Table of contents

A critical modelling of the nonequilibrium thermodynamic properties of nanocavities and nanoparticles is presented. We first correlate our newly observed experimental results about nanocavities in silicon that can shrink during energetic-beam irradiation with the available experimental phenomena of nanoparticle instabilities. Several new concepts, which challenge our current understanding of science, are put forward and a novel universal antisymmetry relationship between a nanoparticle and a nanocavity in condensed matter is revealed.

The intermetallic bronze YbPd₃S₄ is shown to be a heterogeneous mixed-valence system, by means of ¹⁷⁰Yb Mössbauer spectroscopy and x-ray L_III-edge absorption and magnetic measurements. Two valence states coexist in this compound: Yb³⁺ and close-to-divalent Yb. The trivalent fraction (about 50%) undergoes a transition to magnetic ordering at about 2 K, with the Γ₇ doublet as the ground crystal field state. The possibility of charge (or valence) ordering is discussed.

Hydrogen is a ubiquitous impurity in diamond but in contrast to other group IV materials the microscopic structure adopted in bulk material has largely remained elusive. It has therefore been the role of modelling to predict the properties of H in bulk diamond, as well as the interactions with impurities and other defects. Presented here is an account of the current theoretical understanding of hydrogen in diamond.

The transport of biopolymers through large membrane channels is a ubiquitous process in biology. It is central to processes such as gene transfer by transduction and RNA transport through nuclear pore complexes. The transport of polymers through nanoscopic channels is also of interest to physicists and chemists studying the effects of steric, hydrodynamic, and electrostatic interactions between polymers and confining walls. Single-channel ion current measurements have been recently used to study the transport of biopolymers, and in particular single-stranded DNA and RNA molecules, through nanometre-size channels. Under the influence of an electric field, the negatively charged polynucleotides can be captured and drawn through the channel in a process termed 'translocation'. During translocation, the ion current flowing through the channel is mostly blocked, indicating the presence of the polymer inside the channel. The current blockades were found to be sensitive to the properties of the biopolymers such as their nucleotide composition, length, and secondary structure, and to physical parameters such as the driving field intensity, temperature, and ionic strength. These blockades are therefore a rich source of information regarding the dynamics of polynucleotides in the pore. The translocation process is separated into its two main steps: (a) polymer 'capture' in which one of the polymer's ends is threaded a small distance through the channel, and (b) polymer sliding through the channel. The experimental and theoretical efforts to elucidate polymer capture and the transport dynamics of biopolymers in nanoscopic pores are reviewed in this article.

We show theoretically the fingerprints of short-range spiral magnetic correlations in the photoemission spectra of the Mott insulating ground states realized in the √3 ×√ 3 triangular silicon surfaces K/Si(111)–B and SiC(0001). The calculated spectra present low-energy features of magnetic origin with a reduced dispersion ∼10–40 meV compared with the centre-of-mass spectra bandwidth ∼0.2–0.3 eV. Remarkably, we find that the quasiparticle (QP) signal survives only around the magnetic Goldstone modes. Our findings position these silicon surfaces as new candidates for investigation in the search for non-conventional QP excitations.

We have performed an ab initio study of the stability, atomic geometry and electronic structure of the Bi-covered (√3 ×√3) reconstructed Si(111) surface. We find that the energetically stable structure changes from the milkstool model (for 1 monolayer (ML) coverage) to the T₄ model (for 1/3 ML coverage), without going through a stable structure for the honeycomb model (2/3 ML coverage). Our theoretical scanning tunnelling microscopy (STM) simulation for the 1 ML coverage reveals the formation of Bi trimers for occupied states, and a honeycomb image for empty states. This result, together with the energetically unstable structure for 2/3 ML coverage, suggests that the experimentally observed STM image in the form of the honeycomb structure does not mean that the minimum energy configuration corresponds to Bi coverage of 2/3 ML, but rather represents current tunnelling into the empty states localized between Bi trimers for the milkstool model with 1 ML coverage.

Spin-wave excitations in ferromagnetic layered composite (AB · · · BA; A and B being different homogeneous ferromagnetic materials) are analysed theoretically, by means of the transfer matrix approach. The properties of multilayer spin-wave mode profiles are discussed in relation to multilayer characteristics, such as the filling fraction and the exchange or magnetization contrast; also, surface spin pinning conditions and dipolar interactions are taken into account. The interface conditions are satisfied by introducing an effective exchange field expressed by interface gradients of the exchange constant and the magnetization. This approach provides an easy way to find frequencies and amplitudes of standing spin waves in the multilayer. The developed theory is applied to interpretation of spin wave resonance (SWR) spectra obtained experimentally by Chambers et al in two systems: a bilayer Fe/Ni and a trilayer Ni/Fe/Ni, in perpendicular (to the multilayer surface) configuration of the applied magnetic field. By fitting the SWR spectra obtained experimentally and those found numerically, the surface anisotropies are estimated on multilayer surfaces; then, the observed resonance lines are identified as associated with bulk, surface or interface modes. The theory can be extended to a general case of any multi-component layered system.

