Table of contents

Conditions in favour of a realistic multilevelled description of a decohering quantum system are examined. In this regard the first crucial observation is that the thermal effects, contrary to the conventional belief, play a minor role at low temperatures in the decoherence properties. The system–environment coupling and the environmental energy spectrum dominantly affect the decoherence. In particular, zero temperature quantum fluctuations or non-equilibrium sources can be present and influential on the decoherence rates in a wide energy range allowed by the spectrum of the environment. A crucial observation against the validity of the two-level approximation is that the decoherence rates are found to be dominated not by the long time resonant but the short time off-resonant processes. This observation is demonstrated in two stages. Firstly, our zero temperature numerical results reveal that the calculated short time decoherence rates are Gaussian-like (the time dependence of the density matrix is led by the second time derivative at t = 0). Exact analytical results are also permitted in the short time limit, which, consistent with our numerical results, reveal that this specific Gaussian-like behaviour is a property of the non-Markovian correlations in the environment. These Gaussian-like rates have no dependence on any spectral parameter (position and the width of the spectrum) except, in totality, the spectral area itself. The dependence on the spectral area is a power law. Furthermore, the Gaussian-like character at short times is independent of the number of levels (N), but the numerical value of the decoherence rates is a monotonic function of N. In this context, we demonstrate that leakage, as a characteristic multilevel effect, is dominated by the non-resonant processes.

The long time behaviour of decoherence is also examined. Since our spectral model allows Markovian environmental correlations at long times, the decoherence rates in this regime become exponential independently from the number of levels. The latter and the coupling strengths play the major role in the quantitative values of the rates and the rates are independent of the other spectral parameters.

The validity of the presented results is restricted only by their reliance on the Born–Oppenheimer approximation. This approximation is strongly dependent on the external observational time and its reliability depends on an additional timescale. In the rest of the work, the crossover between the short and the long time behaviour of the density matrix of the multilevelled system is examined using an intuitive argument. It is shown that the Born approximation weakens as the resonant couplings become more effective at long times. This implies that, in calculations made with this approximation in the long time regime, a need for a justification arises for the reliability of the results. This justification is made for the present work.

In this paper we study the effects of intermetallic nanoparticles like Ni₃Al on the evolution of vacancy defects in the fcc Fe–Ni–Al alloy under electron irradiation using positron annihilation spectroscopy. It was shown that the nanosized (∼4.5 nm) intermetallic particles homogeneously distributed in the alloy matrix caused a several-fold decrease in the accumulation of vacancies as compared to their accumulation in the quenched alloy. This effect was enhanced with the irradiation temperature. The irradiation-induced growth of intermetallic nanoparticles was also observed in the pre-quenched Fe–Ni–Al alloy under irradiation at 573 K. Thus, a quantum-dot-like positron state in ultrafine intermetallic particles, which we revealed earlier, provided the control over the evolution of coherent precipitates, along with vacancy defects, during irradiation and annealing. Possible mechanisms of the absorption of point defects by coherent nanoparticles have been discussed too.

The emission spectra and the excitation spectra of various emissions have been measured in LiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence of self-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and 5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single exciting photon of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondary exciton. The tunnel phosphorescence has been detected after the irradiation of LiF by an electron beam or x-rays at 6 K, and several peaks of thermally stimulated luminescence (TSL) at 12–170 K appeared at the heating of the sample. It was confirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes (V_K centres). The TSL peak at ∼160 K is ascribed to the thermal ionization of F^' centres. The TSL at 20–30 K and 50–65 K is caused by the diffusion of interstitial fluorine ions (I centres) or H interstitials, respectively. The TSL peak at ∼13 K, the most intense after electron or x-irradiation, cannot be detected after LiF irradiation by VUV radiation, selectively forming excitons or e–h pairs. The creation of a spatially correlated anion exciton and an e–h pair is needed for the appearance of this peak: an exciton decays into an F–H pair, a hole forms a V_K and an electron transforms H into I (an F–I–V_K group is formed) or an F centre into a two-electron F^' centre (an F^'-H -V_K group). The analysis of the elementary components of the 9–16 K TSL showed that a phonon-induced radiative tunnel recombination of F^'–V_K (5.6 eV), F–H (∼3 eV) and F–V_K (3.4 eV) occurs within these groups.

