Table of contents

We study the effect of large swelling in irradiated zircon. We perform molecular dynamics simulation of the overlap of two radiation events and find that the damage produced in the second event scatters away from the densified boundary of the damaged region implanted previously. This serves as the microscopic mechanism of the increase of volume occupied by the damage. The additive nature of this effect results in large swelling observed experimentally. We translate the damage accumulation into the percolation problem, and show that volume swelling is a percolation phenomenon, with the swelling curve increasing rapidly at the percolation threshold.

The magnetic dynamics of the spin ice material Ho₂Ti₂O₇ in its paramagnetic ('hot') phase have been investigated by a combination of neutron spin echo and ac-susceptibility techniques. Relaxation at high temperatures (T > 15 K) is proved to occur by a thermally activated single-ion process that is distinct from the process that dominates at lower temperatures (1 K < T < 15 K). It is argued that the low-temperature process must involve quantum mechanical spin tunnelling, as quasi-classical channels of relaxation are exhausted in this temperature range. Our results resolve a mystery in the physics of spin ice: why has a 15 K ac-susceptibility peak been observed in Dy₂Ti₂O₇ but not in Ho₂Ti₂O₇ or Ho₂Sn₂O₇?

The oxygen-isotope (¹⁶O/¹⁸O) effect (OIE) on the in-plane penetration depth λ_ab (0) in underdoped Y_1−x Pr_x Ba₂ Cu₃ O_7−δ was studied by means of muon-spin rotation. A pronounced OIE on λ_ab⁻² (0) was observed with a relative isotope shift of Δλ_ab⁻² /λ_ab⁻² = −5(2)% for x = 0.3 and −9(2)% for x = 0.4. The OIE exponents of T_c and of λ_ab⁻² (0) exhibit a relation that appears to be generic for cuprate superconductors.

LiNbO₃ (LN) films with a high degree of (006) texture were deposited on Si-based dense SiO₂ layers by pulsed laser deposition. After annealing, the LN/SiO₂/Si structures were revealed to have ultraviolet-, green-, and red-emitting properties related to self-trapped excitons and E' defect pairs in the SiO₂ surface, which are induced by the photorefractive effect of the LN films. The emission wavelength can be tuned by introducing different dopants into the LN films. Waveguiding properties of the structures were demonstrated. The results obtained indicate that the LN/SiO₂/Si structures could be expected to have important applications in modern optoelectronic integration.

In this letter, a new hybrid structure, which comprises of a ferromagnet (FM) deposited on a two-dimensional electron gas (2DEG), is proposed. We present numerical calculations of the ballistic spin-dependent transport properties of the structure in the presence of spin–orbit coupling as described by the Rashba Hamiltonian. It is shown that the gate electrode on top of the structure can be used to control the Rashba coupling and hence the spin injection in the 2DEG. For the typical InAs system assumed in our numerical calculations, the spin polarization in the 2DEG reaches a ratio of up to ∼90%. The structure is also predicted to show magnetoconductance behaviour upon switching the magnetization direction of the FM stripe. The large spin polarization and MC of the proposed structure demonstrate the potential of the device as a spin injector in spin-logic devices as well as a magnetic sensor with ultra-high density storage.

In this letter a theory is developed for the treatment of the ejection of two correlated electrons from binary alloys with substitutional disorder upon the impact of fast electrons. It is shown that, under certain conditions specified in this letter, the target's electronic properties can be disentangled from the correlated two-electron scattering. For a numerical realization we employ the Korringa–Kohn–Rostoker coherent potential approximation and the virtual crystal approximation for the description of respectively the bound states and the high-energy scattering states of the electrons. Numerical results are presented and analysed for the energy correlation within an electron pair emitted from a copper–nickel alloy surface.

