Table of contents

A simplified model for the surface-to-volume atomic ratio dependent cohesive energy of nanocrystals is obtained in this letter. On the basis of the model, we predict that the cohesive energy can not only decrease but also increase with the increasing surface-to-volume atomic ratio under different conditions. The model predictions are found to be in agreement with the experimental values of Mo and W nanoparticles. The conditions on the cohesive energy of an embedded nanocrystal increasing or decreasing with the increasing surface-to-volume atomic ratio are discussed. The relations between the cohesive energy of nanoparticles, nanowires and nanofilms, and the shape effect on nanoparticles are also discussed. Since the cohesive energy is regarded as being directly related to the nature of the thermal stability of nanocrystals, the relationship to the phenomenon of melting and superheating given by our cohesive energy model seems sensible.

We have investigated the C₆₀ monolayer on Al(110) with angle-dependent photoelectron spectroscopy. We find that orbital components have different angular distributions. Calculations of cross sections of the highest occupied molecular orbital components for free, oriented C₆₀ are found to describe the experimental data quite well. The observed band splitting is attributed to intramolecular electronic correlations, due to the different coupling of the orbital components to the substrate conduction band.

The atomic structure and physical properties (dynamic viscosity, electrical conductivity, and thermopower) of liquid Pb–Bi alloys have been investigated in a wide temperature range. Gradual and reversible changes of the physical properties during heating and cooling of the Pb–Bi liquid alloys have been observed. No drastic structural transformations or atomic rearrangement with temperature variation have been found.

Structural and dynamical investigations on concentrated aqueous solutions of aluminium chloride and bromide in liquid and glassy states have been performed by means of Raman and inelastic neutron scattering spectroscopy and by x-ray diffraction. Inelastic neutron scattering results for solutions of beryllium chloride in a glassy state are compared with the results for the aluminium cation solutions. Isotopic changes on going from H₂O to D₂O of the Raman spectra assigned to vibrations of water molecules in the cation hydration shells are discussed. Inelastic neutron scattering data complement and corroborate the Raman spectral study. Further structural information is obtained from x-ray diffraction experiments, and its interpretation is supported by calculations based on ad hoc molecular models. The existence of a liquid-type quasi-close packing is suggested for concentrated aqueous solutions of aluminium halides, in agreement with previous studies.

A density functional perturbation approximation (DFPT), which is based both on the fundamental-measure theory (FMT) to the hard-sphere repulsion and on the weighted-density approximations (WDAs) to the attractive contribution, has been proposed for studying the structural properties of model fluids with an attractive part of the potential. The advantage of the present theory is the simplicity of the calculation of the weight function due to the attractive contribution. It has been applied to predict the equilibrium particle density distributions and adsorption isotherms of Lennard-Jones fluids at interfaces. The theoretical results show that the present theory describes quite well the adsorption isotherms of a Lennard-Jones ethane in a graphite slit pore as well as the equilibrium particle density distributions of a Lennard-Jones fluid near a planar slit pore.

Electronic structures near the Fermi level of polymer/1T-Li_xTaS₂ layered nanocomposites have been studied by tunnelling spectroscopy. Polymer/1T-Li_xTaS₂ layered nanocomposites were synthesized by using the exfoliation–adsorption technique. Single crystals of 1T-TaS₂ were used as host materials. Poly(ethylene oxide) (PEO) and poly(ethylenimine) (PEI) with different molecular weights were adopted as guest intercalants. Powder x-ray diffraction patterns showed that all samples of the polymer/1T-Li_xTaS₂ layered nanocomposites contain organic polymer between all individual 1T-TaS₂ sheets. Although 1T-TaS₂ single crystal is well known to show quite unique temperature dependences of the resistivity due to the charge density wave (CDW), the resistivities of all polymer/1T-Li_xTaS₂ nanocomposites showed semiconductor-like temperature dependences. The tunnelling spectra of polymer/1T-Li_xTaS₂ nanocomposites revealed that the CDW gap disappears in the density of states near the Fermi level of polymer/1T-Li_xTaS₂ nanocomposites and their electronic structures show a metallic behaviour.

