Table of contents

First-principles computational methods are employed to investigate the structural, vibrational, optical and electronic properties of the self-interstitial aggregate, I₄ in silicon. We find the defect to be electrically active and identify it with the B3 EPR centre. We also show that its properties are consistent with DLTS and optical spectra observed following implantation of silicon.

The interaction of cavities with mobile interstitial clusters is believed to be responsible for the saturation of swelling in irradiated materials. We show that the saturation limit depends on the microstructure of the material, and that the origin of the effect is associated with the violation of the low-density approximation in the case of scattering of one-dimensionally moving interstitial clusters by grain boundaries. For large grains, swelling is enhanced near the boundaries, while for small grains swelling is maximum in the grain interior. The maximum size of cavities corresponding to the saturation of swelling is found to be many times the mean-field estimate.

We confirmed bulk-superconductivity of a ferromagnet UGe₂ by the specific heat measurement, together with the measurements of the electrical resistivity and ac susceptibility, in a pressure range from p = 1.0 to 1.5 GPa, where the Curie temperature T_C( = 22-36 K) is still high, but another characteristic temperature T* is close to zero. In this pressure range, the heavy fermion state is found to be formed at low temperatures.

We have carefully reinvestigated the superconducting heavy-electron system CeCu₂Si₂ by preparing very homogeneous polycrystalline samples at many different compositions in the homogeneity range by levitation melting and discovered in the Cu-deficient region a weakly ferromagnetic phase, presumably due to heavy quasiparticles of the Kondo compound. We, in addition, confirmed that the superconducting phase emerges at the fading end of the novel ferromagnetic phase in a very limited region of the phase diagram, which has revealed some salient superconducting properties.

Multidomain samples of ferroics (ferroelectrics, ferroelastics, and related materials) with fixed geometrical distribution of domains can offer new macroscopic properties required for particular applications. Two extreme cases of such applications are defined. In domain-geometry-engineered samples of ferroic crystals, the spatial distribution of domains and thus the spatial distribution of tensorial properties is tuned to correspond to the k-vectors of applied electric, optical or acoustic fields. For a given wavelength, the size, geometry, and distribution of domains give rise to a qualitatively new kind of response specified by the symmetry of the multidomain system. In domain-average-engineered samples of ferroic crystals, the specimen is subdivided into a very large number of domains, representing µ domain states where µ is smaller than the theoretically allowed maximum number, and forming a regular or irregular pattern. Its response to external fields is roughly described by tensorial properties averaged over all of the domain states involved. The effective symmetry of the domain-average-engineered system is given by a point group H and we show how it can be determined. As an example, all groups H are specified for domain-average-engineered samples which can arise in a material undergoing the phase transition with symmetry change from mm to 3m.

Water molecule (H₂O) adsorption on Si_8-10 clusters has been studied using the full-potential linear-muffin-tin-orbital method. The calculations show that water molecules can be adsorbed on some sites of the cluster surfaces. Their adsorption energies are situated in the ranges 0.01-0.49 eV for Si₈, 0.14-0.24 eV for Si₉, and 0.15-0.42 eV for Si₁₀. The results suggest that the clusters are relatively unreactive with water. After an H₂O molecule adsorbs on a site, the structures of the silicon cluster and the water molecule change slightly.

Pulsed laser deposition of La_0.5Sr_0.5MnO₃ (LSMO) thin films on quartz wafers at different deposition temperatures has been carried out. The microstructural, electrical and low-field magnetotransport properties of these films are evaluated as functions of the deposition temperature. The film crystallinity depends substantially on the deposition temperature. Significantly enhanced low-field magnetoresistance for the samples deposited from 570 °C to 600 °C, in which amorphous phase and polycrystalline phase coexist, is observed. The electrical and low-field magnetotransport properties of the thin films are explained by the two-channel model where insulating channels of variable-range-hopping conduction and metallic ones coexist.

The complex perovskite lead iron niobate, Pb(Fe_1/2Ta_1/2)O₃ (PFT), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. A straightforward unit cell of symmetry R3m was obtained for the 300 K structure, with the same symmetry, and a similar unit cell also obtained at the low temperature. In both cases, the iron and titanium ions were found to be disordered over the perovskite B-sites. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic arrangement. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFT are discussed by comparison with the related system of Pb(Fe_1/2Nb_1/2)O₃.

