Table of contents

Nanoparticles of barium hexaferrite, with an average size of 12 nm, were prepared by a hydrothermal route at relatively low temperatures (140–180 °C). The effects of reaction temperature and time on the particle size and magnetic properties were discussed. The nanoparticles show a soft magnetic feature with a saturation magnetization of 1.1 emu g⁻¹ and coercivity of 221.0 Oe, rather than the hard magnetic characteristic that the corresponding bulk material exhibits. Annealing treatment in air at 800 °C led to an order-of-magnitude increase of the saturation magnetization (67.3 emu g⁻¹) and coercive force (4511 Oe). It is suggested that the oxygen vacancies should be responsible for the soft magnetic characteristic that appeared for the as-prepared barium hexaferrite nanoparticles.

Careful neutron diffraction measurements on deuterated low density amorphous (LDA) ice confirm that at 120 K it can be considered a fully 'annealed' structure, as no significant changes are observed in the amorphous spectra until crystallization occurred over time at 130 K. On this basis, the measurement of structural differences between the hydrogenated and deuterated forms of LDA ice at 120 K, have been carried out using 98 keV electromagnetic radiation diffraction techniques. The maximum observed isotope effect in LDA ice is ∼ 3.4% at 40 K when compared to the magnitude of the first peak in the electronic structure factor at Q = 1.70 Å⁻¹. This compares to a maximum effect of ∼ 1.6% previously measured in liquid water at room temperature (Tomberli et al 2000 J. Phys.: Condens. Matter.12 2597). The isotope effect is shown to be similar to a temperature shift in the structure of light LDA ice. However, the existence of a first sharp diffraction peak at Q = 1.0 Å⁻¹ in the isotopic difference function is not reproduced in the temperature difference and suggests that additional longer-range correlations are present in the more ordered deuterated form.

We have studied dielectric and magnetic losses in granular structures constituted by ferromagnetic nanoparticles (Co, Fe, B) in an insulating amorphous a-SiO₂ matrix at microwave frequencies, in relation to metal concentration, substrate temperatures and gas content, in the plasma atmosphere in sputtering and annealing. The magnetic losses are due to fast spin relaxation of nanoparticles, which becomes more pronounced with decreasing metal content and occur via simultaneous changes in the granule spin direction and spin polarization of electrons on exchange-split localized states in the matrix (spin-polarized relaxation mechanism). The difference between the experimental values of the imaginary parts of magnetic permeability for granular structures prepared in Ar and Ar + O₂ atmospheres is determined by different electron structures of argon and oxygen impurities in the matrix. To account for large dielectric losses in granular structures, we have developed a model of cluster electron states (CESs). Cluster states are formed by s-electrons of nanoparticles and by electrons in localized states in intergranule regions of the matrix. The observed sharp increase in dielectric losses in the percolation region, the different values of losses for structures sputtered in the Ar and Ar + O₂ atmospheres and reduction of dielectric losses upon annealing, are caused by electric dipole polarization of electrons in CESs.

The Si-B3 electron spin resonance (ESR) signal from agglomerates of self-interstitials was detected for the first time in hydrogen-doped float-zone-grown silicon samples subjected to annealing after electron irradiation. Previously this signal had been detected only in neutron- or proton-irradiated silicon samples. The absence of obscuring ESR peaks for the investigated samples at applied measurement conditions allowed an investigation of the hyperfine structure of the Si-B3 spectra. The analysis supports assignment of a tetra-interstitial defect as the origin of the signal.

Strain fields in truncated and un-truncated InAs quantum dots with the same height and base length have been compared numerically when the dots are vertically stacked in a GaAs matrix at various stacking periods. The compressive hydrostatic strain in truncated dots decreases slightly as compared with the un-truncated dots without regard to the stacking period studied. However, the reduction in tensile biaxial strain, compressive radial strain and tensile axial strain was salient in the truncated dot and the reduction increased with decreasing stacking period. From such changes in strain, changes in the band gap and related properties are anticipated.

The electronic structure of rare earth bismuthides—Gd₄Bi₃, Tb₄Bi₃ and R₅Bi₃ (R = Gd, Tb, Dy, Ho, Er)—has been investigated with use of x-ray and ultraviolet photoelectron spectroscopies and calculated with the tight-binding linear muffin-tin orbital method. The spectra simulated on the basis of ab initio results reproduce correctly the experimental ones. This enabled analysis of the character of the electronic states, their hybridization and influence on magnetic properties. The temperature dependence of the valence band photoemission of ferromagnetic Gd₄Bi₃ and Tb₄Bi₃ has been studied and compared to the results obtained with the spin-polarized, non-polarized and open core methods of calculation.

