Table of contents

The quality of SiC crystals grown by the physical vapour transport (PVT) method was studied by means of optical microscopy and scanning electron microscopy (SEM) observations with the aid of etching by molten KOH. New types of defects were found, including triangular etching pits, shallow hexagonal etching pits and dendritic silicon inclusions in 4H-SiC. The triangular etching pits usually appear on the C face with a size comparable with the irregular dark etching pits due to the micropipes, while the shallow hexagonal etching pits were observed on the Si face with a size comparable with that due to the micropipes. The silicon inclusions exhibit dendritic shape up to several microns like those usually observed in metal alloys. In addition, 4H-SiC and 6H-SiC domains with different polarities in the growth surface were found to develop from the same seed. The interface of 4H-SiC and 6H-SiC was one of the sources for inducing the micropipes.

The Bean–Livingston barrier at the interface of type-II superconductor/soft-magnet heterostructures is studied on the basis of the classical London approach. This shows a characteristic dependence on the geometry of the particular structure and its interface as well as on the relative permeability of the involved magnetic constituent. The modification of the barrier by the presence of the magnet can be significant, as is demonstrated for a cylindrical superconducting filament covered with a coaxial magnetic sheath. Using typical values of the relative permeability, the critical field of first penetration of magnetic flux is predicted to be strongly enhanced, whereas the variation of the average critical current density with the external field is strongly depressed, in accord with the observations of recent experiments.

The dynamical density functional theory of Marconi and Tarazona (1999 J. Chem. Phys. 110 8032), a theory for the non-equilibrium dynamics of the one-body density profile of a colloidal fluid, is applied to a binary fluid mixture of repulsive Gaussian particles confined in a spherical cavity of variable size. For this model fluid there exists an extremely simple Helmholtz free energy functional that provides a remarkably accurate description of the equilibrium fluid properties. We therefore use this functional to test the assumptions implicit in the dynamical density functional theory, rather than any approximations involved in constructing the free energy functional. We find very good agreement between the theory and Brownian dynamics simulations, focusing on cases where the confined fluid exhibits phase separation in the cavity. We also present an instructive derivation of the Smoluchowski equation (from which one is able to derive the dynamical density functional theory) starting from the Liouville equation—a fully microscopic treatment of the colloid and solvent particles. This 'coarse graining' is, of course, not exact and thus the derivation demonstrates the physical assumptions implicit in the Smoluchowski equation and therefore also in the dynamical density functional theory.

We report a study on several structural and dynamical properties of the liquid Li–Mg and Li–Ba alloys. The study has been carried out by means of the orbital free ab initio molecular dynamics method, combined with local ionic pseudopotentials constructed within the same framework. The results show good agreement with the available experimental data, accounting fairly well for the different ordering tendencies exhibited by these alloys. We analyse the dependence of the longitudinal and transverse collective modes on the concentration and the mass ratio of the alloy, with the latter ranging from to .

Dielectric susceptibility spectra of liquids cooled towards the liquid–glass transition often exhibit secondary structure in the frequency region between the α peak and the susceptibility minimum, in the form of either an 'excess wing' or a secondary peak—the Johari–Goldstein β peak. Recently, Götze and Sperl (2004 Phys. Rev. Lett. 92 105701) showed that a simple schematic mode coupling theory model, which incorporates rotation–translation (RT) coupling, successfully describes the nearly logarithmic decay observed in optical Kerr effect data. This model also exhibits both excess wing and β peak features, qualitatively resembling experimental dielectric data. It also predicts that the excess wing slope decreases with decreasing temperature and gradually evolves into a β peak with increasing RT coupling. We therefore suggest that these features and their observed evolution with temperature may be consequences of RT coupling.

