Table of contents

The partition of hydrous species in radiation-damaged zircon was investigated using infrared spectra from a large range of samples with different degrees of radiation damage and a variety of geological conditions. The results of the present study showed uniformly a partition coefficient of K = 0.3. This indicates that OH is enriched in the damaged crystalline phase and not in the amorphous phase. The uniformity of the results for all samples indicates equilibrium rather than a frozen-in kinetic behaviour. It is possible that the enrichment of OH in the damaged crystalline phase of zircon indicates a catalytic effect of hydrogen during recrystallization events.

The diffusion of implanted Be in GaAs at 100 keV for doses of 1 × 10¹³ cm⁻² and 1 × 10¹⁴ cm⁻² has been investigated. The observed secondary ion mass spectrometry profiles, obtained for annealing temperatures of 700–900 °C and anneal times from 60 to 240 s, were simulated using different models of the kick-out mechanism and taking into account the 'plus one' approach for Ga self-interstitial generation after implantation as well as the local Ga self-interstitial sink phenomenon. The diffusion differential equations for Be and Ga mobile species with initial and boundary conditions were solved numerically for each model by using the explicit finite-difference method.

Measurements of the two-dimensional angular correlation of the electron–positron annihilation radiation have been done in the past to detect the momentum spin density and the Fermi surface. We point out that the momentum spin density and the Fermi surface of ferromagnetic metals can be revealed in great detail owing to the large cancellation of the electron–positron matrix elements which in paramagnetic multiatomic systems plague the interpretation of the experiments. We prove our conjecture by calculating the momentum spin density and the Fermi surface of the half metal CrO₂, which has received large attention due to its possible applications as a spintronics material.

Ice-rule breaking spin-flip in a pyrochlore spin ice compound Dy₂Ti₂O₇ has been studied in a field applied near the [112] direction, by means of angle-resolved magnetization measurements. We obtained strong evidence that the magnetization component perpendicular to the kagomé plane undergoes a first-order transition below ∼0.26 K. This transition is driven by a ferromagnetic ordering in the field-decoupled spins on the triangular lattice in Dy₂Ti₂O₇, as predicted theoretically.

We demonstrate field-induced formation of stripe domains in phase separation accompanying orientational ordering, taking a mixture of an isotropic liquid and a nematic liquid crystal as an example. We can classify spinodal decomposition, which is governed by coupling between conserved compositional and non-conserved orientational ordering, into two types: isotropic and anisotropic spinodal decomposition. In the former, phase separation occurs prior to orientational ordering. Thus, an isotropic phase-separated pattern is formed in the early stage even under an external field. In the latter, on the other hand, a homogeneous oriented phase is formed prior to phase separation under an external field. Then anisotropic domains grow by phase separation. We reveal that the kinetic pathway in the two-dimensional order parameter space is key to field-induced stripe formation. This principle may be commonly applied to a mixture, one of whose components has orientational ordering such as spin, dipolar or orbital ordering. We also confirm that the anisotropic pattern has a memory of orientation of the applied external field, which may be used for device applications.

A slab of material with a negative permeability can act as a super-lens for magnetic fields and generate images with a sub-wavelength resolution. We have constructed an effective medium using a metamaterial with negative permeability in the region of 24 MHz, and used this to form images in free space of radio frequency magnetic sources. Measurements of these images show that a resolution of approximately λ/64 has been achieved, consistent with both analytical and numerical predictions.

The unique potential of scanning tunnelling microscopy (STM) as a tool for determining the elementary steps of surface catalytic reactions at an atomic scale is highlighted using selected representative results obtained in studies of adsorption and reactions on the Rh(110) surface. The Rh(110) surface was chosen as a prototype of a flexible catalyst, due to its propensity to reconstruct in the presence of adsorbates. Both dissociative adsorption of simple molecules and oxidation reactions involving adsorbed oxygen layers are considered. It was demonstrated that a combined approach where STM was used in conjunction with other experimental techniques and ab initio calculations yields a thorough description of the underlying reaction mechanism.

