Table of contents

We report the multiferroic behaviour of MnWO₄, a magnetic oxide with monoclinic crystal structure and spiral long-range magnetic order. Based upon recent theoretical predictions, MnWO₄ should exhibit ferroelectric polarization coexisting with the spiral magnetic structure. We have confirmed the multiferroic state below 13 K by observing a finite electrical polarization in the magnetically ordered state via pyroelectric current measurements.

Zn_1−xMn_xO alloys, with different Mn concentrations, were prepared by the hydrothermal method. X-ray diffraction and electron paramagnetic resonance spectra demonstrate that Zn²⁺ ions are homogeneously substituted by Mn²⁺ ions without changing the ZnO wurtzite structure. The x = 0.02 and 0.04 samples are paramagnetic. When the Mn concentrations are increased to x = 0.08 and 0.10, the samples exhibit some ferromagnetism due to a secondary phase, (Zn,Mn)Mn₂O₄.

Motivated by observing the buckling of glassy crusts formed on evaporating droplets of polymer and colloid solutions, we numerically model the deformation and buckling of spherical elastic caps controlled by varying the volume between the shell and the substrate. This volume constraint mimics the incompressibility of the unevaporated solvent. Discontinuous buckling is found to occur for sufficiently thin and/or large contact angle shells, and robustly takes the form of a single circular region near the boundary that 'snaps' to an inverted shape, in contrast to the externally pressurized shells case. Scaling theory for shallow shells is shown to approximate well the critical buckling volume, the subsequent enlargement of the inverted region and the contact line force.

This paper introduces a simple model for an atomic nanocontact, where its mechanical properties are analysed by calculating numerically the local spectral properties at the contact atom and the nearby atoms. The standard methodology for calculating phonon spectral densities is extended to enable the calculation of localized contact modes and local density of states (DOS). The model system considered for the nanocontact consists of two sets of triple parallel semi-infinite atomic chains joined by a single atom in between. The matching method is used, in the harmonic approximation, to calculate the local Green's functions for the irreducible set of sites that constitute the inhomogeneous nanocontact domain. The Green's functions yield the vibration spectra and the DOS for the atomic sites. These are numerically calculated for different cases of elastic hardening and softening of the nanocontact domain. The purpose is to investigate how the local dynamics respond to local changes in the elastic environment. The analysis of the spectra and of the DOS identifies characteristic features and demonstrates the central role of a core subset of these sites for the dynamics of the nanocontact. The system models a situation which may be appropriate for contact atomic force microscopy.

Atomic-scale surface structure plays an important role in describing many properties of materials, especially in the case of nanomaterials. One of the most effective techniques for the determination of surface structure is low-energy electron diffraction (LEED), which can be used in conjunction with optimization to fit simulated LEED intensities to experimental data. This optimization problem has a number of characteristics that make it challenging: it has many local minima, the optimization variables can be either continuous or categorical, the objective function can be discontinuous, there are no exact analytical derivatives (and no derivatives at all for categorical variables) and function evaluations are expensive. In this study we show how to apply a particular class of optimization methods known as pattern search methods to address these challenges. These methods do not explicitly use derivatives, and are particularly appropriate when categorical variables are present, an important feature that has not been addressed in previous LEED studies. We have found that pattern search methods can produce excellent results compared to previously used methods, both in terms of performance and in locating optimal results.

We propose a nonequilibrium Green's function approach to calculate the ac conductance of various finite-length carbon nanotubes. The simulated ac conductance differs significantly from that for infinite-length carbon nanotubes. At the low-frequency limit, the profiles of the quantized conductance are still observable in the finite-length carbon nanotubes, but many more peaks appear on the conductance curves. We also show that the conductance of finite-length carbon nanotubes oscillates as a function of the ac frequency. The dependence of the oscillation on the lengths, helicities and defects of the carbon nanotubes are also investigated. The knowledge we gain from this research will help us make carbon-nanotube-based interconnects or other ac devices in the future.

