J. Mesot

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.

Neutron Spectroscopic Evidence for Cluster Formation and Percolative Superconductivity in Pr2-xCexCuO4-δ (0 ≤ x ≤ 0.2)

We used neutron crystal field spectroscopy as a local probe to investigate cluster formations in the electron-doped compound Pr2-xCexCuO4-δ. The observed energy spectra were indeed found to result from a superposition of three different components which we interpret in terms of local regions of undoped, intermediately doped and highly doped character. The volume fraction of the doped clusters is shown to reach the percolation limit for x ≈ 0.14 where the onset of superconductivity is observed. The highly doped clusters are presumably controlled by a charge transfer mechanism.

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.

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.

Crystal-field excitations in high-T

Abstract The discovery of high-Tc superconductivity in the perovskite-type copper-oxide compounds has given rise to a large amount of materials research. Of particular interest in determining the nature of the superconductivity is the effect of substituents at various sites in these compounds. It has been realized that the superconducting transition temperature is essentially unchanged upon replacing the Y and La ions by “magnetic” rare-earth (R) ions. This surprising observation is in contrast to conventional super-conductors, for which paramagnetic ions usually have a large detrimental effect on superconductivity. It is therefore important to achieve a detailed understanding of the low-energy electronic properties which define the magnetic ground state of the R ions. In particular, information on the crystalline-electric-field (CEF) interaction at the R site is highly desirable, for the following reasons: For many high-Tc compounds superconductivity and long-range three-dimensional magnetic ordering of ...