Table of contents

We investigate the morphology and kinetics of microphase separation of diblock copolymers by means of a mode expansion method. In a weak segregation limit, we employ two-wavenumber approximation to study the formation and stability of a double gyroid structure and derive the phase diagram of the microphase separated states. We have studied the kinetics of structural transitions due to temperature change by solving the coupled set of amplitude equations for the fundamental modes. We focus our attention on the morphological evolution from a double gyroid to a lamellar structure and to a hexagonal structure.

To find the origin of the diffusion channels observed in sodium silicate glasses, we have performed classical molecular dynamics simulations of Na₂O–4SiO₂ during which the mass of the Si and O atoms has been multiplied by a tuning coefficient. We observe that the channels disappear and that the diffusive motion of the sodium atoms vanishes if this coefficient is larger than a threshold value. Above this threshold the vibrational states of the matrix are not compatible with those of the sodium ions. We thus interpret the decrease of the diffusion by the absence of resonance conditions.

Silicon microphotonics, a technology which merges photonics and silicon microelectronic components, is rapidly evolving. Many different fields of application are emerging: transceiver modules for optical communication systems, optical bus systems for ULSI circuits, I/O stages for SOC, displays, .... In this review I will give a brief motivation for silicon microphotonics and try to give the state-of-the-art of this technology. The ingredient still lacking is the silicon laser: a review of the various approaches will be presented. Finally, I will try to draw some conclusions where silicon is predicted to be the material to achieve a full integration of electronic and optical devices.

The phase transition in NpO₂ at T₀ ≈ 25.5  K is accompanied by the onset of superlattice reflections in the x-ray Bragg diffraction pattern, with intensity enhanced by an electric dipole (E1) event. Additional experiments using other techniques indicate no ordering at T₀ of Np magnetic moments. Absence of long-range magnetic order below T₀ fits with the outcome of a polarization analysis of superlattice intensities at 12 K; signals are observed in both the unrotated (σ'σ) and rotated (π'σ) channels of scattering while magnetic (dipole) moments would contribute only in the rotated channel. We demonstrate that these empirical findings, together with a narrow energy profile of the Bragg intensity at the Np M₄ edge, are consistent with magnetic and charge contributions to the E1 Bragg amplitude described by Np 5f multipoles of ranks 3 (octupole) and 4 (hexadecapole). Key to our understanding of the x-ray diffraction data gathered in the vicinity of the Np M₄ edge is recognition of an exchange field creating 3d_3/2 core-level structure. The particular importance of the exchange field at the Np M₄ edge is emphatically revealed in calculations of the corresponding x-ray absorption spectrum with and without the core–valence interaction. From the experimental information about NpO₂ we can infer a ground-state wavefunction for the Np 5f³ valence shell and estimate saturation values for the octupole and hexadecapole. We are led to null values for Np multipoles of ranks 2 (quadrupole) and 5 (triakontadipole).

The mapping transformation technique is applied to obtain exact results for the spin-1/2 and spin-S (S = 1/2, 1) Ising–Heisenberg antiferromagnetic chain in the presence of an external magnetic field. Within this scheme, a field-induced first-order metamagnetic phase transition resulting in multiplateau magnetization curves is investigated in detail. It is found that the scenario of the plateau formation depends fundamentally on the ratio between Ising and Heisenberg interaction parameters, as well as on the strength of the XXZ Heisenberg exchange anisotropy.

In this paper doubly degenerate defect states in the band gap of the two-dimensional photonic crystal are studied. These states can be split by a convenient distortion of the lattice. Through analogy with the Jahn–Teller effect in solids, we present a group theoretical analysis of the lifting of the degeneracy of doubly degenerate states in a square lattice by different vibronic modes. The effect is supported by the supercell plane-wave model and by the finite difference time domain technique. We suggest ways for using the effect in photonic switching devices and waveguides.