In this work we study the electronic structure of doped multi-quantum wires formed by steps in the vicinal surfaces of GaAs. It is verified that multi one-dimensional electronic systems can exist, which contrasts with a previous conclusion in the case of a single wire where it was demonstrated that the presence of residual acceptors inhibited the formation of such a type of structure. The effects of dopant diffusion, residual density of acceptors and exchange–correlations contributions have been addressed by the self-consistent solution of the Schrödinger equation treated in the effective mass approximation.

Numerical calculations have been employed to study the mechanical deformation of two semi-crystals of different metallic species colliding at relatively low temperature. The roughness of the semi-crystal surfaces induces local shear events that mediate the formation of an interface between the two metallic species. The surface asperities represent the points of local contact between the semi-crystals. The atomic species located in such regions experience a sudden mechanical load and an unusual localization of kinetic energy, resulting in an enhancement of their mobility. Correspondingly, significant atomic migration processes take place at relatively high rates. Atomic mixing phenomena originated by the relative motion of the colliding surfaces are also observed at the interface. After the removal of mechanical loads, the atoms involved in interfacial displacement processes are not able to recover a crystalline structure and keep a disordered arrangement despite the occurrence of thermally induced relaxation processes.

Optical transmission spectra in quasiperiodic multilayered photonic structures, composed of both positive (SiO₂) and negative refractive index materials, are calculated by using a theoretical model based on the transfer matrix approach for normal incidence geometry. The quasiperiodic structures are substitutional sequences, characterized by the nature of their Fourier spectrum, which can be dense pure point (e.g. Fibonacci sequence) or singular continuous (e.g. Thue–Morse and double-period sequences). The transmission spectra for the case where both refractive indices can be approximated by a different constant show a unique mirror symmetrical profile, with no counterpart for the positive refractive index case, as well as a striking self-similar behaviour related to the Fibonacci sequence. For a more realistic frequency-dependent refractive index, the transmission spectra are characterized by a rich transmission profile of Bragg peaks with no more self-similarity or mirror symmetry.

Considering the effect of spin–orbit coupling, we have studied the equation of state, structural parameters and electronic structures of osmium under high pressure by first-principles plane-wave basis pseudopotential calculations. We find the following: (1) the calculated bulk modulus is less than that of diamond, and the value becomes smaller after taking into account the spin–orbit coupling; (2) the proposed isostructural phase transition (c/a ratio anomaly) around 25 GPa is not supported by the present calculations. By analysing the band structure, the density of states at the Fermi surface, and the equation of state for the cases with and without spin–orbit coupling, we find no electronic topological transition up to 150 GPa.

Results from high-pressure synchrotron x-ray diffraction and high-temperature x-ray diffraction measurements on polycrystalline samples of the tetragonal perovskites Sr₂GaSbO₆, Sr₂NiMoO₆, and Sr₂FeNbO₆ are reported. A phase transition, where the unit cell changes symmetry from tetragonal to cubic, is observed for each compound at elevated temperatures. The phase transition changes the structure from one exhibiting an octahedral tilting distortion at ambient temperature to one that is untilted above the transition temperature. At elevated pressures the lattice parameter ratio increases, indicating that the magnitude of the octahedral tilting distortion is increasing as a function of pressure. In the pressure range studied, up to ∼6 GPa, no phase transitions were observed.

Metastable body centred cubic (bcc) Co has been grown as an epitaxial thin film on GaAs, using molecular beam epitaxy. In this paper, epitaxial Co films were grown on GaAs(100) substrates with 1 × 1 surface construction. The film thicknesses cover the bcc–hcp (hexagonal close packed) Co transition. The magnitudes and symmetries of the anisotropy for these films were estimated from normalized magnetization data measured using a magneto-optic Kerr effect magnetometer. It was determined for these films that there was an early onset of the hcp phase and of the uniaxial anisotropy component. The magnetostriction constants were determined by straining the films over a series of bend radii and determining the change in the anisotropy field (the Villari effect). The large uniaxial anisotropy observed in one case may be ascribed to a large magnetostriction constant for that film.

