A. Furrer

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.

Introduction to Neutron Scattering

Among most other methods, neutron scattering allows a detailed understanding of the static and dynamic properties on an atomic scale of materials that occur in our environment. Combined with x-ray scattering a very large range of momentum and energy transfers can be covered thanks to the high complementarity of both techniques. The most relevant, unique character of neutrons that cannot be matched by any other technique, can be summarized as follows: n n nThe neutron interacts with the atomic nucleus, and not with the electrons as x-rays do. This hats important consequences: i) the response of neutrons from light atoms like hydrogen or oxygen is much higher than for x-rays, ii) neutrons can easily distinguish atoms of comparable atomic number, iii) neutrons distinguish isotopes: For example, deuteration of macromolecules allows to focus on specific aspects of their atomic arrangement or their motion. n n nFor the same wavelength as hard x-rays the neutron energy is much lower and comparable to the energy of elementary excitations in matter. Therefore, neutrons do not only allow the determination of the “static average” chemical structure, but also the investigation of the dynamic properties of the atomic arrangements that are directly related to the physical properties of materials.

Neutron crystalline-electric-field spectroscopy of (R = Ce, Pr, Nd)

The ternary intermetallic compounds (R = light rare earth) crystallize in the hexagonal -type structure, like the heavy-fermion superconductor . In this paper we present powder neutron scattering and single-crystal magnetic susceptibility experiments on , which provide sufficient information for us to unambiguously determine the crystalline-electric-field (CEF) splitting of the multiplet of . The CEF parameters obtained for are extrapolated to those for and . For , the powder neutron scattering measurement of the two strongest CEF excitations confirms the reliability of the extrapolated CEF level diagram with an accuracy of better than 10% for the excitation energies. For , with a rather strong Kondo effect, the extrapolation yields a CEF level sequence similar to that derived from single-crystal susceptibility data, and the predicted excitation energy lies well inside the width of the broad inelastic magnetic peak observed by neutron scattering. The CEF parameters yield a magnetic anisotropy which is compatible with the magnetic structures observed in and , and predict no magnetic ordering for down to the lowest temperatures.

Time versus monochromatic focusing on a cold neutron time-of-flight spectrometer

Abstract In spring 99 the SINQ time-of-flight (TOF) spectrometer FOCUS became operational. The instrument can be used either in time focusing or monochromatic focusing conditions. In this contribution we want to report on recent measurements exploring the elastic and inelastic energy resolution of the instrument for the various settings. It was found that especially the usage of higher-order reflections of the double focusing pyrolitic graphite monochromator allows for energy resolutions in the order of 0.3xa0meV at a neutron energy loss in the order of 8–14xa0meV.

Magnetic ordering and crystalline electric field splitting in Nd3Pd20Si6

Abstract At the temperatures T N1 ≈2.4 K and T N2 ≈0.7 K, the cubic intermetallic compound Nd 3 Pd 20 Si 6 undergoes second order magnetic phase transitions. Here we report on the magnetic structures obtained from low-temperature neutron diffraction data by using a He dilution refrigerator, and on the crystalline electric field splitting of the Nd 3+ ground state multiplet 4 I 9/2 obtained from inelastic neutron scattering data.

Intermultiplet crystal field transitions in EuNiO3

Abstract We determined the energy splitting of the 7 F 1 multiplet of Eu 3+ in EuNiO 3 using inelastic neutron scattering. We were able to retrieve accurate values for the second order crystalline electric field parameters, which will allow a more reliable determination of the crystalline electric field potential in the RNiO 3 (R=rare earth) series. This is essential for a possible observation of a charge transfer, which is proposed by one of the models describing the observed metal-to-insulator transition in these compounds.

Crystal-field interaction and oxygen stoichiometry effects in strontium-doped rare-earth cobaltates

Inelastic neutron scattering was employed to study the crystal-field interaction in the strontium-doped rare-earth compounds R(x)Sr(1-x)CoO(3-z) (R=Pr, Nd, Ho, and Er). Particular emphasis is laid on the effect of oxygen deficiencies which naturally occur in the synthesis of these compounds. The observed energy spectra are found to be the result of a superposition of crystal fields with different nearest-neighbor oxygen coordination at the R sites. The experimental data are interpreted in terms of crystal-field parameters which behave in a consistent manner through the rare-earth series, thereby allowing a reliable extrapolation for rare-earth ions not considered in the present work.

The effect of Ca and Th substitution on the crystal-field spectrum of the high-Tc superconductor HoBa2Cu3Ox

The inelastic neutron scattering technique was employed to study the transitions between the low-lying levels of the ground-state J-multiplet of the Ho3+ ions in Ho1-yCayBa2Cu3Ox (x≈6.3, 7.0; y≈0.1, 0.2) and Ho1-yThyBa2Cu3Ox (x≈6.3; y≈0.07, 0.13) split by the crystalline electric field. For oxygen-rich Ca-doped samples, the observed energy spectra exhibit a fine structure, which reflects the behaviour of the local hole density near the doping centre. For the first time this experimental observation unambiguously confirms that the charge inhomogeneity, which was found to be a characteristic feature of the underdoped cuprates, still exists in the deeply overdoped regime with Tc = 56 K. On the other hand, the modification of the crystal-field spectra observed for the oxygen-deficient samples with both Ca2+ and Th4+ substitutions for Ho3+ is found to result from distortions of the copper-oxygen polyhedron around the Ho3+ ion in the direct neighbourhood of the implanted ions, rather than from the doping-induced variation of the carrier concentration in the CuO2 planes.

New excitement with crystal-field excitations

The crystal field is one of the major interactions in rare-earth compounds. Neutron spectroscopy has become the key tool to measure the crystal-field transitions in metallic systems. This has been demonstrated for almost 1000 metallic rare-earth compounds in the past 30 years which resulted in a detailed understanding of the various physical effects caused by the crystal-field splitting. One may conclude that the determination and description of crystal fields in metallic rare-earth systems is now well established and has become a standard technique. Yet the past years have seen exciting developments in different applications where the crystal-field concept attained increasing and sometimes even crucial importance. This is exemplified for two applications: Firstly, the novel principle for cooling by adiabatic pressure application which is based on the occurrence of a pressure-induced structural and/or magnetic phase transition where the point symmetry at the rare-earth site is changed involving a change in the degeneracy of the crystal-field states. Secondly, the observation of anomalies in the linewidth of crystal-field transitions in high-temperature superconductors which reveals direct information on the doping and isotope dependence of the pseudogap.

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.