Table of contents

The geometrically frustrated antiferromagnet Gd₂Ti₂O₇ exhibits magnetic behaviour of such complexity that it poses a challenge to both experiment and theory. Magnetic ordering commences at T_N = 1.1 K and there is a further magnetic phase transition at  K. Here we use neutron diffraction to definitively establish the nature of the phase transition at and the magnetic structure adopted below this temperature. Between and T_N the structure is partly ordered, as previously reported. Below the remaining spins order, but only weakly. The magnetic structure in this temperature range is shown to be a 4-k structure, closely related to the 1-k structure previously suggested. The 4-k and 1-k variants of the structure are distinguished by analysis of the diffuse scattering, which we believe represents a new method of solving the 'multi-k' problem of magnetic structure determination.

The specific heat and entropy of (wurtzite) GaN are not well known experimentally. The literature values for C_p (T>298 K) are based on a fit to an analytic function. The parameters include C_p (298 K) and the melting temperature T_m, both of which were poorly known when the parameters were first determined. The value of C_p (298 K) disagrees with the data measured in the range 5–305 K. Our first-principles calculations allow the values of the parameters to be established and lead to a more accurate prediction of specific heat and entropy of GaN.

We have studied the magnetic and thermal properties of a single crystal of CePt₃Si, which is a recently reported heavy-fermion superconductor with a superconducting transition temperature T_c = 0.75 K and a Néel temperature T_N = 2.2 K. The overall experimental data are principally explained on the basis of the crystalline electric field (CEF) scheme. Even in the antiferromagnetic state, the CEF model applies well to the characteristic features in the magnetization curve. These results indicate the existence of a localized magnetic moment at the Ce site, with a considerably reduced ordered moment of 0.16 μ_B/Ce, and the strongly correlated conduction electrons are condensed into the superconducting state. We have also constructed the magnetic phase diagram including the superconducting phase for and [001].

We review recent developments in the area of self-assembled monolayers (SAMs) and their applications. First, we discuss issues related to the structure, the phase transitions, the phase diagram, and the growth dynamics. We explain how the internal degrees of freedom and the multiple interactions involved can lead to a fairly rich phase behaviour even for systems which are commonly considered 'simple' model systems. Then we discuss selected problems for more complex SAM-based systems, including SAMs as substrates for growth, SAMs and molecular electronics, electrochemical applications, and 'switchable' SAMs, as well as the use of SAMs for biofunctionalized surfaces and lateral structuring.

Prepeaks are observed in the x-ray diffraction patterns of molten Fe₃Si and FeSi alloys and the mechanism of their microstructures is discussed. The distance in real space corresponding to the prepeak positions resembles that of the Si–Si distances in the D0₃ type crystal of Fe₃Si within a deviation of 2.4%. Furthermore, by the reverse Monte Carlo (RMC) simulation, the prepeak is only found in the partial structure S_SiSi(Q) for molten Fe₃Si alloy, while in molten FeSi alloy prepeaks are found in both S_SiSi(Q) and S_FeFe(Q) with similar height. The Gaussian distribution is found in the partial coordination number distribution of Fe atoms around a Si atom in the two alloys. The dominant 7-coordination in molten FeSi alloy suggests that Fe₇Si and FeSi₇ type clusters are kept in the molten state from FeSi crystal. Si–Si coherent packing should be responsible for the medium range order (MRO) of molten Fe₃Si while Fe–Fe and Si–Si coherent packing for that of molten FeSi.

We investigate the wetting of planar, nonselective solid substrates by symmetric binary mixtures where the attraction strength between like molecules of components A and B is the same, that is _AA = _BB<0. Mixture properties come about by varying , that is by varying the attraction between a pair of unlike molecules. By means of mean-field lattice density functional calculations we observe a rich wetting behaviour as a result of the interplay between _AB and the attraction of fluid molecules by the solid substrate _W. In accord with previous studies we observe complete wetting only above the critical end point if the bulk mixture exhibits a moderate to weak tendency to liquid–liquid phase separation even for relatively strong fluid–substrate attraction. However, in this case layering transitions may arise below the temperature of the critical end point. For strongly phase separating mixtures complete wetting is observed for all temperatures along the line of discontinuous phase transitions in the bulk.