The remanent magnetization and the magnetic anisotropy energy of 5–16 monolayer (ML) epitaxial Fe films grown on GaAs(001) were quantitatively monitored in situ by superconducting quantum interference device magnetometry and ferromagnetic resonance as a function of oxygen exposure at 300 K. We find that at an oxygen dose of 10 L the perpendicular uniaxial anisotropy decreases by almost 50%, whereas the magnetization is decreased by 8% only in the case of a 5 ML Fe film. The fourfold anisotropy constant K₄ and in-plane uniaxial anisotropy constant , however, remain unchanged. Low-energy electron diffraction spots of the Fe surface disappear after 6 L of oxygen, indicating a disordered Fe oxide growth. For exposures below 70 L we observe a reduction of the remanent magnetization irrespective of the film thickness. At exposures >70 L we find a faster decrease of the remanent magnetization for the thinner films which is attributed to the finite size effect and a thickness dependent surface roughness of the iron films, respectively. As evidenced by Auger electron spectroscopy and x-ray diffraction, the iron oxide is found to be FeO in the first stage of oxidation, transforming into γ-Fe₂O₃ at higher dosages (>1000 L).

By using first-principles DMol and the discrete-variational method (DVM) based on density functional theory, we investigated the effect of some light impurities, H, B, C, N and O, on the electronic structure of their corresponding different impurity-doped systems in γ-TiAl. The impurity formation energy, Mulliken occupation, bond order and charge density difference have been calculated to study the impurity-induced changes in the energetics and electronic structure. According to the impurity formation energy, it is found that the impurities energetically prefer to occupy the Ti-rich octahedron interstitial sites in the order H<B<O<N<C. Charge transfer suggests that the effect of the impurities is localized and the bond order as well as charge density difference results show that B, C and N are beneficial for the ductility of TiAl, while H and O are not.

A study of the magnetic structure of YFe₄Al₈ is presented, based on the reported magnetic structure, as given by a neutron diffraction study; cycloid, with moments in the a–b plane and with two Fe sublattices with a 140° phase difference between them. Calculations were performed, using density functional theory, for cycloids with , with τ varying between 0 and 1. The calculated magnetic structure agrees with the structure deduced from neutron diffraction results, however with a simpler description. The YFe₄Al₈ magnetic structure can be described by a single Fe lattice with a cycloid propagation vector with τ = 0.865; there is no need for a second Fe lattice with an arbitrary phase difference. We show that the same description can be applied to DyFe₄Al₈ and HoFe₄Al₈.

We report sharp spin-dependent energy loss features in electron scattering from bcc Fe(100) thin films grown on Ag(100). Majority spin features are observed at ∼1.8 and 2.5 eV energy loss, and a minority spin feature is observed at ∼2.0 eV energy loss. The majority spin peaks are attributed to spin-flip exchange scattering from the Fe films. The minority spin peak is attributed to non-flip exchange scattering with an energy corresponding to the separation between occupied and unoccupied minority spin bands. The observed peak energies compare favourably with electronic structure calculations for Fe and with spin-resolved photoemission and inverse photoemission results.

Nanophasic La_0.7Sr_0.3MnO₃ (LSMO) samples were prepared by the sol–gel method. The samples were sintered at different temperatures ranging from 600 to 1000 °C. It is shown that the transport and magnetoresistive properties of LSMO samples strongly depend on the sintering temperature (T_s). A substantial decrease in the insulator–metal transition temperature (T_IM) and an enhancement in resistivity are found on lowering the sintering temperature. Furthermore, a reduction in magnetization and a slight decrease in paramagnetic–ferromagnetic (PM–FM) transition temperatures (T_c) have been observed as the sintering temperature decreases. The magnetoresistance (MR) at T<T_c increases on decreasing the sintering temperature as well as increasing the applied magnetic field. The enhancement in MR on decreasing the sintering temperature is explained by enhanced spin-polarized tunnelling by assuming an increase of the grain boundary contribution as the sintering temperature decreases.

