Table of contents

The partial structure factors of liquid thallium selenide, TlSe, have been determined using a combination of two neutron diffraction measurements using isotopic substitution and one x-ray diffraction measurement. We show that the statistical accuracy obtained using this method is considerably higher than that achievable using neutron diffraction and isotopic substitution alone. The results unambiguously show the formation of Se polyanionic species in the liquid.

Recent developments in off-lattice self-consistent-field theories for inhomogeneous complex fluids are reviewed. Particular emphasis is given to the treatment of intermolecular interactions and compressibility, to the role of fluctuations, and to the discussion of the coarse-graining length which is inherent to the theory. Valuable insight can be gained from the comparison of self-consistent-field calculations with Monte Carlo simulations. Finally, some applications of the theory to orientational properties of polymers and copolymers at interfaces, and to the phase behaviour of amphiphiles at surfaces, are presented.

The structure of the ionic glasses , where x = 0.35, 0.40, 0.45, has been measured at 20 K by using neutron diffraction. The results show that units exist as the predominant structural motifs at all compositions, give a Bi-Cl nearest-neighbour distance of 2.53(2) Å, and an angle Cl-Bi-Cl of . The nearest-neighbour K-Cl correlations occur at about 3.1 Å. The first sharp diffraction peak observed in the total structure factors is attributed to Bi-Bi correlations and the shift in its position to smaller scattering vector values with increased x is attributed to an enlarged separation of the units with enhanced KCl content.

The structure of expanded mercury along its liquid-vapour coexistence curve has been studied in detail by applying the reverse Monte Carlo modelling technique to the x-ray diffraction data of Tamura and Hosokawa (Tamura K and Hosokawa S 1994 J. Phys.: Condens. Matter 6 A241). The models suggest that as the density decreases there is an initial change from a more close-packed to a less close-packed structure. Whilst most atom pairs retain the same near-neighbour distance, of about 3.0 Å, a few change to having a slightly larger neighbour distance of about 3.7 Å. At the lower densities many atoms have a very low coordination, indicating that they are on the edges of clusters, or on `pathways' between clusters.

Ethanol is known to form two different kinds of glassy state, namely a structural glass and a glassy crystal, both showing the same glass transition temperature . The molecular dynamics in the different phases (supercooled liquid/plastic crystal) and during the corresponding phase transition is studied by dielectric spectroscopy in the frequency range (30 K-250 K). Extracting the time constant, width parameter and relaxation strength of the main relaxation (the -process), very similar dielectric loss is found in both phases, including also the high-frequency wing. Comparing the temperature dependence of the time constants , the plastic crystal exhibits a less fragile behaviour. Additionally, a secondary relaxation is detected, again essentially the same in the two glassy states. We conclude that the motional mechanisms probed in the plastic crystal and in the supercooled liquid state are very similar.

We report the results of a preliminary investigation by specular neutron reflectivity of adsorption and wetting of alkane + perfluoroalkane binary liquid mixtures in the vicinity of the liquid/liquid critical point against a chemically modified (alkane-like) silicon wall. The results suggest the existence of a surprisingly long-ranged profile , even some degrees Kelvin above the upper critical point, and enable us to detect a wetting layer and its liquid/liquid interface below the critical point. The technique is capable of determining the composition, uniformity, and thickness of the wetting layer.

The core-core structure factor of dense star polymer solutions in a good solvent is shown theoretically to exhibit an unusual behaviour above the overlap concentration. Unlike usual liquids, these solutions display a structure factor whose first peak decreases with increasing density while the second peak grows. The scenario repeats itself with the subsequent peaks as the density is further enhanced. For low enough arm numbers , various different considerations lead to the conclusion that the system remains fluid at all concentrations.

The fluorescence dynamics of the and energy levels of in single crystals codoped or not codoped with and ions was experimentally investigated and compared with classical energy-transfer models. In the and crystals, the state relaxes predominantly by internal cross-relaxation-type energy transfers, whereas in the and crystals the relaxation is dominated by direct energy transfers to the state of the ions. In all of the codoped systems, strong energy transfers from the state of the ions to the and states of the and codopants were evident. The electric dipole-electric dipole coupling parameter was evaluated for several cross-relaxation and direct-energy-transfer processes.

The structural disorder in the two high-temperature, superionic phases of has been fully characterized using neutron powder diffraction and two complementary analyses. Rietveld profile refinement of Bragg scattering has been used to determine the most probable average structures and radial distribution functions, determined by Fourier transformation of the total scattering structure factors, have been used to refine local structural models based on these average structures. It is found that the -phase, which exists between 420 K and 1075 K is best described by a face-centred cubic arrangement of the tellurium ions with the silver ions randomly occupying the tetrahedral and octahedral voids. There is no local ordering of the filled tetrahedral sites and the silver ions in octahedral sites have a broad distribution and preferentially sit off centre in one of the eight directions away from filled tetrahedra. The -phase, which exists between 1075 K and the melt, has a body-centred cubic structure similar to , and the average structure is best described with silver ions distributed over tetrahedral and trigonal sites and positions which join these two sites. The radial distribution functions in both superionic phases clearly show that silver ions are never closer than Å to each other which means that nearest neighbour 12d tetrahedral sites cannot be simultaneously filled in . The differences in quality of fit to the experimental data between different silver ion ordering schemes are slight and it is proposed that single crystal diffuse scattering may be the only way to distinguish clearly between different models.