Magnetic multilayer structures of Co/Cu prepared by dc magnetron sputtering are studied with respect to changing number of bilayers (N) for different thicknesses of the Cu spacer layer corresponding to different coupling conditions according to the oscillatory interlayer exchange coupling. X-ray reflectivity and diffuse scattering show that the multilayers become smoother with increasing N. The growth exponent of the roughness is found to be lower for a multilayer than for a single-layer film of similar thickness. The roughness of subsequent interfaces along the stack is conformal, and the lateral correlation does not change with the period number, but depends on the thickness of the spacer layers. The improved layer structure for larger N increases the antiferromagnetic coupling fraction as inferred from magneto-optic Kerr effect measurements and thereby increases the giant magnetoresistance (GMR) ratio up to 35% for N = 10. Thus, the first few bilayers do not contribute to the GMR but act as a buffer to improve the growth conditions for the following bilayers. The first about five bilayers can be replaced by a bottom Co layer of equivalent thickness which also improves the layer structure for a subsequently deposited lower number of bilayers without much loss in the GMR ratio. This smoothening effect due to the increasing of the thickness of the bottom-most layer is related to the simultaneously decreasing grain size.

Measurements of the electron paramagnetic resonance (EPR) upon photoexcitation are reported on Ni defects in diamonds grown with Ni-containing solvent/catalysts. The temperature dependence of the W8 EPR spectrum photoquenching shows that the relaxation of substitutional Ni_s⁻ upon electron ionization is very small, corroborating the interpretation that the previously reported photoinduced effects with thresholds at 2.5 and 3.0 eV correspond to two complementary photoionization transitions involving Ni_s. Photoinduced behaviour of the NIRIM1 EPR centre favours the interstitial Ni_i⁺ model for this defect and suggests that the Ni_i^0/+ level is located at 1.98 ± 0.03 eV below the conduction band. In N-doped diamond, Ni_i is more likely to appear in the neutral state, undetectable by EPR, whereas at substitutional sites Ni_s⁻ is revealed. Observation of a strong AB2 EPR signal photoquenching and simultaneous detection of different spectral dependencies of the EPR intensity for other defects determine an electron photoionization energy of 1.67 ± 0.03 eV for the AB2. The implications of the obtained data for the identification of the AB defects' structure are discussed. Our study shows that Ni defects exhibit a weak electron–lattice interaction. The importance of the stronger spin–orbit coupling in these centres as compared to other defects in diamond is discussed. Assuming direct intercentre charge transfer from N_s, a theoretical description of the photoionization kinetics is proposed to explain the observed photoresponse of Ni defects.

Fe–Al nanoparticles of eight kinds have been prepared by hydrogen plasma–metal reaction. The morphology, crystal structure, and chemical composition of the nanoparticles obtained were investigated by transmission electron microscopy (TEM), x-ray diffractometry (XRD), and induction-coupled plasma spectroscopy. The particle size was determined by TEM and Brunaumer–Emmet–Teller gas adsorption. It was found that all the nanoparticles have spherical shapes, with average particle size in the range of 29–46 nm. The oxide layer in nanoparticles containing Al after passivation is not observable by XRD and TEM. The Al contents in Fe–Al ultrafine particles are about 1.2–1.5 times those in the master alloys. The evaporation speeds of Al and Fe in Fe–Al alloys are mutually accelerated at a certain composition. The crystal structures of the Fe–Al nanoparticles vary with the composition of the master alloys. Pure Fe₃Al (D0₃) and FeAl (B2) structures are successfully produced with 15 and 25 at.% Al in bulks, respectively. For samples of Fe–Al nanoparticles with Al content over 56.5 at.%, the crystal structures of the nanoparticles do not comply with the equilibrium phase diagram and there is no intermetallic formation except that of Fe₃Al and FeAl.