We investigate the stress–strain relationship and elastic stability of zinc-blende GaP, GaN, InP and BN lattices under hydrostatic pressure by first-principles calculation. A simple and direct ab initio implementation for studying the mechanical properties of cubic crystals is developed. The four phases' full-set stress–strain coefficients in wide pressure ranges are theoretically calculated. The fundamental mechanism of elastic stability and the origin of phase transformation under hydrostatic pressure are explored. We found that the abilities for most of these lattices are enhanced to sustain axial strain but weaken to shear strain under higher pressure. The conditions of lattice stability are analysed using both the thermodynamic work–energy criterion and the elastic-stiffness criteria. We show that the lattice collapse of the perfect crystals is caused by the disappearance of their bulk moduli under volume dilation. Lattice defects are considered to be the main reason causing phase transformation under pressure. The correlation between the phonon softening and the variation of elastic coefficients is studied. The pressure dependence of the Kleinman internal strain parameter and its relationship to elastic stability is also explored.

We have investigated oriented single crystals of the antiferromagnet NpRhGa₅ (T_N≈37 K) by means of magnetization and specific heat measurements. Strong anisotropy effects are observed: when magnetic fields are applied in the basal plane, the Néel temperature is barely affected. In contrast, in fields applied along the c axis, the second-order transition (magnetic ordering) is shifted to lower temperatures and merges into the first-order transition (reorientation of the magnetic moments). A full mapping of this complex magnetic phase diagram as a function of the temperature (down to 1.6 K), applied magnetic field (up to 9 T) and crystal orientation is presented. This study shows that antiferromagnetic order is more fragile when the magnetic field is applied along the c axis, which is similar to the situation observed for the NpCoGa₅ homologue compound.

Previous experimental studies describe an efficient photoresponse in the visible-light region for anion-doped TiO₂. Doping with carbon, nitrogen, as well as sulfur, yields promising second-generation photocatalysis with TiO₂. We present a theoretical investigation of substitutional anion doping in TiO₂ and discuss doping effects on the electronic structure, and subsequently the photoactivity. The resulting bandgap narrowing predicted in this work is consistent with experimental observations. Furthermore, we discuss the effects of doping concentration on the localization properties of the valence band edge. Our systematic study of anion-doped TiO₂ implies that the carbon-doped TiO₂ is the most promising due to a significant overlap between the O 2p state and the carbon states near the valence band edge. Additionally, carbon dopants produce the largest valence band red shift of the three anion-doped TiO₂ studied.

The electronic structure of quasicrystalline Al–Pd–Mn is investigated by means of valence and core level photoelectron spectroscopy. Variations of the photoionization cross section in the constituents' valence electronic levels as a function of photon energy are used to identify contributions from the different atomic species, in particular near the Pd 4d Cooper minimum. Resonant photoemission at the Mn 2p absorption edge shows the contribution of the Mn 3d states to the density of states in a region near the Fermi level. The asymmetry of Pd 3d and Mn 2p core level photoemission lines, and its difference for emission from metallic and quasicrystalline phases, are utilized to infer the contributions of the different constituents to the density of states at the Fermi level.

Temperature dependent photoluminescence excitation spectroscopy is employed to evaluate basic physical properties of the 2.87 eV absorption peak, recently discovered for the GaN_xP_1−x alloys. Whereas the appearance of this transition is found to be facilitated by incorporation of N and also H atoms, its intensity does not scale with the N content in the alloys. This questions the possible association of this feature with an N-related localized state. On the basis of the results of temperature dependent measurements, it is concluded that the state involved has a non-Γ character. Excitation of the known N-related localized states via this state is found to be non-selective, unlike that between the N-related centres. The observed properties are shown to be barely consistent with those predicted for the higher lying localized state of the isolated N atom derived from the Γ conduction band minimum (CBM). Alternative explanations for the '2.87 eV' state as being due to either a t₂ component of the X₃^c (or L₁^c) CBM or a level arising from a complex of N and H (in some form) are also discussed.

