Table of contents

Results from different sets of band calculations for undoped and doped HgBa₂CuO₄ show that small changes in localization can lead to very different ground states. The normal LDA results are compared with 'modified' LDA results, in which different linearization energies make the O p band more localized. The ground states in the normal calculations are far from the anti-ferromagnetic ones, while nearly AFM states are found in the modified calculations. The proximity of an AFM state in the doped system leads to increased λ_sf, and the modified band structure has favourable conditions for spin–phonon coupling and superconductivity mediated by spin fluctuations.

Microwave studies in the temperature range 4–300 K have been made on the different kinds of cobaltites in an attempt to observe a magnetic resonance. In a La_0.9Ca_0.1CoO₃ single crystal a broad resonance absorption line (with ) due to the presence of Co⁴⁺ ions was observed below 40 K. The measured broadening of the linewidth with decreasing temperature can be related to the process of clustering of cobalt ions. In La_0.8Ca_0.2CoO₃ and TbBaCo₂O_5.5 single crystals non-resonance absorption was observed in the temperature ranges 20–90 and 240–260 K, respectively. In order to investigate the nature of this absorption its intensity was measured as a function of external magnetic field and temperature. The results could suggest that the absorption, although similar for both compounds, was caused by two different mechanisms: microwave power losses on metallic/ferromagnetic clusters in La_0.8Ca_0.2CoO₃ and high-frequency fluctuations of the magnetic domain walls in TbBaCo₂O_5.5.

We use temperature-accelerated dynamics to show the importance of exchange mechanisms in surface diffusion and growth of simple oxides. Such mechanisms can dominate transport processes both on terraces and steps for both homoepitaxial and heteroepitaxial growth. We suggest that the mixing inevitable when an exchange mechanism is present must be considered when attempts are made to grow sharp interfaces in oxide nanostructures.

Low energy (<40 eV) electrons may be used to athermally release the compressive stress in a Si(100) surface layer induced by Ar⁺ ions (<100 eV) (Narushima et al 2001 Appl. Phys. Lett.79 605). In particular, we have strong evidence that the stress relaxation depends only on the number of irradiated electrons. This indicates that complete relaxation is not promoted by a thermal activation mechanism, but by a non-thermal mechanism. In this letter, we show using scanning tunneling microscopy (STM) that the underlying cause of the athermal surface stress relaxation is recrystallization of the surface atoms. Our STM observations show characteristic features to support our hypothesis. The electron-irradiated surface does not have the thermally generated 2 × 1 surface reconstruction, but instead a 1 × 1 reconstruction, which is slightly closer to the positions of a 'bulk-terminated' surface.

The optical evolution for monolithic mesoporous silica soaked in Ag⁺ ion solution (0.01–0.06 M) and annealed in air or water vapour is investigated. Heat treatment in air results in the absence of the surface plasmon resonance (SPR) peak of Ag nanoparticles. However, water vapour annealing induces a strong Ag SPR peak, exhibiting a strong reduction effect on Ag⁺ ions and an enhancement effect on the SPR absorption. The enhancement and weakening of the SPR can be controlled through annealing in water vapour and air alternately. This finding indicates the possibility that Ag nanoparticles dispersed in silica may be useful in optical sensors for humid environment use at high temperature.

The chemical and electronic properties of selenium passivated GaAs(001)-2 × 1 surfaces were investigated by a combination of theoretical calculations and core level photoemission experiments. An anion exchange results in gallium-selenide like layers showing a 2 × 1 reconstruction in low energy electron diffraction (LEED). The analysis of the different components in the core level spectra of As 3d, Ga 3d and Se 3d limits the number of possible structural models. The Se/GaAs(001)-2 × 1 reconstruction has been also analysed by means of DFT-LDA calculations and theoretical STM currents. In a first step, different geometries are considered and the most stable one, from the point of view of the thermodynamic potential, is determined. Then, STM currents and the corresponding surface corrugation are calculated and compared with the experimental evidence. We conclude that the Se/GaAs(001)-2 × 1 reconstruction has a single Se atom in the last crystal layer, bonded to two Ga atoms of the second layer, and another Se layer replacing the third As layer of the crystal. These surfaces may be considered as chemically stable because they withstand considerable exposure to air. In terms of electronic passivation, i.e. the removal of any surface band bending, the selenium modification is not successful. Band bending on n-type doped samples is reduced while band bending on the p-type doped samples is further increased.