In order to investigate magnetic phenomena in Nd and Sm layers separately, resonant x-ray magnetic scattering experiments have been performed to study Nd/Sm(001) superlattices with different relative layers thickness. The samples were grown using molecular beam epitaxy, and optimized to yield dhcp Sm growth and thus a coherent dhcp stacking across the Nd/Sm superlattices. The magnetic phases in Sm layers are very close to the ones evidenced in dhcp thick films. In contrast, the magnetism in Nd layers shows strong differences with the bulk case. In superlattices with a large Sm thickness (>8 nm), no magnetic scattering usually associated with Nd magnetic structure was detected. In superlattices with smaller Sm thickness (<4 nm), new Nd magnetic phases have been observed. A detailed analysis of the propagation of the magnetic structures in the cubic and hexagonal sublattices of both Sm and Nd is presented. Both Sm hexagonal and cubic magnetic phases propagate coherently through 3.7 nm thick Nd layers but remain confined in Sm layers when the Nd layers are 7.1 nm thick. In contrast, the critical Sm thickness allowing a coherent propagation of Nd magnetic order is different for the hexagonal and cubic sublattices above 5 K. Finally, we show that: (i) a spin-density wave and a 4f magnetic order with perpendicular polarization are exclusive on a given crystallographic site (either hexagonal or cubic); (ii) a 4f magnetic order on a crystallographic site does not perturb the establishment of a spin-density wave with a perpendicular polarization on the other site.

The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface of icosahedral Al–Pd–Mn leads to the formation of a several monolayers thick Au–Al alloy film. An AlAu film with 1:1 stoichiometry is formed, which shows no evidence of ordered structure, being either amorphous or polycrystalline. Annealing to 325 °C causes more Al to diffuse into the film, producing Al₂Au but still with no indication of structure. Experiments using 0.5 ML of pre-deposited In demonstrated a surfactant effect as the In 'floated' on the surface during growth and produced a reduction in film roughness. However, contrary to previous findings the film was still either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodic structure. Experiments were also conducted using smaller doses of Au to look for the formation of an epitaxial layer and, if formed, determine the registry with the substrate. However, no change in the Pd blocking curves for the surface could be seen, suggesting that the Au does not adsorb in well defined sites. This result is not surprising when considering that even for these low doses Al is drawn into the film, changing the composition and probably the structure of the topmost layers of the substrate, so that the potential adsorption sites on the clean surface may no longer exist.

Nanosized bismuth doped calcium sulfide (CaS:Bi) particles have been synthesized by a wet chemical co-precipitation method. The average size of the nanoparticles was found to be about 30 nm. The particles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis spectroscopy and fluorescence spectroscopy. The effect of particle size on the photoluminescence (PL) of CaS:Bi has been studied. The optimum dopant concentration was found to be 0.025 mol% of bismuth for maximum PL emission intensity. A comparative study between bulk CaS:Bi prepared by a reduction method and nanosized CaS:Bi made by the wet co-precipitation method has been carried out. Increase in band gap with decrease in particle size has been explained on the basis of the quantum size effect. The PL intensity of the nanoparticles was found to increase to almost twice that of the bulk particles. The effect of different dopant concentrations on emission intensity has also been studied.

We investigate the effect of annealing on the Ge nanocrystal formation in multilayered germanosilicate–oxide films grown on Si substrates by plasma enhanced chemical vapour deposition (PECVD). The multilayered samples were annealed at temperatures ranging from 750 to 900 °C for 5 min under nitrogen atmosphere. The onset of formation of Ge nanocrystals, at 750 °C, can be observed via high resolution TEM micrographs. The diameters of Ge nanocrystals were observed to be between 5 and 14 nm. As the annealing temperature is raised to 850 °C, a second layer of Ge nanocrystals forms next to the original precipitation band, positioning itself closer to the substrate SiO₂ interface. High resolution cross section TEM images, electron diffraction and electron energy-loss spectroscopy as well as energy-dispersive x-ray analysis (EDAX) data all indicate that Ge nanocrystals are present in each layer.

We study x-ray diffraction peak profiles from highly mismatched relaxed epitaxial films at momentum transfers exceeding the peak widths. Calculated profiles for misfit dislocations are compared with triple-crystal diffraction profiles from GaAs/Si(001) epitaxial films. We find that the longitudinal and transverse scans have a common q⁻⁴ asymptote but approach it differently, so that their profiles are qualitatively different in the experimentally available momentum range. The possible contribution from threading dislocations is estimated.

We considered the origin of the arc-shaped streaks connecting usual diffraction spots observed in an LEED pattern, for a Li system adsorbed on a Cu(001) surface at low coverage, Θ<1/2. We noted a condition that the natural distance between adsorbed atoms, b_nat, of less than —in particular, —is consistent with the formation of the arced streaks. Given this condition, adsorbed atoms fill the surface and form a structure, which is one of the 'ladder structures', at Θ = 3/5. Even for Θ<1/2, the atoms form the ladder structure locally. We could observe atomic pairs with second-neighbour distance having a c(2 × 2) structure unit in the ladder structure (where the distance is basically d = 2a), that were shrunk and tilted. These shrunk and tilted pairs produce straight streaks extending from the M(± 1/2, ± 1/2) points, with a tilted angle in the vicinity of M points. Considering the other arrangements of the second-neighbour pairs, which have a different inclination, a variety of straight streaks exists. The envelope function of these straight streaks is nothing but the arc shape observed in the experiment.