In the past, the Callen–Callen (1965 Phys. Rev. 139 A455–71; 1966 J. Phys. Chem. Solids27 1271–85) model has been highly successful in explaining the origin and temperature dependence of the magneto-crystalline anisotropy in many magnetic compounds. Yet, despite their high ordering temperatures of ∼650 K, the Callen–Callen model has proved insufficient for the REFe₂ compounds. In this paper, we show that it is possible to replicate the values of the phenomenological parameters K₁, K₂, and K₃ given by Atzmony and Dariel (1976 Phys. Rev. B 13 4006–14), by extending the Callen–Callen model to second order in H_CF. In particular, explanations are provided for (i) the unexpected changes in sign of K₁ and K₂ in HoFe₂ and DyFe₂, respectively, and (ii) the origin and behaviour of the K₃ term. In addition, it is demonstrated that higher order terms are required, and that K₄ exceeds K₃ at low temperatures. Revised estimates of K₁, K₂, K₃, K₄, and K₅ are given. Finally, an alternative 'multipolar' approach to the problem of magnetic anisotropy is also provided. It is shown that the latter confers significant advantages over the older phenomenological method. In particular, all the multipolar coefficients (, N = 4, 6, 8, 10, 12) decrease monotonically with increasing temperature, with decreasing faster than etc. These observations are in accord with expectations based on the original Callen–Callen model.

The magnetic structure of UCu₂Si₂ has been studied by means of neutron scattering. We have observed the first- and very weak third-order incommensurate satellite reflections for 100 K<T<106 K with the propagation q = [0 0 δ], δ = 0.116 at T = 101 K, indicating nearly sinusoidal magnetic modulation with long periodicity of Λ = 85.7 Å. The intensity of the first-order satellite reflections is consistent with a moment direction parallel to the c-axis. The second-order satellite due to lattice modulation accompanied by a charge density wave could not be detected within our experimental sensitivity. The long-period magnetic modulation in UCu₂Si₂ cannot be explained in terms of an ANNNI model based on frustrated antiferromagnetic short-range interactions. The nearly sinusoidal long-period modulation is evidence for a spin density wave state as a consequence of itinerant behaviour of the 5f electrons in UCu₂Si₂. The ferromagnetic ground state and the very weak upper critical field for the incommensurate phase are consistent with the long periodicity.

The temperature dependent resistivity and thermoelectric power of monovalent (Ag) doped La_0.7Sr_0.3−xAg_xMnO₃ polycrystalline pellets (x = 0.05,0.10,0.20,0.25) between 20 and 450 K are reported. Ag substitution enhances the conductivity of this system. The Curie temperature (T_C) also increases from 303 to 364 K with increasing Ag content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing the residual resistivity, a two-magnon scattering term and a term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. The small-polaron hopping mechanism is found to fit well to the thermoelectric power data for T>T_C and in the high temperature limit for the thermoelectric power is primarily defined by the spin contribution. At low temperatures (T<T_C) in the ferromagnetic region, the Seebeck coefficient (S) is well explained with an expression of the type: S_FM = S₀+S_3/2T^3/2+S₄T⁴, and confirms the dominant role of electron–magnon scattering. Both resistivity and thermopower data over the entire temperature range are further examined in the light of a two-phase model based on an effective medium approximation. Thermopower data on La_0.7Sr_0.3−xAg_xMnO₃, computed using measured resistivity and the expression derived using the effective medium approach, give a fairly good description of the observed thermal variation of thermoelectric power in monovalent (Ag) doped La_0.7Sr_0.3−xAg_xMnO₃.