Electron pair emission from ferromagnetic crystalline surfaces, which occurs due to the collision of an incident low-energy electron with a valence electron, is shown to be governed by spatial and spin selection rules. Valence electron states with spatial parts antisymmetric with respect to the reaction plane are not observable for any spin configuration. If there is a spatial symmetry plane normal to the reaction plane, spin-polarized primary electrons selectively probe equal and opposite spin valence electrons. These analytical findings are further elaborated for ferromagnetic cubic (110) surfaces and illustrated by numerical results for Fe(110).

We have determined the nonlocal, dynamic dielectric response properties of a multiple-quantum-wire lattice embedded in a semi-infinite plasma-like host medium, with the progression of parallel wires perpendicular to the interface, while the wires themselves are parallel to the surface. This is carried out within the framework of the random phase approximation, neglecting tunnelling. In this study, we have also investigated the condition for the occurrence of current-driven plasmon instability as a function of z₀, the distance of the first quantum wire from the bounding surface, and as a function of a, the separation of the quantum wires. Furthermore, the coupled mode dispersion relations for the plasmons of multiple-quantum-wire systems in interaction with the surface and bulk plasmons of the host material are analysed for dependences on the geometrical parameters z₀ and a.

The DC conductivity of p-sexiphenyl films 0.2–2.5 µm thick deposited on glass substrates was investigated at temperatures between 4.2 and 300 K to evaluate role of the grain boundary interfaces in the transport properties. Molecular dynamics simulations and quantum chemical modelling of the experimentally observed conductivity have shown that there exist at least three different phases of the films effectively contributing to the observed features of the DC conductivity: conductivity caused by proper crystalline states, conductivity originating from amorphous-like intergrain region and conductivity caused by grain boundaries. A comparison of experimental data with those calculated theoretically shows that the main contribution to the observed DC conductivity is supplied by the boundary region between the crystalline grains and the amorphous-like background, even though this region occupies a relatively low proportion of the total volume.

The photoinduced effect, at different temperatures, on layered perovskite manganite La_1.2Sr_1.8Mn_1.8Co_0.2O₇ thin film deposited on a SrTiO₃(100) substrate has been studied using a He–Ne laser (632.8 nm) with the power of output 25 mW at low temperatures. The electrical transport and magnetic properties of the film are investigated systematically in the temperature region from 20 to 300 K. The resistance shows a significant decrease at low temperatures (T<90 K) under laser irradiation, which can be interpreted in terms of the spin-state transition of Co³⁺ ions. The magnetic field combined with the laser is also used to detect the photoinduced effect of the spin-glass state. The time dependence of the metastable system under several applied fields was also discussed. The system has the ability to keep a persistent memory of the magnetic history imprinted in its zero-field resistance.

Recent experimental measurements of large flexoelectric coefficients in ferroelectric ceramics suggest that strain gradients can affect the polarization and permittivity behaviour of inhomogeneously strained ferroelectrics. Here we present a phenomenological model of the effect of flexoelectricity on the dielectric constant, polarization, Curie temperature (T_C), temperature of maximum dielectric constant (T_m) and temperature of the onset of reversible polarization (T_ferro) for ferroelectric thin films subject to substrate-induced epitaxial strains that are allowed to relax with thickness, and the qualitative and quantitative predictions of the model are compared with experimental results for (Ba_0.5Sr_0.5)TiO₃ thin films on SrRuO₃ electrodes. It is shown that flexoelectricity can play an important role in decreasing the maximum dielectric constant of ferroelectric thin films under inhomogeneous in-plane strain, regardless of the sign of the strain gradient.

Using the adiabatic trajectory method, the migration energy barriers for the migration of Li ions and Cr ions along the one-dimensional diffusion pathway in pure and Cr doped LiFePO₄ are obtained from first principles calculations. The results show that while Li ions can diffuse along the diffusion pathway easily, Cr ions do not easily diffuse away from their initial positions. This means that the heavy Cr ions will block the one-dimensional diffusion pathway of the material. Monte Carlo simulations are performed to evaluate the influences of the blocking behaviours on the electrochemical performance of LiFePO₄ cathode material for Li ion secondary batteries. The results show that the evaluated capacity is highly sensitive to the amount of the dopant, the size of the super-cell being used for simulation (particle size of the powder cathode material) and the Monte Carlo steps for statistics (charge–discharge current density).

The first example of ferromagnetism in a compound crystallizing with the filled beta-manganese structure is reported. Magnetometry and neutron diffraction show that Fe_1.5Pd_0.5Mo₃N is ferromagnetic below 160 K, with a saturation magnetization of 2.44 μ_B/formula unit which is associated with the Fe/Pd sublattice. Self-consistent spin-polarized electronic structure calculations, performed within density functional theory, predict an average magnetic moment of 2.93 μ_B/formula unit at T = 0 K. The moment is shown to be a result of the fulfilment of a local Stoner criterion on the Fe sites.