We report a first-principles study of high-pressure phases of Ge finding that a Cmca phase is stable in the pressure range 91-155 GPa, while at higher pressures the hcp phase has lower enthalpy. The structural parameters of the Cmca phase are similar to those of other Cmca phases found in Si (Si-VI) and Cs (Cs-V). Our results challenge the experimental report of a transition from the simple hexagonal phase to a double hexagonal close-packed phase in this material.

The experimentally observed anomalies in temperature dependences of the Hall coefficients and the electrical resistivities for hot-pressed samples of Ge_1-xAg_x/2Bi_x/2Te with 0.095<x<0.333, due to the ferroelectric lattice instabilities in the phase transition region, are discussed.

The Landau-Lifschitz type criterion for these phenomena shows that the phase transition is of second order. According to the soft-mode conception the main part of the anomalous increase in the resistivity is due to the soft-TO phonon scattering and the curvature caused by the soft-TO phonon near zero wavevector is accelerated with increasing AgBiTe₂ content.

Computer simulation studies of the F^--ion motion in pure and KF-doped α-PbF₂ are described. The aliovalent doping introduces vacancies into the crystal lattice which promote a significant degree of ionic motion, as witnessed by nuclear magnetic resonance and conductivity measurements. The simulations, using polarizable ionic potentials that have already been shown to reproduce the properties of the superionic conducting β-phase of PbF₂, give results which are consistent with these experimental data. A detailed site-hopping analysis is used to clarify the mechanism of the vacancy-promoted ionic motion, which differs markedly from the mechanism responsible for the facile ion diffusion in the β-phase.

Results of first-principles band-structure calculations for the ternary alkaline-earth silicon nitrides M₂Si₅N₈ (M = Ca and Sr) are presented. In the structures of M₂Si₅N₈ (M = Ca, Sr and Ba), the N atoms show connections to two (N^[2]) and three (N^[3]) neighbouring silicon tetrahedral centres. Calculations show that the local electronic structure is strongly dependent on the local chemical bonding. The valence band is dominated by N 2p hybridized with the s, p states of the alkaline-earth-metal and silicon atoms. The upper part of the valence band is dominated by the 2p states of N^[2] atoms, while the N^[3] 2p states lie about 2 eV below the Fermi level. The bottom of the conduction band consists of the N 3s characters hybridized with s orbitals of the alkaline-earth metals, while the s character of Si atoms is higher in energy. Sr₂Si₅N₈ is a semiconductor with a direct energy gap at Γ, while Ca₂Si₅N₈ is an indirect semiconductor. Optical diffuse reflectance spectra show an energy gap of 4.9 eV for Ca₂Si₅N₈, 4.5 eV for Sr₂Si₅N₈, as well as 4.1 eV for Ba₂Si₅N₈, in fair agreement with the calculated values.

Electrical transport measurements have been performed on doped and undoped TiCoSb half-Heusler phases. The semiconducting properties are found to be more robust than those reported for MNiSn (M = Ti, Zr, Hf ). Undoped TiCoSb phases exhibit large n-type Seebeck coefficients and high resistivities that reach -500 µV K^-1 at 300 K and ~1500 Ω cm at 4.2 K, respectively. A tendency towards carrier localization is seen in several disordered phases. The effects due to n-type and p-type dopants are readily manifested in the thermopower, from which moderately heavy electron and hole band masses are inferred. The unusual properties measured are consistent with the prediction of a wide bandgap for the TiCoSb phase. A resistivity minimum is observed at 500-600 K for undoped and V-doped TiCoSb. Consequently, the semiconducting gap has not been determined.

Phase transitions of a four-component lattice gas or spin-3/2 Blume-Emery-Griffiths model, with a single-ion uniaxial anisotropy and nearest-neighbour pair interactions, both bilinear and biquadratic, are investigated for two-dimensional lattices using an approximate renormalization-group approach of the Migdal-Kadanoff type. The set of fixed points and flows provide the characteristic phase diagrams, in the case of repulsive biquadratic interaction, featuring four ordered phases including high-entropy ferrimagnetic and staggered quadrupolar phases. Successive phase transitions and multicritical points are also found.