We develop a generalized real-space effective medium super-cell approximation (EMSCA) method to treat the electronic states of interacting disordered systems. This method is general and allows randomness both in the on-site energies and in the hopping integrals. For a non-interacting disordered system, in the special case of randomness in the on-site energies, this method is equivalent to the non-local coherent potential approximation (NLCPA) derived previously. Also, for an interacting system the EMSCA method leads to the real-space derivation of the generalized dynamical cluster approximation (DCA) for a general lattice structure. We found that the original DCA and the NLCPA are two simple cases of this technique, so the EMSCA is equivalent to the generalized DCA where there is included interaction and randomness in the on-site energies and in the hopping integrals. All of the equations of this formalism are derived by using the effective medium theory in real space.

The magnetovolume effect of itinerant electron magnets is discussed by the explicit examination of the volume dependence of the spin fluctuation free energy. Magnetic Grüneisen parameters are introduced instead of magnetovolume coupling constants, that enable us to describe all the magnetovolume properties in terms of these parameters. We have particularly found the presence of a new thermal expansion, showing T²-like temperature dependence. It explains the appreciable electronic volume thermal expansions observed experimentally. We also show that the magnetovolume coupling constants for spontaneous and forced magnetostrictions, of different magnitudes, are temperature dependent. Analysis of the pressure effect on the Curie temperature T_c and the spontaneous magnetic moment M shows that the linear relation between them, , is generally violated.

Static magnetization for single crystals of insulating Nd_0.85Pb_0.15MnO₃ and marginally conducting Nd_0.70Pb_0.30MnO₃ has been studied around the ferromagnetic to paramagnetic transition temperature T_C. Results of measurements carried out in the critical range |(T− T_C)/T_C|≤0.1 are reported. Critical exponents β and γ for the thermal behaviour of magnetization and susceptibility have been obtained both by modified Arrott plots and the Kouvel–Fisher method. The exponent δ independently obtained from the critical isotherm was found to satisfy the Widom scaling relation δ = γ/β+1. For both compositions the values of exponents are consistent with those expected for isotropic magnets belonging to the Heisenberg universality class with short-range exchange in three dimensions. Correspondingly, the specific heat displays only a cusp-like anomaly at the critical temperature of these crystals which is consistent with an exponent α<0. The results show that the ferromagnetic ordering transition in Nd_1−xPb_xMnO₃ in the composition range 0.15≤x≤0.40 is continuous. This mixed-valent manganite displays the conventional properties of a Heisenberg-like ferromagnet, irrespective of the differing transport properties and in spite of low ordering temperatures T_C = 109 and 147.2 K for x = 0.15 and 0.30, respectively.

We perform first principles calculations based on density functional theory with the generalized gradient approximation (GGA) on the electronic and magnetic properties of carbon dopant in graphitic boron nitride (graphitic-BN) by the supercell method. It is found that carbon substitution for either boron or nitrogen atoms in graphitic-BN favours spin polarization over non-spin-polarization by 0.1 eV. The calculated electronic band structures show a spin-polarized, dispersionless band near the Fermi energy. The spin polarization is found to originate from the carbon dopant and can be attributed to the carbon 2p electron. The lower energies and the resistivity to curvature effect of the spin polarization suggest a possibility of ferromagnetic ordering of the carbon dopants in many realistic BN-based nanostructures which possess a hexagonal network such as nanotubes. Energies required for carbon substitution turn out to be within experimental access. The random substitution and the low doping energy indicate that graphitic-BN-based nanostructures can be viewed as candidates for metal-free magnet applications.