We report an inelastic neutron scattering study of coherent magnetic excitations in powder and single-crystal samples of the model kagome antiferromagnet potassium iron jarosite, KFe₃(OH)₆(SO₄)₂. Initial measurements on a natural single crystal using a triple-axis spectrometer revealed a mode with a zone-centre gap of 7 meV that showed little dispersion within the kagome layers, as well as some indication of a mode with a zone-boundary energy of approximately 20 meV. However, the high background from hydrogen in the sample made it very difficult to search for other excitations. In the absence of suitable deuterated crystals, measurements were performed on deuterated powders using time-of-flight neutron spectrometers over a range of temperatures that include . This confirmed the flat 7 meV mode as well as dispersive modes that reached to higher energies. The origin of these modes is discussed in relation to the most likely Hamiltonian for the magnetic degrees of freedom in this material, and estimates are made of the strength of the nearest-neighbour exchange, J₁, and contributions from a Dzyaloshinsky–Moriya interaction or single-ion anisotropy arising from a crystal field.

The structures and energies of point defects and point defect complexes in B 2 iron aluminium FeAl are calculated using a local density functional theory based method with large supercells. Particular emphasis is given to pseudopotential quality, choice of chemical potentials used to calculate defect formation energies, and how these are affected by magnetism. Both purely native defects and those containing boron atoms are considered. It is found that the relative stabilities of isolated point defects versus defect complexes depends on whether the material contains excess iron or aluminium. The situation in material containing boron is further complicated by the existence of iron borides in more than one form. We propose that the interaction between point defects, dislocations, and antiphase boundaries, where the local atomic environment has some similarities with antisite defects, also depends on the alloy composition. It is likely that these interactions are part of the underlying mechanism responsible for the unusual mechanical properties of iron aluminides.

We have studied the correlation between the valence electron configuration and the electronic structure of M₂AlC(0001) surfaces (M = Ti, V, Cr) using ab initio calculations. Based on our surface energy data, we find that the Al termination is the most stable configuration. As the M valence electron population is increased, the surface energy increases. This can be understood by analysing the valence electron concentration induced changes of the electronic structure. Antibonding states are present as Ti is substituted by Cr in M₂AlC(0001). These results are of relevance for vapour phase condensation as well as oxidation.

The structure, vibrational dynamics and thermodynamics of a chiral surface, Cu(532), have been calculated using a local approach and the harmonic approximation, with interatomic potentials based on the embedded atom method. Relaxation of the atomic positions to the optimum configuration results in a complex relaxation pattern with strong contractions in the bond length of atoms near the kink and the step sites and an equivalently large expansion near the least under-coordinated surface atoms. The low coordination of the atoms on the surface substantially affects the vibrational dynamics and thermodynamics of this system. The local vibrational density of states shows a deviation from the bulk behaviour that persists down to the tenth layer, resulting in a substantial contribution of the vibrational entropy to the excess free energy amounting to about 90 meV per unit cell at 300 K.

⁵⁷Fe Mössbauer spectroscopy, x-ray diffraction and magnetic measurements have been used to study the polycrystalline Er_2−xPr_xFe₁₄B (x = 0.25, 0.5, 0.75, 1.0, 1.5) compounds. Special emphasis was put on the spin reorientation phenomena (change of spin orientation from planar to axial arrangement) occurring in this series. The spin reorientation in each compound has been investigated mainly by means of narrow step temperature scanning in the neighbourhood of the spin reorientation temperature, T_SR.

Initial magnetization versus temperature measurements allowed us to establish the temperature regions of reorientations and also the Curie temperatures of the compounds.

Mössbauer temperature scanning was performed with a step of 2 K in the vicinity of T_SR. The spectra obtained were analysed by using a procedure of simultaneous fitting and the transmission integral approach. Consistent fits were obtained; T_SR and the composition dependences of hyperfine interaction parameters were derived from fits for all compounds studied.

The T_SR values obtained with both methods were compared and the spin phase diagram for the series was constructed.