Polycrystalline samples of the ternary intermetallics RE₃Co₆Sn₅ (RE = Sm, Gd, Tb and Dy) were studied by means of magnetization, dc magnetic susceptibility and electrical resistivity measurements. All these stannides were found to order magnetically at low temperatures. Sm₃Co₆Sn₅ is antiferromagnetic below T_N = 8 K, while Tb₃Co₆Sn₅ exhibits ferromagnetic ordering below T_C = 16 K. The other two compounds show more complex magnetic behaviour with subsequent phase transitions in the ordered regions. For Gd₃Co₆Sn₅ one observes the onset of ferromagnetism at T_C = 25 K, which is followed by a change in the magnetic structure at T₁ = 12 K. In the case of Dy₃Co₆Sn₅ an antiferromagnetic type of order sets in at T_N = 7 K, and then a spin structure rearrangement occurs at T₁ = 3 K that yields a ferromagnetic component at lower temperatures. All the ternaries studied exhibit metallic-like conductivity with pronounced anomalies at the magnetic phase transitions. The thermoelectric power, measured for Gd₃Co₆Sn₅, and for comparison also for Y₃Co₆Sn₅ and Er₃Co₆Sn₅, is for each compound negative and of the order of several µV K⁻¹ at room temperature.

This paper describes a computational study of the mixed metal fluorides LiCaAlF₆ and LiSrAlF₆, which have potential technological applications when doped with a range of elements, especially those from the rare earth series. Potentials are derived to represent the structure and properties of the undoped materials, then defect properties are calculated, and finally solution energies for rare earth elements are calculated, enabling preferred dopant sites and charge compensation mechanisms to be predicted.

I apply the newly developed dynamical cluster approximation (DCA) to the calculation of the electron and phonon dispersions in the two-dimensional Holstein model. In contrast to previous work, the DCA enables the effects of spatial fluctuations (non-local corrections) to be examined. Approximations neglecting and incorporating lowest-order vertex corrections are investigated. I calculate the phonon density of states, the renormalized phonon dispersion, the electron dispersion and electron spectral functions. I demonstrate how vertex corrections stabilize the solution, stopping a catastrophic softening of the (π, π) phonon mode. A kink in the electron dispersion is found in the normal state along the (ζ, ζ) symmetry direction in both the vertex- and non-vertex-corrected theories for low phonon frequencies, corresponding directly to the renormalized phonon frequency at the (π, 0) point. This kink is accompanied by a sudden drop in the quasi-particle lifetime. Vertex and non-local corrections enhance the effects at large bare phonon frequencies.

The optical vibrations of hydrogen in TbNiAlH_1.4 and UNiAlH_2.0 were investigated by means of inelastic neutron scattering. The experimental data were analysed, including multiphonon neutron scattering contributions, calculated in an isotropic harmonic approximation. At least two fundamental H optical peaks were observed in TbNiAlH_1.4, and were assigned to the vibrational modes of hydrogen atoms occupying different interstitial sites in the metal sublattice. The high-energy part of the UNiAlH_2.0 spectra is characterized by strong anharmonicity, and a broad fundamental band. The latter can be accounted for by a large dispersion of phonon modes due to the strong H–H interactions, and/or different metal–hydrogen force constants, which may originate from different metal atoms surrounding the H atoms in the unit cell.

The characterization of rf magnetron-sputtered Fe–Pt thin films at various compositions (Pt = 15, 24, 46 and 78 at%) is reported. X-ray diffraction studies on annealed Fe–46%Pt thin films at 600 °C revealed an ordered L1₀ γ₂-FePt phase with fct structure whereas annealed Fe–24%Pt and Fe–78%Pt films exhibited ordered γ₁-Fe₃ Pt and γ₃-FePt₃ phases, respectively. The effects of argon quenching and rapid thermal annealing (RTA) on the structural and magnetic properties are investigated. When the films are annealed at 600 °C for 1 h and then quenched to room temperature in argon gas, ordered γ₂-FePt with L1₀ phase is obtained. Argon-quenched and rapid thermal annealed films exhibit microtwins in scanning electron microscopy analysis. The appearance of microtwins may be attributed to the planar defects developed in the FePt films due to the release of elastic strain during annealing. The saturation magnetization is found to increase with ferrous content in the films. Argon-quenched Fe–46%Pt films exhibited larger saturation magnetization than RTA-processed films. The large value of saturation magnetization obtained from M– H hysteresis indicates the predominant existence of the hard fct γ₂-FePt phase. The combined effects of twins, long-range order and the hard γ₂-FePt phase on the magnetic properties are discussed.

We present a method for calculating the inverse of the dielectric matrix in a solid using a band Lanczos algorithm. The method produces a multi-pole approximation for the inverse dielectric matrix with an arbitrary number of poles. We discuss how this approximation can be used to calculate the screened Coulomb interaction needed for electron self-energy calculations in solids.

We consider instabilities of two-component Tomonaga–Luttinger (TL) liquids for the one-dimensional extended Hubbard model in a magnetic field. When the density of electrons with spin σ is 1/2, electrons with spin σ tend to occupy every other site, and do not conduct. The instability stems from the umklapp scattering between same-spin electrons. But for the remaining electrons with spin −σ, an energy gap does not appear. We also consider the instability relating to the phase separation. This relates to the intrinsic stability condition of the TL liquid.