In this paper, we study the one-dimensional long-range Ising model with a spin-spin interaction that decays as 1/r^1+σ using the mean-field renormalization group. The critical coupling K_c and critical exponents y_t and y_h are computed using cluster sizes of up to sixteen spins. We then apply the alternating-alpha VBS (Vanden Broeck-Schwartz) transformation to accelerate the convergence of results for different cluster sizes.

Orbital ordering in V₂O₃ appears due to the orbital degeneracy of the doubly degenerate ground state of the V³⁺ ions and causes an antiferromagnetic phase transition in this compound. Assuming a crystal field of symmetry lower than cubic and spin-orbit coupling, the magnetic properties of this compound have been studied. Correlated effective-field theory is used to study the magnetic properties of the insulating phase. The magnetic structure observed at low temperature is also incorporated in the calculation.

We study a three-dimensional anisotropic quantum antiferromagnetic Heisenberg model at finite temperature in which the interplanar coupling can be varied gradually. With the magnon interactions considered, we have calculated the spin-wave excitation and sublattice magnetization. We find that when the interplanar coupling increases, the Néel ordered state is stabilized gradually. Moreover, we find power laws at low temperatures: a T⁴-law for the spin-wave excitation, and a T²-law for the sublattice magnetization, which is consistent with the nuclear magnetic resonance experiments on antiferromagnets. The interplanar coupling dependences of the finite-temperature spin-wave excitation and magnetization are also given.

The Curie phase line T_C(x) of the diamagnetically diluted ferromagnets Eu_xSr_1-xS and the Néel phase line T_N(x) of the diamagnetically diluted antiferromagnets Eu_xSr_1-xTe are analysed empirically on the basis of a mean-field approximation. Experimental data for the transition temperatures are collected from neutron scattering, magnetic specific heat and susceptibility measurements. Neither phase line, T_C(x) or T_N(x), is proportional to the concentration x of the magnetic atoms. It is shown that the deviations from linearity correlate with the fourth-order interaction sum in the paramagnetic phase. We therefore also ascribe the nonlinear x dependence of both phase lines to fourth-order interactions. Furthermore, our zero field neutron scattering measurements show that even for the strongly diluted Eu_xSr_1-xS samples the order parameters exhibit a T² Bloch law at low temperatures instead of a T^3/2 law. The T² law is a characteristic signature of fourth-order interactions in 3D materials with half-integral spin. It is also observed that the critical exponent β of the order parameter changes from β = 0.5 for x = 1 towards β = 1 at about x⩽2/3. This confirms that a new universality class is reached for a random distribution of magnetic moments. However, all phase transitions are first order and the critical power law applies only for the continuous part of the rise in the order parameter.

Magnetic and transport properties of ultrafine La_2/3Ca_1/3MnO₃ (LCMO) powders synthesized by mechanical alloying with a grain size of about 18 nm have been investigated. It is found that the powder sample is superparamagnetic above 95 K. The blocking temperature (T_B) obtained from the characteristic peak in the zero-field-cooled magnetization decreases with the measuring applied field, and can be well fitted by T_B = aln (µ₀H)-b. The spontaneous magnetization of the powder sample is only 38% of that in the corresponding bulk LCMO. Unlike conventional bulk LCMO, the powder compact is insulating from 5 K to 300 K. The low-temperature magnetoresistance (MR), defined as MR = ΔR/R(0) (R(0) being the zero-field resistance), is approximately 100% under a field of 6 T, much higher than that of the conventional bulk LCMO (50-60%). When the temperature is above T_B, the low-field MR associated with the spin-polarized tunnelling between grains cannot be observed; however, the high-field MR increases linearly with the applied field. The small grain size and the surface layer in which many oxygen vacancies and defects induced by high-energy milling exist are suggested to be responsible for the present observations.

The effect of Cooper pairs on Zn impurity centres at copper sites of the high-temperature superconductor Nd_1.85Ce_0.15CuO₄ has been revealed by ⁶⁷Cu(⁶⁷Zn) emission Mössbauer spectroscopy.