Through correlated studies using scanning electron microscopy (SEM), Raman spectroscopy and low-temperature photoluminescence (PL) we have demonstrated that the intercalated PbI₂ with ammonia, poly (vinyl alcohol) and polyacrylamide are characterized by a distinctive signature in the Raman and photoluminescence spectra. After intercalation, the Raman spectrum of PbI₂ reveals an orthorhombic structure, identical with that observed on the micrometric scale, of KPbI₃ rod-like particles resulting from the reaction between Pb(NO₃)₂ and KI, carried out in liquid media such as ethanol and acetonitrile. The rods and the intercalated PbI₂ are characterized by a new and strong emission band at 2.23 eV (about 550 nm) that appears at 77 K under an excitation wavelength of about 340 nm. Modification of the Raman and PL spectra results from a compressing effect produced by the penetration between the PbI₂ layers of different molecular species. The compression acting primarily on the iodine layers has the result of lowering the contribution of the 5p states of I⁻ ions to the constitution of the electronic level situated at the top of valence band of PbI₂, so that the photoluminescence of intercalated PbI₂ acquires the characteristics of Pb²⁺ emission when it is dissolved in an alkali halide crystal.

We present analytical results for the contribution of electromagnetic fluctuations in the distribution of interaction energy and pressure in isotropic systems whose properties depend only on one spatial coordinate. If we neglect the continuous inhomogeneity introduced here and consider the simplest case of two macroscopic homogeneous bodies separated by a homogeneous film our result reduces to the well-known Lifshitz formula. As a first application of theory, a one-dimensional modulated system with a homogeneous layer embedded in it is considered and a suitable perturbation theory for this system is developed. In the main part of this paper we limit the calculations to the non-retarded case, that is only the calculation of van der Waals interaction energy is given. As a second application of theory we consider the van der Waals interaction between two semi-infinite media across a planar region within which there is a thin film having an arbitrary variation of the dielectric permittivity. The importance of the precise evaluation of the transverse magnetic surface mode dispersion relation in inhomogeneous media is elucidated. For concreteness the influence of the transition layer between water and lipid in a symmetrical configuration is considered in some detail. The zero-frequency term and the dispersion-only contribution to the Hamaker coefficient are given analytically using some approximations and modelling of the dielectric constants reasonable for these dielectrics at room temperature. As a whole the results indicate the necessity of performing Lifshitz-type calculations on realistic inhomogeneous layered models, as are the models described here, for accurate interaction energy modelling. Of course further work is needed for real justification of the continuous variation of dielectric permittivities across phase interfaces.

The structural behaviour of Li₃Na₃In₂F₁₂ garnet (, Z = 8) has been studied as a function of temperature (100 K≤T≤579 K) using single-crystal and powder x-ray diffraction at atmospheric conditions. The temperature dependence of the coefficient of thermal expansion is α (K⁻¹) = 2.0 × 10⁻⁵+2.4 × 10⁻⁸ T. The unit-cell volume change on cooling the Li₃Na₃In₂F₁₂ crystal from 298 to 100 K is equivalent to compressing it to about 0.5 GPa at room temperature. The effect of low temperature on the crystal structure is examined on the basis of structural parameters obtained from the refinements of the single-crystal data. The large coefficient of thermal expansion and the small bulk modulus of Li₃Na₃In₂F₁₂, in comparison with the oxide analogues, are due to massive changes in the In–F–Li interpolyhedral angle in the garnet framework at non-ambient conditions.

A strategy for developing ferrimagnetism through B-site cation ordering in thermodynamically stable ferroelectric complex perovskites is described, with the aim of realizing both a spontaneous polarization and magnetization at room temperature. Specifically, solid solutions of the complex perovskites Pb(Fe_2/3W_1/3)O₃, Sr(Fe_2/3W_1/3)O₃, and Ba(Fe_2/3W_1/3)O₃ with BiFeO₃ are considered. Preliminary data on the 0.75Pb(Fe_2/3W_1/3)O₃–0.25BiFeO₃ ceramic show promising results.

