F. Fauth

Antiparallel ordering of Mn and Nd magnetic moments in Nd0.7Ba0.3MnO3

Abstract The crystal and magnetic structures of the giant magnetoresistive compound Nd0.7Ba0.3MnO3 have been investigated by means of neutron powder diffraction. The nuclear structure remains orthorhombic within the temperature range 1.5–300 K. As expected from the higher size of Ba-ion radii, the buckling as well as the distortion of MnO6 octahedra are less pronounced than in the parent compounds Nd0.7A′0.3MnO3 (A′ = Ca, Sr). At 1.5 K, we observe a ferromagnetic ordering of both Nd and Mn magnetic moments along the b-axis. However, Nd and Mn moments point in opposite directions. The saturated magnetic moment of Mn ions is 3.45 μB, whereas the magnetic moment of Nd ions reaches 0.45 μB at 1.5 K. The Curie temperatures are TC(Mn) = 142 K and TC(Nd) = 20 K.

New elaboration technique, structure and physical properties of infinite-layer Sr1−xLnxCuO2(Ln = Nd, Pr)

Abstract The infinite-layer compounds of the (Sr, Ln)CuO 2 family have been prepared by a new synthesis method from the low-pressure phase of SrCuO 2 and NdCuO 2 as starting materials. The crystal structure, microstructure and superconductivity of these samples have been studied with use of neutron powder diffraction, optical micrographs, energy-dispersive X-ray analysis. magnetic-susceptibility and electrical-resistivity measurements. The ratio of the lattice constants a/c as an empirical parameter for the existence of superconductivity in the electron-doped infinite-layer compounds is discussed.

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.

Neutron Spectroscopy in RBa2Cu3Ox (R=Rare Earth, 6≤x≤7) Compounds: Charge Transfer, Phase Separation, Spin Fluctuations

Inelastic neutron scattering has been employed to study the perovskitetype high-T c superconducting compounds RBa2Cu3O x (R = rare earth;6 = x = 7). The variation of the energies and intensities of the observed crystalline-electric-field (CEF) transitions versus the oxygen content x is shown to be predominantly related to a charge transfer process between the chains and the planes. The observed energy spectra are the result of a superposition of two different metallic components and a semiconducting one, i.e., there is clear experimental evidence for phase separation. A two-dimensional bond percolation model explains the appearance of superconductivity as well as the critical oxygen concentrations associated with the two-plateau structure of T c . The line shape of some low-energy R3+ excitations turns out to be highly asymmetric which we interpret in terms of an exchange interaction between the R3+ spins and fluctuating Cu2+ spins. The latter are most likely associated with low-energy spin excitations of spin-polarized polynuclear clusters of Cu2+ ions. From a line-width analysis of the CEF transitions we derive the evolution of the fractal sizes of the clusters versus the oxygen concentration x. Magnetic field-dependent neutron spectroscopic experiments give further evidence for the existence of spin-polarized clusters which are suggested to be the elementary building blocks giving rise to high-temperature superconductivity above the percolation threshold.

Neutron spectroscopy in ErBa2Cu3Ox (6 ⩽ × ⩽ 7) and Pr2 − xCexCuO4 − δ (0 ⩽ × ⩽ 0.2): Charge transfer, cluster formation and percolative superconductivity

Abstract Neutron spectroscopic data obtained for ErBa 2 Cu 3 O x (6 ⩽ × ⩽ 7) and Pr 2 − x Ce x CuO 4 − δ (0 ⩽ × ⩽ 0.2) are discussed. Since the rare-earth ions in these compounds are situated in close proximity to the superconducting CuO 2 -planes, crystal-field spectroscopy constitutes an ideal local probe of the charge distribution and therefore monitors directly changes of the carrier concentration induced by doping. Clear evidence for cluster formation and percolative superconductivity is provided by the observed separation of crystal-field transitions — observed in hole- and electron-doped superconductors — into different local components whose spectral weights depend on the doping level.