The crystal structure of the layered perovskite La_0.6Sr_0.1TiO₃ at room temperature has been solved by synchrotron x-ray powder diffraction in combination with group theoretical analysis. The structure is orthorhombic in Cmmm, on a cell with a = 7.7556(1), b = 7.7349(1) and c = 7.7910(1) Å. It is believed that this is also the structure adopted by La_2/3TiO₃. Pertinent features are the alternation of fully and partly occupied layers of La (Sr) cations, and out-of-phase tilting of the TiO₆ octahedra around an axis perpendicular to the direction of the cation ordering. The compound undergoes a second order transition to a tetragonal structure, the transition temperature being estimated as 360 °C.

We report photo-induced time-dependent photoluminescence (PL) from Ge–S glass that shows not only known PL fatigue but also a PL recovery phenomenon—observed under prolonged irradiation by the excitation light, even after PL fatigue—in Ge_1−xS_x glasses (0.67 ≤ x ≤ 0.90). The two phenomena have different time constants. Also, the fatigue process depends strongly on glass composition, excitation energy and excitation intensity, while the recovery process depends clearly only on excitation intensity. We propose a simplified functional form to describe the time dependence that is based on a model in which non-radiative recombination centres—created by the photo-excitation of the PL centres—revert to radiative centres through thermal reaction. The proposed function reproduces the experimental data for all irradiation time domains.

The intrinsic defects in KMgF₃, such as V_K centres and self-trapped excitons (STEs), are studied by the ab initio method and the extended-ion method. In the ab initio method, the Madelung potentials are introduced into the Fock operator terms to perform calculations on clusters modelling ionic solids. It is found that the V_K centre moves toward the nearby interstitial site, still keeping C_2v symmetry; and the STE is unstable in the on-centre symmetry, undergoing a relaxation consisting of an axial translation superimposed with a rotation. Such a translation plus rotation relaxation of the STE in KMgF₃ is quite different from those in alkali halides and in alkali-earth halides. The calculated results for the excitation energy of the V_K centre and for the emission energy of the STE are in reasonable agreement with experiments.

The temperature dependence of the penetration depth in the presence of nonlocality and impurity in d-wave superconductors is calculated. It is shown that the penetration depth is proportional to the relaxation time at low temperatures, where the nonlocal effects are in fact completely masked by impurities.

We consider a lattice of spin-1 particles with a general pairwise interaction [(cos γ)(S_lS_l+1) + (sin γ)(S_lS_l+1)²]. We show that, for a large class of lattices with even numbers of sites, the ground state for the region −3π 4 < γ < −π 2 has total spin S_tot = 0, whereas the state of minimum excited energy but with finite S_tot has S_tot = 2. These results are contrasted with the generalized Marshall theorems, applicable to a bipartite lattice and for −π 2 < γ ≤ 0.

The electrical resistivity, thermal properties, pressure effect on the Néel temperature and compressibility of Mn_3+xRh_1−x (x = 0.0, 0.12, 0.20 and 0.32) alloys have been investigated in order to discuss the magnetovolume effects in both ordered and disordered states. The electronic specific heat coefficient γ^exp of the Mn₃Rh ordered alloy is slightly smaller than that of the disordered one, qualitatively consistent with the theoretical calculations. A negative magnetovolume effect is observed below the Néel temperature T_N in both the ordered and disordered alloys. Furthermore, the magnetic contribution to the thermal expansion is significant even in the paramagnetic region. That is, the thermal expansion coefficient α in the paramagnetic region shows a large value of about 30 × 10⁻⁶ K⁻¹, because of a significant contribution from spin fluctuations. For the Mn₃Rh ordered alloy, the pressure shift of the Néel temperature, dT_N/dP, is estimated to be about 7.0 K GPa⁻¹. The temperature dependence of α exhibits a pronounced positive peak at T_N in accordance with the positive pressure dependence of T_N. A large compressibility of 1.4 × 10⁻² GPa⁻¹ has been obtained from the change in the lattice constant under pressure.