Magnetic gels with magnetite nanoparticles incorporated in a matrix of poly(acrylamide) gel were studied. Magnetite was synthesized through coprecipitation of Fe(II) and Fe(III) in the gel phase, in the solution of linear polymer and in aqueous solution without polymer in alkaline media. The effects of network structure and of the concentration of iron salts in the swollen networks on the composition, structure and properties of magnetic gels have been studied by electron diffraction, XRD, transmission electron microscopy and vibration sample magnetometry. The average size of magnetite nanoparticles, D, is of the order of 10 nm. It decreases with the increase of polymer concentration in the gel phase. In the dried gels the particles form spherical aggregates (diameter about 150 nm), whereas in the solution of linear polymer, in the aqueous solution of iron salts and in the gel with high content of polymer the aggregates have irregular shape.

The effect of hydrogenation on the local structure of a Ti₂₅Hf₅₀Ni₂₅ amorphous alloy was studied by extended x-ray absorption fine structure (EXAFS) measurements. The samples were loaded to different hydrogen-to-metal ratios, H/M, 1.2 and 1.4. For the non-hydrogenated alloy, the local structure was found to be different from that in the crystalline Ti–Hf–Ni phase. A new icosahedral ordering, similar to that in the Ti₂Ni crystal, is identified around the nickel atoms. Upon hydrogenation, little change is observed in the environment of the nickel atoms. As in the 3/2 phase and in the Ti–Zr–Ni alloys, the perturbation of the local structure with hydrogenation concerns mainly the environments of hafnium (zirconium) and titanium atoms. This indicates that the hydrogen atoms sit preferentially near hafnium (zirconium) and titanium neighbours. Moreover, a drastic decrease of the disorder parameter for the Hf–Ti correlation could be explained by a stiffening of the structure of the glass, possibly due to the formation of a hydrogen network.

The crystallization process of a Ti–Al amorphous alloy during isothermal annealing was studied with molecular dynamics simulations. The structural development and phase transformation were analysed based on the variations of the internal energy, cell volume, radial distribution function, bond pairs, and atomic configuration. The crystal nucleation, grain growth, and grain coarsening during the crystallization process were studied. The three-stage feature of the crystallization process was identified. The simulation results also show that there are transformations from a metastable crystal phase to a more stable crystal phase during the crystallization.

We describe in this paper the likely physical mechanisms that underlie the enormously enhanced electron emission observed from certain carbon field emitters. Our key ideas link the enhancement to dynamic resonant tunnelling and to the image interaction in a more general form than is usual. We have recently proposed a giant enhancement of the dynamic surface potential that could explain the anomalously large field emission currents observed in carbon-based emitters with predominantly non-Fowler–Nordheim I–V characteristics. Here we report further results on this effect which can best be described as an image-induced dynamical resonant tunnelling phenomenon, and in particular the applied field dependence of the inverse potential enhancement factor κ(ω), which seems to be a hallmark of such systems. The latter is studied by a consideration of the non-linear second-order susceptibility γ(ω) which couples the static applied field to the dynamic field. We derive the criterion for this mechanism to operate and demonstrate that it does indeed provide the linear field dependence of κ(ω) found in our earlier work. We further provide a link between γ(ω) and other microscopic parameters of the surface plasmon model, notably the anharmonicity coefficient, via a Duffing oscillator model. Through the use of a one-dimensional fluctuating barrier model with a self-consistent approach, we further assess the significance of other non-linear damping effects.

The structure of amorphous Ge₁₅Te₈₅ has been studied by neutron and x-ray diffraction. Experimental data have been modelled simultaneously with the reverse Monte Carlo simulation method. The contrast between structure factors together with the application of some plausible physical constraints allowed the separation of the three partial pair correlation functions and determination of Ge and Te local environment.

Extended x-ray absorption fine structure measurements at the Mn K absorption edge on MnAs in the temperature range between 250 and 500 K are reported. The temperature dependence of the interatomic Mn–Mn and Mn–As distances is in accordance with the lattice parameter behaviour reported by previous diffraction studies. The Debye–Waller factors of the Mn–Mn and Mn–As first sub-shells are both characterized by an anomaly near the first-order magnetostructural transition at T_C. This critical behaviour is superimposed on a temperature dependence well described by a correlated Debye model and is analysed in terms of a cusp-shaped anomaly strictly related to a critical lowering in the phonon frequencies.