Using the density-matrix renormalization-group method we study the surface critical behaviour of the magnetization in Ising strips in the subcritical region. Our results support the prediction that the surface magnetization in the two phases along the pseudo-coexistence curve also behaves as for the ordinary transition below the wetting temperature for finite values of the surface field.

First-principles total-energy calculations were performed for the trigonal shear elastic constant (C₄₄) of body-centred cubic (bcc) V and Nb. A mechanical instability in C₄₄ is found for V at pressures of ∼2 Mbar which also shows a softening in Nb at pressures of ∼0.5 Mbar. We argue that the pressure-induced shear instability (softening) of V (Nb) is due to the intra-band nesting of the Fermi surface.

The crystal structure of 4-aminopyridinium tetrachlorobismuthate(III), [4-NH₂PyH ][BiCl₄ ], has been determined at 100 and 260 K by the x-ray diffraction method as monoclinic space group, P 2₁/c and C 2/c, respectively. Differential scanning calorimetry and dilatometry reveal one perfectly reversible discontinuous phase transition at 254/255 K (cooling/heating) with ΔS = 10.1 J mol⁻¹ K⁻¹. The ¹H NMR spin–lattice relaxation time (T₁) and second moment (M₂) measurements disclose a significant change in the motional state of the cations near the phase transition temperature. Infrared spectra of polycrystalline [4-NH₂PyH ][BiCl₄ ] have been studied in the temperature range 10–306 K. Substantial changes in the temperature evolution of frequencies of internal modes of the 4-aminopyridinium cations near 255 K are due to the change in the dynamics of cationic moieties. The experimental results indicate that the motion of the organic cations contributes mainly to the mechanism of an 'order–disorder' transition in [4-NH₂PyH ][BiCl₄ ].

Chemically sprayed indium-doped zinc oxide thin films (ZnO:In) were deposited on glass substrates starting from zinc pentanedionate and indium sulfate. The influence of both the dopant concentration in the starting solution and the substrate temperature on the transport, morphology, composition, linear and nonlinear optical (NLO) properties of the ZnO:In thin films were studied. The structure of all the ZnO:In thin films was polycrystalline, and variation in the preferential growth with the indium content in the solution was observed: from an initial (002) growth in films with low In content, switching to a predominance of (101) planes for intermediate dopant regime, and finally turning to a (100) growth for heavily doped films. The crystallite size was found to decrease with doping concentration and range from 36 to 23 nm. The film composition and the dopant concentration were determined by Rutherford backscattering spectrometry; these results showed that the films are almost stoichiometric ZnO. The optimum deposition conditions leading to conductive and transparent ZnO:In thin films were also found. In this way a resistivity of 4 × 10⁻³ Ω cm and an average transmittance in the visible spectra of 85%, with a (101) preferential growth, were obtained in optimized ZnO:In thin films.

The effects of different amounts of segregated Ga (substitutional) on an Al grain boundary have been investigated by using a first-principles pseudopotential method. The segregated Ga is found to draw charge from the surrounding Al due to the electronegativity difference between Ga and Al, leading to a charge density reduction between Ga and Al as well as along the Al grain boundary. Such an effect can be enhanced by increasing the Ga segregation amount. With further Ga segregated, in addition to the charge-drawing effect that occurs in the Al–Ga interface, a heterogeneous α-Ga-like phase can form in the grain boundary, which greatly alters the boundary structure. These effects are suggested to be responsible for Ga-induced Al intergranular embrittlement.

The structural relations between carbon polymorphs are described as reconstructive phase transitions with displacive mechanisms occurring via a common substructure. The polymorphs are shown to correspond to limit states resulting from critical fractional displacements and critical strains. The transition order-parameters are defined as periodic functions of the displacements.

The oxidation state of Fe in highly doped lithium niobate crystals (2 wt% Fe₂O₃) in the as-grown, reduced, and oxidized states is determined by the combination of differentiation and integration methods applied to their Fe K-edge XANES (x-ray absorption near edge spectroscopy) spectra. The obtained valences are confirmed further by absorption measurements in the IR/vis (infrared and visible) spectral range. It is shown that reduction and thermo-electric oxidization of as-grown Fe:LiNbO₃, respectively, leads to a noticeable lowering of the number of Fe³⁺ and to complete oxidization of all Fe²⁺ to Fe³⁺, respectively. It is found that Fe in highly iron-doped lithium niobate samples (0.5, 2, and 4 wt% Fe₂O₃) is incorporated onto the Li site comparing the experimental XANES spectra to the FEFF8 calculations. The difference of the XANES spectrum of the 4 wt% Fe₂O₃ doped sample as compared to those of the 0.5 and 2 wt% doped samples is explained by a co-phase formation in the former one. The results obtained can help to tailor such crystals for non-linear optical applications as well as for photorefraction.