We propose a simplified version of the one-bond two-mode percolation model originally developed for the long wave phonons related to the stiff Be–VI bond in (Zn, Be) chalcogenides, which open the class of random mixed crystals with contrast in the bond stiffness. This is deduced from the comparison between the Raman responses from the stiff Be–VI bond and the soft Zn–VI one in these systems. The simplified version is tested on (Ga, In)As, made of soft-like bonds only and taken here as a representative challenging system. This results in a successful reinterpretation of the puzzling multi-phonon behaviour in the Raman/infrared spectra of this alloy, that has been a subject of debate. The discussion is supported by contour modelling of the TO and LO Raman lineshapes by applying the Hon and Faust treatment to a version of the modified-random-element-isodisplacement model generalized to multi-oscillators. Also, the assignment of the long wave phonons in (Ga, In)As is supported by atomistic calculations of the bond length distributions of the minority bond species in large (Ga, In)As supercells corresponding to alloy compositions close to the In–As (In ∼0.19) and Ga–As (In ∼0.81) bond percolation thresholds. The configurations are analysed to distinguish between isolated and connected bonds, not in the usual terms of next nearest neighbours.

The high resolution and polarized absorption spectroscopic data of Nd³⁺-doped, Mg²⁺-compensated strontium hexa-aluminate Sr_1−xNd_yLa_x−yMg_xAl_12−xO₁₉ (ASL: Nd) crystals in an extended compositional range (0.01≤x≤0.5) are presented. This crystal offers for the Nd³⁺ ions a unique crystallographic site (2d) with 12 O²⁻ coordination and ideal D_3h symmetry. The optical spectra in all this compositional range contain contributions from two families of structural centres, C₁ and C₂ (with two components C₂^' and C₂^''), whose spectral features are mainly dependent on the composition parameter x. The presence of these centres is attributed to the cationic disorder. It is inferred that while the centres C₁ and C₂ are connected with the presence or absence of trivalent ions in the nearest (2d) coordination sphere normally occupied by Sr²⁺, the two components C₂^' and C₂^'' are associated to the existence or lack of Mg²⁺ ions in the nearest sphere of Al³⁺ tetrahedral (Al_tetr³⁺) sites. This structural disorder leads to crystal-field perturbations by electric charge difference, although for C₁ centres the perturbation by ionic size differences is also evident. The polarized spectroscopy indicates departures from the selection rules for D_3h symmetry for C₁ centres, whereas both polarized spectra and crystal-field analysis confirm the near-D_3h symmetry for C₂ centres. The composition dependence of the spectra indicates that the distribution of the La³⁺ and Nd³⁺ ions at the (2d) sites is random, while that of the Mg²⁺ in the Al_tetr³⁺ sites is correlated with the former. The connection of this structural model with the EPR data and with earlier models is discussed.

Temperature and structural disorder can have a strong influence on the stability of solitons excited in protein molecules. This problem has been studied in depth with an improved model, using numerical simulation and the Runge–Kutta method, for biological temperatures. The results obtained show that the soliton is thermally stable in the region of biological temperatures, 300 K<T<320 K, and is very robust against structural disorder, including substantial disorder in the sequence of masses of the amino acids and fluctuations of the spring constant, coupling constant, dipole–dipole interaction constant and ground state energy, due to its larger binding energy which can suppress the destructive effect of the above types of disorder and thermal perturbation. Therefore this soliton is a possible carrier for bio-energy transport in protein molecules. However, very strong structural disorder can also destroy the stability of this soliton.

General distributions of relaxation times are discussed and then specialized to two types associated with Kohlrausch stretched-exponential temporal response, the K0 and K1 models. For the important choice of 1/3 for their beta shape parameters, their specific distributions and different temporal responses are first compared. Then the 16 real and imaginary parts of their dielectric- and conductive-system frequency responses are presented in normalized form. Only eight of these are distinct, however, because of pairing of identical dielectric and conductive responses. There are five different peaked imaginary-part pairs, two of which differ only in scale: the important conductive-system M''(ω) response and the dielectric-system ε''(ω) one. Their near equality explains how the widely used but inappropriate original modulus formalism (OMF) of Moynihan and associates, proposed in 1973, could be implicitly derived from pure dielectric considerations and yet fortuitously yield conductive-system response. The crucial effects of the endemic dielectric quantity on K0- and K1-model responses are illustrated, and they explain why conductive-system shape parameter values derived from data fitting with the OMF model have been misleadingly found to depend on temperature and charge-carrier concentration. Instead, the K1 model fits data for a wide variety of homogenous materials with a value of 1/3 independent of the values of these variables. Finally, different fits of a historic experimental data set are compared to illustrate the present findings.