ZrMn₆Sn_6−xGa_x () compounds of HfFe₆Ge₆-type structure (P6/mmm) have been studied using powder neutron diffraction experiments in the 2–600 K temperature range. Below their magnetic ordering temperatures, these phases adopt commensurate easy-plane magnetic arrangements: +−− + antiferromagnetic AF2 () or ferromagnetic (). At low temperature, the Ga-poor compounds () exhibit a transition to concentration-dependent incommensurate configurations. Upon increasing the Ga content, this incommensurate state evolves from helimagnetic to fan-like through arrangements we termed antifan-like. The Mn moment value is close to for the Ga-rich and Ga-poor compounds and reaches a maximum amplitude of in the (anti)fan-like structures of intermediate concentrations. A tentative (x,T) magnetic phase diagram is presented. The occurrence of Lifshitz point(s) is forecast for 0.60<x<0.70.

A local and medium range atomic structure model for the face centred icosahedral (fci) Mg₂₅Y₁₁Zn₆₄ alloy has been established in a sphere of r = 27 Å. The model was refined by least squares techniques using the atomic pair distribution (PDF) function obtained from synchrotron powder diffraction. Three hierarchies of the atomic arrangement can be found: (i) five types of local coordination polyhedra for the single atoms, four of which are of Frank–Kasper type. In turn, they (ii) form a three-shell (Bergman) cluster containing 104 atoms, which is condensed sharing its outer shell with its neighbouring clusters, and (iii) a cluster connecting scheme corresponding to a three-dimensional tiling leaving space for a few glue atoms. Inside adjacent clusters, Y₈ cubes are tilted with respect to each other and thus allow for overall icosahedral symmetry. It is shown that the title compound is essentially isomorphic to its holmium analogue. Therefore, fci-Mg–Y–Zn can be seen as the representative structure type for the other rare earth analogues fci-Mg–Zn–RE (RE = Dy, Er, Ho, Tb) reported in the literature.

The energy levels and far-infrared absorption spectra of a self-assembled InAs ring with one and two electrons in an external magnetic field are calculated numerically. We use a truly three-dimensional effective mass model which considers finite potential barriers and mass dependence on the energy and position, and includes strain effects. The results obtained indicate that far-infrared spectroscopy of self-assembled rings is very sensitive to electron–electron interactions. The exchange energy leads to aperiodic fractional Aharonov–Bohm oscillations of electronic states and rapid narrowing of the magnetic field windows corresponding to the spin singlet ground state. Our results also suggest that the symmetric form of parabolic confinement potential, which has been widely employed to describe quantum rings, is unsuitable for self-assembled rings as it poorly describes the relevant effects of the inner radius.

Polarized neutron diffraction has been used to determine the spatial distribution of the magnetization in CoS₂ and its temperature dependence. Although the previously reported ground state moment 0.84 μ_B is non-integral, the compound has sometimes been classified as a half metallic ferromagnet. The present results show that the ferromagnetic phase cannot have half metallic properties. The magnetization distribution around the Co²⁺ ions in the ferromagnetic phase at 1.8 K is nearly spherical, indicating that, contrary to electronic structure calculations, electrons both of e_g and t_2g character contribute to the observed magnetic moment. The orbital contribution to the moment is found to be small, a factor of two less than that determined from magnetic circular dichroism measurements. A small moment, approximately 2% of that on the cobalt ions, resides on the sulfur atoms at 1.8 K. The magnetization distribution induced in the paramagnetic phase by a field of 9.6 T at 150 K is significantly different from that in the ferromagnetic state and has the asphericity characteristic of e_g electrons. On heating to 300 K and then to 400 K the t_2g character of the magnetization increases continuously and becomes predominant above the anomaly in the susceptibility at 350 K.