The influence of Pb excess on the properties of lead lanthanum titanate (PLT) ferroelectric thin films on Pt and LaNiO₃ electrodes is studied in this paper. Electron probe analysis results show that there is still quite an amount of excess Pb in PLT films after annealing at 550 °C for one hour. The distribution of excess Pb in the films is investigated by an Auger electron spectroscopy (AES) depth profile. It is found that the bottom electrodes have significant effects on both the content and the distribution of excess Pb in the PLT films. Meanwhile, the bottom electrodes have great effects on the size of the grains and also on the distribution of the excess Pb. The excess Pb and its appearance at grain boundaries and accumulation at the interface between the film and bottom electrodes may act as pinning centres and have a pinning effect on domains, which can be observed by abnormal P-E hysteresis loops and abnormal C-V curves. The excess Pb content in the films and Pb accumulation at the interface is high in the PLT films deposited on Pt/Ti/SiO₂/Si; their pinning is severe. This is one of the primary reasons why the films have a high fatigue rate. While LaNiO₃ has absorbed most of the excess Pb in PLT films, the content of excess Pb in the films deposited on LaNiO₃/SiO₂/Si is very low: the pinning effect is hardly observed. This is one of the primary reasons that the films have good fatigue properties.

Elastic properties of YBa₂Cu₃O_7-δ single crystals and thin epitaxial films were measured at room temperature by means of Brillouin scattering. The elastic stiffnesses, which reflect a pseudo-tetragonal symmetry of the elastic properties, were determined for both kinds of samples. For the single crystals, only small differences from the previously published data were found. For the thin films, a significant hardening of the shear stiffness C₅₅ (15%) was observed, in contrast with the softening usually obtained for many compounds.

Helium-dislocation interaction during the isochronal annealing of high energy He-implanted aluminium is studied using positron annihilation techniques. While helium is strongly trapped by the dislocations, the binding appears not to be strong enough to delay the annealing stages significantly. The nucleation of helium bubbles is delayed but the helium atoms do not hold the dislocations stable against annealing. The estimation of the helium bubble parameters was carried out taking into consideration the expansion of the lattice during the annealing and the bubble pressure is higher than that at room temperature, as expected. A polynomial relation is obtained between the helium melting temperature and the helium atom density. The bubbles are highly overpressurized and could serve as ideal systems for exploring the Simon-Glatzel transition expected at low temperatures.

Distinct species U(III), U(IV), U(V) and U(VI) have been identified when U(VI) or U(IV) species are doped into the cubic elpasolite lattice. The band intensities, and derived vibrational frequencies, in the luminescence and absorption spectra of the uranyl ion situated in this lattice are compared with those of Cs₂UO₂Cl₄. In particular, the intensity enhancement of the B_2g→A_1g(D_4h) hypersensitive transition shows that the uranyl ion occupies a majority site of C_2v or lower symmetry.

Absorption and luminescence measurements on Tm³⁺ and Ho³⁺ ions in LiYF₄:Tm³⁺, LiYF₄:Ho³⁺, and LiYF₄:Tm³⁺, Ho³⁺ crystals were carried out. The data collected were used to determine the Ho³⁺ absorption coefficient integral for ⁵I₈→{⁵I₇} transition, and the integral for the overlap between the normalized Tm³⁺ luminescence spectrum due to the ³F₄→³H₆ transition and the Ho³⁺ absorption band due to ⁵I₈→⁵I₇ transition. The relevant critical transfer distance (R₀) that gives a measure of the Tm³⁺-Ho³⁺ coupling was determined by considering a Förster-type interaction between the ions. It was found to be 22.5 Å at 300 K with the overlap integral 6.8×10^-19 cm⁵, which is 4.41 times higher at 77 K.

A time-dependent model is formulated for the chemical reaction between sodium colloids and gas bubbles, which are brought into contact with each other by the growing voids. It is shown that in this exothermic reaction, heat is released much faster than it is dissipated by conduction to the surrounding salt. This results in a large and sudden temperature and pressure increase in the void, which is analysed in detail. The effect on the mechanical stability of NaCl under irradiation is discussed.