Table of contents

It has been suggested by several authors that SrTiO₃ layers in SrTiO₃ –BaTiO₃ superlattices should be tetragonal and ferroelectric at ambient temperatures, like the BaTiO₃ layers, rather than cubic, as in bulk SrTiO₃, and that free-energy minimization requires continuity of the polarization direction. A recent ab initio calculation constrained solutions to this structure. Surprisingly, our x-ray study shows that the SrTiO₃ layers are orthorhombic with 0.03% in-plane strain, with the BaTiO₃  c-axis matching the SrTiO₃a- and b-axis better than the c-axis; strain energy overcomes the cost in electrostatic energy.

We analyse two-particle renormalizations within the many-fermion perturbation expansion. We show that present diagrammatic theories suffer from a lack of direct diagrammatic control over the physical two-particle functions. To rectify this, we introduce and prove a Ward identity enabling an explicit construction of the self-energy from a given two-particle irreducible vertex. Approximations constructed in this way are causal, obey conservation laws, and offer explicit diagrammatic control of singularities in dynamical two-particle functions.

On the basis of a ladder-diagram approximation, we study the pairing-fluctuation effect in d-wave superconductivity. The single particles and pairs are treated on an equal footing. In the superconducting state, the predominant pairing fluctuation is due to the excitation of pairs to the states of the Goldstone mode. These bosonic degrees of freedom are relevant to the pseudogap physics in high-T_c cuprates. The Green function of electrons is obtained as an analytic solution to a cubic equation. The superconducting order parameter and the transition temperature are substantially reduced from the values of the mean-field theory. The calculated phase boundary of the superconductivity can reasonably describe the experiment results for cuprates.

We investigated the use of ion photodesorption as a tool to monitor the transition from the physisorbed to the chemisorbed state on a surface. The adsorption of benzene on Si(111) 7 × 7 in the temperature range 40–300 K is chosen as a prototype. The D⁺ ion photodesorption yield was monitored as a function of temperature at various benzene exposures. Comparative measurements of the C 1s photoelectron yield in the same temperature range enable the physisorbed to chemisorbed state transition to be distinguished from that of the multilayer to the chemisorbed state. We find the onset at 110 K in the first case, and at 130–140 K in the second case. These results demonstrate that ion photodesorption is a potentially interesting method to identify physisorption to chemisorption transitions of adsorbed molecules on surfaces.

The high-symmetry ring geometry is shown to exhibit a wide range of intriguing magnetostatic and magnetodynamic properties, which we survey in this topical review. We consider first the patterning and deposition techniques, which are used to fabricate ring structures (diameters between 0.1 and 2 µm) and discuss their respective advantages and disadvantages. The results of direct nanoscale imaging of the novel magnetization configurations present in rings with different geometrical parameters (including discs) are discussed. These results give valuable insight into the influence of the magnetic anisotropies governing the magnetic states. The different types of domain walls that arise are compared quantitatively to micromagnetic simulations. The magnetodynamic switching between the different magnetic states is described in detail. In particular we elaborate on the different geometry-dependent magnetic switchings, since the different transitions occurring allow us to determine which energy terms govern the reversal process. We discuss a process by which fast (sub-ns) and controlled switching can be achieved, therefore making rings an attractive geometry for applications, in addition to studying fundamental issues of nanomagnetism.

Surface-sensitive spectroscopic techniques, namely metastable deexcitation spectroscopy (MDS) and ultraviolet photoemission (UPS), have been applied to investigate the effects of chlorine chemisorption on the electronic properties (surface density of states and charge density) of Ag(100), Ag(110) and Ag(111) surfaces. Initial stages of chemisorption, up to the formation of a saturated Cl overlayer, have been examined. In particular, MDS permitted us to observe at low Cl gas exposure a progressive depletion of the Ag (5s) charge due to transfer and bonding with Cl atoms. From both MDS and UPS it was possible to observe the development of Cl (3p) bonding and anti-bonding states, the amount of their splitting increasing with coverage. Differences between chemisorption at the three surfaces have been noticed and they have been justified in terms of the different adatom packing and possible formation of small AgCl clusters (especially for the Ag(111) surface).