The equilibrium shapes of a simple cubic crystal in contact with a planar chemically patterned substrate are studied theoretically using an effective interface model. The substrate is primarily made of lyophobic material and is patterned with a lyophilic (easily wettable) stripe domain. Three regimes can be distinguished for the equilibrium shapes of the crystal. The transitions between these regimes as the volume of the crystal is changed are continuous or discontinuous depending on the strength of the couplings between the crystal and the lyophilic and lyophobic surface domains. If the crystal grows through a series of states close to equilibrium, the discontinuous transitions correspond to growth instabilities. These transitions are compared with similar results that have been obtained for a volume of liquid wetting a lyophilic stripe domain.

We present the Hugoniots of Al, Ta, Mo and W in their solid as well as liquid phases. The liquid phase calculations are carried out on the basis of the corrected rigid spheres (CRIS) model. The 0 K isotherm of the solid phases, which are the necessary inputs for our computations, have been obtained by full potential first principles electronic structure calculations with generalized gradient approximation (GGA) for the exchange–correlation terms. The melting curve as a function of pressure was obtained according to the recently published model based on dislocation mediated melting, and also compared with that using Lindemann criterion. Though the adiabatic pressure–volume curve is affected little by melting, the pressure–temperature curve shows substantial change.

It has been proposed that short-lived quantum entanglement of protons in condensed matter systems would result in anomalous inelastic scattering cross-sections at electronvolt energy transfers. This proposal seems to be confirmed by neutron measurements on the VESUVIO spectrometer at ISIS and by measurements using other techniques. However, there have been a number of published suggestions of ways in which the observed effects on VESUVIO could be introduced by assumptions used in the data analysis. In this paper it is shown using experimental data and Monte Carlo simulations that these suggestions cannot explain the observed cross-section anomalies. The other assumptions of the data analysis are also examined. It is shown that the assumption of a Gaussian peak shape for the neutron Compton profile can introduce significant errors into the determination of cross-section ratios, but also cannot explain the observed anomalies.

The nonlocal weighted density approximation (WDA) is compared with the local density approximation (LDA) and generalized gradient approximation (GGA) for a broad range of inhomogeneous electron gas densities that are confined in three dimensions (uniform confinement) and along just one dimension (non-uniform confinement), using a model external potential. All three functionals display similar properties in the uniformly confined case; however LDA and GGA energies diverge with respect to the WDA in the non-uniform strongly confined case. This is caused by the strong anisotropy in the XC hole, demonstrated by the WDA.

We investigate the interplay of phonons and correlations in superconducting pairing by introducing a model Hamiltonian with on-site repulsion and couplings to several vibration branches having the Cu–O plane of the cuprates as a paradigm. We express the electron–phonon (EP) coupling through two force constants for O–Cu and O–O bond stretchings. Without phonons, this reduces to the Hubbard model, and allows purely electronic W = 0 pairing. A W = 0 pair is a two-body singlet eigenstate of the Hubbard Hamiltonian, with no double occupancy, which gets bound via interactions with background particles. Indeed, this mechanism produces a Kohn–Luttinger-like pairing from the Hubbard repulsion, provided that its symmetry is not severely distorted. From the many-body theory, a canonical transformation extracts the effective two-body problem, which lends itself to numerical analysis in case studies. As a test, we use as a prototype system the CuO₄ cluster. We show analytically that at weak EP coupling the additive contributions of the half-breathing modes reinforce the electronic pairing. At intermediate and strong EP coupling and , the model behaves in a complex and intriguing way.