We have analysed the complex dielectric-function spectra ε(E) = ε₁ (E) + i ε₂ (E) of cubic (c-)ZnS in the full spectral range (E = 0–20 eV) using a classical harmonic oscillator and a simplified interband transition model. The experimental ε(E) spectra reveal the reststrahlen band, distinct critical-point structures and cation d-band excitations in the spectra. The critical points are assigned to specific points in the Brillouin zone with the aid of the band-structure calculation. They are E₀ doublet at ∼ 3.8 eV; E₁ at ∼ 6.4 eV; E₂ at ∼ 7.0 eV; E₂ + δ at ∼ 7.4 eV; E₀ ' at ∼ 7.9 eV and E₁ ' at ∼ 9.4 eV. Excellent agreement is also achieved between the modelled and experimental ε(E) spectra over the entire range of photon energies. The sum rules are used to extract more detailed information. The high-frequency and static dielectric constants of c-ZnS are determined to be ε_∞ = 5.1 and ε_s = 8.0, respectively. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient and normal-incidence reflectivity, of c-ZnS are also presented.

The complex band structure of an isolated polyethylene chain is calculated within density functional theory (DFT). A plane wave basis and ultrasoft pseudopotentials are used. The results are compared with those obtained via a local basis set. We obtain a gap between the highest occupied molecular orbital (HOMO) and the antibonding unoccupied molecular orbitals of 9.3 eV and a non-resonant tunnelling β parameter of 0.9 per monomer, in reasonable agreement with experiment and with results obtained via local basis. Polyethylene is a negative electron affinity material and the actual gap should be the energy of the HOMO with respect to the vacuum level (in DFT approximation only about 5.14 eV). The Bloch states at imaginary k are mainly free-electron-like parabolic bands which are missing in the local basis. We present also the complex bands of bulk polyethylene in order to estimate the effects of the chain–chain interactions on the complex band structure. The relevance of these results for the tunnelling conduction of n-alkane chains is discussed.

Inelastic neutron scattering experiments were performed on single crystals of the heavy-fermion compound CeIn₃ for temperatures below and above the Néel temperature, T_N. In the antiferromagnetically ordered phase, well-defined spin-wave excitations with a bandwidth of 2 meV are observed. The spin waves coexist with quasielastic (QE) Kondo-type spin fluctuations and broadened crystal-field (CF) excitations below T_N. Above T_N, only the QE and CF excitations persist, with a weak temperature dependence.

N.B. Owing to an error in the publishing process, figure 1 of this paper was identical to figure 2. This was corrected on 03/06/03.

We consider inelastic light scattering (a Raman process) in non-fully symmetrical channels in metals with disorder and doped semiconductors. Due to the screened Coulomb interaction between electrons moving in the random field of impurities, the electronic excitations with relatively large momenta contribute to the Raman spectra. The physics of this effect is related to the physics of the inelastic collisions of electrons that arises due to the same interaction. We show that the contribution of the finite-momentum excitations can be regarded as a manifestation of the weak localization of carriers due to the disorder.

The influence of electron correlations on spin-polarized transport is examined within a phenomenological approach based on the Landau theory of Fermi liquids. The inclusion of a spin-dependent interaction in the drift terms of the Boltzmann transport equation satisfied by the spin-dependent distribution function n_σ leads to an 'apparent' charge current that is different from the spin σ current associated with the quasiparticle momentum flow. This effect originates in the back-flow current of opposite spins and its magnitude is proportional to the Landau phenomenological parameter that describes the angular average of the opposite-spin interaction and with the relative drift velocity of the two spin species.

Results of electrical resistivity, magnetization and susceptibility measurements are reported for the pseudo-ternary alloys (Ce_1−xGd_x)Pt₂Si₂. The end-members of the alloy series are the non-magnetic Kondo lattice compound CePt₂Si₂ and the antiferromagnet GdPt₂Si₂. The resistivity results reveal a rapid evolution to Kondo behaviour beyond small Ce substitution in GdPt₂Si₂ (for 1 − x ≥ 0.2). Coherent Kondo scattering is observed for larger Ce substitution (1 − x ≥ 0.75). The low-temperature resistivity of CePt₂Si₂ in the coherent state is dominantly a T² Fermi-liquid behaviour. The magnetization results suggest metamagnetic behaviour at Gd-rich concentrations. From the susceptibility results, the Gd and Ce moments seem to contribute independently to the magnetism of the alloy samples.

