Table of contents

We analyse the problem of the electronic correlations in V₂O₃, focusing on its low-temperature antiferromagnetic phase, in the light of recent experimental findings. In fact, the implications of the vanadium K-edge linear dichroism data demand further theoretical investigations to explain the consequences of the non-reciprocal gyrotropic effect, which has not been considered by previous theoretical models. The analysis of all the available experimental data indicates 2/m as a magnetic space group for the ground state, instead of C_2h⊗. Such a reduction of the symmetry can be given by the onset of an orbital ordering in the system. We illustrate the merits and drawbacks of the proposed solution.

The electron-momentum-density distributions (Compton profiles) in decagonal Al₇₂Ni₁₂Co₁₆ have been measured along the [00002] and [11000] directions with a momentum resolution of 0.14 au. The experimental valence-electron profile is decomposed into two partial profiles: an inverted parabola-like profile and a broad Gaussian-like profile. By fitting the broad partial profile with a profile made up of atomic 3d wavefunctions, it is found that a considerable portion of the 3d-electron holes of Ni and Co are filled with electrons. The directionally averaged radius of the Fermi sphere is estimated from the number of electrons under the parabolic partial profile. The radius is larger than that of the inscribed sphere of the quasi-Brillouin zone. This leads to the Hume-Rothery mechanism does not play an important role in explaining the stability of the decagonal quasicrystal. Slight anisotropies of the Compton profile are observed.

As a result of important advances over the last decade, block copolymer melts have become an excellent model system for studying fundamental phenomena associated with molecular self-assembly. During this time, good quantitative agreement has been achieved between theory and experiment in regards to equilibrium phase behaviour, and with it has emerged a thorough understanding in terms of simple intuitive explanations. The theoretical contributions to this effort are largely attributed to mean-field calculations on a standard Gaussian model. Here, we review this present understanding of block copolymer phase behaviour, the model and its underlying assumptions, the mean-field approximation and its limitations, and the attempts to incorporate fluctuation corrections. Rather than following the traditional rigorous derivations, we present slightly more intuitive and transparent ones in such a way to stress the close connection between the related calculations. In this way, we hope to provide a valuable introduction to block copolymer theory.

A high-throughput and systematic study of complex material systems is described. Initial efforts using thin-film discrete material chips are briefly reviewed from the perspective of materials discovery and optimization, and the limitations of the method are discussed. We then focus our discussion on the experimental techniques based on `continuous phase diagrams' (CPDs) for epitaxial growth of thin films, and mapping their physical properties in two different material systems: perovskite manganese oxides and Ni_1-xFe_x metal alloys. For the former, we discuss the results of mapping optical, electrical, and magnetic properties of manganese oxides as functions of doping concentration, ionic radius, etc. We discuss the surprising evidence that suggests various electronic phase transitions, such as orbital orderings and smectic phase formation, in this highly correlated electronic system. For Ni_1-xFe_x metal alloy, we discuss a different application of the CPD in describing the structure-property relation of traditional materials science.

The temperature dependence of the domain structure associated with the ferroelastic phase transition (Fmm↔Rm) in ZrO₂ doped with 11% Sc₂O₃ has been determined from a peak shape analysis of high-resolution synchrotron x-ray powder diffraction data. In the temperature region of coexisting phases the observed characteristic anisotropic broadening and asymmetry of the lines is modelled by three different phases: a main rhombohedral phase, a distorted rhombohedral phase with a smaller c/a ratio, and a cubic phase. The latter two are assigned to the internal structure of the domain walls between two adjacent twin domains. The size and amount of the cubic phase show an initially slow increase with temperature followed by a very steep increase and a slow one after that. The size of the (main) rhombohedral domains remains nearly constant, while (micro-) strain in the distorted regions gradually decreases.

Intrinsic localized modes in the gap of a diatomic chain with free ends are discussed in detail by going beyond the rotating wave approximation. We include in the time dependence of the displacements terms up to cos (2ωt). We consider a finite chain of particles interacting with nearest-neighbour interactions. We study amplitudes of the intrinsic localized modes smaller than 0.25 Å. In this range of amplitudes the full potential can be well represented by an expansion in powers of the displacements up to fourth-order terms. The use of a force constant model allows us to simplify the problem. As a test case we consider a chain of LiI atoms. We found intrinsic localized modes in the gap. The amplitudes of the first harmonic term (cos (ωt)) are of even or odd parity, whereas we prove that the amplitudes of the static part and those of the second harmonic can have only even symmetry. The main result of the paper is that the amplitudes associated with the second harmonic are two or three orders of magnitude less than those of the first harmonic. Furthermore, the frequency of the localized modes are modified by less than 1% by the inclusion of the second harmonic.