In the R₃Ga₅SiO₁₄ compounds, the network R of rare earth cations forms well separated planes of corner sharing triangles topologically equivalent to a Kagomé lattice. Powder samples and single crystals with R = Nd and Pr were prepared and magnetostatic measurements were performed under magnetic field up to 10 T in the temperature range from 1.6 to 400 K. Analysis of the magnetic susceptibility at the high temperatures where only the quadrupolar term of the crystal electric field prevails suggests that the Nd and Pr magnetic moments can be modelled as coplanar elliptic rotators perpendicular to the threefold axis of the crystal structure that interact antiferromagnetically. Nonetheless, a disordered phase that can be ascribed to geometric frustration persists down to the lowest temperature, which is about 25 times smaller than the energy scale for the exchange interactions.

A theoretical study of the electronic structure and magnetic properties of the half-metallic ferromagnet CrO₂ is performed by means of the relativistic full-potential linear muffin-tin orbital method within the generalized gradient approximation (GGA) to the exchange correlation potential. The calculated spin and orbital magnetic moments at the chromium site agree well with existing experimental and theoretical results. It is found that the easy magnetization is along the [001] axis in agreement with experiment, which is confirmed by a noticeable anisotropy between the calculated chromium L_2,3 x-ray magnetic circular dichroism (XMCD) spectra for the [100] and [001] quantization directions. Furthermore, the oxygen K edge XMCD spectrum clearly shows an induced magnetism at the oxygen site, in agreement with experiment. The XMCD sum-rule-computed spin and orbital magnetic moments from the experimental spectra are close to the self-consistent results and are in good agreement with most experimental results.

The electrical conductivity (DC, AC) and dielectric properties' dependence on temperature (293–393 K) and on frequency (0.1 Hz–100 kHz) of thermally deposited thin films of N-(p-dimethylaminobenzylidene)-p-nitroaniline (DBN) have been reported. The DC conductivity indicates a thermally activated carrier hopping rate; it increases with increasing temperature. The electronic parameters such as activation energy and room temperature conductivity are in the regime of semiconductors. The obtained experimental results of the AC conductivity have been analysed with reference to various theoretical models. The analysis shows that the correlated barrier hopping (CBH) model is the appropriate mechanism for the electron transport in DBN film. Application of the CBH model reveals that the electronic conduction takes place via bipolaron hopping processes in the whole temperature range of study. Both the dielectric constant and the dielectric loss showed a decrease with increasing frequency while they increased with increasing temperature. The barrier height, W_M, between charged defect states was calculated according to the theory of hopping of carriers over a potential barrier.

First-principles calculations were carried out to study the functionalization of single wall carbon nanotubes by the chemical absorption of F and Cl atoms. Our results confirmed that the band gap of semiconductor zigzag carbon nanotubes is reduced on addition of F or Cl atoms on the walls of the nanotubes. For metallic armchair nanotubes, the doubly degenerate states crossing the Fermi level were separated by the introduction of F or Cl atoms. An additional energy level emerged near the Fermi level, due to coupling between the carbon nanotube and the F or Cl atom. For zigzag nanotubes, charge transfers of 0.27e from the tube to the Cl atom and of 0.41e to the F atom took place, while for armchair nanotubes, the charge transfers from the nanotube to Cl and F are 0.25 and 0.42e, respectively. The Cl–C and F–C bond lengths were found to be 2.09 and 1.49 Å, respectively. The systems show semiconducting behaviour when charged with one electron per halogen atom, but remain metallic under hole injection, regardless of the chirality of the carbon nanotubes.

Raman scattering spectra of a KH₂PO₄ (KDP) single crystal have been obtained under various temperature and pressure conditions. It was observed that (i) the peak around 1310 cm⁻¹ is not a single peak as reported previously but consists of two components and (ii) in the ferroelectric phase the 1013, 1306 and 1315 cm⁻¹ peaks become obscure near the ferroelectric–paraelectric phase boundary. Furthermore, they show positive pressure shifts and the pressure coefficients are obtained. An interpretation is proposed for the origins of these Raman peaks.

