Table of contents

A short introduction to the powerful techniques of neutron diffraction and spectroscopy is illustrated largely with achievements by Swedish researchers in the past few years. Background material on sources and instrumentation is included, together with a directory of facilities routinely available to the Swedish scientific community.

Neutron diffraction is often an unequivocal method for determining magnetic structures. Here we present some typical examples, stressing the sequence through experiments, data analysis, interpretation and modelisation.

Two series of compounds are chosen: (a) RT₂X₂ (R = Rare Earth, T = 3d metal and X = Si or Ge), (b) RBe₁₃ (R = Rare Earth).

Depending on the nature of the elements R, T, X, the magnetic structures produced can be commensurate, incommensurate or even show a transition between two such phases as a function of temperature.

A model, taking magnetic exchange and anisotropy into account, will be presented in the case of commensurate-incommensurate magnetic transitions in RBe₁₃.

The nature of the magnetism of 5f (actinide) compounds is dominated by the large spin-orbit interaction (because l = 3) and the hybridisation of the 5f electrons with other electron states (because of the large spatial extent of the 5f electrons). A key question is what happens to the individual spin and orbital moments. We report on neutron experiments to measure these components.

In this paper we will discuss the influence of structure on the superconductivity and T_c of oxides, from the 'low T_c' materials such as Ba(Pb, Bi)O₃, (Ba, K)BiO₃ and Ba(Pb, Sb)O₃ through the CuO-layer materials such as "123", "124", "127" etc. to the (Bi, Pb, Tl)-oxide layer materials. Neutron diffraction is a unique method of studying the oxygen structure in these heavy metal oxides, especially at low temperature and high pressure. We will see that superconductivity depends strongly on oxygen content and cation doping, but also on the details of the Cu-O co-ordination and bond lengths. These neutron results have lead to the concept of T_c being controlled by charge transfer between "charge reservoir" layers and conducting CuO-planes.

Cross-sections for total and Compton scattering of circularly polarized photons by magnetic materials are presented. The expressions include scattering from both the spin and orbital magnetization densities, together with inelasticity corrections.

The structure of deuterated liquid iso-propanol has been studied with neutron diffraction at the LAD diffractometer at the ISIS spallation source. Measurements were performed at temperatures 190, 220, 250 and 275 K. To correct for inelastic effects a model for the dynamic structure factor which obeys detailed balance and included recoil effects was used. The static molecular structure factor S_M(Q) exhibits a pre-peak at about 0.75 Å⁻¹. The origin of the pre-peak, which increases in amplitude with temperature, is presently unknown. The structure of the iso-propanol molecule was obtained from the total pair distribution function and from a fit of the intramolecular form factor f₁(Q) to the measured S_M(Q) at large Q. The obtained values of the bond length and Debye-Waller factors are in good agreement to those obtained from lower alcohols.

Neutron incoherent quasielastic scattering experiments on the liquid crystal MBBA in the nematic phase are reported. Taking advantage of published NMR results it is shown that the motion of an oriented MBBA molecule is composed of two parts. The 12 protons of the butyl end and alkoxy chains perform uniaxial rotational diffusion with a diffusion constant of 10¹⁰s⁻¹ while the 9 protons of the rigid benzylideneaniline core are restricted to a uniaxial simple rotational diffusion on a segment of a circle of an apex angle of 57° with an associated rate constant of 2.3 × 10¹⁰s⁻¹.

The upper critical field H_c2 of four Bi-(Pb)-Sr-Ca-Cu-O superconductors with the 110K phase has been measured as a function of temperature. H_c2 exhibited a (T_c - T)^α temperature dependence with α > 1 for two impure samples, but showed a linear temperature dependence for the purest sample, which is not expected according to current theories and other experiments.