An irradiation with the third-harmonic wave of a Nd:yttrium–aluminium–garnet (YAG) laser with a 5 ns pulse width successfully transformed colloid–chemically synthesized FePt nanoparticles (NPs) from disordered face-centred cubic (fcc) into chemically ordered L 1₀ (face-centred tetragonal: fct) phase. Granular films and hexane dispersions of fcc-FePt NPs were irradiated by the pulsed laser beam. In the case of the granular film, a coercivity of 1 kOe was achieved after the laser irradiation; meanwhile, the dispersion sample exhibited superparamagnetism, although the crystalline structure was transformed into fct phase. X-ray diffractometry and transmission electron microscopy revealed that coalescence between NPs was fairly well suppressed in the case of the dispersion sample. The superparamagnetism observed in the dispersion sample was due to insignificant coalescence and the resulting small magnetic domain.

The affinity-induced positron state was detected in 'defect-free' intermetallic particles using angular correlation of annihilation radiation. It was found that approximately 98% of the positrons annihilate within nanosized and subnanosized intermetallic particles embedded in an fcc Fe–Ni–Al alloy. These particles were created by means of thermal ageing. The ultrafine particles had a structure of the Ni₃Al type with addition of Fe atoms. The nanoparticles were three-dimensional and had no open-volume defects at the interfaces, which could trap a positron. These observations suggest that this positron state promises to be a powerful tool for studies of mesoscopic systems in condensed matter.

First-principles total-energy calculations with WIEN2k using a procedure that finds the equilibrium states of hcp structures under pressure from minima of the Gibbs free energy have found a structural anomaly in hcp Zn and in hcp Cd under pressure. The calculated small but definite anomaly in the pressure dependences of the structural parameters of hcp Zn allows a reinterpretation of the data to show the anomaly. We find a similar but stronger anomaly in Cd than in Zn. The calculated anomaly in Zn contradicts a recent theoretical conclusion that found that the anomaly disappears at a large number of k-points in the Brillouin zone. The calculation also shows that the uncertainty in locating equilibrium found in another recent paper is absent here. Reasons are given for computational differences from previous work. Evaluation of the pressure dependence of various elastic quantities which are much more sensitive to the anomaly shows the anomalies in hcp Zn and hcp Cd exist over a considerable range of pressure; several abrupt changes in the electron distribution are thereby indicated in that pressure range.

Elementary magnon, libron and vibron excitations as well as combined two-libron excitations of solid α oxygen have been investigated by means of Raman scattering at several isobars in the pressure range up to 1.25 GPa. We deduced the band frequency, bandwidth and relative band intensity of all modes as a function of temperature and pressure. On the basis of these results we can exclude the possibility of all second-order phase transitions in the low temperature, low pressure range of oxygen stated in the literature. The disappearance of the sublattice magnetization σ could be estimated from the frequency of the higher energy magnon mode at the critical pressure which is in the vicinity of the phase transition from the antiferromagnetic ordered δ phase to the non-magnetic ε phase. The change of several spectroscopic features under increasing pressure clearly indicates that anharmonic contributions in the libron potential are altered.

The rearrangement of the Fermi surface of a homogeneous Fermi system upon approach to a second-order phase transition is studied at zero temperature. The analysis begins with an investigation of solutions of the equation (p) = μ, a condition that ordinarily has the Fermi momentum p_F as a single root. The emergence of a bifurcation point in this equation is found to trigger a qualitative alteration of the Landau state, well before the collapse of the collective degree of freedom that is responsible for the second-order transition. The competition between mechanisms that drive rearrangement of the Landau quasiparticle distribution is explored, taking into account the feedback of the rearrangement on the spectrum of critical fluctuations. It is demonstrated that the transformation of the Landau state to a new ground state may be viewed as a first-order phase transition.

The self-consistent harmonic approximation is extended in order to account for the existence of Klein factors in bosonized Hamiltonians. This is important for the study of finite systems where Klein factors cannot be ignored a priori. As a test we apply the method to interacting spinless fermions with modulated hopping. We calculate the finite-size corrections to the energy gap and the Drude weight and compare our results with the exact solution for special values of the model parameters.