Mössbauer spectroscopy has been applied to study the magnetic hyperfine interactions of ⁵⁷Fe and ¹¹⁹Sn probe atoms within TlFeO₃ and TlFe_0.99Sn_0.01O₃ ferrites. According to the magnetic measurements, the magnetic ordering temperature T_N (= 560 K) for TlFeO₃ is much lower than that for any other orthoferrites RFeO₃ (R = rare earth). It is suggested that such difference in T_N may be explained by different characteristics of the Tl–O and R–O chemical bonds involved and the induced competition with the Fe–O bonds. The effects of covalency and overlap distortion on the spin and charge densities at ⁵⁷Fe nuclei are discussed in relation with the hyperfine interactions observed. By using cluster molecular orbital (MO) calculations, it was shown that the low value of the hyperfine magnetic field H_Fe(= 538 kOe) at ⁵⁷Fe nuclei in TlFeO₃ is strongly associated with a decrease of the covalent mixing parameter (b_σ)² = 0.048 and (b_π)² = 0.009 values, which characterize the Fe–O bond covalence. It was elucidated that the value of super-transferred hyperfine field (H_STHF) at ¹¹⁹Sn nuclei in TlFe_0.99Sn_0.01O₃ is sensitive mainly to the angle of the Sn–O–Fe bonds.

By means of the nonequilibrium Green function and equation of motion method, the electronic transport is theoretically studied through a parallel-coupled double quantum dot (DQD) in the presence of on-dot Coulomb interaction U. With focus on the quantum interference in the U-dominant parallel-coupled DQD, we find two types of Fano interferences in the conductance spectra. If the one-particle DQD bonding and antibonding bands are well separated from their Coulomb blockade counterparts, the main features of Fano interference in usual DQD systems are recovered with minor revisions. The most interesting is the hybridization between the antibonding state and the Coulomb counterpart of the bonding state, which gives rises to two new channels for Fano resonance. The Fano interference in the Coulomb hybridized systems can be controlled by the electrostatic and magnetic approaches, and exhibits properties quite different from what are reported in the noninteracting Fano–Anderson model.

We perform Monte Carlo simulation on the dielectric relaxation behaviour of a model relaxor ferroelectric, based on the Ginzburg–Landau theory of ferroelectrics with dipole-defect induced random field. The coexistence of ferroelectric nanoclusters and paraelectric matrix is demonstrated. It is found that the time-domain spectrum for dielectric relaxation below the ferroelectric transition point exhibits a multi-peaked pattern rather than the diffusive single-peaked pattern, indicating the existence of multi-characteristic times for the dielectric relaxation. The extended multi-peaked time-domain spectrum is responsible for the diffusive ferroelectric transitions and frequency dispersion of the dielectric relaxation, usually observed for relaxor ferroelectrics.

Based on one-dimensional single-electron infinite periodic models of trans-polyacetylene, we show analytically that the overall permutation symmetry of nonlinear optical susceptibilities is, although preserved in bound-state molecular systems, no longer generally held in periodic systems. The overall permutation symmetry breakdown provides a natural explanation of the widely observed large deviations of Kleinman symmetry in off-resonant regions of periodic systems. Finally, physical conditions to experimentally test the overall permutation symmetry breakdown are discussed.

The time-dependent non-crossing approximation is used to calculate the transient currents through a quantum dot in the Kondo regime subject to a sudden change in its coupling to the leads. The currents are found to display transient non-universal behaviour immediately after the perturbation and then to follow a slow universal increase toward equilibrium. The timescales for the approach to equilibrium are shown to be the same as those recently identified in a study of transient currents in a quantum dot subject to a sudden change in the energy of the dot level (Plihal et al 2005 Phys. Rev. B 71 165321). We present improved numerical algorithms which enable relatively fast calculation of the transient response of quantum dots to sudden perturbations.