We report on electrical resistivity measurements performed on polycrystalline samples of UCu_5−xNi_x  (x = 0.25, 1). In order to extract the Kondo contribution to the resistivity, the experiments were carried out over a wide temperature range (0.4–800 K). From the analysis of our results, we conclude that the Kondo temperature takes values of T_K ∼ 240 K for x = 1 and T_K ∼ 245 K for x = 0.25, and that for both Ni concentrations the dominant part of the remarkably high residual resistivity (ρ(0) ∼ 400 µΩ cm) corresponds to the Kondo contribution. These results are discussed in comparison with previous analysis of specific heat and magnetic susceptibility data that produced similar values of T_K. We interpret our results in terms of disorder-driven non-Fermi liquid behaviour for UCu₄Ni, as indicated by the anomalous temperature dependences of the electrical, thermal and magnetic properties previously observed in this compound.

Using the fully relativistic, full potential, spin-polarized, linearized augmented-plane-wave (RSPFLAPW) method, the energy differences, the electronic structure and magnetic moments of alpha- and delta-plutonium with different alignments of the atomic magnetic moments have been calculated. We have found in our calculations that both α-Pu and δ-Pu structures have an antiferromagnetic order at their experimental equilibrium volumes. Similar results have also been obtained by other researchers for δ-Pu. An important conclusion from the present work is that taking the spin polarization into account improves the calculated density of states for α-Pu. This fact may be considered as an argument for the importance of magnetic interactions in the low temperature phase of plutonium.

We have studied the structural and electronic properties of YN in rock salt (sodium chloride), caesium chloride, zinc blende and wurtzite structures using first-principles total energy calculations. Rock salt is the calculated ground state structure with a = 4.93 Å, B₀ = 157 GPa. The experimental lattice constant is a = 4.877 Å. There is an additional local minimum in the wurtzite structure with total energy 0.28 eV/unit cell higher. At high pressure (∼ 138 GPa), our calculations predict a phase transformation from a NaCl to a CsCl structure.

The infrared optical absorption (0.1 eV < ℏω < 1.5 eV) in La_0.7Ca_0.3MnO₃ films on LaAlO₃ substrates exhibits a drastic temperature evolution of the spectral weight, evidencing an insulator-to-metal transition. Single-crystal films were found to reveal strong linear dichroism with anomalous spectral oscillations and fairly weak temperature dependence. Starting from the concept of phase separation, we develop an effective medium model to account for these effects. The optical anisotropy of the films is attributed to the texturization of the ellipsoidal inclusions of the quasimetal phase caused by a mismatch of the film and substrate and the twin texture of the latter.

In many compounds the broadband emission of Eu²⁺ and Yb²⁺ is subject to a very large (0.6–1.2 eV) Stokes shift and it behaves peculiarly with temperature change. Conduction band states of the host compound are involved in this 'anomalous' emission. Cases of anomalous emission are identified and the conditions for it to occur studied. Clear trends with the size of the lanthanide ion, the size of the site occupied, the size of anions in the compound, and the binding strength of oxygen ligands were found. The trends are interpreted by models involving the Madelung potential and Pauling repulsion at the lanthanide site together with the Coulomb and isotropic exchange interactions within the lanthanide ion. The results provide information on the approximate location of the lowest 4fⁿ⁻¹5d level relative to the bottom of the conduction band. The systematic variation with type of lanthanide and host lattice is discussed. Combining the results with information on the systematic variation in the fd transition energies, all energy levels for each divalent lanthanide can be approximately positioned relative to the conduction and valence band.

We study a two-dimensional (2D) spin-half Heisenberg model related to the quasi-2D antiferromagnets (Ba, Sr)₂Cu₃O₄Cl₂ by means of exact diagonalization and spin-wave theory. The model consists of two inequivalent interpenetrating square-lattice Heisenberg antiferromagnets A and B. While the antiferromagnetic interaction J_AA within the A subsystem is strong the coupling J_BB within the B subsystem is much weaker. The coupling J_AB between A and B subsystems is competing, giving rise to interesting frustration effects. In dependence of the strength of J_AB we find a collinear Néel phase, non-collinear states with zero magnetizations as well as canted and collinear ferrimagnetic phases with non-zero magnetizations. For not too large values of frustration J_AB, which correspond to the situation in (Ba, Sr)₂Cu₃O₄Cl₂, we have Néel ordering in both subsystems A and B. In the classical limit these two Néel states are decoupled. Quantum fluctuations lead to a fluctuational coupling between both subsystems ('order from disorder') and select the collinear structure. For stronger J_AB we find evidence for a novel spin state with coexisting Néel ordering in the A subsystem and disorder in the B subsystem.