We study the effects of Coulomb interaction and inter-grain quantum tunnelling in an array of metallic grains using the phase-functional approach for temperatures T well below the charging energy E_c of individual grains yet large compared to the level spacing in the grains. When the inter-grain tunnelling conductance , the conductivity σ in d dimensions decreases logarithmically with temperature () (Panyukov and Zaikin 1991 Phys. Rev. Lett. 67 3168, Goppert and Grabert 2000 Eur. Phys. J. B 16 687, Efetov and Tschersich 2002 Europhys. Lett. 59 114), while for , the conductivity shows simple activated behaviour (). We show, for bare tunnelling conductance , that the parameter determines the competition between charging and tunnelling effects. At low enough temperatures in the regime , a charge is shared among a finite number of grains, and we find a soft activation behaviour of the conductivity, , where z is the effective coordination number of a grain.

Ab initio simulations are performed for Cu atoms adsorbed on the perfect MgO(001) substrate, with an ordered metal coverage varied from monolayer (ML), i.e. almost single atoms, up to 1 ML. A strong dependence of the adhesion energy and the sub-monolayer film distance from the substrate on the surface coverage and adsorbate positions (Mg²⁺ or O²⁻) is discussed. The nature of interfacial bonding at all coverages is physisorption. When increasing Cu atomic fraction, a decrease of the substrate-induced polarization of adatoms accompanied by an increase of both in-plane metallic bonding and the interfacial distance has been found. Combining results of ab initio calculations with thermodynamic theory (taking into account the lattice mismatch), we show that the metal cluster formation becomes the predominant growth mode even at low Cu coverages, in agreement with experiment.

The main aim of this work is to investigate the electronic structure, specific heat and magnetic properties of Ce_1−xLa_xRhSn compounds near the critical concentration . Near x₀ the magnetic properties for x>x₀ correspond to spin fluctuations, while for the Ce-rich samples (x<x₀) the mictomagnetic-like behaviour indicates the formation of magnetic clusters. Both the magnetic susceptibility and electrical resistivity data obtained for the Ce_0.55La_0.45RhSn, Ce_0.5La_0.5RhSn and Ce_0.45La_0.55RhSn compounds obey a non-Fermi liquid temperature dependences due to an atomic disorder and/or the presence of spin fluctuations.

Photon- and electron-stimulated luminescence assessments have been quantitatively used in assessing Er³⁺-doped silica-glass fibre to obtain information about (i) diffusional processes taking place at the core/clad interface and (ii) residual stresses piled up (during fibre manufacturing) both in the clad and in the filamentary core. The refractive index of the core in an optical fibre was found to be affected by residual stress; therefore, information about the micro/nanoscopic stress/structure of the optical fibre device, which is obtained through piezo-spectroscopic assessments on the fibre section, has been routinely used for inspection before and after preform drawing. A calculation of the thermal contribution to residual stress and a comparison between experimental and theoretical results are also offered in the discussion section.

We present the one-dimensional Korringa–Kohn–Rostoker (KKR) method with the aim of elucidating its linear features, particularly important in optimizing the numerical algorithms in energy band computations. The conventional KKR equations based on multiple scattering theory as well as novel forms of the secular matrix with nearly linear energy dependency of the eigenvalues are presented. The quasi-linear behaviour of these eigenvalue functions appears after (i) re-normalizing the wavefunctions in such a way that 'irregular' solutions vanish on the boundary of the 'muffin-tin' segments, and (ii) integrating the full Green function over the whole Wigner–Seitz cell. In addition, using the aforementioned approach we derive a one-dimensional analogue of the generalized Lloyd formula.

The novel KKR approach illustrated in one dimension can be almost directly applied to higher dimensional cases. This should open prospects for the accurate KKR band structure computations of very complex materials.

