 A along the c=axis. The observed temperature dependence of the magnetic order parameter is in excellent agreement with mean-field calculations taking into account crystal field, exchange and hyperfine interactions.

Neutron spectroscopic studies of crystalline electric fields in high-Tc ErBa2Cu3O7 doped with zn and Ni

Abstract The crystalline electric field (CEF) splitting of the ER 3+ 4f-shell in the high- T c superconductor ErBa 2 Cu 3 O 7 doped with Zn and Ni has been investigated by the inelastic neutron scattering tecnique. the observed exhibit a large number of well resolved CEF transitions between 8 and 80 meV, so that we have been able to determine all nine CEF parameters required for the orthorhombic symmetry. The results are used to derive information on the effective charge associated with the superconducting CuO 2 planes.

Combined electronic‐nuclear magnetic ordering of the Ho3+ ions and magnetic stacking faults in HoBa2Cu3Ox (x=7.0,6.8,6.3)

Magnetic ordering of HoBa2Cu3Ox (x=7.0,6.8,6.3) was investigated by means of neutron diffraction. Long‐range antiferromagnetism is found for x=7.0 below TN=190 mK and for x=6.8 below TN=100 mK corresponding to propagation vector K=[0,1/2,1/2] with magnetic moment directions parallel to the a axis. The observed temperature dependence of the magnetic order parameter is in excellent agreement with mean‐field calculations taking into account crystal field, exchange, and hyperfine interactions. No magnetic superlattice reflections were detected for x=6.3. A highly unusual ordering behavior is observed in HoBa2Cu3Ox (x=7.0,6.8) where the three‐dimensional magnetic ordering is imperfect, due to finite correlation length ξc=29 and 129 A, respectively, along the c axis.

Magnetic properties of Nd3+ in Nd−Ba−Cu−O-compounds

Neutron diffraction, neutron spectroscopy and magnetization measurements have been employed to study the structural, electronic, and magnetic behavior of eleven compounds with the general formula Nd1+yCavBa2−y−vCu3Ox (0≦y≦0.5; 0≦v≦0.25; 6≦x≦7). The structure turned out to react to oxygen reduction similar as other 123-compounds, yielding discontinuities close to the metal-insulator-transition and the well-known relations of bond lengths as a function ofTc. The crystalline electric field (CEF) interaction, splitting the 10-fold degenerate ground-state J-multiplet of the Nd3+-ions into five doublet states, was investigated by neutron spectroscopy. The derived CEF parameters have been used to determine changes in the electronic surroundings of the Nd3+ ions. In addition, with the help of the CEF parameters the thermodynamic magnetic properties of these compounds were calculated which turn out to be in good agreement with the experimental data.

Collective magnetic excitations of Ho3+ ions in grain-aligned HoBa2Cu3O7

Inelastic neutron scattering has been employed to study the magnetic excitations of Ho3+ in a grain-aligned sample of HoBa2Cu3O7. The lowest-lying branch exhibits dispersion for spin waves propagating in the (a, b)-plane, whereas excitations along the c-axis remain constant in energy. The data are analysed in the random phase approximation in terms of the Heisenberg model, resulting in an enormous spatial anisotropy of the magnetic coupling parameters, which are weakly ferromagnetic, antiferromagnetic and almost zero for nearest-neighbor Ho3+ pairs along the a, b, and c axes, respectively.

Ho3+ dimer excitations in Ho0.1Y0.9Ba2Cu3Ox (6.6≤x≤7.0)

Abstract The inelastic neutron-scattering technique was used to measure the magnetic excitations in Ho 0.1 Y 0.9 Ba 2 Cu 3 O x (6.6≤ x ≤7.0) at T = 1.5 K. The observed energy spectra correspond to a superposition of mainly Ho 3+ monomer and dimer excitations. From the latter we derive the magnetic coupling strengths of Ho 3+ pairs in the ( a, b )-plane. The lineshape of the magnetic excitations turns out to be highly asymmetric for which we discuss various mechanisms in terms of fluctuating fields at the Ho 3+ sites.

Crystal-field splitting and temperature dependence of two-dimensional antiferromagnetism in the high-Tc compound DyBa2Cu4O8

By means of a quantitative analysis of extensive elastic neutron scattering experiments performed on theTc=78 K superconductor DyBa2Cu4O8 it is shown that two-dimensional (2D) antiferromagnetic Dy ordering in the (a,b)-plane remains stable in the whole temperature range fromTN=(1.10±0.02) K down to 7 mK. The magnetic difference pattern with good angular resolution fits well to the Warren equation for scattering on 2D systems with powder averaging. Consequently, the fitted sublattice magnetisation is found to have a 2D Ising character. Moreover, the Dy3+ crystal-field levels were determined by inelastic neutron scattering. From the resulting crystal-field parameters we calculate the sublattice magnetisation of Dy3+ at saturation to beμDy,CEF=6.0μB in good agreement withμDy,obs=(5.9±0.5)μB as measured by neutron diffraction.

Collective magnetic excitations of Ho3+ ions in grain‐aligned HoBa2Cu3O7

Inelastic neutron scattering has been employed to study the magnetic excitations of Ho3+ in a grain‐aligned sample of HoBa2Cu3O7. The lowest lying branch exhibits dispersion for spin waves propagating in the (a,b) plane, whereas excitations along the c axis remain constant in energy. The data are analyzed in the random phase approximation in terms of the Heisenberg and an anisotropic model, resulting in an enormous spatial anisotropy of the magnetic coupling parameters, which are weakly ferromagnetic, antiferromagnetic, and almost zero for nearest‐neighbor Ho3+ pairs along the a, b, and c axes, respectively. In addition, we have been able to separate the spin wave excitations in longitudinal and transverse parts.

Neutron scattering studies of crystal structure and crystalline electric field in high-Tc ErBa2Cu3Ox disordered by fast neutron irradiation

Abstract The crystal structure and the crystalline-electric-field splitting of the Er 3+ 4f-shell in the high- T c superconductor ErBa 2 Cu 3 O x ( x = 6.95, 6.18) disordered with fast neutron irradiation has been investigated by elastic and inelastic neutron scattering techniques, respectively. The results yield evidence for the absence of a charge transfer between the planes and chains under disorder and point to the atomic displacements being the main radiation-induced defects in the CuO 2 planes.