YBa₂Cu₃O_7-y compounds with 0.5% of the Cu atoms replaced by ⁵⁷Fe and with y = 0.15, 0.48, 0.96 and 1.01 have been synthesized. X-ray diffraction showed that the structures are orthorhombic for y < 0.50, tetragonal for larger y and that the volume of the unit cell increases with y. The orthorhombic compounds are superconducting with critical temperatures T_c = 90.7 K and 49 K for y = 0.15 and 0.48 respectively. The tetragonal compounds are semiconducting with band gaps ΔE ≈ 0.15eV for y ≈ 1.0. The Mössbauer spectra give some hints to the site preference of the iron atoms from which we suggest that more than 84% of the Fe atoms substitute for Cu(1) and the rest for Cu(2). Charge states of +3 and +4 for the Fe(2) and the Fe(1) species are consistent with the values of the hyperfine parameters.

We report on Mössbauer spectroscopy of GdBa₂(Cu_1-x⁵⁷Fe_x)₃O_7-y superconductors with iron contents x = 0.005, 0.01 and 0.02. The critical temperatures were determined to be T_c = 78, 65 and 56 K respectively for increasing x. Mössbauer spectra were recorded at room temperature and at 5 K. No magnetic ordering appears. All spectra can be analyzed using three quadrupole split doublets whose relative intensities and hyperfine parameters vary with x and with temperature. From the intensities we conclude that roughly 90% of the Fe substitutes for Cu(1) and the rest for Cu(2).

Recently the theory of dilation analyticity (or the Complex Scaling Method, CSM) was applied, for the first time, to the domain of quantum statistics far from equilibrium. As a result a novel kind of universal coherence has been revealed from first principles. The corresponding irreducible structures, called coherent-dissipative structures, represent a short-lived and spatially restricted cooperative phenomenon. The crucial points of the theory are stressed and similarities as well as differences with (1) the BCS-states of superconductivity and (2) the dissipative structures of the Brussels school are pointed out. Here the general theory is applied to quantum correlations in high-T_c superconductors and the large energy gap compared to standard BCS results, as well as the recently found universal linear relation between T_c and the carrier density/effective mass, will be discussed. The conditions for these correlations to occur in the Cu-O based HSTC's are discussed in terms of a specific model for the superconductivity mechanism, involving the motion of charge carriers surrounded by a cluster of frustrated spins (created by the introduction of O-associated holes in an otherwise antiferromagnetic ground state). The motion and lifetime of these magnetic clusters are limited by the "magnetic wall" conditions (including exchange and anisotropy terms). An effective mass as well as a thermal deBroglie wavelength can be estimated for these objects. By referring back to the general theory it is possible to check the consistency of the description and discuss the conditions for short and long range coherence in the motion of the carriers.

Synthetic and process studies of the new high-T_c super-conductors are carried out using the methods of powder technology. The equipment for these studies at the Institute of Chemistry, University of Uppsala is described. The laboratory is equipped with jaw crusher, various mills, powder mixer, spray dryer and several sintering furnaces. The laboratory has excellent facilities for powder characterization, such as a semi-automatic image analyzer, a system for X-ray line broadening analysis, BET apparatus, sample divider and a sieving machine. Single-crystal growth experiments are also reported. Single crystals of YBa₂Cu₃O₇ and Bi₂Sr₂CaCu₂O₈ were grown by the off-stoichiometric method. The largest crystals of YBa₂Cu₃O₇ were obtained with the soaking temperature of 980°C and a cooling rate of 2°C h⁻¹. A soaking temperature of 925°C and a cooling rate of 40°C h⁻¹ produced the largest Bi₂Sr₂CaCu₂O₈ single crystals. A single crystal examination showed that the structure of Bi₂Sr₂CaCu₂O₈ is incommensurate along the b axis. The lattice parameters were a = 5.40 Å, b = 25.25 Å, c = 30.62 Å.

The thermal expansion coefficient of a sample of the YBa₂Cu₄O₈ phase superconductor was measured using a low temperature X-ray diffractometer. The thermal expansivity was found to be highly anisotropic. The average coefficient of thermal expansion over the temperature range of 150 to 450 K was approximately equal for the a and b axes at 9 × 10⁻⁶/°C, vs a coefficient of about 15 × 10⁻⁶/°C for the c-axis. This would indicate that the bonding is weaker along the c-axis than in the ab-plane and that the specific heat of single crystals of the high T_c superconductors can depend on their geometry. The low thermal expansivity in the ab-plane relative to typical metal conductors as copper and silver could lead to compressive stresses in the superconductor when cooling a composite metal-superconductor wire having good alignment of the ab-plane along the wire axis.