Structure, transport and magnetic properties have been investigated for the layered materials Na_γCo_1−xMn_xO₂ (). With increase of the Mn concentration, the in-plane lattice parameter a decreases while the lattice parameter c increases. 3% Mn doping can result in a metal–insulator transition. The temperature dependence of the magnetization of the samples obeys the Curie–Weiss law and the effective moment of the magnetic ions increases with doping. Electron energy-loss spectroscopy measurement shows that the Co valence is about 3.3 and remains unchanged upon doping. However, the valence of Mn increases rapidly with doping and reaches 3.7 ± 0.1 for the sample with x = 0.5. The results are discussed in terms of the disorder effect induced by doping of magnetic ions. This work demonstrates that the structure, transport and magnetic properties of Na_γCoO₂ are very sensitive to doping with magnetic ions.

Magnetophonon resonance in quantum wells in a tilted magnetic field B is investigated. Measurements of the Hall coefficient and correspondingly of the carrier concentration as a functions of magnetic field and temperature are simultaneously performed. It is shown that the experimental data can be interpreted in terms of a great sensitivity to the effects of varying the two dimensional carrier concentration n_s in a certain concentration interval. In other words, the observed angular dependence of the MPR amplitudes is a manifestation of dependence of n_s on the magnitude of the magnetic field B. We believe that such a dependence can be relevant in general for the interpretation of magnetotransport in nondegenerate 2D electron gas.

X-ray powder diffraction, AC susceptibility and differential scanning calorimetry (DSC) studies were performed on the polycrystalline PrMn₂Ge_2−xSi_x () compounds. All compounds investigated crystallize in the body-centred tetragonal ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Si for Ge leads to a linear decrease of the lattice constants and the unit cell volume. The lattice constants and the unit cell volume obey Vegard's law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for PrMn₂Si₂ as a function of Si concentration x. At low temperatures, the Pr sublattice also orders for . The samples with are ferromagnetic and have spin reorientation temperatures just below the Curie temperature. In the cases of x = 1.2 and 1.3, re-entrant ferromagnetism is observed. The samples with are antiferromagnetic below the Néel temperature T_N(Mn). By comparing our results to earlier neutron diffraction and Mössbauer studies, the x–T magnetic phase diagram has been constructed.

The effects of iron substitution on the crystal structure and magnetic properties of SmCo_6.8−xFe_xHf_0.2 (x = 0.1, 0.3, 0.5, and 0.7) compounds were investigated by means of x-ray powder diffraction and magnetic measurements. The as-cast SmCo_6.8−xFe_xHf_0.2 compounds crystallize in the TbCu₇-type structure with the space group P6/mmm. The lattice parameters a and c increase with the Fe content. The SmCo_6.8−xFe_xHf_0.2 compounds exhibit ferromagnetic order with a strong room temperature uniaxial magnetocrystalline anisotropy. The Curie temperature and saturation magnetization increase with the Fe content. The magnetic anisotropy field of the compounds reaches a maximum value at x = 0.5. The anomalous concentration dependence of the anisotropy field is explained by considering the contribution of individual site anisotropy in combination with the preferential occupation of 3d ions on different sites.

The dielectric constant of the proton conductor SrZr_1−xYb_xO₃ (x = 0–0.1) was measured as a function of temperature and frequency. Two well-defined relaxation peaks were observed in SrZrO₃ doped with more than 1 mol% of Yb. The assignment of the two dielectric relaxations is discussed in terms of IR spectra and by free energy calculation for a miscibility of dopant Yb ions. The Yb concentration dependence of the relaxation strength of the two dielectric relaxations is in agreement with the results calculated from the free energy. The two relaxations can be assigned to a reorientation of a single Yb–OH dipole and of Yb–OH dipoles associated with Yb-clusters. The attractive energy for Yb-clustering in SrZrO₃ isevaluated at about −85 meV.

Systematic effort was made to optimize the quality of ferroelectric Bi_3.25La_0.75Ti₃O₁₂ thin films, that were pulsed laser deposited onto indium–tin-oxide coated glass plates, by varying the conditions of laser fluence density, partial oxygen pressure and post-annealing temperature. Characterizations from the dielectric and ferroelectric measurements concluded that the films deposited with a laser power of 2.5 J cm⁻², a deposition pressure of 200 mTorr and an annealing temperature of 650 °C showed the finest electrical properties, consisting of remnant polarization (2P_r) and coercive field (E_c) values of 28–32 µC cm⁻² and 90–100 kV cm⁻¹, respectively. No significant reduction in switching polarization magnitudes were observed until 1 × 10¹¹ switching cycles, indicating that these electrical properties and the relevancy of the film with metallic-oxide ITO electrodes have satisfied the necessary requirements for applications in nonvolatile ferroelectric memory devices.