Conventionally, the addition of sesquioxide cation dopants to ceria has been thought of as a class of almost model systems. The most important defect mechanism involves simple anion vacancy charge compensation with those vacancy defects associating themselves with the trivalent cation and being distributed randomly through the lattice. However, this simple model has been significantly challenged in recent years and it seems possible that these associated defects might cluster in ordered arrangements. Whilst evidence has been provided by theoretical work, only limited experimental data are available. This letter reports the first observation of local ordering in these systems as observed by careful powder x-ray diffraction studies. In detail, it is shown that measurements of the lattice parameter do not vary monotonically with dopant concentration. It is also shown that far from being ideal systems with very high dopant solubilities and true solid-state solutions, these systems have complex solubility.

The x-ray resonant scattering cross-section of K₂ReCl₆ has been studied for energies near the L edges of rhenium. Below the Néel temperature of T_N ∼ 11 K, additional peaks appear when the photon energy is tuned to the L_II or L_III edges. The peaks are found to rotate the incident (linear) polarization, and have a Lorentzian lineshape in energy. The peaks are ascribed to resonant magnetic scattering, where a dipolar transition connects 2p–5d states. At resonance the scattering amplitude is of order ∼ r₀ per atom, roughly two orders of magnitude greater than that of the bare magnetic scattering.

The problem of multiband k · p Hamiltonians describing the hole energy structure of semiconductor nanosystems in a magnetic field is addressed. The approximate formulation given previously by Luttinger is revisited. We show that interaction with a magnetic field enters into the multiband equations for the envelope function components through the usual quadratic term and two linear Zeeman terms. The first linear term corresponds to the envelope angular momentum, while the other corresponds to the Bloch band-edge angular momentum. Several approximate ways of including the magnetic field in a four-band valence Hamiltonian are discussed and numerically compared.

Piezoelectric constant and temperature-dependent dielectric constant measurements have been performed on ⟨110⟩-oriented (1 − x)Pb (Mg_1/3 Nb_2/3)O₃–xPbTiO₃ crystals with different compositions under different poling fields. The width of the morphotropic phase boundary region (0.30 < x < 0.35) is determined on the basis of two abnormal regions of the dielectric and piezoelectric properties. An irreversible rhombohedral–monoclinic M_A–monoclinic M_C–tetragonal phase transition sequence was observed directly from the dielectric constant versus temperature results for ⟨001⟩-poled rhombohedral crystals with compositions near the rhombohedral–monoclinic phase boundary. The structure of the morphotropic phase is shown to be monoclinic with space group P_m.

When the size of particles varies over the entire realm of nanotechnology, we find that surface forces can change by orders of magnitude depending on how surface interactions are influenced by the irregular fluctuations of rough surfaces at all length scales from atoms to microns. The length scales and roughness exponent define the distribution of asperities at points of contact and the adhesion that is maintained by van der Waals forces and contact deformation via asperities.

Using density-functional theory we have calculated the equilibrium geometries and binding energies of a CO monolayer adsorbed on the nonpolar (1010) and the polar (0001)-Zn and (0001)-O surfaces of ZnO. Different adsorption sites and CO orientations were considered, and for the polar surfaces the influence of a hydrogen coverage upon CO adsorption was studied. For the clean surfaces we find that CO exclusively binds to Zn ions with a binding energy of 0.24 and 0.37 eV for the nonpolar (1010) and the polar (0001)-Zn surface, respectively. A purely repulsive interaction of CO with surface oxygen ions is obtained. On the other hand, if the polar surfaces are hydrogen saturated, we predict a weak chemisorption of CO to the OH-terminated (0001) surface with a binding energy of 0.20 eV but no CO adsorption for the ZnH-terminated (0001) face.

In this article, we review recent progress in the exploration of the complex magnetic phases of the fcc Fe/Cu(111) system. In particular, we emphasize the magnetic properties realized by the synthesis of novel nanostructures of Fe on Cu(111). These include monolayer films, one-dimensional stripe arrays and nanodot arrays. The effects of spatial confinement, together with strong spin–lattice correlations, result in dramatically different magnetic behaviour for the various manifestations of the Fe/Cu(111) system. Multi-scale theoretical calculations have been used to provide an understanding of the magnetic behaviour in each case.