Biological functions and genetic features of DNA, such as duplication, transcription and gene expression, are mainly determined by its structure, but depend also on the temperature and features of solution, such as salt concentration. We study the influence of temperature and salt concentration on the conformation changes and transcription of DNA by using a new dynamical model. This new model admits three degrees of freedom per base-pair: two displacement variables related to the vibrations of hydrogen atom in the hydrogen bonds and base (nucleotide), respectively, and an angular variable related to the rotation of base. The important role of motion of hydrogen atom in the hydrogen bonds is specially stressed in this model. This is helpful to reveal the mechanism of transcription of DNA. According to their properties of motion, we first give the Hamiltonian of the system, corresponding equations of motion and their soliton-solutions. The solitons are the excitation states formed by the displacements of hydrogen atoms and bases and the rotations of bases, arising from the energy absorbed by DNA, in the systems, respectively. By applying the transfer integral method we obtain the thermodynamic properties (e.g. free energy and entropy) of the thermal excitation state of DNA at the biological temperature in this model. According to the properties of these thermodynamic functions obtained we study the mechanism and processes of melting and transcription of DNA with the aid of the transforms of energy carried by the soliton in such a case. We further give the properties of the transcription of DNA with the help of the average value of the mean square of displacement of hydrogen atom, and the values of subcritical temperature and force of the phase transition are also found. Finally, we conclude that the transcription of DNA not only depends directly on the properties of its structure and of energy absorbed by it, but also is influenced by the temperature and salt concentration in the solution of DNA, which is consistent with experimental data. Therefore this new model not only can predict the transcription of DNA, but also can give the relationship among the conformation changes and the temperature and salt concentration. Finally we discuss in simple terms the influences of water or solution around DNA on its dynamics and further point out the defects and limitations of the new model of the dynamics of DNA and its direction of development.

The crystal structures and magnetic properties of Ln₄Re₆O₁₉ (Ln = La,Pr,Nd) are reported. They crystallize in a cubic structure with space group I 23 and have a three-dimensional network of Re₂O₁₀ dimers consisting of two edge-sharing ReO₆ octahedra. Measurements of the magnetic susceptibility, specific heat, and electrical resistivity for Ln₄Re₆O₁₉ and the calculation of the electronic structure for La₄Re₆O₁₉ reveal that the 5d electrons of the Re ion are delocalized and that these compounds are metallic. For Ln₄Re₆O₁₉ (Ln = Pr,Nd), a Schottky-type specific heat anomaly is observed at 2 K for Ln = Pr and 5 K for Ln = Nd, which is due to the short-range magnetic orderings in the Ln₄O tetrahedral cluster. Furthermore, for the Nd₄Re₆O₁₉ compound, the long-range magnetic ordering via conduction electrons is found at 0.45 K.

We study theoretically the plasmon mediated propagation of light through a chain of metallic nanoparticles along which a small chain (called a resonator) is attached vertically. The effect of this vertical resonator is to induce peaks and zeros in the transmission power. We show that, with a resonator constituted of two metallic clusters, an appropriate choice of the geometrical parameters can lead to a narrow peak in the transmission spectrum. This may be realized for this device by adjusting the distances between the two resonator clusters and the chain cluster to which they are attached. This enables us to get, in particular, a sharp peak between two zeros of transmission close to each other or sharp dips between two transmission ones. Such a device can be useful as a selecting or rejecting plasmon filter.

Highly crystalline, nanometre sized ZnFe₂O₄ particles with different diameters, 6.6 and 14.8 nm, were prepared by forced hydrolysis in a polyol medium. The DC magnetic properties exhibit a strong dependence on the particle size as a result of the unusual cation distribution. They clearly establish their superparamagnetic character at room temperature and the occurrence of ferrimagnetic or ferromagnetic ordering at low temperature. The magnetization is found to increase with grain size reduction. The ⁵⁷Fe Mössbauer spectra were recorded at 300 and 4.5 K. There is no evidence for the presence of the Fe²⁺ charge state, confirming the perfect stoichiometry of the two samples. At 300 K, the Mössbauer spectra consist of doublets due to the superparamagnetic behaviour whereas at 4.5 K they reveal a magnetically blocked state. Mössbauer spectra at 10 K in an external 6 T magnetic field applied parallel to the direction of the gamma rays clearly show a close to collinear Néel-like ferrimagnetic ordering for the 6.6 nm particles and a canted Yafet–Kittel-like ferrimagnetic ordering for the 14.8 nm ones.

Semiconductor quantum dots (QDs) have been gaining much attention because of their outstanding properties for multiple-photon microscopy applications. By solving nonperturbatively the time-dependent Schrödinger equation, it has been shown that the large number of energy states densely compacted in both the conduction and valence bands of the QD greatly enhance the inter-band and intra-band optical couplings between two energy states induced by multiple photons from ultra-fast and ultra-intense lasers. The multiphoton absorption processes are further enhanced by many energy relaxation processes in commonly used semiconductors, which are generally represented by the relaxation energy in the order of tens of meV. Numerical calculation of multiphoton processes in QDs agrees with experimental demonstration. After proper designing, QDs can be activated by infrared radiation to emit radiation in the visible optical regime (up-conversion) for bioimaging applications.