We studied the structural and electrical properties of TiO₂ thin films grown by thermal oxidation of e-beam evaporated Ti layers on Si substrates. Time of flight secondary ion mass spectroscopy (TOF-SIMS) was used to analyse the interfacial and chemical composition of the TiO₂ thin films. Metal oxide semiconductor (MOS) capacitors with Pt or Al as the top electrode were fabricated to analyse electrical properties of the TiO₂ thin films. We show that the reactivity of the Al top contact affects electrical properties of the oxide layers. The current transport mechanism in the TiO₂ thin films is shown to be Poole–Frenkel (P–F) emission at room temperature. At 84 K, Fowler–Nordheim (F–N) tunnelling and trap-assisted tunnelling are observed. By comparing the electrical characteristics of thermally grown TiO₂ thin films with the properties of those grown by other techniques reported in the literature, we suggest that, irrespective of the deposition technique, annealing of as-deposited TiO₂ in O₂ is a similar process to thermal oxidation of Ti thin films.

Molecular dynamics simulations are conducted to investigate disordering of stishovite (a high-pressure polymorph of silica) and the role of defects including grain boundaries, vacancies and free surfaces. It is shown that pre-existent defects initiate or facilitate solid-state disordering and melting. As illustrated in the case of vacancies, melting precedes solid-state disordering for stishovite with low defect concentrations, while defect-rich stishovite may transform into high-density vitreous silica which then undergoes a (quasi-)continuous transition into melt. In sharp contrast, melting of ordinary glass at the same heating rate is sluggish yet evidently first order. Disordering on free surfaces is anisotropic, being more pronounced on (101), (011) and (111) than on (100), (010), (001) and (110) crystallographic planes.

We report ⁵⁹Co NMR studies on magnetically oriented powder samples of Co oxide superconductors, Na_xCoO₂·yH₂O, with T_c∼4.7 K. From the two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated from the magnetic field dependence of the second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility χ. From the analysis of the so-called K–χ plot, the spin and the orbital components of K and the positive hyperfine coupling constant were estimated. The onset temperature of the superconducting transition in the Knight shift does not change much in an applied magnetic field up to 7 T, which is consistent with the reported high upper critical field H_c2. The Knight shift at 7 T shows an invariant behaviour below T_c. No coherence peak just below T_c was observed in the temperature dependence of the nuclear spin–lattice relaxation rate 1/T₁ in either case (NMR, nuclear quadrupole resonance). We conclude that the invariant behaviour of the Knight shift below T_c and unconventional behaviours of 1/T₁ may indicate spin triplet superconductivity with p- or f-wave symmetry.

The cohesive energetics of three phases of solid cubic rubidium chloride, the zinc blende structured 4:4 phase, the 6:6 sodium chloride polymorph and the 8:8 phase with the cesium chloride structure, are computed using a non-empirical fully ionic model. The rearrangement energies needed to convert free anions to their optimal states in-crystal, two-body inter-ionic potentials, plus the further contributions arising from electron correlation, are reported. The 'optimal' anion–anion potentials, computed by using at each geometry the optimal wavefunction, are compared with the 'frozen' potential using the same wavefunction at all geometries.

The lattice energy of the 4:4 structure is predicted to be some 40 kJ mol⁻¹ smaller than that of either the 6:6 or the 8:8 phases. Introduction of the Axilrod–Teller triple dipole dispersion interactions and the vibrational zero point energy predicts the 8:8 phase to lie 3.2 kJ mol⁻¹ lower in energy than the 6:6 structure. This is both consistent with radius ratio arguments and supported by two separate experiments that strongly suggest that the 8:8 phase is favoured over the 6:6 structure at low temperatures even though the latter is more stable at ambient temperatures.