Lloyd's formula is an elegant tool in multiple-scattering theory. It implicitly provides an analytical integration over energy and over all space and directly gives the number of states as a function of energy. The usual derivations of Lloyd's formula are involved and the range of their applicability is not obvious. It is the purpose of this paper to give a derivation which requires only elementary mathematical manipulations. The result is valid for potentials of general shape and for arbitrary complex energies.

γ-Si₃N₄ is a nitrogen-based ultra-hard ceramic with Si atoms occupying both tetrahedral and octahedral sites in a spinel structure. Soft-x-ray emission and absorption spectra of γ-Si₃N₄ are presented, which together provide an experimentally determined bandgap of 4.30 ± 0.25 eV. Information about localized partial density of states (LPDOS) of the non-equivalent Si sites is presented.

We have prepared a series of polycrystalline samples La_0.5Sr_0.5CoO_3−δ with and characterized their oxygen content, crystal structure, and magnetic properties. While the fully oxygenated samples are good ferromagnets, samples with larger δ values display increasingly broad magnetic transitions. The saturation magnetization at 5 K falls rapidly as δ increases. First-principles electronic structure calculations provide insights into the magnetic behaviour of the fully oxygenated compound, and the manner in which ferromagnetic ordering is affected by increasing oxygen non-stoichiometry.

We have measured the electrical resistivity of RFe_11.5Ta_0.5 and RFe_11.3W_0.7 (R = Tb, Dy, Ho, Er, and Lu) polycrystalline systems over a temperature range of 4–700 K. The overall behaviour of the resistivity is determined mainly by the transition-metal ions. However, some interesting features are related to rare-earth atoms. At low temperatures, the resistivity, ρ, increases with temperature as AT², where the coefficient A varies systematically with the total spin of rare-earth ions. We also find that ρ depends on the M element in our RFe_12−xM_x iron-rich compounds. Close to the Curie temperature, the resistivities of the alloys studied clearly show a change in the slope of ρ versus T. Saturation values of the high temperature magnetic resistivity follow the prediction by de Gennes and Friedel for spin disorder scattering.

The validity of a simple band description for explaining the unexpectedly large thermopower in Na_1+xCo₂O₄ was investigated. Non-spin-polarized and spin-polarized electronic structures of layered cobalt oxides Na_1+xCo₂O₄ are studied by means of first-principles density functional theory calculations. The fractional occupancy of Na is described via two approaches—the virtual crystal approximation and the rigid band model. We also found that the band structure of the valence band is very sensitive to the geometry of the CoO₆ 'octahedron', and cannot be described by a simple crystal field model. The Seebeck coefficients are calculated from the standard kinetic theory. The calculated thermopower demonstrates the importance of a high density of states close to the Fermi level. The calculated Seebeck coefficient based on the paramagnetic band structure is apparently in good agreement with experimental observation although it has been suggested that spin entropy may dominate the thermopower. However, the calculation failed to account for the observed negative in-plane Hall coefficient.

The thermoelectric power and electrical resistance of pure bismuth nanowires have been measured in longitudinal magnetic fields up to B = 20 T at temperatures between 4.2 and 26 K. At B = 0, the 200 nm samples exhibit large values of thermopower (about +110 μV K⁻¹ at 25 K), which are dominated by diffusion with no phonon drag being evident. Both the magnetoresistance and magnetothermopower show well-pronounced features generated by the diffuse surface scattering of hole carriers. In particular, the magnetothermopower offers the possibility of identifying the characteristic magnetic field, where the diameter of the hole Larmor orbit equals the wire diameter, as an extremum, in distinction from the magnetoresistance data, where the corresponding feature manifests as an inflection point. The frequencies of Shubnikov–de Haas oscillations were found to be consistent with the Fermi-surface parameters of bulk Bi. The contribution of holes to the charge transport in pure Bi nanowires is more significant than generally thought.

Under selective photo-excitation, the capacitance response of internal tunnelling coupling in quantum-dots-imbedded heterostructures is studied to clarify the electronic states and the number densities of electrons filling in the quantum dots (QDs). The random nature for both optical transitions and the filling in a QD assembly makes highly resolved capacitance peaks appear in the C–V characteristic after turning off the photo-excitation.