Understanding the size-dependent electronic, structural and chemical properties of metal clusters on oxide supports is an important aspect of heterogeneous catalysis. Recently model oxide-supported metal catalysts have been prepared by vapour deposition of catalytically relevant metals onto ultra-thin oxide films grown on a refractory metal substrate. Reactivity and spectroscopic/microscopic studies have shown that these ultra-thin oxide films are excellent models for the corresponding bulk oxides, yet are sufficiently electrically conductive for use with various modern surface probes including scanning tunnelling microscopy (STM). Measurements on metal clusters have revealed a metal to nonmetal transition as well as changes in the crystal and electronic structures (including lattice parameters, band width, band splitting and core-level binding energy shifts) as a function of cluster size. Size-dependent catalytic reactivity studies have been carried out for several important reactions, and time-dependent catalytic deactivation has been shown to arise from sintering of metal particles under elevated gas pressures and/or reactor temperatures. In situ STM methodologies have been developed to follow the growth and sintering kinetics on a cluster-by-cluster basis. Although several critical issues have been addressed by several groups worldwide, much more remains to be done. This article highlights some of these accomplishments and summarizes the challenges that lie ahead.

We report a study on the structural and magnetic properties of iron–vanadium thin films grown in multilayer form and mixed by thermal treatment. The multilayer samples were annealed at 610^°C for times ranging from 10 to 540 min. The samples were structurally characterized by means of x-ray diffraction (XRD) and by x-ray absorption spectroscopy (XAS). The magnetic characterization was carried out with a conventional alternating gradient magnetometer (AGM) and by conversion electron Mössbauer spectroscopy (CEMS). The XRD result for the as-deposited multilayer shows a high degree of crystallinity while the CEMS result suggests an abrupt interface, since no significant contribution from vanadium in iron is observed. After the thermal treatment, the results from XRD show a phase transformation of the disordered body-centred-cubic structure (α-phase) into a tetragonal structure (σ-phase) and a subsequent return to the α-phase. This α–σ–α oscillation is not reported in the literature available to the authors.

Mesoporous SiO₂ composite films with small Ag particles or clusters dispersed in them were prepared by a new method: first the matrix SiO₂ films were prepared by the sol–gel process combined with the dip-coating technique; then they were soaked in AgNO₃ solutions; this was followed by irradiation with γ-rays at room temperature and ambient pressure. The structure of these films was examined by high-resolution transmission electron microscopy, and their optical absorption spectra were examined. It has been shown that the Ag particles grown within the porous SiO₂ films are very small and are highly dispersed. On increasing the soaking concentration and subjecting the samples to an additional annealing, a different peak-shift effect for the surface plasmon resonance was observed in the optical absorption measurement. Possible mechanisms of this behaviour are discussed in this paper.

We have performed the atomistic simulations of the adhesion process of a boron atom on a tungsten(110) surface on the basis of the generalized simulation annealing formalism. The interatomic potentials used in these simulations were obtained from ab initio total energy calculations on the basis of the recursion procedure. The nonempirical calculations have been carried out in the framework of density functional theory in the coherent potential approximation.

Magnetocrystalline anisotropy (MA) energy of (001) face-centred-cubic Co(N) films is calculated for film thicknesses N = 1–28 in a realistic tight-binding model with and without sp–d orbital hybridization included. The obtained results show that the average MA energy is not largely influenced by the sp–d hybridization. On the other hand, the oscillation pattern is remarkably changed when the sp–d hybridization is included: in this case the MA energy has oscillations with a clear period of 2 atomic layers (AL), similar to the previous ab initio calculations (Szunyogh L, Újfalussy B, Blaas C, Pustugova U, Sommers C and Weinberger P 1997 Phys. Rev. B 56 14036). A careful analysis in k- and N-spaces reveals that the total MA oscillations are a superposition of two oscillatory contributions: one coming from the neighbourhood of the -point with period close to 2 AL (regardless whether the sp–d hybridization is present or not) and the other originating in the region around the -point. The -point contribution has a larger period and its amplitude is significantly smaller than that of the -point contribution when the sp–d hybridization is included so that the 2 AL -point contribution is dominant in this case. The two oscillatory MA contributions are attributed to quantum-well states and the corresponding oscillation periods are related to the extremal radii of the minority-spin bulk Co Fermi surface.