single crystals were grown which appear to be different from and . The space group is monoclinic Cm, the lattice constants are a = 14.916(3) Å, b = 24.860(3) Å, c = 4.6419(8) Å , Z = 16, Å, , T = 293 K (room-temperature phase), R = 0.076 and wR2 = 0.174 for 3450 observed reflections. The structure consists of five crystallographically independent molecules in the asymmetric unit. Each of the independent sulphate or selenate tetrahedron is linked to a translationally equivalent sulphate or selenate by acentrically ordered hydrogen bonds. The separated chains of asymmetric hydrogen bonds run parallel to the c-axis. The distance O-H...O varies between 2.48 Å and 2.60 Å.

We report preliminary results on the production of colour centres in crystals, subjected to electron irradiation. The dependence of the colour-centre creation on the temperature and dose of the irradiation, as well as on the direction of crystal growth, was studied. Our experimental results concerning absorption, emission and excitation spectra measurements show that defect formation in crystals depends strongly on the conditions of crystal growth and preparation of the samples. By comparing with and LiF crystals, we were able to attribute the absorption bands at 260, 420, 632, 386 and 480 nm to F, and centres. Theoretical evaluations of the absorption energies for these defects were made, and they supported these assignments. Room temperature electron irradiation of crystals grown in and directions favours the formation of different kinds of F-aggregate centre for each of these two cases. In particular, for crystals we were able to observe the formation of centres with one absorption band at 632 nm and a corresponding emission at 702 nm.

Tight-binding (TB) methods are designed to work for neutral systems. When an electron is added to or subtracted from the system, or when an external field is applied to the system, one should handle the problem of screening and charge redistribution in a self-consistent manner. Here, we have proposed a simple way to calculate the potential due to an external field and the corrected Hartree potential. A straightforward way to compute the Coulomb (Hubbard) integrals for any atom is also demonstrated. This formalism has been tested against ab initio calculations for carbon and silicon clusters. Good agreement between ab initio and self-consistent TB results is found.

Mössbauer spectroscopy is used to monitor the quadrupole interaction for the superconductor as a function of temperature. Qualitative comparison with other results supports earlier claims that the rank-2 crystal-field contribution lies between that for and that for . However, there is quantitative disagreement with crystal-field parameters determined elsewhere for .

Electrical conductivity and thermopower data are presented for sintered semiconducting compositions , , where charge transport is thought to be effected by electron hopping . The extrapolated crystallite DC conductivity was measured between and 300 K using impedance spectroscopy (20 Hz-1 MHz), with typical values of . The results can be approximated reasonably well by the relation at not too low temperatures. The thermopower is negative (n-type conduction) in the range and shows below a temperature variation of approximately , while it becomes largely temperature independent above . From the latter data the concentration ratio for Fe ions taking part in charge transport can be derived; this is compared with that determined from Mössbauer spectra at low temperatures which encompasses all Fe cations. There appears to be a certain correlation with the change at , which is possibly related to electron correlation effects for larger x-values.

Conductive polyaniline (PANi) films have been prepared via a new route comprising 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPSA) as both the protonating acid and the solvating group, and dichloroacetic acid (DCA) as a solvent. The AMPSA content was varied so that between 30 and 100% of the nitrogen sites on polyaniline could be protonated. The temperature dependence of the conductivity of the films was measured between 15 and 300 K. Above 75-115 K, depending upon the protonation level, a negative temperature coefficient was observed. At 240-260 K a new transition was observed, essentially independent of the protonation level, above which the negative temperature dependency becomes much stronger. The conductivity data were fitted using a simple model employed previously for films of polyaniline camphorsulphonate, and comparisons made between the two systems.

Differential scanning calorimetry measurements on a series of films showed an endothermic transition centred at 240-254 K, whose magnitude is dependent upon the AMPSA content. This thermal transition is ascribed to increased motion in the AMPSA anions when heated to above the transition temperature. The AMPSA anion may be considered to behave as a side chain ionically bonded to the PANi backbone, whose particular structure will affect the intrachain molecular dynamics, and hence electrical transport properties of the bulk polymer. The observed thermal transition correlates well with the new transition seen in the conductivity data.

It was possible to draw films of uniaxially both at room temperature and at 363 K. The room temperature conductivity along the stretch direction was increased to a maximum value of for a film drawn at 363 K, compared to for an undrawn sample. The films are compared to previous results obtained with polyaniline films protonated with 10-camphorsulphonic acid (CSA).