Eighteen selected two-photon absorption (TPA) transition line strengths with polarization angles θ = 0° and 45°, spanning several orders of magnitude, have been calculated for the Tb³⁺ ion in the cubic host Cs₂NaTbCl₆. The results are in reasonable agreement with experimental results in the literature. The calculation utilized the crystal field (CF) wavefunctions for the initial and final states of the 4f⁸ configuration, and utilized free ion or CF wavefunctions (with the corresponding energies) for 4f⁷ core states of the whole intermediate 4f⁷ 5d configuration comprising 34 320 states. The intensities of certain transitions were found to be very sensitive to the inclusion of the CF interaction within the 4f⁷ core. In contrast to previous fourth- or third-order calculations of the TPA transition line strength of the strong transition (⁷F ₆)A_1g → (⁵D ₄)A_1g using pure Russell–Saunders (RS) wavefunctions for the |⁷F ₆ ⟩ initial and ⟨⁵D ₄ | final states, our second-order direct calculation shows that the admixed RS wavefunctions |[⁷F ₆ ]⟩ and ⟨[⁵D ₄ ]| must be used to account for its high intensity. The effects of CF interactions within the 4f⁷ core, i.e. J-mixing and CF energy level splitting, upon the (⁷F ₆)A_1g → (⁵D ₄)E_g TPA transition line strength have been separated, and the latter effect is shown to be more important for the transition investigated.

We study the disorder-induced quantum Hall-to-insulator transition in a lattice model with only off-diagonal disorder. The localization length of the system is calculated by using the finite-size scaling method combined with the transfer-matrix technique. By increasing the off-diagonal disorder strength we find that the extended states do not float up in energy, but move towards the direction of lower energy. As a consequence, a direct transition from a higher integer quantum Hall state to an insulator becomes possible, in agreement with previous studies based on a tight-binding model with only diagonal disorder.

The Pr_0.5Ca_0.5Mn_1−xCr_xO₃ series has been investigated up to x = 0.5. For low doping content (x ≤ 0.04), the introduction of Cr³⁺ favours the creation of ferromagnetic (FM) regions leading to phase-separation phenomena. Steps in the magnetic and transport properties measured versus field are observed and these properties strongly depend on the thermal history of the samples. For x = 0.04–0.06, the samples are almost 100% FM. As the Cr³⁺ concentration increases beyond this optimum doping level of 0.04–0.06, the FM fraction x_FM slowly decreases, reflecting the fact that Cr³⁺ is antiferromagnetically coupled to the Mn network, and reaches almost 0 for x = 0.5. Beyond the optimum doping, the steps disappear. In a martensitic-like picture, the presence of steps for x < 0.04 is consistent with the existence of phase separation. On the other hand, the disappearance of jumps beyond the optimum doping shows that the nature of phase separation, if it exists for x > 0.04–0.06, is completely different from the low-x side.

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO₂ ] hexagonal layer. Both cobaltites exhibit large thermopower values (S_300 K ∼ 140 µV K⁻¹), low resistivity values (ρ_300 K = 40–60 mΩ cm) and small thermal conductivities (κ_300 K ∼ 1 W K⁻¹ m⁻¹). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = ρ_H − ρ_{H = 0}/ρ_{H = 0}), reaching, at 2.5 K, − 85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

An analytical approach to the problem of a negatively charged donor in an infinitely deep quantum well (QW) in the presence of parallel electric and strong magnetic external fields both directed perpendicular to the heteroplanes is developed. The double adiabatic approximation is employed. The dependences of the binding energy on the field strengths, the width of the well and the position of the impurity within the well are derived in explicit form. The effect of the inversion of the electric field is investigated. It is shown that the combined potential acting on the 'outer' electron resembles that of a double QW. When the levels associated with the two effective QWs anticross, a resonant structure arises. The explicit dependence of the resonant splitting on the width of the QW, the strength of the electric field and the position of the impurity are obtained. Using the parameters associated with the GaAs QW, estimates of the inversion shift of the binding energy and the frequency of the emitted resonant radiation induced by the electric field are made.

Equations for the temperature-(T-) dependent superconducting (Δ(T)) and dielectric (Σ(T)) order parameters are solved self-consistently in the partial dielectric gapping model of Bilbro and McMillan for superconductors with charge-density waves (CDWs). It is shown that for the close enough structural phase transition temperature, T_s, and superconducting one, T_c, with T_s > T_c, Σ below T_c may become smaller than Δ. The electronic heat capacity C(T) is calculated. It is shown that the discontinuity ΔC at T = T_c is always smaller than the Bardeen–Cooper–Schrieffer value. The effect is detectable over a wide range of the model parameters. Experimental implications for CDW superconductors, such as A15 compounds, high-T_c cuprates, and MgB₂, are suggested and discussed.