The crystal structure of the orthorhombic phase Al₄(Cr,Fe) with a composition of Al_80.6Cr_10.7Fe_8.7 has been determined by single-crystal x-ray diffraction (table 2). There are 11 crystallographically independent Cr/Fe sites, of which 10 are icosahedrally coordinated. Of the 28 crystallographically independent Al sites 5 are icosahedrally coordinated. However, owing to the presence of many partial occupancies, caused by split atoms, some of these coordinations are only pseudo-icosahedral with effective coordination numbers (section 4.3). The icosahedral and pseudo-icosahedral clusters are oriented with a two-fold axis parallel to [100]. They are linked either by vertex-, edge- or face-sharing or by mutual interpenetration, thus forming large clusters of icosahedra in the (100) layers with one of the five-fold axes in the [010] direction. Four of the 12 atoms defining the vertices of an icosahedral cluster lie in a mirror plane (F plane), while the other atoms are in puckered layers below and above the mirror plane. Thus, the icosahedral cluster consists of layers of three atoms thick. Two such layers stack within one lattice constant  nm. The interconnection of icosahedra in the layer block forms large triangular and hexagonal structural motifs, though they have only two-fold rotational symmetry. Similar geometrical motifs but displaying three- or six-fold symmetry do exist in the structure of the hexagonal μ-Al₄Mn, κ-(AlCrNi), λ-Al₄Mn and ν-Al₁₂Cr₂Fe phases.

The Raman- and IR-active phonons were studied in ordered Ba(Y_2/3Mo_1/3)O₃, Ba(Y_2/3W_1/3)O₃, Ba(Gd_2/3W_1/3)O₃, Ba(Sm_2/3W_1/3)O₃, Ba(Dy_2/3W_1/3)O₃, Ba(Dy_2/3Mo_1/3)O₃ and Ba(In_2/3W_1/3)O₃ as well as disordered Ba(In_2/3Mo_1/3) O₃ and Sr(In_2/3W_1/3)O₃. The assignment of the observed modes was given on the basis of lattice dynamical calculations. The studies performed revealed the presence of forbidden by the selection rules Raman bands for the Ba(In_2/3Mo_1/3)O₃ sample. This result suggests that the Ba(In_2/3Mo_1/3)O₃ contains 1:1 ordered domains of symmetry embedded in the disordered matrix and that Ba(In_2/3Mo_1/3)O₃ and Ba(In_2/3W_1/3)O₃ may be obtained in both ordered and disordered structures, depending on the thermal treatment. This behaviour resembles that of well known Pb(In_1/2Nb_1/2)O₃, Pb(Sc_1/2Nb_1/2)O₃ and Pb(Sc_1/2Ta_1/2)O₃ relaxors.

We calculate the Hartree–Fock energy of a density-wave state in a spin-polarized two-dimensional electron system with short-range repulsive interactions. We find that the ground state is always either normal (nonmagnetic) or ferromagnetic. However, the energy of a density-wave state approaches the energy of the ferromagnetic ground state by a factor proportional to (1−ζ²) (ζ is the polarization of the electron gas). Based on this result, we comment on the possible existence of a density-wave ground state when a more realistic interaction that includes electron correlations is considered.

We have studied the electronic and magnetic properties of Fe–Pt, Co–Pt and Ni–Pt alloy systems in ordered and disordered phases. The influence of various exchange–correlation functionals on the values of equilibrium lattice parameters and magnetic moments in ordered Fe–Pt, Co–Pt and Ni–Pt alloys have been studied using the linearized muffin-tin orbital method. The electronic structure calculations for the disordered alloys have been carried out using an augmented space recursion technique in the framework of the tight binding linearized muffin-tin orbital method. The effect of short-range order has also been studied in the disordered phase of these systems. The results show good agreement with available experimental values.

Structural, mechanical and elastic behaviour, density of states, electronic charge density and band structure of V B₂ and ZrB₂ are studied by the ab initio density functional method. The calculations carried out using the gradient-corrected approximation of the density functional theory are in excellent agreement with the experimental results where available. The five independent elastic constants have been calculated for the first time for V B₂ and ZrB₂. The elastic behaviour of these compounds is compared with the measured values for ZrB₂ and TiB₂. The origin of the pseudogap in the total density of states, a common feature of the compounds, is interactions between boron p states. The analyses of calculated density of states and band structure do not indicate superconductivity in the compounds.