The stoichiometries and geometric structures formed by the segregation of C, Al and S on a Fe(100) surface have been investigated by Auger electron spectroscopy and quantitative low-energy electron diffraction (LEED). Step-wise annealing of a sputtered surface with increasing annealing temperature reveals the successive segregation of C, Al and S. According to quantitative LEED analyses, each segregand forms a distinct c(2 × 2) long-range ordered structure. Also, each segregand removes the preceding one from the surface entirely, i.e. segregation in the Fe(100)–(C, Al, S) system is purely competitive with no ordered co-segregation regimes involving two or even three elements. The c(2 × 2) phases of segregated carbon and sulfur consist of elemental surface adlayers with the adatoms residing in four-fold symmetric hollow sites of the iron substrate. This is in contrast to segregated Al which, according to an earlier analysis, forms a c(2 × 2)-symmetric surface alloy layer with iron. In all cases there is some chemical disorder within subsurface layers with Fe atoms substituted by Al. The bond lengths between the segregated adatoms and iron neighbours are close to the sum of the covalent radii of the elements involved whereby carbon appears to be five-fold coordinated, in contrast to four-fold coordinated sulfur.

The thermal stability of nanocrystalline clusters, the phase evolution, and their effects on magnetic properties were studied for as-cast Nd₆₀Al₁₀Fe₂₀Co₁₀ alloy using differential scanning calorimetry curves, x-ray diffraction patterns, scanning electron microscopy, and high-resolution transition electron microscopy. Thermomagnetic curves and hysteresis loops of the bulk metallic glass were measured during the annealing process. The high thermostability of the hard magnetic properties of the samples observed is attributed to the stability of the nanocrystalline clusters upon annealing, while the slight enhancement in the magnetization is due to the precipitation of some Nd-rich metastable phases. The mechanism of thermostability of the nanocrystalline clusters and the formation of the metastable phases are discussed.

A theoretical model is suggested which describes the generation and evolution of misfit dislocations in composite solids containing nanowires with rectangular cross-section. In the framework of the model, the ranges of the geometric parameters (nanowire sizes, misfit parameter, interspacing between the nanowire and the free surface of the composite) are calculated at which the generation of various misfit dislocation configurations (loops, semi-loops and dipoles) is energetically favourable. Transformations of these dislocation configurations and their specific features are discussed.

In order to study the mechanism and parameters of hydrogen diffusion in the cubic (C15-type) Laves phase YMn₂, we have performed quasielastic neutron scattering measurements in YMn₂H_x (x = 0.4, 0.65 and 1.26) over the temperature range 30–395 K. It is found that the diffusive motion of hydrogen in this system can be described in terms of two jump processes: the fast localized H motion with the jump rate τ_l⁻¹ and the slower process with the rate τ_d⁻¹ associated with H jumps leading to long-range diffusion. The ratio τ_d /τ_l at room temperature is close to 10². Our results suggest that the localized H motion in YMn₂H_x corresponds to back-and-forth jumps of hydrogen atoms within pairs of interstitial g (Y₂Mn₂) sites. The parameters of the long-range diffusion of hydrogen in the samples with different H content are found to be close to each other. In the range 210–395 K, the temperature dependences of τ_d⁻¹ and the hydrogen diffusivity are reasonably described by the Arrhenius law with activation energies 0.18–0.22 eV.

The energy–momentum resolved valence band structure of beryllium oxide has been measured by electron momentum spectroscopy (EMS). Band dispersions, bandwidths and intervalence bandgap, electron momentum density (EMD) and density of occupied states have been extracted from the EMS data. The experimental results are compared with band structure calculations performed within the full potential linear muffin-tin orbital approximation. Our experimental bandwidths of 2.1 ± 0.2 and 4.8 ± 0.3 eV for the oxygen s and p bands, respectively, are in accord with theoretical predictions, as is the s-band EMD after background subtraction. Contrary to the calculations, however, the measured p-band EMD shows large intensity at the Γ point. The measured full valence bandwidth of 19.4 ± 0.3 eV is at least 1.4 eV larger than the theory. The experiment also finds a significantly higher value for the p-to-s-band EMD ratio in a broad momentum range compared to the theory.

Pd/Ru and Pd/Rh alloys have been loaded with hydrogen in high-pressure conditions. The resulting hydrogen contents were close to the stoichiometric composition, H/(Pd + Me) = 1. Lower hydrogen contents have been obtained by successive partial desorptions. The thermoelectric power and electrical resistance of one- and two-phase alloys have been measured simultaneously in the temperature range between 80 and 300 K. A Nordheim–Gorter type correlation of the two quantities has been observed in many cases and the partial thermopowers corresponding to electron–phonon scattering and lattice disorder could be determined. The observed anomalous behaviour of the total and partial thermopowers is attributed to virtual bound states of ruthenium or rhodium.