By analysing an effective Hamiltonian for spin polarons forming in weakly doped antiferromagnets represented by the t– J model we demonstrate that the driving mechanism which gives rise to superconductivity in such a system is the lowering of the kinetic energy, which is consistent with recent experimental observations. That source of attraction between holes is effective if the antiferromagnetic correlation length is longer than the radius of polarons. Notwithstanding that the attraction is strongest in the undoped system with long-range order, the superconducting order parameter vanishes when the doping parameter decreases, which can be attributed to emptying the spin polaron band and approaching the Mott insulator phase. Since the hypothetical normal phase of a low-density gas of fermions is unstable against formation of bound hole pairs the intensity of low-energy excitations is suppressed and a pseudogap forms in the underdoped region.

An ab initio approach to the magnetic properties of bulk hexagonal Gd is developed that is based on the local spin-density approximation with the 4f electrons treated as localized core electrons. The effective one-electron problem is solved using the tight-binding linear muffin-tin orbital method in the atomic-sphere approximation with the valence basis consisting of s-, p-and d-type orbitals. The approach leads to a correct description of the ground-state properties like the stability of the ferromagnetic structure, the magnetic moment and the equilibrium lattice constant. Application of a real-space Green-function formalism yields the exchange pair interactions between distant neighbours that are inevitable for quantitative studies of magnetic excitations. The distance dependence and anisotropy of the exchange pair interactions are presented and the Curie temperature in the mean-field approximation is evaluated. The obtained value of 334 K is in much better agreement with the experimental value of 293 K than previous theoretical results. Depending on the atomic volume we find an unusually large dependence of the Curie temperature on the c/a ratio, which bears important consequences for the critical temperatures of thick strained Gd films as grown on various substrates.

The magnetic systems described by a two-spin-per-site Heisenberg-like Hamiltonian are investigated in detail. When there are two sub-spins in one site, the magnetic behaviour becomes more complicated than usual Heisenberg systems due to the internal spin fluctuation. Spontaneous magnetization with the variation of temperature is calculated. The quantitative phase diagrams are given for ferromagnetic and antiferromagnetic states and qualitative phase diagrams are shown for mixed states. The roles played by sub-spin quantum number values, four exchange parameters and single-ion anisotropy are studied. The research gives us a comprehensive understanding of the magnetic systems with internal spin fluctuation.

The present work is devoted to the derivation of an effective magnon–paramagnon theory starting from a microscopic single-band lattice model of ferromagnetic metals. For some values of the microscopic parameters it reproduces the Heisenberg theory of localized spins. For small magnetization the effective model describes the physics of weak ferromagnets. It allows us to account for the magnon–magnon and magnon–paramagnon interactions going beyond Moriya's theory. The effective theory is written in a way which keeps O(3) symmetry manifest, and describes both the ordered and disordered phases of the system.

To derive the effective model a Schwinger-bosons–slave-fermions representation of the operators is used. Within this approach the local Coulomb repulsion is treated exactly, and as a result, the constants in the effective theory are finite and well defined for all values of the magnetization.

An equation for the Curie temperature, which takes the magnon fluctuations into account exactly, is obtained. For weak ferromagnets, in the spin-wave approximation, the critical temperature scales like T_c ∼ m^5/3. It is well below the Stoner critical temperature T_c ∼ m and the critical temperature obtained within Moriya's theory T_c ∼ m^3/2.

The susceptibility and high-field magnetization of single-crystalline Yb_1−xY_xInCu₄ (x = 0, 0.2 and 0.3) samples have been measured for different field orientations at ambient and high pressures. The compounds with x = 0 and 0.2 undergo a first-order valence transition from the intermediate-valence state to the trivalent state on increasing either temperature or magnetic field. The magnetization and susceptibility of these compounds have appreciable anisotropy in both states. The magnetic phase diagram of Yb_1−xY_xInCu₄ determined at ambient pressure is also anisotropic, which is explained by the crystal-field calculations for the free Yb ion in the high-temperature phase. Moreover, the low-temperature magnetization process for x = 0.2 and 0.3 has been measured in low fields under high pressure; it shows anisotropic ferromagnetic ordering.