In this paper a detailed study of the La_1−xCa_xMnO₃ () phase diagram using powder x-ray diffraction and magnetization measurements is presented. The lack of well characterized samples has resulted in confusion as regards the underlying physics of this complicated system. As the present study reveals, two distinct families of samples exist. The first family consists of samples prepared in an air atmosphere (designated as AP; P(O₂) = 0.2 atm). These are ferromagnetic with their Curie temperatures increasing with x. The second family of samples consists of samples prepared in air and then post-annealed under a reducing nearly zero oxygen partial pressure (designated R). These samples show a canted antiferromagnetic structure for below T_N, while for an unconventional ferromagnetic insulated phase is present below T_c. Most probably, the difference between the two families of samples, R and AP, lies in their cation stoichiometries, the AP samples being stoichiometric and the AP samples being non-stoichiometric. The most important difference between AP and R samples is the anisotropy in the exchange interactions of the R samples. This leads us to suggest the possibility that a new orbital ordered phase is responsible for the ferromagnetic insulating regime in the La_1−xCa_xMnO₃ compounds.

The temperature dependences of the Raman spectrum and lattice parameters of polycrystalline MgB₂ have been investigated by means of Raman spectroscopy and x-ray diffraction. It is found that the lattice parameters show an approximately linear change with the temperature decrease, giving different thermal expansions along the a- and c-axes, which is caused by the comparatively weak metal–boron bonding in MgB₂. The grain size of MgB₂ determined by means of x-ray diffraction is around 45 nm for both [100] and [001] directions. There is no evidence for any structural transition while the temperature changes from 300 K down to 12 K. An anomalous Raman band at 603 cm⁻¹ is observed, which is consistent with the theoretical prediction for the E_2g in-plane boron stretching mode. The Raman frequency increases and the linewidth decreases as the temperature decreases. A possible origin of the temperature dependences of the Raman frequency and the linewidth is discussed. It is suggested that the grain size effect of MgB₂ on the nanometric scale will have a clear influence on the frequency and the linewidth of the Raman spectrum.

The La-2125 type La_2−xDy_xCa_2xBa₂Cu_4+2xO_z (; LDBO) compounds have been synthesized and studied as regards their structural and superconducting properties using room temperature neutron diffraction, high field dc magnetization, four-probe resistivity and iodometric double titration. The Rietveld analysis of the neutron diffraction data reveals tetragonal structure for all the samples, which crystallizes into La-123 type tetragonal structure in the P4/mmm space group. Iodometric double titrations were performed to determine the oxygen content values and calculate the mobile charge carrier (hole) density. The superconducting transition temperature (T_c) increases from for x = 0.1 to a maximum of 75 K for x = 0.5. The flux pinning force (F_p) and critical current density (J_c), calculated from the low temperature hysteresis loops, also increase with increasing dopant concentration. This paper presents studies on the structure and superconducting properties of all LDBO compounds in relation to the role of calcium in inducing superconductivity in the tetragonal non-superconducting oxide.

The magnetization process and the magnetic domains of the FeNi (100 nm)/Cu (2.5 nm)/FeNi (100 nm)/Cu (480 nm)/FeNi (100 nm)/Cu (2.5 nm)/FeNi (100 nm) structure were studied. This geometry consists of two FeNi/Cu/FeNi trilayers with a thick in the direction perpendicular to the plane of the sensitive element and narrow in the direction of the flowing current Cu electrode in the centre. Ferromagnet/conductor/ferromagnet is the typical geometry of magnetoimpedance thin-film-based sensitive elements used to detect small magnetic fields. Multilayered structures were prepared by rf-sputtering in a magnetic field of 100 Oe applied perpendicular to the Cu electrode in order to induce transverse magnetic anisotropy. The magnetic measurements and magnetic domain structure observations were made in magnetic fields applied one at a time parallel or perpendicular to the Cu electrode. Different magnetization processes with non-homogeneous rotations in the first case and dominant multiple nucleation and merging of domains in the second one were observed.