Absorption spectra of chemically synthesized uniform and multilayer semiconductor nanocrystals in a magnetic field are investigated theoretically. The nanocrystals are modelled by spherical barrier/well potentials. The electron states are calculated within the effective mass model. A four-band k · p Hamiltonian, accounting for the valence subband mixing, is used to obtain the hole states. The magneto-optical transition spectrum depends strongly on the size and composition of the nanocrystals. In the case of small uniform quantum dots, only the linear Zeeman splitting of the electron and hole energy levels is observed even for very strong magnetic fields. In larger nanocrystals, the quadratic magnetic interaction turns out to be important and the transition spectrum becomes complicated. The most complicated influence of the magnetic field is found in quantum dot–quantum well systems in which the lowest electron and hole states are localized in a thin spherical layer. It is shown that transitions that are optically active when no magnetic field is applied remain strong even in very strong fields.

We present a theoretical and experimental study of a quasi-one-dimensional zigzag antiferromagnet (N₂H₅)CuCl₃, which can be viewed as weakly coupled Heisenberg chains with a frustrated interaction. We first discuss generic features of the magnetic properties of the zigzag spin chain between the nearly single chain case and the nearly double chain case, on the basis of the finite temperature density-matrix renormalization group (DMRG) calculations. We next show the experimental results for the magnetic susceptibility and the high-field magnetization of a single crystal of (N₂H₅)CuCl₃ above the Néel temperature T_N = 1.55 K. By comparing the experimental data with the DMRG results carefully, we finally obtain the ratio of the nearest and next-nearest exchange couplings as J₁/J₂ = 0.25 with J₂/k_B = 16.3 K. We also investigate the three-dimensional (3D) coupling J' effect by using mean-field theory combined with the DMRG calculations. The estimated value J' ∼ 0.04J₂ supports the one-dimensionality of (N₂H₅)CuCl₃ above T_N.

The electronic structure of invar alloys (i.e. materials in which the near absence of thermal expansion is observed) has been the focus of much study, owing both to the technological applications of these materials and interest in the fundamental mechanism that is responsible for the effect. Here, calculations of the magnetic Compton profiles are presented for ordered and disordered Fe₃Pt alloys. Using linear muffin-tin orbital and KKR methods, the latter incorporating the coherent potential approximation to describe the substitutional disorder, the electronic band structure and measurable quantities such as the Fermi surface topology are presented.

Lead zinc niobate–lead titanate (PZN–PT) single crystals show very large piezoelectric strains for electric fields applied along the unit cell edges e.g. [001]_R. It has been widely reported that this effect is caused by an electric field induced phase transition from rhombohedral (R3m) to monoclinic (Cm or Pm) symmetry in an essentially continuous manner. Group theoretical analysis using the computer program ISOTROPY indicates phase transitions between R3m and Cm (or Pm) must be discontinuous under Landau theory. An analysis of the symmetry of a strained unit cell in R3m and a simple expansion of the piezoelectric strain equation indicate that the piezoelectric distortion due to an electric field along a cell edge in rhombohedral perovskite-based ferroelectrics is intrinsically monoclinic (Cm), even for infinitesimal electric fields. PZN–PT crystals have up to nine times the elastic compliance of other piezoelectric perovskites and it might be expected that the piezoelectric strains are also very large. A field induced phase transition is therefore indistinguishable from the piezoelectric distortion and is neither sufficient nor necessary to understand the large piezoelectric response of PZN–PT.

Si layers were developed on pre-oxidized Si wafers by decomposition of silane in a low pressure chemical vapour deposition reactor. By keeping the deposition time constant (2 min) three sets of samples were fabricated at deposition temperatures equal to 580, 610 and 625 °C. The deposited Si layers were thinned by high temperature dry oxidation thus forming SiO₂/Si/SiO₂ structures. Room temperature photoluminescence (PL) measurements showed that for those samples in which the thickness of the remaining Si layer was greater than ∼ 6 nm, the spectra exhibited a peak at ∼ 650 nm. Prolonged oxidations led to the formation of SiO₂/nanocrystalline-Si/SiO₂ structures in which the thickness of the remaining nanocrystalline Si (nc-Si) layer was smaller than 3 nm. The PL spectra obtained from these structures were at least ten times stronger compared to the previous ones. The PL peak wavelength exhibited a weak dependence on the nc-Si layer thickness shifting from 800 to 720 nm for nc-Si thickness decreasing from ∼ 3 to ∼ 0.5 nm. In this publication we present our experimental findings, which indicate that the intensity of the 720–800 nm PL band is influenced by the deposition temperature of the initial Si layer and by the thickness of the oxide layer between the nc-Si layer and the Si substrate.

The phase diagram of nanoparticles is known to be a function of their size. In the literature, this is generally demonstrated for cases where their shape is spherical. Here, it is shown theoretically that the phase diagram of non-spherical particles may be calculated from the spherical case, at the same surface area/volume ratio, both with and without surface segregation, provided the surface tension is considered to be isotropic.