Fe/In₂O₃ granular films have been prepared by the radio frequency sputtering method. The magnetic and transport measurements of a representative sample, Fe_0.35/(In₂O₃)_0.65, showed that there exist different magnetic states in different temperature regions. At room temperature, the film shows superparamagnetic behaviour, and a 5.2% magnetoresistance (MR) ratio was obtained. The susceptibility measurements showed that the blocking temperature is 50 K. Below a certain freezing temperature T_f of about 10 K, the film transits from the ferromagnetic state to the particle-spin-cluster state. In this event, the MR ratio of the film increases dramatically with decreasing temperature. A maximum giant magnetoresistance (GMR) ratio up to 506% is obtained at the metal–semiconductor transition temperature of about 2.2 K. The mechanism of this GMR is related to the interaction with the impurities influencing the local magnetization which is quite different to the spin-dependent tunnelling effect at room temperature.

We have investigated an 850 nm GaAs/GaAlAs (001) vertical-cavity surface-emitting laser (VCSEL) structure using angle- and temperature-dependent surface photovoltage spectroscopy (SPS). The SPS measurements were performed as functions of angle of incidence (0^° ≤ θ ≤ 60^°) and temperature (25^° C ≤ T ≤ 215^° C) for both the metal–insulator–semiconductor (MIS) and wavelength-modulated MIS configurations. Angle-dependent reflectance (R) measurements have also been performed to illustrate the superior features of the SPS technique. The SPS spectra exhibit both the fundamental conduction to heavy-hole excitonic transition of quantum well and cavity mode (CM) plus a rich interference pattern related to the mirror stacks, whereas in the R spectra only the CM and interference features are clearly visible. The variations of SPS spectra as functions of incident angle and temperature enable exploration of light emission from the quantum well confined in a microcavity with relation to the Fabry–Pérot cavity mode. The results demonstrate considerable potential of SPS for the contactless and nondestructive characterization of VCSEL structures.

We present a first-principles study on the structural stability of Co silicide phases and their magnetic properties for 1–2 monolayers (ML) of Co deposited on Si(001). The Co–Si interaction between the nearest neighbouring sites at the surface layer is strongly attractive. The formation of CoSi in the subsurface layer is energetically more favourable than that in a surface layer. The interdiffusion of a Co atom to the fourfold (tetrahedral) site is found to be energetically favourable. For surface alloy films of 1 and 2 ML Co on Si(001), there are no Co atoms at the surface due to the interdiffusion of Co atoms. The structural stability of the 'fourfold Si surface' model with the CoSi₂ phase is compared with that of the sixfold model. Our result for the surface and interface of a thin CoSi₂/Si(001) film is consistent with experimental and other theoretical data.

The structural properties of CeNiGe₃ have been investigated via electron diffraction and neutron powder diffraction (NPD). This ternary germanide crystallizes in the orthorhombic SmNiGe₃-type structure (Cmmm space group). Electrical resistivity, ac- and dc-magnetization measurements show that CeNiGe₃ orders antiferromagnetically below T_N = 5.5(2) K and exclude the occurrence at low temperatures of a spin-glass state for CeNiGe₃ as previously reported. Specific heat measurements and NPD both reveal two magnetic transitions, observed at T_N1 = 5.9(2) K and T_N2 = 5.0(2) K. Between T_N1 and T_N2, the Ce magnetic moments in CeNiGe₃ are ordered in a collinear antiferromagnetic structure associated with the k₁ = (100) wavevector and showing a relationship with the magnetic structure of the Ce₃Ni₂Ge₇ ternary germanide. Below T_N2, this k₁ = (100) commensurate magnetic structure coexists with an incommensurate helicoîdal magnetic structure associated with k₂ = (00.409(1)1/2). This last magnetic structure is highly preponderant below T_N2 (93(5)% in volume). At 1.5 K, the Ce atoms in CeNiGe₃ carry a reduced ordered magnetic moment (0.8(2) μ_B). This value, smaller than that obtained in Ce₃Ni₂Ge₇, results from an important hybridization of the 4f(Ce) orbitals with those of the Ni and Ge ligands.