Measurements of the hydrogen diffusivity in the hydrogen-stabilized Laves phase compound C15-Hf Ti₂H_x (3.9⩽x⩽4.2) have been performed by means of the pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) between 220 and 500 K. The activation enthalpies for hydrogen diffusion obtained by fitting an Arrhenius expression to the diffusivities measured at a given concentration x are H_a = 240 meV (x = 3.9), 210 meV (x = 4.0) and 235 meV (x = 4.2). The proton spin-lattice relaxation rates Γ₁ have been measured on the same samples over wide temperature ranges and at different resonance frequencies. Above about 250 K, the Γ₁ data are well represented by a relaxation model for jumps between tetrahedral [Hf Ti₃] sites. A comparison with the PFG results shows that these jumps correspond to long-range diffusion. At lower temperatures, the Γ₁ data indicate localized motion of hydrogen, presumably within hexagons formed by six tetrahedral [Hf₂Ti₂] sites, which has no effect on the PFG measurements. Similar diffusion mechanisms have been observed previously in C15-ZrTi₂H_x (x≈4).

We study the adsorption of a Lennard-Jones gas in a slit-like pore formed by two walls exhibiting different adsorbing properties. The calculations are carried out by means of a density functional approach. We show that changes in the potential field exerted by one wall can lead to substantial modifications in the phase behaviour of confined fluids.

Investigations of structural and transport features of La_1-xBa_xMnO₃ with x = 1/3 and 2/3 are performed. The x = 2/3 sample is shown to be phase separated into a mixture of La_2/3Ba_1/3MnO₃ and BaMnO₃ with the same volume fractions, ~50%. In this two-phase system, La_2/3Ba_1/3MnO₃ regions are connected in a percolative manner, so the electrical transport is dominated by flow along these percolative paths. Using the recently proposed random resistor network based on electronic phase separation between ferromagnetic metallic and paramagnetic insulating domains, we show that the model can yield results in quantitative agreement with the resistance versus temperature dependence measured for the x = 1/3 and 2/3 samples by using the metallic number density as a fitting parameter. This approach suggests a simple quantitative picture that can be used to explain the insulator-metal behaviour in La_1-xBa_xMnO₃ with higher x.

We have studied the crystal structure, the magnetic and electron transport properties and the magnetoresistance effect of nitrogenation of La_2/3Ca_1/3MnO₃ by x-ray powder diffraction, magnetic measurement and standard four-probe resistance-temperature measurement. Nitrogenation results in an increase in the lattice parameter and unit cell volume, and induces the formation of a superlattice. The N atom prefers to occupy the 4c crystal position in the superlattice structure. Nitrogenation weakens the ferromagnetic ordering and results in the decrease of Curie temperature and the saturation magnetic moment but increases the electrical resistivity significantly. The electrical resistivity exhibits a metal-insulator (MI) transition, but the MI transition temperature decreases after nitrogenation. The behaviour of the electrical resistivity follows the equation ρ = ρ₀(1-aT + bT² + cT⁴) in the metal regime. However, it follows the formula ρ = ρ₀exp (E/k_BT) in the insulator regime. The magnetoresistance ratio measurement shows an increase of 15% after nitrogenation.

Some granular disordered superconductors, namely diamond-like carbon-silicon films containing tungsten, show one or more small, clearly detectable peaks in resistivity curves at lower temperatures than that of the superconducting main transition, T_c. Based on the concept of cluster superconductivity and on the percolation theory, we carried out extensive numerical simulations that relate the appearance of the peak-type re-entrant superconductivity to the absence of superconducting percolative paths crossing the entire sample. The numerical determination of such paths allowed us to compute the maximum supercurrent density j_c to flow through the sample in the temperature region below T_c.

The anisotropic properties of the 30% hole-doped tri-layered manganite film La_2.1Ca_1.9Mn₃O₁₀ have been investigated by means of magnetic transport measurements and electron spin resonance (ESR) experiments carried out in magnetic fields with direction perpendicular and parallel to its surface (the ab plane). The results show that the magnetoresistance in a parallel magnetic field is much more prominent than that in a perpendicular one. In addition, the temperature dependence of the resonance field revealed by ESR experiments with respect to the magnetic field directions behaves in different ways. Both the c-axis and the ab plane anisotropic magnetic interactions and strong demagnetization in the c-axis direction should be taken into account to explain these features. The magnetic anisotropy is manifested by the fact that the magnetizations in the parallel and perpendicular magnetic fields deduced from ESR signals are different; the former, which agrees well with a direct dc magnetization measurement in a parallel magnetic field, is much larger than the latter.

