Evidence of short-range order in the disordered Cu-Ti alloys

Diffuse X-ray scattering measurements have been carried out on a series of Cu-Ti alloys, in order to determine the dependence of short-range order (SRO) parameters on the composition and annealing temperature. The diffuse scattering intensity (DSI) curves contain a maximum at the superstructural position 100 which provides evidence of SRO in this alloy. The degree of SRO has been estimated by calculating the Warren-Cowley parameter ₁ (WCP) and size-effect parameter ₁ from the DSI curves. The large negative values of WCP for all samples show the presence of heterogeneous SRO in these alloys. Comparison is also made between the values of experimental WCP and theoretical WCP calculated in the framework of electronic theory of short-range order and the high-temperature approximation of Clapp and Moss. A fairly good agreement between theory and experiment is observed.     

Fullerene superconductivity by short-range order instability

For the fullerene-group (C₆₀ group) of metastable materials a novel model is conceptualized on the basis of a special dynamic clustering. This clustering is realized by a short-range instability through a collective, cooperative, and coherent displacement of the ions to seek for the most dense and most stable cluster formation. In the C₆₀ superconductivity, mediation by harmonic lattice vibrations (phonons) is replaced by a pairing mechanism with anharmonic collective and coherent cluster vibrations. The suggested model fits the experimental data surprisingly well.     

Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH _c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH _c2 (T) in case of NdRh₄B₄.     

Effects of the angle of deposition on short-range order in amorphous germanium

A comparison of the atomic arrangements in germanium films deposited at normal and oblique incidence is made. It is shown that the parameters of short-range order are sensitive to the angle ? of deposition. The oblique incidence effects manifest themselves both in the intensity function and in the radial distribution function (RDF). The RDF of a film deposited at oblique incidence has an additional correlation peak at r₀ = 0.174 nm which is not observed for normal incidence. Its location corresponds to the length of the GeO bond. It arises because of the increase in the oxygen content of the filn with increasing obliqueness. The coordination number consistent with the GeO correlations is proportional to the oxygen content of the film as estimated by Auger electron spectroscopy. The incorporated oxygen and the structural anisotropy of obliquely deposited films also influence the shape of the second structural peak of the RDF. The reasons for the different oxygen contents in films prepared under the same vacuum conditions are discussed.     

Spin-glass with short-range interaction in a magnetic field

The properties of spin-glasses with nonmagnetic defects are considered in the vicinity of the transition point in a finite magnetic field, by means of a percolational approach. It is shown that the dependence of the magnetic specific heat C(h) = C(h)–C(o) or of the magnetic susceptibility M(h)/h on field and temperature allows us to define the distribution function of finite clusters with respect to their size. In addition to general formulas resulting from the scaling laws, more detailed formulas are obtained for two model distributions: (a) the Bethe lattice distribution, (b) a special model distribution with arbitrary indices.     

Size-effect induced short-range magnetic ordering in germanium nanostructures

Formation of ordered magnetic states in germanium nanostructures embedded in SiO2 has been investigated. Samples with the nanostructures were prepared by sputtering deposition on Si(100) substrates, followed by thermal annealing in vacuum. Transmission electron microscopy, energy dispersive X-ray spectrometry, and Raman spectroscopy have been used to characterize the samples. Magnetic measurements were performed using a superconducting quantum interference device. Size-effect induced magnetic orderings in the germanium nanostructures were found to be present at room temperatures and below. Superparamagnetic behavior was observed at temperatures above 230 K, whereas thermal excitation of spin reorientation and magnetic coupling has been revealed at temperatures below 60 K. Inverted hysteresis loops with negative remanences and multiple plateaus revealed the ferri- or antiferromagnetic nature of the coupling. Inter-domain coupling and effect of magnetic anisotropy will be discussed based on the experimental results and simulations with a spin reorientation model.     

Superconductivity characterization by a magnetic pendulum

A magnetic pendulum device was designed, developed, and used to characterize high-T_c materials. A correlation between the volume fraction of superconducting phase in a composite and the force of expulsion was found. The device is sensitive enough to detect a volume of 0.5% superconducting phase in a composite. The application of the device for characterizing composites, incipient superconductivity, and films has been discussed.     

Investigation of short-range atomic order in glasses from the MoO3-TeO2 system

Short-range atomic order in the MoO₃-TeO₂ glassy system has been studied by the neutron diffraction method. With increase of the amount of MoO₃ from 20 to 50 mol %, a sequential transition 4 (3 + 1) 3 of the coordination state of tellurium with respect to oxygen was observed. Throughout this process, a formation of MoO₅ and MoO₄ groups was detected. The interpretation of the experimental radial distribution functions was performed by comparison with quasi-crystalline model curves (structure diffusion model).     

The influence of short-range magnetic correlations on the superconducting transition temperature

Taking into account correlations between well-defined localized magnetic moments in a dilute magnetic system, we derive an expression for the super-conducting transition temperature which is different from Bennemann's theory. In particular it is shown that the influence of short-range magnetic correlations in the vicinity of a magnetic phase transition on the superconducting transition temperature is twofold: (1) There are large corrections to the Abrikosov-Gor'kov theory. (2) For a certain choice of parameters the phase diagram in the temperature versus concentration plane shows a break (where the system is normally conducting) between two superconducting regions.     