The specific heat corresponding to the tetragonal-to-cubic transition in Ca_0.04Sr_0.96TiO₃ perovskite has been measured by conduction calorimetry. The order parameter of the transition has been obtained by means of neutron diffraction at low temperatures. Comparison of calorimetric data with the evolution of the order parameter indicates that this transition seems to follow a mean field Landau potential as in SrTiO₃. The linear behaviour of the excess of entropy versus temperature suggests that a 2–4 Landau potential is sufficient to describe the transition.

The pressure dependence of the acoustic velocities of a Pd₃₉Ni₁₀Cu₃₀P₂₁ bulk metallic glass have been investigated up to 0.5 GPa at room temperature with the pulse echo overlap method. Two independent second-order elastic coefficients C₁₁ and C₄₄ and their pressure derivatives are yielded. The vibrational anharmonicity is shown by calculating both the acoustic mode Grüneisen parameters in the long-wavelength limit and the thermal Grüneisen parameter, and this result is compared with that for the Pd₄₀Ni₄₀P₂₀ bulk glass.

By using the interatomic pair potential obtained with the lattice inversion method, the stability of RT_13−xM_x (R = La, Ce, Pr and Nd; T = Co and Fe; M = Si, Al, Cr, V and Ti) of the NaZn₁₃ type and its derivative structure are studied. The structural transition of LaT_13−xSi_x (T = Co and Fe) between the cubic one with the space group Fm3c and the tetragonal one with I4/mcm is imitated from the viewpoint of energy. As for the function of the third elements, Al and Si are beneficial to the phase stability of RT_13−xM_x, whereas Cr, Ti and V are unfavourable to the stability. In the calculation, the range of x, with which RT_13−xM_x could crystallize in the cubic or tetragonal structures, agrees with the experiments very well. The calculated crystallographic parameters coincide with the experimental observation. In the cubic structure, Si and Al prefer the 96i site, and in the tetragonal structure Si first occupy the 16l(2) site, then the 16k site. In addition, all the site positions of the compounds with either the cubic or tetragonal structure are really congruent with the experimental one.

A series of GaAs/InGaAs quantum wells with a silicon δ-doped layer in the top barrier was investigated by Shubnikov–de Haas measurements as a function of the illumination time of the samples. During the illumination process strong modifications of the electronic density and the quantum mobility of each occupied subband were observed. Based on self-consistent calculations, the dominant mechanism which caused the changes in the subband quantum mobilities with illumination was elucidated.

Strain distributions around a Ge quantum dot (QD) buried in a Si spacer layer are investigated theoretically by means of classical molecular dynamics simulations using the Tersoff potential. Applying periodic boundary conditions laterally, two-dimensional superlattices of QDs are obtained. Strain distributions in systems of different sizes and lattice misorientations are computed in order to explain possible vertical correlations in self-organized three-dimensional QD superstructures. Generally, the strain of relaxed systems displays an oscillatory behaviour as a function of the distance from the QD. For QD systems with growth direction [001], a simple fitting function is used to describe the strain along a vertical path above the QD by an oscillation and a decay according to a power law. For QDs with the shape of a truncated pyramid, the planar strain decays by a power of approximately −3. The period of the oscillation is nearly proportional to the QD superlattice constant and decreases with increasing coordination number of the QD superlattice. In misoriented systems with a small tilt angle about the [110] axis, the region of tensile planar strain above the QD is bent in the direction opposite to the misorientation causing a vertical correlation with lateral shift. For a tilt angle ≈55^°, no strain oscillation is found which implies a perfect vertical correlation.