Scanning electron microscopy, Raman scattering, UV–VIS absorption spectroscopy and low temperature photoluminescence (PL) were used to examine small particles produced by the chemical reaction between Pb(NO₃)₂ and KI in different liquid media: water, methanol, ethanol and acetonitrile. By stoichiometric changes in the synthesis reaction, platelets of PbI₂ and rods probably of KPbI₃ are produced. Regardless of shape and size, these particles exhibit almost the same PL, which consists of two intense bands centred around the 2.5 (E band) and 2.0 eV (G band), in turn similar to that of a crystalline slide or a micrometric powder, both prepared from a PbI₂ single-crystal grown from the melt. Crystalline PbI₂ platelets exhibit an E band with two components, at 2.49 (E_F band) and 2.47 eV (E_T band), originating in the recombination of the free and trapped excitons produced by inter-band irradiation. A close relation between the enhancement of the E_T and G band reveals that they are linked to the surface defects. For the rod-like particles, the PL spectrum is somewhat similar to that of a Pb²⁺ ion introduced into an alkaline halide lattice, which as for any ns² ion displays two emission bands, A_T and A_X, whose correspondents are E and G bands.

A triple of quantum numbers of conventional zone theory, namely the quasi-momentum, described in terms of Bloch functions, the lattice coordinate, described in terms of Wannier functions, and the associated energy band index, are modified to describe the space of states of a charged particle in magnetic field, in a translation covariant manner. A quartet of Bloch functions that belong to an invariant subspace, and define an irreducible representation of an anticommutative subgroup of the magnetic translation group, lead to Wannier function sets at a single Landau level that satisfy the exponential localization criterion. The degrees of freedom within the invariant subspace of this physically irreducible representation match those of spin 1/2. With a view to possible experimental studies, it is shown how a set of four regular arrays of connected wires or a similar quantum device may be used to access the pseudo-spin degree of freedom described, by observation of the effects of a controlled doubly periodical electric potential applied to a 2DEG in a strong and commensurate magnetic field.

We consider the effective mass model of spinless electrons in single-wall carbon nanotubes that is equivalent to the Dirac equation for massless fermions. Within this framework we derive all possible energy independent hard wall boundary conditions that are applicable to metallic tubes. The boundary conditions are classified in terms of their symmetry properties and we demonstrate that the use of different boundary conditions will result in varying degrees of valley degeneracy breaking of the single-particle energy spectrum.

The crystal and magnetic structures of the manganites Pr_1−xSr_xMnO₃ (x = 0.5, 0.56) have been studied by means of powder neutron diffraction at high external pressures up to 4.8 GPa. At ambient pressure both Pr_0.44Sr_0.56MnO₃ and Pr_0.5Sr_0.5MnO₃ have a tetragonal structure (space group I4/mcm). At  K in Pr_0.44Sr_0.56MnO₃ the onset of A-type antiferromagnetic (AFM) state occurs, which is accompanied by a structural phase transformation to the orthorhombic structure (space group Fmmm). Pr_0.5Sr_0.5MnO₃ at 175 K<T< T_C = 265 K exhibits an intermediate tetragonal ferromagnetic (FM) phase followed by the onset of the orthorhombic A-type AFM phase at  K. Under high pressure, in Pr_0.44Sr_0.56MnO₃ a tetragonal C-type AFM phase ( K) appears and the phase separated state is formed, consisting of its mixture with the initial orthorhombic A-type AFM phase ( K). In Pr_0.5Sr_0.5MnO₃ the application of high pressure leads to a noticeable increase of the FM–A-type AFM transition temperature from up to 230 K and formation of a phase separated state below 150 K, consisting of a mixture of orthorhombic A-type AFM phase and tetragonal phase without long range magnetic order. The stability of different magnetic states of Pr_0.44Sr_0.56MnO₃ and Pr_0.5Sr_0.5MnO₃ under high pressure is discussed.

The susceptibility and thermal expansion of Yb_1−xR_xInCu₄ (; R =  Y, La and Ce) have been measured at ambient and high pressures. These compounds undergo a first-order valence transition from an intermediate-valence state to a nearly trivalent state with increasing temperature. The valence change at the transition is for YbInCu₄. It decreases on both alloying and applying pressure for all the compositions studied. From the pressure effect on the susceptibility of YbInCu₄, the Grüneisen parameter for the Kondo energy Ω_K is determined to be −34.5 and −20 for the low-temperature and the high-temperature state, respectively. The Γ coefficient characterizing the hybridization strength between the 4f and conduction electrons for the intermediate-valence state is nearly five times larger than that for the high-temperature local-moment state. The obtained results suggest that the change of the 4f–conduction band electron hybridization rather than the volume dependence of the Kondo interaction is responsible for a large modification of the physical properties of the YbInCu₄-based compounds at the valence transition.