Piezomagnetic materials are characterized by strong coupling between magnetization and acoustic vibration and the piezomagnetic coefficients change sign with respect to domain polarization. For piezomagnetic superlattices (PMSL) with periodically up and down polarized domain structures, the piezomagnetic coefficients are modulated periodically, and the resulting phonon–polaritons form a band-like structure. This study, using the generalized transfer matrix method, shows that a polariton-like band-gap can be realized in piezomagnetic material with the 6mm point group under a certain configuration of magnetic field, lattice displacement and domain polarization, but the mid-gap frequency is restricted to the range of a few GHz or below if the domain wall effect is taken into consideration.

Magnetization and Mössbauer studies have been performed on the polymer coated magnetite nanoparticles with particle size from 5.1 to 14.7 nm. The maximum in the temperature dependence of magnetization (T_M) is found to be inconsistent with the particle size (D_TEM). The effective magnetic anisotropy (K_an) is found to increase with the decrease of D_TEM, which is attributed to the increase of surface anisotropy. The absence of coercivity and remanence of magnetization noticed well above T_M indicate superparamagnetic behaviour, which has also been observed in the temperature dependent Mössbauer results. The temperature dependence of hyperfine field is found to follow a similar dependence to saturation magnetization for bulk magnetite.

We provide evidence for the existence of an antiferromagnetic coupling between iron epilayers separated by a wedge-like ZnSe crystalline semiconductor by magnetometric and ferromagnetic resonance experiments. The coupling strength of 46 µJ m⁻² for t_ZnSe = 25 Å is strongly reduced as the barrier thickness is increased. The coupling increases linearly with temperature from 5 to 300 K, with a 5.5 × 10⁻⁹ J m⁻² K⁻¹ rate. Thermally induced effective exchange coupling mediated by spin-dependent tunnelling of electrons via localized mid-gap defect states in the ZnSe spacer layer appears to be the most plausible mechanism to induce the antiferromagnetic coupling.

We report a classical molecular dynamics isothermal–isobaric ensemble (NPT) implementation for the simulation of pressure effects on finite systems. The method is based on calculating the enclosed surface area by means of the Delauney triangulation method, which results in a fairly accurate description of the surface and the system volume. The external pressure is applied to the system by external forces acting on the triangulated surface covering the nanostructure. Pressure is exerted perpendicularly to every one of the Delauney triangles, by equally distributing the force to every corner of a triangle. We applied the method to finite single wall capped carbon nanotubes (SWCNTs) with different chiralities and different tube lengths ranging from 4 nm up to 30 nm. Pressure effects are studied as a function of the radii and the nanotube length, as well as as a function of temperature. Our results are in very good agreement when compared with both experimental and other theoretical results.

The dark conductivity and photoconductivity of Se_80−xTe₂₀Bi_x (0≤x≤1) thin films have been studied. The values of dc activation energy, optical band gap and photoconduction activation energy are found to decrease with the addition of bismuth content. The optical energy gap has been found to be approximately equal to twice the dark thermal activation energy in all the compositions studied. The variation of photocurrent with illumination intensity indicates that the recombination mechanism is bimolecular.

We have observed strongly spin-dependent quantum transport in nanowires created by bringing macroscopic Cu wires into and out of contact under an applied magnetic field when exposed to air. A 70% increase in the strength of the first spin split conduction mode obtained from conductance histograms is observed for a field of 2 mT at room temperature, implying the existence of a robust spin polarization. Density functional full potential linearized augmented plane wave (FLAPW) calculations reveal that a CuO chain when stretched becomes spin polarized and has half-metallic character. We therefore attribute the surprisingly strong magnetic effect on the conductance to the formation of spin polarized CuO chains within the nanowire by spontaneous atmospheric oxidation.