The site preference of Ir, the temperature dependence of magnetic susceptibility and the low-temperature specific heat of β-Mn_1−xIr_x alloys have been investigated. In Mn_1−xIr_x alloys, the β-phase exists below x = 0.102. From x-ray diffraction patterns, almost all Ir atoms are substituted at site 1 in the β-Mn lattice. The electronic specific heat coefficient γ is proportional to the Néel temperature T_N in the form T_N^3/4 in regions of low-Ir concentration. With increasing x, however, the gradual decrease in γ leads to a relative increase in γ from the straight line. Therefore, it is concluded that the magnetic state of the β-Mn_1−xIr_x alloys varies from a weak itinerant-electron antiferromagnet to an intermediate antiferromagnet with increasing Ir concentration. The point to observe is that the γ versus T_N^3/4 plots give a universal line for all the β-Mn alloy systems over the entire concentration range.

We have made extensive studies of a dilute U doped YRu₂Si₂ alloy using several experimental techniques such as ac and dc magnetic susceptibility, dc resistivity and microstructural investigations. The real part of the ac susceptibility shows a maximum at around 6.75 K for f = 7 Hz. With increasing frequency, the temperature of this maximum increases while, at the same time, the magnitude of the maximum decreases, which is a typical spin glass behaviour. The magnitude of the imaginary part shows frequency-dependent behaviour too. We show that most of the imaginary part of the susceptibility, at least over the range of the frequencies we used, comes from the conduction electrons. Zero-field cooling and field cooling dc magnetization measurements also indicate the presence of a spin glass transition at about the same temperature. We also found that relaxation processes below the freezing temperature show ln(t + t_O) behaviour. We discuss our findings in light of the origin of the spin glass behaviour and compare them with other similar studies.

20 nm diameter Fe–Co nanowire arrays with different composition were fabricated by electrodeposition. A nanoporous anodized aluminium oxide film was used as the substrate. The microstructure and magnetic properties were studied by x-ray fluorescence, transmission electron microscopy, x-ray diffraction and vibrating sample magnetometry. Both the coercivity H_c and the spontaneous magnetization M_s increase when the Co content in Fe–Co alloy increases from 0 to about 30 at.%, and then decrease with further increase in the Co content. A model called the 'chain of crystals' is developed and solved to explain the experimental results.

In the garnet-structure compound Yb₃Ga₅O₁₂, the Yb³⁺ ions (ground-state effective spin S' = 1/2) are situated on two interpenetrating corner-sharing triangular sublattices such that frustrated magnetic interactions are possible. Previous specific heat measurements have evidenced the development of short-range magnetic correlations below ∼0.5 K and a λ-transition at 0.054 K (Filippi et al 1980 J. Phys. C: Solid State Physics13 1277). From ¹⁷⁰Yb Mössbauer spectroscopy measurements down to 36 mK, we find that there is no static magnetic order at temperatures below that of the λ-transition. Below ∼0.3 K, the fluctuation frequency of the short-range correlated Yb³⁺ moments progressively slows down and, as T → 0, it tends to a quasi-saturated value of 3 × 10⁹ s⁻¹. We also examined the Yb³⁺ paramagnetic relaxation rates up to 300 K using ¹⁷²Yb perturbed angular correlation measurements: they evidence phonon-driven processes.

The two most important molecular movements which bring about the order–disorder ferroelectric phase transition in the hydrogen-bonded ferroelectric triglycine sulfate (TGS) are the swinging of the amino group (−NH₃⁺) of one of its three glycine ions, namely GI, and the tunnelling of hydrogen in the hydrogen bond between its other two glycine ions, GII and GIII (GII–H–GIII). The potential function for bent hydrogen bonds is used along with the structural parameters of the TGS crystal to model the double-well potential (U) seen by the amino group (−NH₃⁺) of GI in TGS. The ferroelectric phase transition in TGS is investigated from the point of view of the double-well instability. Results obtained are in good agreement with those obtained earlier using the Ising-type theoretical model. Correlation between the two crucial molecular movements in TGS, namely swinging of the −NH₃⁺ group of GI and tunnelling of hydrogen in the hydrogen bond GII–H–GIII of TGS, is established.

We propose a scheme for generating three primary colours with a 1064 nm pump wave. Three quasi-phase-matched parametric processes, i.e., second-harmonic generation, optical parametric generation, and sum-frequency generation, are coupled in a single optical superlattice. On the basis of the plane-wave approximation, the coupling process is investigated theoretically. And a special design for a quasiperiodic structure is presented.