The phase diagram of (PbMg_1/3Nb_2/3O₃)_1−x–(PbTiO₃)_x (PMN–PT) has been determined to high accuracy over the whole compositional range using birefringence imaging. A steep change has been detected in the thermal hysteresis of phase transitions in the morphotropic phase boundary region at x = 0.295 at the boundary between relaxor and non-relaxor phases. At the same composition at a temperature about 30 °C below T_c a monoclinic phase appears. The phase transition between non-relaxor and the monoclinic phase is characterized by a pronounced thermal hysteresis and a large jump in birefringence, showing that this is of first order. A significant change in slope for the compositional dependence of T_c has been found for , suggesting a boundary between the tetragonal phase and a phase of lower symmetry. The temperature dependence of the birefringence has been measured for a number of different compositions in the range 0.3<x<1, in which it is found that the birefringence of the monoclinic phase is much higher than that seen in the other phases for x<0.5.

The structural, magnetic and transport properties of the LaMn_1−xCo_xO₃ system were investigated over a wide temperature range, 10–900 K. Two structural types, depending on x, were detected—orthorhombic Pbnm () and rhombohedral (x>0.6), separated by the bi-phasic sample x = 0.6. At both ends of the LaMn_1−xCo_xO₃ system, the respective substituents are unambiguously characterized as Co²⁺ () and Mn⁴⁺ (). On the Mn-rich side, up to x<0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of the Co²⁺ valence towards the prevailing Co³⁺, while for the Co-rich samples (), the Co³⁺ states tend to disproportionate at high temperatures to Co²⁺+Co⁴⁺, which probably controls both the transport and magnetic properties. At low temperatures, the long range ferromagnetic order was confirmed by means of neutron diffraction for the x = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up to x = 0.8 and was associated with FM clusters that are formed below  K and coagulate below  K.

The influence of the Fano effect and the Kondo and antiferromagnetic (AF) correlations on the transport properties through a double quantum dot (DQD) structure is investigated by the finite-U slave-boson mean field method. In the singly-occupied regime, with weak AF correlation, the Fano–Kondo effect greatly reduces the conductance G no matter whether the transmissivity through the direct channel is high or low, whereas the strong parity splitting leads to the blockade of the channel through the DQDs. At the particle–hole symmetric point, when the Kondo and AF correlations are comparable with each other, their competition results in a resonant conductance peak, which is accompanied with a transmission zero due to the Fano effect. This peak-zero structure is a counterpart of the characteristic asymmetric line shape and governs the variational trend of G with the energy levels on dots. The qualitative relations between the positions of those peaks and zeros with the transmissivity through the direct channel are obtained via a fitting method.

The hysteresis curves of systems composed of small interacting magnetic particles, regularly placed on stacked layers, are obtained with Monte Carlo simulations. The remanence as a function of temperature, in interacting systems, presents a peak that separates two different magnetic states. At low temperatures, small values of remanence are a consequence of antiferromagnetic order due to the dipolar interaction. At higher values of temperature the increase of the component normal to the lattice plane is responsible for the small values of remanence. The effect of the number of layers, coordination number and distance between particles are investigated.

Elastic neutron scattering measurements have been performed under hydrostatic pressure on single-crystalline MnSi. The scaling between the staggered moment and the transition temperature of the helical magnetic structure does not follow predictions from spin-fluctuation theories. For pressures below –13 kbar, where the transition is second order, the length of the helix decreases with increasing temperature. Such a behaviour is not expected for a ferromagnetic Dzyaloshinsky–Moriya instability. With increasing pressure, the length of the helix also decreases, but there is a lock-in to an incommensurate temperature-independent value above p^*, where the phase transition is first order.