A asymmetric double-quantum-well structure exhibits a metastability in the quantum Hall regime when the system is illuminated, under a high magnetic field sufficient to depopulate the excited-state subband. The photo-generated electrons are transferred to the excited-state subband when its bottom Landau level falls below the Fermi level at lower magnetic fields. The metastability is attributed to the potential renormalization associated with the charge transfer. The amount of the transferred charge can be controlled by varying the minimum magnetic field and remains unchanged as long as the magnetic field stays above the previous minimum value.

A theoretical and experimental analysis of the heat conductivity in metal-dielectric point contacts is given. The contribution of the phonon diffraction effects to the heat conductivity is investigated. The phonon heat transport through the point contacts is measured from 0.1 K to 100 K using the anvil-needle technique. In KBr-KBr and KBr-Cu point contacts, substituting a Cu needle for the dielectric cold edge does not disturb the heat conductivity pattern. Measurements for Si-Cu point contacts reveal well-defined diffraction maxima of reduced heat conductivity at temperatures in the range 0.5 K-1 K.

=1 We report an unusual history dependence of the field-cooled magnetization in and around the peak-effect regime of the superconducting mixed state of . Such history effects in magnetization are shown to be absent in the peak-effect regime of the high- superconductor Bi-2212. Some qualitative similarities with and at the same time considerable quantitative differences from relatively old studies of the history dependence of the transport critical current density in single-crystal Nb and and amorphous films are examined. Our work indicates that the peak effect in is probably due to a first-order phase transition unlike the peak effect in Bi-2212, and that the latter could be due to a continuous or second-order phase transition. Differences in details of the history effects suggest that the origin of the peak effect in is likely to be different from those for these hard type-II superconductors.

The valence hyperfine fields of atoms in fcc iron and nickel containing 2p interstitials X ( X = B, C, N, O and F) have been investigated by applying the spin-polarized LMTO method. The results on the valence hyperfine field and the hyperfine coupling constants indicate that the Fe-Fe and Ni-Ni interactions are sensitively dependent on which interstitial atom is present, and also that Fe-X interactions dominated by bonding states are distinguishable from those between Ni and X atoms which `prefer' anti-bonding states. The volume (distance) dependences of the valence hyperfine fields in Fe-X are also investigated, and the results reveal an anomalous distance dependence of the three hyperfine coupling constants. It is concluded that it is the unusual distance dependence of the strong interactions between face-centred Fe and X atoms that gives rise to these anomalies.

We study here the dynamic susceptibility of a model quantum glass. A statistical mechanical treatment, based on a mean-field analysis of the transverse Ising model, is able to account for the competition between disorder and frustration, on the one hand, and quantal fluctuations on the other. Our results derived for the dynamic susceptibility are in qualitative agreement with experimental measurements on a recently studied dipolar-coupled quantum glass, namely , whose quantum term can be manipulated in the laboratory.

We present a new method for calculating directly the one-particle self-energy of an impurity Anderson model with Wilson's numerical renormalization group method by writing this quantity as the ratio of two correlation functions. This way of calculating turns out to be considerably more reliable and accurate than that via the impurity Green's function alone. We give results for the self-energy for the case of a constant coupling between the impurity and the conduction band and the effective arising in the dynamical mean-field theory of the Hubbard model. The implications of the problem of the metal-insulator transition in the Hubbard model are also discussed.

PAC measurements on show that the U moments are oriented mainly along [001] in an AFII type ordering. A small scale deviation from this ordering must be present, for which two possibilities are proposed. There are several indications that on top of this small scale deviation another not yet understood perturbative effect occurs.

Measurements of the dielectric permittivity are reported for a deuterated solid solution of antiferroelectric betaine ferroelectric betaine at frequencies MHz. The freezing phenomena in the ferroelectric phase of shows a typical dielectric behaviour of a transition into a dipolar glass state. Using the Kutnjak model, a glass temperature of 155 K is estimated.

Two Cu-17 at.% Mn single crystals quenched to room temperature after aging at 483 and 553 K, respectively, were investigated by small-angle neutron scattering between room temperature and 15 K. While nuclear small-angle scattering is low and increases only slightly towards the direct beam, strong magnetic small-angle scattering is observed. Its intensity increases with increasing degree of chemical short-range order and decreasing temperature. The correlation lengths for chemical and (static) ferromagnetic fluctuations are comparable.

This paper reports spectroscopic and scintillation studies of the well established scintillator material . Standard measurements of luminescence emission and excitation spectra have been accompanied by investigations of thermoluminescence and scintillation light yield over a wide temperature range, and by decay measurements under pulsed gamma and VUV excitations at various temperatures. These measurements are interpreted in the framework of a model that includes a recombination centre and a number of electron traps. We demonstrate that some unusual and largely unexplained features of the scintillator, such as a substantial disparity between scintillation and radiative decay times, the presence of slow components in scintillation decays, and a strong temperature variation of scintillation light yield between 150 and 300 K, have their origin in the processes of charge carrier capture and emission by electron traps. Although the nature of these traps remains elusive, most of the trap parameters, such as frequency factors, energy depths, and relative populations, have been estimated. This makes it possible to predict the characteristics of trap-free material and thereby the potential improvements that could be achieved.