We have carried out a nuclear magnetic resonance study on pseudo-binary spinel compounds (Cu_xCo_1-x)Co₂S₄ with x = 0-1.0 to deduce the varied impact of variations in the antiferromagnetic (AF) spin correlation and density of states (DOS) that can clearly define the combination of magnetism and superconductivity in the system. The Curie-Weiss-type behaviour of the Knight shift K for both ⁵⁹Co and ⁶³Cu on the tetrahedral A site and the temperature-independent K for ⁵⁹Co on the octahedral B site indicate that the magnetism of the system originates from 3d bands associated with the transition-metal elements on the A site. With the Cu substitution x for Co on the A site, the negative Weiss temperature θ deduced from the K data initially decreases, takes a deep minimum around x = 0.7 followed by a rapid increase for x→1. For the compounds with x⩽0.3 and x>0.8, the nuclear spin-lattice relaxation rate T₁^-1 for the nuclei on both A and B sites has a T^1/2 dependence at high temperatures, which is characteristic of three-dimensional itinerant weak antiferromagnets. In the Cu-rich region (x>0.7) the compounds transform into a superconducting state below T_S with no long-range magnetic ordering. From a T₁^-1 data analysis, we find that the increase of T_S correlates not only with the development of the AF spin correlation but also with the significant increase in the DOS at the Fermi level of the 3d bands N_d(E_F) associated with Co on the B site. T₁^-1 in the superconducting state of CuCo₂S₄ (T_S = 4.4 K), measured for ⁵⁹Co on the B site utilizing the pure quadrupole resonance spectrum, has a coherence peak followed by an exponential decrease, indicating that CuCo₂S₄ is an s-wave superconductor with an energy gap of 2Δ = 4.14k_BT_S. We conclude that the appearance of superconductivity for the compounds in the Cu-rich region originates from the large increase in N_d(E_F) associated with Co on the B site, and the development of the AF spin correlation originates from the increase in N_d(E_F) associated with Cu on the A site.

We have developed a simulation model of the implantation of a negative charge into an insulating target by a fixed and well-focused high-energy electron beam. We are particularly interested in the evolution of the distribution of the charges trapped during the bombardment.

Our simulation is based on a Monte Carlo method permitting us to account for the various electron-insulator interactions. The charge carriers, unless they are emitted into vacuum, are followed until they have lost most of their kinetic energy. After that, they drift along the internal electric field lines before getting trapped. The field generated by these trapped charges is calculated self-consistently by solving the appropriate Poisson equation.

When the trapping site density is sufficiently high, the dynamics of the charge is principally governed by the self-regulation of the total secondary emission yield. The total number of implanted charges is therefore limited and a quasi-stationary regime arises.

The charge distribution builds up, forming a negative semi-ellipsoidal shell whose extent is directly related to the maximum penetration of the primary electrons. The internal region corresponds to a mixing zone with a weak positive mean charge. This characteristic distribution appears at all the primary beam energies considered.

On the other hand, when the trapping site density is too low, the whole region under the beam is saturated and the mixing zone is completely occupied by electrons before the self-regulation of the total secondary yield acts.

Ferroelectric BaFe_0.5Nb_0.5O₃ (BFN) ceramic is synthesized by the solid-state reaction technique for the first time. The x-ray diffraction of the sample at room temperature shows a monoclinic phase. Dielectric studies of the sample show a frequency dependence of the temperatures at which the dielectric permittivity (real and imaginary) peaks. The temperature variations of the real and imaginary components of the dielectric permittivity show broad maxima. There is evidence for Vogel-Fulcher-type relaxational freezing. The analysis of the real and imaginary parts of the dielectric permittivity with frequency has been performed assuming a distribution of relaxation times as confirmed by Cole-Cole plots as well as the scaling behaviour of the dielectric loss. All these observations clearly suggest that BFN is a relaxor ferroelectric. The Mössbauer spectrum of the sample at room temperature shows a symmetric doublet, with the iron being trivalent.

The polarization effects of in-plane electric fields and eccentricity on electronic and optical properties of semiconductor quantum rings (QRs) are discussed within the effective-mass approximation. As eccentric rings may appropriately describe real (grown or fabricated) QRs, their energy spectrum is studied. The interplay between applied electric fields and eccentricity is analysed, and their polarization effects are found to compensate for appropriate values of eccentricity and field intensity. The importance of applied fields in tailoring the properties of different nanoscale materials and structures is stressed.

Single crystals of the Li₃Na₃In₂F₁₂ fluoride garnet have been studied by Fourier-transform infrared spectroscopy and Raman scattering. Polarized Raman spectra were recorded at low temperatures showing that the room-temperature phase is stable down to 7 K. Selected scattering geometries allow us to identify the Raman-active phonons based on the factor group analysis. The infrared reflectance spectrum was measured at room temperature and analysed on the basis of the four-parameter semi-quantum model.

Nanocrystalline CdS and CdS-ZnO thin films were deposited at room temperature using high-pressure radio-frequency magnetron sputtering. The CdS-ZnO nanocomposite film was made up of particles smaller than 2 nm and showed a composite band gap of 2.72 eV. Both samples showed appreciable photoconductivity, extremely fast response and high photostability. We show that the photoresponse in these samples arises mainly from the band-gap transition and excitonic contributions are insignificant. A comparison of the photocurrent response with absorption and photoluminescence data indicates that photocurrent spectroscopy is a simple and extremely useful technique for characterizing systems (such as nanoparticles) with low radiative cross sections.