The crystal structure of the U(Pd_1−xFe_x)₂Ge₂ compounds with Fe content x = 0– 0.03 and the crystal and magnetic structure of U(Pd_0.98Fe_0.02)₂Ge₂ at high external pressures up to 4.5 GPa were studied by means of powder neutron diffraction in the temperature range 1.5–300 K. With increasing Fe content the values of the lattice parameters and interatomic distances change only slightly, but it is known from previous experiments that the magnetic structure changes drastically for x ≥ 0.015. In contrast to this, high external pressure modifies the crystal structure more significantly while the magnetic structure remains unchanged. The results obtained allow one to infer that drastic changes in the magnetic structure of the U(Pd_1−xFe_x)₂Ge₂ compounds with increasing Fe content are a consequence of modification of the RKKY-type (RKKY standing for Ruderman, Kittel, Kasuya and Yosida) indirect exchange interaction due to the variation of the number of conduction electrons per U atom rather than a result of lattice contraction.

SrF₂ and BaF₂ crystals, doped with the Yb³⁺ ions, have been investigated by electron paramagnetic resonance and optical spectroscopy. As-grown crystals of SrF₂ and BaF₂ show the two paramagnetic centres for the cubic (T_c) and trigonal (T₄) symmetries of the Yb³⁺ ions. Empirical diagrams of the energy levels were established and the potentials of the crystal field were determined. Information was obtained on the SrF₂ and BaF₂ phonon spectra from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyse the crystal lattice distortions in the vicinity of the impurity ion and the F⁻ ion compensating for the excess positive charge in T₄. Within the frames of a superposition model, it is shown that three F⁻ ions from the nearest surrounding cube, located symmetrically with respect to the C₃ axis from the side of the ion-compensator, approach the impurity ion and cling to the axis of the centre when forming T₄. The F⁻ ion located on the axis of the centre between the Yb³⁺ ion and ion-compensator, also approaches close to the impurity ion.

The transverse-field Ising model is successfully applied to the Ba_xSr_1−xTiO₃ system. An impurity-induced paraelectric–ferroelectric phase transition is found for proper parameters. An explanation is offered for the results of the susceptibility χ(x, T), the transition temperature T_m (x), the spontaneous polarization ⟨P ⟩ versus x and versus T, the field dependence of χ(x, T) and that of the polarization ⟨P ⟩ versus E for x, 0.2 ≤ x ≤ 0.95.

The phonon Raman spectra of Bi₂Sr₂Ca_1−xR_xCu₂O_8+δ (R = Pr and Gd) single crystals are systematically investigated. The experimental results show that the O(2)_Sr A_1g mode softens with Pr and Gd doping, while the O(1)_Cu B_1g mode softens with Pr doping but hardens with Gd doping. The changes of average ionic radius on the Ca site in Bi-based cuprates can account well for the Raman frequency shifts of the O(1)_Cu mode, but have little influence on the O(2)_Sr mode. The frequency softening of the O(2)_Sr mode in Pr-and Gd-doped Bi2212 crystals mainly results from contraction of the BiO bilayers with doping content. The correlation between the O(2)_Sr mode frequency and the c-axis parameter as well as the incommensurate modulation wavelength is discussed.

We report the phenomenon that the intensity of the ultraviolet (UV) photoluminescence (PL) from ZnO was greatly enhanced by incorporating ZnO into the SiO₂ matrix. PL excitation results show that both the ZnO nanoparticles and the SiO₂ matrix in the nanocomposites contribute to the luminescence process for the UV band. On the basis of the x-ray photoelectron spectra, we suggest that interface energy states are formed due to the presence of Zn–O–Si bonds between ZnO nanoparticles and the SiO₂ matrix. A tentative model concerning the contribution of the ZnO nanoparticles, SiO₂ matrix, and ZnO–SiO₂ interface is suggested to explain the PL enhancement effect.

The mineral zircon, ZrSiO₄, belongs to a class of promising materials for geochronometry by means of thermoluminescence (TL) dating. The development of a reliable and reproducible method for TL dating with zircon requires detailed knowledge of the processes taking place during exposure to ionizing radiation, long-term storage, annealing at moderate temperatures and heating at a constant rate (TL measurements). To understand these processes one needs a kinetic model of TL. This paper is devoted to the construction of such a model. The goal is to study the qualitative behaviour of the system and to determine the parameters and processes controlling TL phenomena of zircon. The model considers the following processes: (i) Filling of electron and hole traps at the excitation stage as a function of the dose rate and the dose for both (low dose rate) natural and (high dose rate) laboratory irradiation. (ii) Time dependence of TL fading in samples irradiated under laboratory conditions. (iii) Short time annealing at a given temperature. (iv) Heating of the irradiated sample to simulate TL experiments both after laboratory and natural irradiation.