EPR measurements have been made at room temperature on Tl₂ZnF₄ crystals doped with Gd³⁺ and co-doped with Gd³⁺ and Li⁺. For crystals doped only with Gd³⁺, a spectrum with tetragonal symmetry (A centre) is observed. For co-doped crystals new spectra with tetragonal (B centre) and monoclinic (C centre) symmetries are observed in place of the spectrum of the A centre. The A centre is identified as the substitutional Gd³⁺ ion at a Zn²⁺ site in six-fold coordination without any local charge compensation in its immediate neighbourhood. On the basis of spin Hamiltonian separation analysis, the separated parameter b_2a(1) for the C centre has a value close to the b₂⁰ parameter for the B centre. The B and C centres in co-doped crystals are ascribed to a Gd³⁺ ion substituted for a Tl⁺ site in nine-fold coordination, where the divalent excess positive charge on Gd³⁺ is compensated by a Li⁺ ion at the Zn²⁺ site along the c axis. For the C centre, another association with a Li⁺ ion is found at the nearest Zn²⁺ site along the [111] direction. The other separated parameter b_2a(2) for the C centre has an opposite sign and six times the magnitude of b₂⁰ for the Gd³⁺ centre with trigonal symmetry along the [111] direction in RbCaF₃. The large magnitude negative value may be due to the deviation of the ligands toward the Li⁺ ion at the nearest Zn²⁺ site.

A new crystal of 2-aminopyridine phosphate (NC₄H₄NH₂)·H₃PO₄ has been grown and its x-ray structure and physical properties were studied. At room temperature the crystals are monoclinic, space group C2/c. The flat 2-aminopyridine cations are hydrogen bonded to the anionic [PO₄ ] groups. The interesting feature of the crystal structure is the three-dimensional network of hydrogen bonds including, among others, two strong, symmetrical O · · · H, H · · · O interactions with disordered proton locations. Symmetrically related PO₄ anions linked through these protons form infinite (PO₄)_∞ chains along the crystal a-axis. The anomalies in the temperature dependence of the electric permittivity showed that the crystal undergoes ferroelectric phase transition at T_c = 103.5 K. The spontaneous polarization takes place along the crystal a-axis, being parallel to the chains of the hydrogen-bonded PO₄. The disordered protons, thermally activated at room temperature, can be frozen at their positions in the ferroelectric phase. The order–disorder continuous type of the transition has been evidenced on the basis of the temperature dependences of electric permittivity and spontaneous polarization measurements.

We report our first-principles studies on the linear and non-linear optical properties of ferroelectric polyvinylidene fluoride crystal. Calculated values of the refractive indices agree well with experiments. As regards the second-harmonic-generation coefficients, theoretical values are larger than experimental ones. Possible reasons for the discrepancy are discussed.

The optical properties of a one-dimensional polar semiconductor in a strong electric field are considered. This class of materials includes non-centrosymmetric III–V inorganic quantum wires but also polar conjugated polymers such as polymethineimine. The polar Franz–Keldysh effect is derived via an analytic expression for the complex dielectric constant including line broadening and linear field terms. Results for the high-field non-perturbative regime as well as the low-field expansion are presented.

Energy transfer from Ce³⁺ to Mn²⁺ ions in a single crystal of CaF₂ co-doped with cerium and manganese ions has been analysed. The spectroscopic data obtained clearly indicate that sensitized luminescence with cerium ions as sensitizers and manganese ions as activators takes place between Ce³⁺ – Mn²⁺ clusters formed in the crystalline matrix. The number of such clusters was estimated using a simple model in which both ions are treated as two-energy-level systems. Some insight into the possible nature of the Ce³⁺ – Mn²⁺ complexes as well as of the Ce³⁺ → Mn²⁺ energy transfer mechanism taking place inside them were gathered using Dexter's theory for energy transfer phenomena.

The effect of atomic order on the martensitic phase transition and magnetic properties of stoichiometric Ni₂MnGa has been investigated in a sample quenched from 1000 °C. Magnetization, resistivity and x-ray diffraction measurements indicate that the structural phase transition occurs at ∼ 103 K, substantially lower than the value reported for samples quenched from 800 °C and ordered in the Heusler L2₁ structure. A small reduction in the ferromagnetic moment was also observed, although the Curie temperature remained largely unaffected. The electronic Sommerfeld coefficient obtained from heat capacity measurements is enhanced but smaller than that observed for the 800 °C quenched sample. The results are consistent with band structure calculations and the electronic changes brought about by atomic disorder.