Evidence for electron-lattice coupling in RNiO3 perovskites

Abstract In order to get additional insight into the origin of the metal-insulator transition in RNiO3 perovskites we have investigated (a) the stability of the magnetic structure against the proximity of the metallic state and (b) the effect of the oxygen isotope mass on the metal-insulator transition temperature TMI. The observation of the same kind of magnetic ordering in the four nickelates studied (PrNiO3, NdNiO3 with TMI = TN, and 154SmNiO3, 153EuNiO3 with TN

Neutron investigation of Nd{sub 2-x-y}Ce{sub x}La{sub y}CuO{sub 4} (0{le}x{le}0.2; y=0.5, 1)

Samples of the electron-doped superconductor precursor Nd{sub 2{minus}x{minus}y}Ce{sub x}La{sub y}CuO{sub 4} with 0{le}x{le}0.2 and y=0.5, l have been synthesized with a sol-gel method. While the as-prepared samples are in the normal state, oxygen reduction results in superconducting samples. The dependence of T{sub c} upon Ce-doping x and oxygen content (4{minus}{delta}) was determined in detail. It turned out that the internal pressure exerted by the La ions results in an extended superconducting range in x and {delta} compared to Nd{sub 2{minus}x}Ce{sub x}CuO{sub 4{minus}{delta}}. Elastic neutron scattering was used to investigate the dependence of structural features upon doping. Upon oxygen reduction a non-isotropic shrinking of the unit cell (lattice parameter a increases, c decreases) and indications for interstitial oxygen were found. The local electronic surrounding of the Nd ions was observed via the crystal-field splitting by inelastic neutron scattering and turned out to be inhomogeneous. Such electronic inhomogeneities were previously measured for other high-T{sub c} superconductors.Samples of the electron-doped superconductor precursor Nd2-x-yCexLayCuO4 with 0≤x≤0.2 and y=0.5, I have been synthesized with a sol-gel method. While the as-prepared samples are in the normal state, oxygen reduction results in superconducting samples. The dependence of Tc upon Ce-doping x and oxygen content (4-δ) was determined in detail. It turned out that the internal pressure exerted by the La ions results in an extended superconducting range in x and δ compared to Nd2-xCexCuO4-δ Elastic neutron scattering was used to investigate the dependence of structural features upon doping. Upon oxygen reduction a non-isotropic shrinking of the unit cell (lattice parameter a increases, c decreases) and indications for interstitial oxygen were found. The local electronic surrounding of the Nd ions was observed via the crystal-field splitting by inelastic neutron scattering and turned out to be inhomogeneous. Such electronic inhomogeneities were previously measured for other high-Tc superconductors.

Neutron crystal-field spectroscopy of RNi2 11B2C (R=Ho, Er, Tm)

Inelastic neutron scattering experiments of the crystal-electric-field (CEF) splitting of the R3+-ions (R=Ho, Er, Tm) in the superconductor RNi211B2C have been performed in the paramagnetic as well as in the magnetically ordered state in order to deduce the CEF parameters which in turn determine the magnetic properties of these systems. Both the easy axis and the size of the zero-field moments are correctly reproduced by our calculations. Our CEF parameter set also reproduces the behaviour of specific heat and M/H measured for single crystals.

Neutron and X-ray diffraction study of superstructure and localized magnetic moments in Cu0.5Fe0.5Cr2S4 and Cu0.5In0.5Cr2S4 compounds

The superstructure parameters for the Cu0.5Fe0.5Cr2S4 and Cu0.5In0.5Cr2S4 compounds have been determined by neutron and X-ray diffraction. The localized magnetic moments in different sublattices measured for Cu0.5Fe0.5Cr2S4 are equal to 3.06 ± 0.17 μB for Fe3+ ions in the A-site and 2.76 ± 0.22 μB for Cr3+ ions in the B-site (Cu+ possess no magnetic moment), which are much less than the magnetic moments for the ions in the purely ionic state.

Peculiarities of crystal structure and crystal-field excitations in “1212” AlSr2RCu2O7 (R=Y, Er)

Abstract Investigation of substitution effects for various sites in the “1212” perovskite-like systems with a general formula R(Sr,Ba) 2 (Cu 3− y M y )O 7− δ (M=Fe, Co, Hg, Al, Ga, Zn, and Ni and R=rare earth) is of great interest in connection with the problem of high- T c superconductivity. The crystal structure of the “1212” AlSr 2 RCu 2 O 7 (R=Y, Er) compounds has been studied using high-resolution neutron powder and electron microscope diffraction measurements. Inelastic neutron scattering has been employed to search for crystalline-electric-field (CEF) transitions. The structural results yield evidence for superlattice ordering with orthorhombic symmetry ( Pm2a space group). The CEF spectrum turns out to be a superposition of two components, which can be related to a local inhomogeneity of charge states within the CuO 2 planes.