We discuss the influence of the electron–electron interaction on transport properties of open quantum dot systems. Based on the idea of the Anderson model, we present interaction-induced temperature-dependent corrections to the conductance beyond the single-particle picture.

A series of first-principles calculations were performed for ferromagnetic Ni₂MnGa using density functional theory and PAW potentials. Theoretically, a tetragonal crystal structure homogeneous lattice-distortive strain is stabilized around c/a = 0.94 with respect to the L2₁ structure when, in addition, modulation shuffles with a period of five atomic planes are taken into account. This is in agreement with the observed structures in experimental works. The modulation appears to be critically important for stability of the tetragonal structure with c/a < 1. Here, we report a new feature which is related to the optimum amplitudes of the modulation in different atomic planes. Related to this are systematic changes in the minority spin density of states near the Fermi surface, like in the formalism of a pseudo-gap.

Maximally localized Wannier functions are the basis of a new technique for resolving ambiguous bonding issues for amorphous materials. Geometrical methods using the Wannier function representation provide an insightful chemical picture of local bonding and hybridization in disordered structures. Central to these methods is the notion of treating the Wannier function centres as a virtual atomic species with a well-defined degree of localization. Using Wannier function methods, we classify and quantify the types of bonding present in a sample of the ternary alloy hydrogenated amorphous silicon carbide, C₂₂Si₂₂H₂₀. In addition to the bonding previously observed for this material, we see three-centre bonding and flipping bonds. We identify a cluster defect in our sample associated with these flipping bonds, and observe a temperature dependence of the bond flipping. This effect may be observable using temperature-dependent Raman spectroscopy.

Wannier excitons confined in an InP/InAs inhomogeneous quantum dot (IQD) have been studied theoretically in the framework of the effective mass approximation. A finite-depth potential well has been used to describe the effect of the quantum confinement in the InAs layer. The exciton binding energy has been determined using the Ritz variational method. The spatial correlation between the electron and the hole has been taken into account in the expression for the wavefunction. It has been shown that for a fixed size b of the IQD, the exciton binding energy depends strongly on the core radius a. Moreover, it became apparent that there are two critical values of the core radius, a_crit and a_2D, for which important changes of the exciton binding occur. The former critical value, a_crit, corresponds to a minimum of the exciton binding energy and may be used to distinguish between tridimensional confinement and bidimensional confinement. The latter critical value, a_2D, corresponds to a maximum of the exciton binding energy and to the most pronounced bidimensional character of the exciton.

Metamagnetic behaviour has been observed in an LiV₂O₄ powder sample around 38 T at 4.2 K. On the other hand, the magnetization for oxygen deficient LiV₂O_3.92 shows no indication of metamagnetism up to 40 T, and shows substantially reduced magnetic moment compared to that of LiV₂O₄. These results suggest that ferromagnetic interaction is strongly enhanced by magnetic fields in LiV₂O₄, whereas antiferromagnetic interaction is dominant in LiV₂O_3.92.

This paper reports the decay kinetics of the intrinsic blue luminescence from self-trapped excitons in lead tungstate excited by a Nd:YAG laser. The kinetics exhibits some complicated features, depending strongly on the experimental conditions, such as laser power, sample temperature, and one- or two-photon excitation. The result indicates that the self-trapped excitons interact with each other under high-density excitation, thereby shortening the decay time of the blue luminescence. Furthermore, a noticeable rise component is observed under two-photon excitation in the temperature range of 50–150 K. The contribution of radiation-induced defect centres in the process of energy transfer from Pb sites to WO₄ sites is discussed.

The stage-2 FeBr₂ graphite intercalation compound (GIC) was prepared by reacting FeBr₂ powder and highly oriented pyrolytic graphite in a bromine atmosphere at 500^°C for 40 weeks. The dc magnetization, ac susceptibility, specific heat, resistivity and Hall effect were measured. The GIC is paramagnetic at temperatures above 14.5 K. There is short-range ordering at 14.5 K and longer-range magnetic ordering at 8.5 K. There is a spin glass phase below 3.2 K in which the ac susceptibility is frequency dependent. The in-plane and c-axis resistivities result from in-plane and out-of-plane electron–phonon scattering. The Hall coefficient is independent of temperature between 4.2 and 300 K and is explained by the single-carrier model.