A shell model description is presented for the ion-induced dipole interactions that arise both in small clusters and in crystals encapsulated in nanotubes. The elastic constants and entropy at 300 K predicted for the 6:6 phase from this model by using the GULP program agree well with experiment. A smaller entropy is predicted for the 8:8 structure.

Measurements of the magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, specific heat, thermoelectric power and neutron powder diffraction on a polycrystalline sample of β-UB₂C are reported. It is found that this compound with enhanced electronic specific-heat coefficient is a ferromagnet below T_C = 74.5(0.5) K. At 1.5 K the ordered magnetic moment of the uranium ions is found to be 1.12(1) μ_B (U1 in 3b) and 1.03(1) μ_B (U2 in 6c), respectively. The uranium moments are close to the ab-plane (Φ = 47° and Θ = 84°) forming ferromagnetic chains parallel to the hexagonal c-axis. In addition to the ferromagnetic transition, we found a characteristic temperature T^* = 37 K, at which both the electrical resistivity and specific heat show anomalies. The thermoelectric power is positive and displays a maximum at 12 K. The observed features are compared to those of ferromagnetic UGe₂ and UIr, known as superconductors under pressure. We discuss the experimental data in terms of strong electron correlation effects.

The properties of the electron paramagnetic resonance spectra of Gd³⁺ ions in PbWO₄ single crystals have been investigated in the X-band microwave frequency region, in the 1.5 to 290 K temperature range. The observed S₄ symmetry of the local crystal field at the Gd³⁺ impurity ions strongly suggests that the Gd³⁺ ions substitute for the Pb²⁺ lattice cations, with charge compensation at a distance. The spin Hamiltonian parameters are comparable with those of the Gd³⁺ ions in other isomorphous tungstates. The temperature variation of the fine structure parameters B₂⁰ and B₄⁰ points to an energy transfer between the impurity ions and the host lattice, which takes place mainly through a local vibrational mode of frequency ω = 3.1 × 10¹³ rad s⁻¹.

The theory of transversal light forces on electron–hole pairs in semiconductors has been formulated recently (Lindberg and Binder 2003 J. Phys.: Condens. Matter15 1119), but only light forces from single Gaussian beams were considered. In the present paper, it is shown theoretically that Hermite–Gaussian beams can be used to reduce and even reverse natural wavepacket spreading. Furthermore, a spatially moving beam can be used to displace and accelerate an electron–hole plasma, in analogy to well-known optical tweezers in atomic systems. The light forces exerted by Hermite–Gaussian beams appear to be robust and therefore of possible practical importance.

The transport properties of the tetravalent ion-doped La_0.8Zr_0.2MnO₃ (LZMO) film, with strong magnetic–resistive correlation, have been studied. Vacuum annealing was adopted to modify the valence of Mn of the film. During the vacuum annealing process, the resistivity of the film increased overall and the metal–semiconductor transition shifted to lower temperature and finally vanished. The Hall effect studies revealed that the charge carriers were hole-like and the density of the carriers decreased with the oxygen escaping from the film. The x-ray photoelectron spectroscopy showed that the Mn ions in the film were driven from a mixture of Mn³⁺ and Mn⁴⁺ to that of Mn²⁺ and Mn³⁺. When the Mn³⁺ and Mn²⁺ ions were dominant in the film, no resistive–magnetic correlation was observed. These results indicated that the magnetic–resistive correlation in the LZMO film was attributable to the interaction between Mn³⁺ and Mn⁴⁺ ions.

The results of studies of physical properties of Ni fcc lattice obtained with the help of the reduced all-neighbour approximation of the self-consistent phonon theory are presented. The interatomic interactions are described by the modified form of the generalized Morse potential, proposed lately by Akgün and Uğur, with parameters derived from the experimental data for the lattice constant, cohesive energy, bulk modulus and elastic constants. As a test of the validity of both the model of interatomic interactions and the model of lattice dynamics the temperature variations of selected physical properties of Ni are given and compared with experimental and other theoretical data.