High quality superconducting YBa₂Cu₃O_7-δ (YBCO) thin films were fabricated with ex situ co-evaporation on SrTiO₃ and in situ laser deposition on (001) MgO substrates. The co-evaporated films gave a superconducting transition temperature, T_c ∼ 90 K, and a critical current density, j_c ∼ 5 × 10⁵ A/cm² at 77 K after an ex situ post anneal at 890°C. For laser deposited films, T_c was above 90 K, and j_c, about 10⁶ A/cm² at 77 K. Two kinds of Josephson weak links were developed. The first kind consisted of a 0.8 μm or wider constriction in a co-evaporated YBCO film. A lift-off stencil was used to pattern the film before the post anneal. The second kind of weak link utilized the inherent grain boundaries in the laser deposited YBCO film. A 70-130 nm wide strip in A1 was deposited across a 4-6 μm wide YBCO bridge. After a 10-20 min long anneal at 300-450°C, the A1 had diffused into the film. We believe that the diffusion occurs mainly along the grain boundaries. By the proximity effect, the normal metal weakened the superconductivity at the grain boundary, and a weak link was formed with I_cR_n ∼ 0.9 mV at 77 K. Dry an wet etching were also used to pattern some structures.

Potentialities of Solid Phase Intergrowing (SPI) of crystals [5] in conformity with the investigations of HTSC are discussed. In particular the application of SPI for creation of dielectric crystalline substrates containing model boundaries (low- or high-angle, single or multiple) perpendicular to their surfaces is analyzed. Both HTSC film structures with a single boundary of a given crystallographic orientation and plain shape, and systems of such boundaries in parallel rows or with triple joints are constructed on a base of this type substrates.

The response from a high T_c superconducting multielement optical detector made of a laser deposited Y-Ba-Cu-O thin film has been evaluated. Several microscopic and spectroscopic techniques were used to establish the presence of the correct phase of the thin film. We have used optical pulses from a laser diode at 830 nm and from a Q-switched CO₂-laser at 10.6μm. The detector responded to 50 ps (FWHM) pulses. A comparison between dR/dT of the film and the response amplitude as a function of temperature indicated a bolometric response.

The temperature and magnetic field dependences of the magnetization and the electromagnetic shielding efficiency in polycrystalline samples of the high-T_c superconductor YBa₂Cu₃O_7-δ have been studied. At all temperatures investigated, two field regimes can be identified: A low field regime with an ideal diamagnetic behaviour and a large shielding factor, and an intermediate field regime, typically above a few mT, with a reduced diamagnetic susceptibility and a poor shielding efficiency. Using arguments from a modified critical state model, with a strongly field dependent critical current density, the transition to the intermediate field regime is attributed to the decoupling of the superconducting grains. It is argued that this effect is caused by the weakening of the intergrain Josephson junctions in the presence of a magnetic field.

FeCl⁻ doped polythiophene, iodine doped Shirakawa polyacetylene and an ultrathin film of undoped poly(4,4'-dibutyl-2,2'-bithiophene) have been studied by scanning tunneling microscopy (STM). The polythiophene shows a lamella-like structure with a thickness of approximately 50 Å. Distribution of dopants has been studied by means of STM. The results indicate the existence of low doped or undoped islands surrounded by highly doped regions.

Molecular-dynamics simulations have been carried out on a two-component Lennard-Jones system, quenched into supercooled and amorphous states. Careful attention is paid to proper equilibration of the system in the supercooled liquid regime and long production runs are performed in order to reveal slow structural relaxation processes. The results for the time-dependence of the self-part of the density autocorrelation function F_q^s(t) show two different slow relaxation processes, where the slowest (α relaxation) can be represented by a stretched exponential, A exp[– (t/τ_rel)^ß]. In frequency domain this gives rise to a quasi-elastic peak and it is found that its area, the nonergodicity parameter f_q^s, shows an anomalous decrease when increasing the temperature towards a critical value T_c. This happens in the supercooled liquid regime and it is one of the basic predictions of the recent mode-coupling theory for the liquid-glass transition problem.