We study the coherent dynamics of one- and two-electron transport in a linear array of tunnel-coupled quantum dots. We find that this system exhibits a rich variety of coherent phenomena, ranging from electron wavepacket propagation and interference to two-particle bonding and entanglement. Our studies, apart from their relevance to the exploration of quantum dynamics and transport in periodic structures, are also aimed at possible applications in future quantum computation schemes.

This is a tutorial paper on the properties of partially coherent hard x-ray beams and their use in the structural analysis of condensed matter. The role of synchrotron radiation in the generation of coherent x-ray beams is highlighted and the requirements on the source properties are discussed. The technique of phase contrast imaging is briefly explained, as well as diffraction in the Fresnel and Fraunhofer regimes. The origin of speckle is elucidated and it is shown how oversampling of the diffraction pattern by at least a factor of two enables retrieval of the phases of the waves scattered from different parts of the object. This in turn allows for a direct reconstruction of the object's structure. One-dimensional objects, such as a fluid confined between two surfaces, cannot be unambiguously reconstructed by phase retrieval without additional assumptions. A trial-and-error method based on the analysis of waveguiding modes within the confined geometry is discussed.

We used molecular dynamics simulations based on a potential model, in analogy to the tight binding scheme in the second moment approximation, to simulate the effects of aluminium icosahedral grains (dispersoids) on the structure and the mechanical properties of an aluminium matrix. First we validated our model by calculating several thermodynamic properties referring to the bulk Al case and we found good agreement with available experimental and theoretical data. Afterwards, we simulated Al systems containing Al clusters of various sizes. We found that the structure of the Al matrix is affected by the presence of the dispersoids resulting in well ordered domains of different symmetries that were identified using suitable Voronoi analysis. In addition, we found that the increase of the grain size has a negative effect on the mechanical properties of the nanocomposite as manifested by the lowering of the calculated bulk moduli. The obtained results are in line with available experimental data.

We report in this paper the spectra properties of Cr-doped and Cr, La-co-doped PbWO₄ (PWO) crystals grown by the Czochralski method. It is found that low-field Cr³⁺ ions and Cr⁴⁺ ions in tetrahedral sites co-exist in the crystals studied. In the case of Cr-doped PWO samples, Cr³⁺ is supposed to reside primarily at the Pb-site, and this results in the relaxation of lattice, which is confirmed by FT-IR analysis. The crystal-field parameters of Cr³⁺ in a distorted octahedral environment are estimated, and they are Dq = 1351 cm⁻¹ and Dq/B = 2.11, respectively. The NIR absorption of the crystals confirms the formation of Cr⁴⁺ and it is suggested that this is situated in the W⁶⁺ position. In particular, the co-addition of La³⁺ can induce a remarkable increase of absorption coefficient in the NIR region, on the grounds that La³⁺ ions preferably occupy the Pb²⁺ sublattice and force Cr to enter the W⁶⁺ position in the form of Cr⁴⁺. Cr-doped PWO crystals may be of interest as a new passively Q-switched laser material considering their high absorption coefficient in the NIR spectrum.

We study the transmission coefficient and thermal conductivity for acoustic phonons crossing a T-shaped quantum waveguide at low enough temperatures by use of the scattering-matrix method. Our results show that relatively small changes in the stub length and width can induce strong variations in the phonon transmission and thermal conductivity. Compared with the electron case in such a structure, acoustic phonon transmission and thermal conductivity exhibit some novel and interesting features. The phonon transmission coefficients and thermal conductivity can be artificially controlled by adjusting the parameters of the proposed microstructure.