The magnetic ordering of the compound CeCoAl₄ (orthorhombic; Pmma space group) has been investigated by means of neutron diffraction from powder and single-crystal samples. Only the Ce moments order antiferromagnetically below T_N = 12.8(1) K, with a propagation vector . Their collinear magnetic moments point along the b axis of the magnetic unit cell, which is doubled along both the nuclear b and c axes. At 1.5 K the refined magnetic moment value is 1.29(3) μ_B/Ce atom.

A Cu:Hf sample with 1 wt% Hf as prepared by arc melting is characterized by TEM and microdiffraction analysis to contain HfC precipitates. HfC precipitates in a Cu matrix bind vacancies and divacancies strongly in the quenched Cu:Hf sample as deduced by time differential perturbed angular correlation (TDPAC) studies. Isochronal annealing studies using TDPAC and positron lifetime measurements indicate the stability of these vacancy complexes in the quenched sample for annealing treatments up to 1200 K, beyond which the de-trapping of the vacancies from HfC precipitates is observed to occur. This shows that HfC precipitates present in Cu inhibit the formation of voids by strongly binding quenched vacancies.

In a recent paper (Infeld and Senatorski 2003 J. Phys.: Condens. Matter15 5865) we confirmed Feynman's hypothesis on how circular vortices can be created from an oppositely polarized linear pair in a Bose–Einstein condensate. This was done by perturbing the original pair numerically, so that a circular vortex (or array of identical circular vortices) was created as a result of reconnection. These circular vortices were then checked against known theoretical relations binding velocities and radii. Agreement to a high degree of accuracy was found. Here in part II, we give examples of the creation of several different vortices from one linear pair. All are checked as above. We also confirm the limit of separation of the line vortices below which mutual attraction, followed by annihilation, prevents the Feynman metamorphosis. Other possible modes of behaviour are illustrated.

We analyse the slowing down of the structural relaxation dynamics of polymers in terms of the Adam and Gibbs theory. We consider a previously derived general relation between the configurational and the excess entropy, which was used to derive an analytical equation for the dependence of the structural relaxation time from the pressure and temperature. The model proved to successfully fit the relaxation dynamics of poly(methyl methacrylate), poly(propylene glycol) and poly(propylene glycol dimethylether), of different molecular weights, over a wide region of temperature and pressure values above the glass transition.

The finite length of polymer chains affects both the static and the relaxation properties of the density of the melt state. These have been investigated by molecular-dynamics simulations of a Lennard-Jones model with fixed bond length. Under isothermal–isobaric conditions the density increases with the molecular weight. A suitable Voronoi tessellation reveals the extra free volume around the chain ends and shows that it is strongly localized within the first end monomer. Simple arguments are given for interpreting the main changes of the monomer radial distribution function and the corresponding static structure factor when the chain length is increased. As to the relaxation aspects of the density, it is found that the structural relaxation time increases with the molecular weight, which is interpreted as a signature of the well-known corresponding increase of the glass transition temperature.

X-ray absorption near-edge structure (XANES) signals at the oxygen K edge in polycrystalline α-MoO₃ and amorphous a-MoO₃ thin film were analysed within the full-multiple-scattering (FMS) formalism. Significantly different XANES signals were found for non-equivalent oxygen atoms in low-symmetry layered-type α-MoO₃ structure. The obtained results are in agreement with the experimental data and allow us to interpret all XANES peaks for α-MoO₃. Besides, the FMS XANES simulations, performed for several fragments of α-MoO₃ structure, allowed us to explain the O K-edge XANES in amorphous a-MoO₃ thin film. We found that although the crystallographic structures of α-MoO₃ and a-MoO₃ are strongly different, a cluster consisting of six MoO₆ octahedra joined by vertices produces the main contribution to both XANES signals.

The feasibility of optical amplifying waveguides is investigated through the elaboration of silica films codoped with erbium ions and silicon nanograins. The waveguiding function is provided by the index contrast between the active and cladding layers whereas the amplification is achieved by an efficient interaction between silicon nanograins and erbium ions. Based on experimental results and theoretical studies, this paper aims at providing a detailed analysis of the optical gain expected in slab waveguides taking into account a lateral pumping scheme of the silicon nanograins, the energy transfer mechanism from silicon grains to erbium ions and finally the propagation equations within the active layer. The main parameters—such as erbium and silicon concentrations, device length and pumping power—have been examined.