We present results for the temperature dependence of the mobility for elastic scattering in a two-dimensional electron gas at low temperatures. Due to anomalous screening in two-dimensional systems the mobility varies linearly with temperature. We discuss many-body effects and spin-polarization effects and compare with some recent experimental and theoretical results. We show that the sign of the temperature dependence may change in spin-polarized systems.

In this paper, the phonon spectrum of YBCO is obtained from experimental specific heat data by an exact inversion formula with a parameter for eliminating divergences. The results can be compared to those of neutron inelastic scattering, which can only be carried out in a few laboratories. Some key points of specific heat-phonon spectrum inversion (SPI) theory and a method of asymptotic behaviour control are discussed. An improved unique existence theorem is presented, and a universal function set for numerical calculation of SPI is calculated with high accuracy, which makes the inversion method applicable and convenient in practice. This is the first time specific heat-phonon SPI has been realized for a concrete system.

Deploying a recently developed semiclassical theory of quasiparticles in the superconducting state we study the de Haas–van Alphen effect. We find that the oscillations have the same frequency as in the normal state but their amplitude is reduced. We find an analytic formula for this damping which is due to tunnelling between semiclassical quasiparticle orbits comprising both particle-and hole-like segments. The quantitative predictions of the theory are consistent with the available data.

We present a study of the magnetic properties of the electron doped manganites Ca_1−x Y_x MnO₃ (for 0 ^≤x^≤ 0.25) in the paramagnetic regime. For the less doped samples (x^≤ 0.1) the magnetic susceptibility, χ(T), follows a Curie–Weiss (CW) law only for T > 450 K and, below this temperature, χ⁻¹ (T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn³⁺ and Mn⁴⁺ ions, competes with classical superexchange. For higher levels of doping (x^≥ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (T_mo) and a better description of χ(T) was obtained by assuming full delocalization of the e_g electrons. In order to explore the degree of delocalization as a function of T and x, we analysed the problem through Monte Carlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when T_mo is approached, they form magnetic polarons of large spin that cause the observed curvature in χ⁻¹ (T) for x^≤ 0.1.

Electrical resistivity, magnetoresistance and magnetic susceptibility were measured for ceramic (La_1−y Pr_y)_0.7Ca_0.3MnO₃ samples (y = 0.75 and 1) with different content of ¹⁸O isotope. All samples were paramagnetic insulators in the high-temperature range. Some of them became ferromagnetic (FM) metals at temperatures below 60–80 K. The high-temperature behaviour of the resistivity, magnetoresistance and magnetic susceptibility was practically identical for all samples in spite of the significant difference in their low-temperature properties. In particular, the magnetoresistance was proportional to the magnetic field squared and decreased approximately as 1/T⁵ in a wide magnetic field and temperature range. The results were interpreted based on the concept of an inhomogeneous state with pronounced FM correlations in the paramagnetic phase.

The magnetic properties of the nanocomposite permanent magnetic materials Sm–Fe–Ga–C have been investigated. As-quenched Sm₂Fe₁₈Ga₂C_2.2 samples prepared with a substrate velocity v_s = 18.5 and 19.5 m s⁻¹ contain some amorphous intergranular phase, together with Sm₂Fe₁₅Ga₂C_2.2 (Th₂Zn₁₇-type structure) and α-Fe. On the basis of the compositional dependence of the Curie temperature in Sm₂ Fe_15+x Ga₂C_2.2 amorphous ribbons, an enhancement of the Curie temperature of the amorphous intergranular phase (aip) was found in nanocomposite alloys with respect to amorphous ribbons of the same composition.⁵⁷Fe magnetic hyperfine field distributions for the amorphous intergranular amorphous phase are narrower than those for the amorphous ribbon. No difference was found in average hyperfine field, and hence in Fe magnetic moment, within the experimental error. We also excluded the effect of compositional difference on Curie temperature. The enhancement of the Curie temperature is thus concluded to originate from the penetration of the molecular field of crystallized magnetic phases into the amorphous intergranular region. Finally, a theoretical model was proposed to explain the mechanism of 'extra' enhancement of the Curie temperature.