The electronic structure of some europium chalcogenides and pnictides is calculated using the ab initio self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both a localized description of the rare earth f-electrons and an itinerant description of s-, p-, and d-electrons. Localizing different numbers of f-electrons on the rare earth atom corresponds to different nominal valencies, and the total energies can be compared, providing a first-principles description of valency. All the chalcogenides are found to be insulators in the ferromagnetic state and to have a divalent configuration. For the pnictides we find that EuN is half-metallic and the rest are normal metals. However, a valence change occurs as we go down the pnictide column of the periodic table. EuN and EuP are trivalent, EuAs is only just trivalent, and EuSb is found to be divalent. Our results suggest that these materials may find applications in spintronic and spin filtering devices.

We apply the Wigner function formalism to derive drift-diffusion transport equations for spin-polarized electrons in a III–V semiconductor single quantum well. The electron spin dynamics is controlled by the spin–orbit interaction which is linear in the momentum. In the transport regime studied, the electron momentum scattering rate is appreciably faster than the spin dynamics. A set of transport equations is defined in terms of a particle density, a spin density, and the respective fluxes. The model developed allows study of the coherent dynamics of a non-equilibrium spin polarization. As an example, we consider a stationary transport regime for a heterostructure grown along the (0, 0, 1) crystallographic direction. Due to the interplay of the Rashba and Dresselhaus spin–orbit terms, the spin dynamics strongly depends on the transport direction. The model is consistent with the results of pulse–probe measurements of the spin coherence in strained semiconductor layers. It can be useful in studying properties of spin-polarized transport and modelling spintronic devices operating in the diffusive transport regime.

The magnetoimpedance effect and its nonlinear terms are analysed for a (Co_0.94Fe_0.06)_72.5Si_12.5B₁₅ amorphous wire. In order to enhance the nonlinear contribution the sample was previously subjected to current annealing (Joule heating) to induce a circumferential anisotropy. The effect of the application of a torsional strain on the nonlinear magnetoimpedance is analysed in terms of the torsional dependence of the magnetic permeability, evaluated through experimental circumferential hysteresis loops. The results obtained clearly confirm the direct correlation between the asymmetric circumferential magnetization process and the occurrence of nonlinear second-harmonic terms in the magnetoimpedance voltage.

The physical properties of single crystals of filled skutterudite compounds EuT₄Sb₁₂ (T = Fe, Ru, Os) have been investigated by means of x-ray diffraction, electrical resistivity, specific heat, magnetization, and x-ray absorption spectroscopy measurements. The Eu-based materials crystallize in the LaFe₄P₁₂-type structure (space group ). A small Eu deficiency is encountered for Eu_0.95Fe₄Sb₁₂, while the homologous compounds, Eu_1.0Ru₄Sb₁₂ and Eu_1.0Os₄Sb₁₂, reveal full occupancy of the Eu site. Eu_0.95Fe₄Sb₁₂ appears to exhibit either canted ferromagnetic or ferrimagnetic order at T_C = 88 K whereas EuRu₄Sb₁₂ and EuOs₄Sb₁₂ are ferromagnets with Curie temperatures of T_C = 4 and 9 K, respectively. X-ray absorption near edge spectroscopy measurements reveal a nearly divalent Eu electronic configuration for EuT₄Sb₁₂ (T = Fe, Ru, Os) with no significant change with T atom or temperature.

We report ac magnetic susceptibility data measured on two samples of Co nano-particles, with average diameters and 3.1 nm, respectively. The temperatures T_max of the maxima of the real and imaginary components of the susceptibility shift in opposite directions when an increasing bias magnetic field H is applied. We show that the increase of T_max of with H is caused by the non-linear field dependence of the equilibrium magnetization and that only provides reliable information about the magnetic relaxation mechanisms in a bias field. Our data show that the magnetization reversal takes place via a classical thermally activated relaxation process. The influence that inter-cluster dipolar interactions have on the relaxation time depends on H. At zero field, the magnetic relaxation is slower than in the limit of no interactions. By contrast, as the magnetic moments of the particles become gradually polarized by H, the relaxation approaches the theoretical predictions for non-interacting particles.