In the dipolar approximation, we present an analytical study of the magnetic modes occurring in a stripe submitted to a magnetic field H parallel to a principal axis of its elliptical cross section. The studied excitations oscillate within the cross section. The results differ greatly from those obtained in the previously treated case of H parallel to the stripe direction. The dependences of the frequencies upon the applied field are opposite below and above the saturation field H_s, where H_s defines the limit between the oblique phase (H<H_s)and the aligned one (H>H_s): the frequencies slow down until H_s, they vanish at H_s and they increase above. This softening was previously observed on stripes with a rectangular cross section and the measured critical field lies near the saturation field, in agreement with our calculations.

We find that the statistics of levels undergoing the metal–insulator transition in systems with Gaussian disorder and non-interacting electrons behaves in a way similar to that of the single parametric Brownian ensembles (Dyson 1962 J. Math. Phys.3 1191). The latter appear during a Poisson Wigner–Dyson transition, driven by a random perturbation. The analogy provides analytical evidence for the single-parameter scaling of the level correlations in disordered systems as well as a tool to obtain them at the critical point for a wide range of disorders.

We report the observation of a velocity overshoot phenomenon for electrons as well as holes in a GaAs-based p–i–n nanostructure by using transient picosecond Raman spectroscopy. Under the picosecond laser excitation, we have found that the extent of velocity overshoot for electrons is comparable to that of holes. These experimental results have been explained in terms of various carrier scattering processes. Comparisons with results obtained from other III–V semiconductors are also made and a comprehensive discussion is given.

In order to tailor diamond synthesized through chemical vapour deposition (CVD) for different applications, many diamond films of different colours and variable quality were deposited by a 5 kW microwave plasma CVD reactor under different growth conditions. The morphology, quality and hydrogen incorporation of these films were characterized using scanning electron microscopy (SEM), Raman and Fourier-transform infrared (FTIR) spectroscopy, respectively. From this study, a general trend between hydrogen incorporation and film colour, morphology and quality was found. That is, as the films sorted by colour gradually become darker, ranging from white through grey to black, high magnification SEM images illustrate that the smoothness of the well defined crystalline facet gradually decreases and second nucleation starts to appear on it, indicating gradual degradation of the crystalline quality. Correspondingly, Raman spectra evidence that the diamond Raman peak at 1332 cm⁻¹ becomes broader and the non-diamond carbon band around 1500 cm⁻¹ starts to appear and becomes stronger, confirming increase of the non-diamond component and decrease of the phase purity of the film, while FTIR spectra show that the CH stretching band and the two CVD diamond specific peaks around 2830 cm⁻¹ rise rapidly, and this indicates that the total amount of hydrogen incorporated into the film increases significantly.

Laser ablated Cr-doped SnO₂ thin films grown on various kinds of substrates all show ferromagnetism well beyond room temperature. Surprisingly, films of Sn_0.95Cr_0.05O₂ grown on LaAlO₃ substrates have a giant magnetic moment of 6 μ_B/Cr, which is 20–30 times larger than that of films grown under the same conditions on SrTiO₃ and R-cut sapphire substrates. All films are highly transparent.

The local atomic order of an amorphous Ni₆₀Ti₄₀ alloy produced by mechanical alloying was studied by x-ray diffraction (XRD) and neutron diffraction techniques (ND). The total structure factors derived from the XRD and ND measurements were simulated by using the reverse Monte Carlo (RMC) method. The simulations were used to compute the partial pair distribution functions G_ij^RMC(r) andthe partial structure factors S_ij^RMC(K). From these functions, coordination numbers and interatomic distances for the first neighbours were calculated. There is a shortening in the heteropolar distance and a preference in forming unlike pairs, indicating a relatively strong chemical short-range order in the alloy.