Neutron diffraction study on the medium and short-range order of ternary chalcogenide glasses

Glasses from Ge_xAs_40–xS₆₀, Ge_xAs_40–xSe₆₀, Ge_xSb_40–xS₆₀ and Ge_xSb_40–xSe₆₀ families have been investigated by neutron diffraction. The well expressed first sharp diffraction peak of the neutron spectra at Q = 10–14 nm^–1 has been explained by a pseudo-binary model of the structure of studied chalcogenide glasses. The amplitude of this interference maximum increases with Ge-content in all investigated samples. Experimental radial distribution functions show that the basic structural units, GeS(Se)₄ tetrahedra and As(Sb)S₃ or As(Sb)Se₃ pyramids persist in glassy state for the whole range of studied compositions. Crystal-like model distribution functions have been used to describe the short-range order observed. A shortening of heteropolar bonds in S-containing glasses as well as an increase of heteropolar bonds length in Se-containing glasses has been found.     

Stationary and nonstationary absorption in lead silicate glasses with short-range order inversion

The methods of stationary and pulsed absorption spectroscopy were used to study the optical properties of xPbO·(1-x)SiO₂ glasses produced by cooling of a molten mixture of chemically pure oxides. The spectral dependence of the absorption in the range of the short-wavelength edge obeys the Urbach rule. As the PbO concentration increases, a red shift of the optical transparency cutoff is observed. At x = 0.45-0.50 the amorphous matrix undergoes a structural inversion, which is due to a transition from a silicate to a lead-oxygen glass-forming network. This transition shows up as an abrupt change in the type of optical transitions, the width of the optical gap E_g, and the Urbach energy E_U. The short-range order inversion in the glass is accompanied by an increase in the atomic correlation radius R₀ characterizing the size of the medium-range order in the system. It was found empirically that R₀ has a linear relationship with a continuum-disorder parameter E_U.It was found that pulsed electron irradiation produces short-lived color centers, which absorb at 1.65 and 2.30 eV. The relaxation of unstable absorption centers is characterized by microsecond kinetics. The nature of unstable absorption centers and their relationship with a short-range order inversion and the structure function of lead atoms have been discussed. The kinetic dependences have been interpreted in the context of a mechanism responsible for diffusion-controlled tunneling recombination of radiation-induced electronic and hole states of the matrix.     

Basic parameters for the short-range order of amorphous ternary alloys with applications to practical cases

A procedure was considered for determining theoretical expressions of the area under the first radial distribution function (RDF) peak, and of the relative co-ordination numbers, n _ii which depend on the co-ordination hypotheses and on the numbers of bonds between elements of a single type of pair. In this method, two important facts were taken into account. First, the products of atomic factors are functions of s (the scattering vector module) and so they cannot always be considered constant; they were therefore approximated by polynomic functions which best fitted the results obtained from the atomic factors given in international tables. Secondly, consideration of the influence of the structural hypotheses (co-ordinations, existence of certain types of bond) on the area and co-ordination numbers mentioned enabled the most probable local order to be postulated. In order to test the reliability of this method, the method was applied to a set of alloys, quoted in the literature, and the theoretical results obtained agreed very satisfactorily with the experimental values.     

Dipolar magnetic order in the rare earth ethyl sulfates

The magnetic ground states of the ethyl sulfates of Nd, Gd, Ho, and Er have been determined by dc susceptibility measurements and found to agree with those predicted using (where possible) the classical Luttinger-Tisza theory. Ytterbium ethyl sulfate is not observed to order at temperatures above 6.5 mK. The observed transition temperatures scale roughly with the magnitude of the calculated ground state energy. The low-temperature paramagnetic susceptibility of Gd ethyl sulfate agrees qualitatively with a theoretical prediction based on experimental crystal-field parameters.     

Crystallographic order in multi-walled carbon nanotubes synthesized in the presence of nitrogen

Multi-walled carbon nanotubes were synthesized by chemical vapor deposition from pure toluene and toluene/diazine mixtures using ferrocene as a catalyst precursor at 760 degrees C. As recently announced, characterization of the resulting nanotube films showed that, unlike pure carbon nanotubes, those grown in the presence of nitrogen have an extremely high degree of internal order, both in terms of the uniform chirality in the nanotube walls and of the crystallographic register between them. Here, the structure, defects, and morphology of the nanotubes were analyzed in depth using advanced electron microscopy techniques, and compared with existing models and observations. Nitrogen, which seems to be responsible for the dramatic structural order, was found to segregate preferentially within the core of the nanotubes.     

Nuclear magnetic order in solid 3He

Over the past 20 years our understanding of cooperative magnetic phenomena in solid ³He has expanded greatly. Starting from the concept of atomic exchange, the author explains how early experiments led to the abandonment of the naive Heisenberg nearest neighbor antiferromagnetic Hamiltonian, in favor of a multiple exchange model. He then describes the detailed experimental picture of the spin-ordered phases which has developed over the past decade, and the successes and failures of the multiple exchange model to explain the experimental results. Today solid ³He is a powerful model magnetic system for extending our general understanding of magnetic order, and should continue to play an important role for many years to come.