In this paper we use the pseudofermion dynamical theory (PDT) to study the singular spectral features due to one-electron removal within the one-dimensional Hubbard model. The PDT reveals that in the whole (k, ω)-plane such features are of the power-law type and correspond to well defined lines of three types: charge singular branch lines, spin singular branch lines and border lines. One of our goals is the study of the momentum and energy dependence of the distribution of the spectral weight in the vicinity of such lines. We find that the charge and spin branch lines correspond to the main tetracyanoquinodimethane (TCNQ) peak dispersions observed with angle-resolved photoelectron spectroscopy in the quasi-1D organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). Our expressions refer to all values of the electronic density and on-site repulsion U. The weight distribution in the vicinity of the singular spectral lines is fully controlled by the overall pseudofermion phase shifts. Moreover, the shape of these lines is determined by the bare-momentum dependence of the pseudofermion energy dispersions.

We report a comparative study by electron paramagnetic resonance (EPR) of the E'_δ point defect and of the 10 mT doublet in γ-ray irradiated amorphous silicon dioxide. Our experimental investigation, carried out on three distinct materials and for an overall variation of one order of magnitude of the concentration of defects, shows a quite general linear correlation between the two EPR signals. This result definitively supports the attribution of the 10 mT doublet to the hyperfine structure of the E'_δ centre, originating from the interaction of the unpaired electron with a nucleus of ²⁹Si (I = 1/2).

An analysis of the energy level structure of Cr³⁺ ions in Cs₂NaAlF₆ crystal is performed using the exchange charge model (ECM) together with the crystal field analysis/microscopic spin Hamiltonian (CFA/MSH) computer package. Utilizing the crystal structure data, our approach enables modelling of the crystal field parameters (CFPs) and thus the energy level structure for Cr³⁺ ions at the two crystallographically inequivalent sites in Cs₂NaAlF₆. Using the ECM initial adjustment procedure, the CFPs are calculated in the crystallographic axis system centred at the Cr³⁺ ion at each site. Additionally the CFPs are also calculated using the superposition model (SPM). The ECM and SPM predicted CFP values match very well. Consideration of the symmetry aspects for the so-obtained CFP datasets reveals that the latter axis system matches the symmetry-adapted axis system related directly to the six Cr–F bonds well. Using the ECM predicted CFPs as an input for the CFA/MSH package, the complete energy level schemes are calculated for Cr³⁺ ions at the two sites. Comparison of the theoretical results with the experimental spectroscopic data yields satisfactory agreement. Our results confirm that the actual symmetry at both impurity sites I and II in the Cs₂NaAlF₆:Cr³⁺ system is trigonal D_3d. The ECM predicted CFPs may be used as the initial (starting) parameters for simulations and fittings of the energy levels for Cr³⁺ ions in structurally similar hosts.

Raman spectroscopy was used to investigate the structure of ion-irradiated α-SiC single crystals at room temperature and 400 °C. Irradiations induce a decrease of the Raman line intensities related to crystalline SiC, the appearance of several new Si–C vibration bands attributed to the breakdown of the Raman selection rules, and the formation of homonuclear bonds Si–Si and C–C within the SiC network. For low doses, the overall sp³ bond structure and the chemical order may be almost completely conserved. By contrast, the amorphous state shows a strong randomization of the Si–Si, Si–C and C–C bonds. The relative Raman intensity decreases exponentially versus increasing dose due to the absorption of the irradiated layer. The total disorder follows a sigmoidal curve, which is well fitted by the direct impact/defect stimulated model. The chemical disorder expressed as the ratio of C–C bonds to Si–C bonds increases exponentially versus the dose. A clear correlation is established between the total disorder and the chemical disorder. The increase of temperature allows the stabilization of a disordered/distorted state and a limitation of damage accumulation owing to the enhancement of the dynamic annealing.

Nanoparticles of Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared by the standard co-precipitation method. The formation of nanocrystalline mixed spinel phase has been confirmed by x-ray diffractograms. The sizes of the nanoparticles were estimated in the range 7–30 nm, which was confirmed by transmission electron microscopy. Thermal variations of the real part of AC magnetic susceptibilities measured from 450 K down to 80 K and Mössbauer effect measurements at room temperature and down to 20 K clearly indicate the presence of superparamagnetic particles in all the samples. Specific saturation magnetizations measured by VSM are found to increase steadily with the increase of average particle size. The coercive field obtained from low frequency measurements shows that in all the samples a small fraction of particles is not relaxed within the measuring time. For samples showing a less dominating superparamagnetic behaviour, AC magnetic susceptibility data showed the expected increase of blocking temperature with increase in particle size. Magnetic anisotropy energy constants of the nanoparticles were estimated from the blocking temperature and the values cannot be directly correlated with their particle sizes.