Tin K-edge EXAFS recorded in situ at 498 K from tin-doped Fe₃O₄ show that the octahedral coordination of tin in the inverse spinel-related structure remains essentially unchanged as it is oxidized to structurally-related γ-Fe₂O₃. Changes in the magnetic hyperfine field distributions in the tin-119 Mössbauer spectra recorded in situ between 285 and 500 K show that at the higher temperatures some of the tin-doped Fe₃O₄ is converted to structurally-related tin-doped γ-Fe₂O₃. Comparison with data recorded from similar materials by ⁵⁷Fe Mössbauer spectroscopy show that the phase transition is sensitive to the partial pressure of oxygen and the period of time at the elevated temperatures. The changes in the magnetic hyperfine field distributions show that conversion of tin-doped Fe₃O₄ to tin-doped γ-Fe₂O₃ is largely completed at 600 K and occurs without the segregation of tin. At 700 K some tin segregates to form tin dioxide as the conversion of tin-doped Fe₃O₄ to tin-doped γ-Fe₂O₃ is completed. At higher temperatures the inverse spinel-related γ-Fe₂O₃ phase is converted to the corundum-related α-Fe₂O₃ structure.

The optical properties of the isoelectronic compounds LaCoO₃ and HoCoO₃ have been experimentally and theoretically investigated. We have measured the real (ε₁(ω)) and imaginary (ε₂(ω)) parts of the dielectric function, the reflectance R(ω) and the optical conductivity at room temperature. A shift of the most pronounced spectral features to the high energy region by 0.3 eV associated with larger distortions due to the smaller rare earth ionic radii in HoCoO₃ in comparison with LaCoO₃ was observed. Also there was found an enhancement of the absorption intensity in the range 1.3–2.3 eV in all kinds of spectra of HoCoO₃, which can be attributed on the basis of the results of LDA +U calculations to the different spin states of the Co³⁺ ion in these compounds. The shift of the onset of the absorption from less than 0.1 eV in LaCoO₃ to 0.7 eV in HoCoO₃ and an absorption intensity enhancement in a narrow spectral range 1.2–2.6 eV in HoCoO₃ are clearly seen from the calculated convolution of partial densities of states obtained in the LDA +U approach. Such changes are assumed to be induced by the different Co³⁺ spin states in these compounds at room temperature.

The electronic structure of LiBC has been studied using soft x-ray fluorescence measurements. Resonant inelastic x-ray scattering (RIXS) spectra were measured with the excitation energy tuned to the boron and carbon K-edges. RIXS spectra show dispersive features, which were assigned to the calculated energy bands using the site-selective quantitative band mapping method based on the concept of k-momentum conservation. It is concluded that while the electronic structure calculations are in general agreement with experimental spectra, carbon and boron K-emission bands show some deviation due to an incomplete hybridization of the C 2p–B 2p states.

F-passivated ZnO nanocrystalline films were prepared by thermal oxidation of ZnF₂ films. ZnF₂ films were deposited on a Si wafer by the electron beam evaporation technique. X-ray diffraction and x-ray photoelectron spectroscopy were used to study the structural changes of ZnF₂ as a function of oxidation temperature. When the ZnF₂ film was oxidized at 400 °C for 30 min, a polycrystalline hexagonal wurtzite structure of ZnO:F was obtained. The room temperature photoluminescence spectrum of the ZnO:F film showed a strong near band edge ultraviolet emission located at 379 nm with a narrow linewidth of 70 meV and a very weak visible emission associated with deep level defects. The results demonstrated that the presence of residual F ions in ZnO nanocrystalline film can dramatically decrease the visible emission and increase the ultraviolet emission of ZnO. On the basis of the experimental findings, two possible mechanisms are proposed: (1) the residual F ions in the film occupy the lattice sites of V o^* centres (the oxygen vacancies with one electron) inside the ZnO nanocrystals, which results in an appreciable decrease in visible emission and (2) some of the F ions also passivate ZnO nanocrystal surface states, which prevents the holes in the valence band from being trapped in surface states and then tunnelling back into nanocrystals to combine with V o^* to form V o^** centres () which are another source of visible emission.