Magnetic domain walls in thin films can be well analysed using polarized neutron reflectometry. Well defined streaks in the off-specular spin-flip scattering maps are explained by neutron refraction at perpendicular Néel walls. The position of the streaks depends only on the magnetic induction within the domains, whereas the intensity of the off-specular magnetic scattering depends on the spin-flip probability at the domain walls and on the average size of the magnetic domains. This effect is fundamentally different and has to be clearly distinguished from diffuse scattering originating from the size distribution of magnetic domains. Polarized neutron reflectivity experiments were carried out using a ³He gas spin filter with a analysing power as high as 96% and a neutron transmission of approximately 35%. Furthermore, the off-specular magnetic scattering was enhanced by using neutron resonance and neutron standing wave techniques.

ZnO films were grown by atmospheric metal–organic chemical vapour deposition and annealed at 900 °C in an oxygen environment. The annealing properties of the films have been characterized by means of x-ray diffraction, Raman scattering, Rutherford backscattering (RBS), elastic recoil detection analysis (ERDA) and photoluminescence spectra. The results indicate that high crystal quality ZnO film has been obtained after annealing. The full width at half-maximum of ω rocking curves is only 369 arcsec. The Raman spectra show a strong high frequency E₂ mode peak comparable to that for bulk ZnO. The intensity ratio of the E₁(LO) peak to E₂^high peak before annealing is 0.81 and after annealing 0.75. RBS and ERDA spectra indicate that a stoichiometric ZnO film is formed and the annealing only changes the H content in the ZnO film. After annealing all emission lines become sharper, as expected, which means a higher quality film has been obtained.

We have conducted the first single-crystal resonant soft x-ray diffraction study of the magnetic order in holmium at the Ho M_V edge. Both the (0,0,τ) and (0,0,2τ) satellite reflections can be accessed at the M_V edge. Energy scans through these reflections display very different line shapes. The behaviour of the integrated intensity and wavevector modulation was similar to that observed in earlier neutron and resonant x-ray diffraction studies at the L edges, which showed that valid qualitative results can be obtained using the soft x-ray diffraction technique. The sample absorption at the M_V edge is considerable, reducing the penetration depth to approximately 100 Å.

We give a theoretical treatment of the interaction of electronic excitations (excitons) in biomolecules and quantum dots with the surrounding polar solvent. Significant quantum decoherence occurs due to the interaction of the electric dipole moment of the solute with the fluctuating electric dipole moments of the individual molecules in the solvent. We introduce spin boson models which could be used to describe the effects of decoherence on the quantum dynamics of biomolecules which undergo light-induced conformational change and on biomolecules or quantum dots which are coupled by Förster resonant energy transfer.

The free-energy of discrete nonlinear Klein–Gordon (NKG) systems with anharmonic interparticle interactions is derived by means of the transfer integral operator method, with the first lattice corrections and kink–kink interactions taken into account. Two particular substrate potentials are considered: the ϕ−four and the sine-Gordon (sG). We show that, in the general case where the system exhibits the kink soliton like excitations, the correction factors, due to the lattice discreteness, appearing in the free-energy and in the lattice corrected static kink soliton energy, depend on the temperature through a coupling of the interparticle anharmonicity strength to the temperature. Similarly, in the purely anharmonic NKG systems, characterized by the absence of the linear dispersion, where thermodynamic properties are sensitive to kink compactons, we find also that the correction factors are temperature dependent. In both cases, they decrease with increasing temperatures, although the correction factors verify different temperature laws.

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Optical poling in photopolymers has several principal differences compared to other guest–host composites. There exists a principal difference between aligning the DC electric field presented in the article and interior DC electric strength. Also, the use of the hyperpolarizabilities of different orders is clarified.

On p 238, the last sentence of the second paragraph `From our experiment we have estimated the value of the electric field, which corresponds to the value χ⁽²⁾, to be ~10² - 10⁴ V cm^-1.' should be replaced by the following more precise and detailed sentence.

`From our experiment on electric-field poling we have found that to achieve the same nonlinear optical susceptibility as during the optical poling, it is necessary to apply an external aligning (not effective dc-internal, which will be different) electric field of 10² - 10⁴ V cm^-1, depending on the degree of photosolidification.'