The input parameters of the model, such as the types and concentrations of the TL centres and the energy distributions of the hole and electron traps, were obtained by analysing the experimental data on fading of the TL-emission spectra of samples from different geological locations. Electron paramagnetic resonance (EPR) data were used to establish the nature of the TL centres. Glow curves and 3D TL emission spectra are simulated and compared with the experimental data on time-dependent TL fading. The saturation and annealing behaviour of filled trap concentrations has been considered in the framework of the proposed kinetic model and compared with the EPR data associated with the rare-earth ions Tb³⁺ and Dy³⁺, which play a crucial role as hole traps and recombination centres. In addition, the behaviour of some of the SiO_mⁿ⁻ centres has been compared with simulation results.

Ions are commonly believed to be detrimental to diamond growth because of the high degree of lattice disorder induced by ion bombardments. In this paper, we examine the possibility of preparing diamond using thermally evaporated C₆₀ and simultaneous bombardment with Ne⁺ ions. It is found that the diamonds can be grown on Si wafers in the appropriate substrate temperature and ion energy ranges. Micro-Raman spectroscopy, x-ray diffractometry, and scanning electronic microscopy were employed to characterize the deposited specimen. These measurements provide definite evidence of the structure of nanosized hexagonal diamond. The mechanism responsible for the diamond formation is discussed.

Ruthenium-doped γ-Fe₂O₃ has been synthesized and examined by x-ray powder diffraction, XANES, EXAFS and by ⁵⁷ Fe Mössbauer spectroscopy. Ruthenium K-edge x-ray absorption spectroscopy shows that ruthenium adopts a fully occupied octahedral site in the spinel related γ-Fe₂O₃ structure as Ru⁴⁺. The ⁵⁷ Fe Mössbauer spectra recorded in the presence of a longitudinal magnetic field of 6 T confirmed the octahedral coordination of the tetravalent ions and canting angles for the Fe³⁺ ions were determined as 24° for those in octahedral sites and 33° for those in tetrahedral sites. The ⁵⁷ Fe Mössbauer spectra recorded in situ from ruthenium-doped γ-Fe₂O₃ showed parameters typical of maghemite up to 600 K but with a magnetic hyperfine field distribution suggesting an inhomogeneous distribution of ruthenium within particles of varied size around about 15 nm. At 700 K a phase transition from γ-Fe₂O₃ to α-Fe₂O₃ was observed and further studies showed the ruthenium-doped α-Fe₂O₃ to have a Morin transition temperature of about 400 K.

Ordered TiO₂ nanowire arrays have been successfully fabricated into the nanochannels of a porous anodic alumina membrane by sol–gel electrophoretic deposition. After annealing at 500°C, the TiO₂ nanowire arrays and the individual nanowires were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and x-ray diffraction (XRD). SEM and TEM images show that these nanowires are dense and continuous with a uniform diameter throughout their entire length. XRD and SAED analysis together indicate that these TiO₂ nanowires crystallize in the anatase polycrystalline structure. The optical absorption band edge of TiO₂ nanowire arrays exhibits a blue shift with respect of that of the bulk TiO₂ owing to the quantum size effect.

We present a study of Fano-type resonances in high quality boron-doped silicon as a function of boron content. The resonance (antiresonance) in the infrared absorption spectra occurs close to the ≈ 0 optical phonon at 519 cm⁻¹. The interaction between the otherwise infrared-forbidden optical phonon and the continuum states of the acceptor was analysed based on a modified Fano model that involves the interaction of a discrete state with two continua.

A nonlinear system possessing a natural forbidden band gap can transmit energy of a signal with a frequency in the gap, as recently shown for a nonlinear chain of coupled pendulums (Geniet and Leon 2002 Phys. Rev. Lett.89 134102). This process of nonlinear supratransmission, occurring at a threshold that is exactly predictable in many cases, is shown to have a simple experimental realization with a mechanical chain of pendulums coupled by a coil spring. It is then analysed in more detail. First we go to different (nonintegrable) systems which do sustain nonlinear supratransmission. Then a Josephson transmission line (a one-dimensional array of short Josephson junctions coupled through superconducting wires) is shown to also sustain nonlinear supratransmission, though being related to a different class of boundary conditions, and despite the presence of damping, finiteness, and discreteness. Finally, the mechanism at the origin of nonlinear supratransmission is found to be a nonlinear instability, and this is briefly discussed here.

In this work, we suggest an interatomic potential for iodine applicable to the simulation of the condensed phases of the halogen within the temperature and density range accessible to experiments. The potential includes an attractive term that is partitioned into directional chemical bonding with a many-particle character and a pairwise interaction. Despite its simplicity, the potential reproduces the crystal structure of solid iodine, the presence of atomic phases with increasing pressure, and the metallic or insulating character of the solid phases. Finally, we present preliminary simulation results for fluid iodine.