The high-pressure Raman spectra of ferroelastic NaAl(MoO₄)₂ have been measured at room temperature. The studies indicated that this crystal exhibits two pressure-induced phase transitions at about 1.1 and 3.3 GPa. The first transition is connected with slight rotation of the MoO₄⁻² tetrahedra with loss of the inversion centre. The second transition is connected with significant distortion of the MoO₄²⁻ tetrahedra and Al³⁺ coordination sphere. By performing the lattice dynamics calculations in the starting phase (monoclinic C_2h⁶) we have been able to make an assignment of the Raman modes of this material. The deep knowledge of the modes helped us to get fundamental insights into the mechanism driving the structural changes occurring in NaAl(MoO₄)₂. The two phase transitions observed in the 0.0–4.2 GPa pressure range are completely reversible.

The key parameters that characterize the long wavelength behaviour of correlation functions, including the Fermi momenta and the correlation length, are calculated using the transfer matrix renormalization group for the one-dimensional Kondo lattice model. The Fermi momentum varies slowly at high temperatures, but changes sharply at low temperatures in association with the formation of the Kondo singlets. By comparison with the temperature dependence of the conduction electron density, we find that the long wavelength correlations of conduction electrons are strongly affected by the localized spins at low temperatures. At high temperatures, the conduction electrons behave as for a system with a small Fermi surface. However, at low temperatures, the Fermi momentum of the conduction electrons is altered by the coupling with the localized spins.

The photoluminescence and Raman spectra of poly (para-phenylenevinylene) (PPV) have been investigated at low temperatures down to 83 K. With decreasing temperature the photoluminescence spectra peaks are shifted to the lower energy side while the peak intensities are enhanced. The fitted Huang–Rhys parameter for the conjugated length has indicated that the conjugated length is elongated by 2.2 repeat units from 294 to 83 K. Though a distinct shift for the Raman band frequencies was not found, the Raman band intensities are, however, intensified at low temperatures. The Raman band intensity ratio I₁₅₄₈/I₁₆₂₆ is enhanced with decreasing temperature, indicating that the disorder is reduced and the conjugated length is increased at low temperatures.

Energy band diagrams of LaCuOCh (Ch = S, Se and Te) were calculated by a full-potential linearized augmented plane-wave method. The calculations, based on the local density approximation with/without an on-site Coulomb repulsion parameter, were to examine the energy levels of La 4f states. The results of the calculations showed that the on-site correlation parameter is necessary for evaluating the energy levels of La 4f states appropriately. The calculations for LaCuOCh with the on-site correlation parameter revealed that LaCuOS and LaCuOSe have almost the same energy band structure with a direct allowed-type band gap, while LaCuOTe has significantly different conduction band structure that exhibits an indirect-type band gap. This difference in electronic structure between LaCuOCh (Ch = S, Se and Te) is consistent with the observed optical properties of these materials.

The Seebeck coefficient of a metal is expected to display a linear temperature dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1 K. We put under scrutiny the magnitude of this term in different families of strongly interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalization contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly correlated electron systems.

Metastable phases of silver and gold in hexagonal close-packed structures are investigated by means of first-principles total-energy calculations. Two different methods are employed to find the equilibrium states: determination of the minima along the hexagonal epitaxial Bain path, and direct determination of minima of the total energy by a new minimum-path procedure. Both metals have two equilibrium states at different values of the hexagonal axial ratio c/a. For both metals, the elastic constants show that the high-c/a states are stable, hence, since the ground states are face-centred cubic, these states represent hexagonal close-packed metastable phases. The elastic constants of the low-c/a states show that they are unstable.