We consider small symmetric clusters of magnetic atoms (spins) with anisotropic exchange interaction between the atoms in a magnetic field at zero temperature. The inclusion of the anisotropy leads to a wealth of different phases as a function of the applied magnetic field. These are not phases in the thermodynamic sense with critical properties but rather physical structures with different arrangements of the spins and hence different symmetries. We study the spatial symmetry of these phases, for the classical and quantum cases. Results are presented mainly for three frustrated systems, the triangle, the tetrahedron and the five-atom ring, which have many interesting features. In the classical limit we obtain phase diagrams in which some of the phase changes occur because of energy crossings and others due to energy bifurcations, corresponding to 'first-' and 'second-order' changes. In the quantum case we show how the symmetries of the states are related to the corresponding classical symmetries.

Monocrystalline CeCu₂ has been studied by muon-spin rotation and relaxation (μSR) spectroscopy, using transverse- and longitudinal-field techniques between 5 and 300 K for the paramagnetic state, and in zero field (ZF) for the ordered antiferromagnetic (AF) phase,  K. As in the isostructural orthorhombic compounds GdCu₂ and PrCu₂, in CeCu₂ the μ⁺ is found at a 4e site. Bulk magnetic susceptibility and local susceptibility (extracted from the μ⁺ Knight shift) are found to differ, most probably because of a change in crystalline electric field splitting of the Ce³⁺ ions next to the muon. The relaxation of the μ⁺-spin polarization shows heavily anisotropic magnetic field fluctuations, explained by a dominant moment fluctuation amplitude along the crystallographic b axis. Below T_N, ZF measurements reveal a spontaneous μ⁺ precession frequency. The corresponding static magnetic field pattern is consistent with the known AF structure; fields at the μ⁺ and Ce moments are parallel to the ± c axis. The temperature dependence of the μSR frequency is fitted by a two T-regime model, reflecting critical behaviour near T_N and magnon excitation at low T. The oriented Ce moment values extracted from neutron diffraction measurements are not consistent with the μSR results. Part of the discrepancy could possibly be explained by a change of the contact-hyperfine coupling parameter A₀ at low temperature.

The anomalous temperature dependence of the magnetostriction in single-crystal DyFe₂ has been an unsolved problem in magnetism for some 27 years. In this paper, it is shown that an explanation can be provided, within the constraints of the Callen and Callen model, provided changes in the second-, fourth- and sixth-order crystal field energies of the rare earth ion are all taken into account. In particular, it is shown that the peculiar temperature dependence of the magnetostriction in DyFe₂ is due to competition between n = 2, 4 and 6 crystal field distortions, whose temperature dependence can be accurately modelled using the Callen and Callen model.

Weak field magnetoresistance was explored in GaAs/AlGaAs superlattices with different strengths of disorder produced either by the random variation of the well thicknesses or by the interface roughness. Depending on the disorder strength, three different regimes of the quantum transport were distinguished: the regimes of weak localization identified as the regimes of propagative and diffusive Fermi surfaces and the strongly localized insulating regime. Our results imply different dephasing mechanisms in the weak and strong localization limits. The vertical coupling energies in the superlattices determined with the random variation of the well thicknesses revealed a significant decrease with increasing disorder strength.

This paper reports results on the crystallographic study of heterophase structures in relaxor–ferroelectric (1− x)Pb (Zn_1/3Nb_2/3)O₃–xPbTiO₃ single crystals with 0.06<x<0.11. Possible variants of the elastic matching of three ferroelectric polydomain or twinned phases are considered with due regard for the relief of internal stresses at various volume fractions of these phases near the morphotropic phase boundary. It is found that two-phase regions, 'rhombohedral–monoclinic' and 'monoclinic–tetragonal', can be elastically matched along so-called zero-net-strain planes, parallel to the (100) or (010) domain (twin) walls in the intermediate monoclinic phase, where the Miller indices (hkl) are written in terms of the perovskite axes of the cubic unit cell. Calculated volume fractions of the three phases coexisting at 0.06<x<0.11 agree well with recent experimental results obtained by Bertram et al (2003 J. Cryst. Growth253 212).

Samples of Cr³⁺ doped porous anodic alumina (PAA:Cr) with different crystal structures were prepared and very strong photoluminescence from a PAA:Cr template calcined at temperature higher than 900 °C was obtained. The crystallization of the PAA:Cr and its influence on the photoluminescence were studied. This kind of PAA:Cr with the template property and strong luminescence may be beneficial in the preparation of composite luminescent nanomaterial.