The electron paramagnetic resonance spectrum of trigonal Fe³⁺ centres has been investigated and parameters of the spin Hamiltonian obtained for nominally pure congruent LiTaO₃ crystals annealed at ∼1500 K under Li₂O vapour pressure corresponding to the pressure over the stoichiometric lithium tantalate. The possibility of calculating the zero-field splitting of the ground state of the impurity ion on the basis of the superposition model is discussed.

The optical spectroscopy of Cr³⁺ ions doped into near-stoichiometric LiNbO₃ crystals, pure and co-doped with MgO, has been investigated.

In the near-stoichiometric LiNbO₃:Cr(0.2 mol%):Mg(2 mol%) crystal, the optical spectra resemble those previously observed for congruent LiNbO₃:Cr:MgO samples when the total MgO content exceeds the 4.6 mol% threshold. The coexistence of two types of Cr³⁺ centre ([Cr]_Li and [Cr]_Nb) characterized the optical and luminescence spectra of this sample. The concentration equilibrium between the two types of centre is strongly displaced towards the [Cr³⁺ ] _Nb centre, permitting us to obtain with accuracy the parameters of the broad bands. The R-line associated with the [Cr]_Nb centre is only observable in the low-temperature emission spectrum. The Fano anti-resonance lines present have been observed to be more pronounced for the near-stoichiometric samples than for congruent ones.

We present the results of the modelling of proton translocation in finite H-bonded chains in the framework of the two-stage proton transport model. We explore the influence of reorientation motion of protons, as well as the effect of electric field and proton correlations on system dynamics. An increase of the reorientation energy results in the transition of proton charge from the surrounding to the inner water molecules in the chain. Proton migration along the chain in an external electric field has a steplike character, proceeding with the occurrence of electric field threshold-type effects and drastic redistribution of proton charge. Electric field applied to correlated chains induces first a formation of ordered dipole structures for lower field strength, and then, with a further field strength increase, a stabilization of states with Bjerrum D-defects. We analyse the main factors responsible for the formation/annihilation of Bjerrum defects showing the strong influence of the complex interplay between reorientation energy, electric field and temperature in the dynamics of the proton wire.

Nanosized beta carbon nitride (β-C₃N₄), of grain size several tens of nanometres, has been synthesized by mechanochemical reaction processing. The low-cost synthetic method developed facilitates the novel and effective synthesis of nanosized crystalline β-C₃N₄ (a = 6.36 Å, c = 4.648 Å) powders. The graphite powders were first milled to a nanoscale state, then the nanosized graphite powders were milled in an atmosphere of NH₃ gas. It was found that nanosized β-C₃N₄ was formed after high-energy ball milling under an NH₃ atmosphere. After thermal annealing, the shape of the β-C₃N₄ changes from flake-like to sphere-like. The nanosized β-C₃N₄ formed was characterized by x-ray diffraction, Fourier transformation infrared spectroscopy, and transmission electron microscopy. A solid–gas reaction mechanism was proposed for the formation of nanosized β-C₃N₄ at room temperature induced by mechanochemical activation.

The temperature evolution of the static order parameter of α-quartz and its soft-mode frequencies were determined at temperatures below 300 K. While these parameters follow classic Landau theory at higher temperatures, quantum saturation was found below room temperature with a characteristic quantum temperature of 187 K. A quantitative analysis gave a good agreement with the predictions of a Φ⁶ model close to the displacive limit and a rather flat dispersion of the soft-mode branch. No indication of any effect of strong mode–mode coupling on the saturation behaviour was observed.