Table of contents

While the principal features of the exchange bias between a ferromagnet and an antiferromagnet are believed to be understood, a quantitative description is still lacking. We show that interface spin disorder is the main reason for the discrepancy of model calculations versus experimental results. Taking into account spin disorder at the interface between the ferromagnet and the antiferromagnet by modifying the well known Meiklejohn and Bean model, an almost perfect agreement can be reached. As an example this is demonstrated for the CoFe/IrMn exchange biased bilayer by analysing the azimuthal dependence of magnetic hysteresis loops from MOKE measurements. Both exchange bias and coercive fields for the complete 360° angular range are reproduced by our model.

Low-temperature powder neutron diffraction measurements are performed in the ordered magnetic state of the pyrochlore antiferromagnet Gd₂Sn₂O₇. Symmetry analysis of the diffraction data indicates that this compound has the ground state predicted theoretically for a Heisenberg pyrochlore antiferromagnet with dipolar interactions. The difference in the magnetic structure of Gd₂Sn₂O₇ andof nominally analogous Gd₂Ti₂O₇ is found to be determined by a specific type of third-neighbour superexchange interaction on the pyrochlore lattice between spins across empty hexagons.

The non-local terms previously disregarded in the Landau free energy expansion of a melt of polydisperse heteropolymers are shown to dramatically change the appearance of the phase diagram of a heteropolymer liquid.

A new method of advanced quantitative analysis of x-ray absorption near edge structure (XANES) is described. The method is based on the fitting of experimental XANES data using multidimensional interpolation of the spectra as a function of the structural parameters and full multiple scattering calculations. Such an approach has several advantages in comparison with schemes that existed before, in particular a very small number of ab initio calculations, significant reduction of required computational time, and the possibility to see immediately the spectrum that corresponds to any set of structural parameters. A first application of the method to determine the parameters of the local geometry of a real system, namely Pd-diethynylbiphenyl, is reported. The procedure of polynomial construction is described in detail for this complex of palladium. The best-fit three-dimensional geometry obtained has the following bond distances: Pd–P 2.36 ± 0.02 Å, Pd–C 2.06 ± 0.02 Å and C–C 1.1 ± 0.03 Å, in agreement with results of previous EXAFS studies.

We have carried out differential scanning calorimetric and dielectric studies on composites of hydrophilic aerosil with a liquid crystal that does not possess a terminal polar group. While the shift in the nematic–isotropic transition temperature is in agreement with the general behaviour observed for such composites, the dielectric studies show, contrary to the commonly observed feature, that there is a systematic increase in the relaxation frequency associated with the rotation of the molecules around their short axis, as the aerosil concentration in the composite is increased.

Static and dynamic properties of liquid and amorphous GeO₂ have been simulated by a molecular dynamics method in a model containing 3000 particles under periodic boundary conditions. We have proposed for the first time new interatomic potentials for liquid and amorphous GeO₂ which have a Morse-type potential for short range interaction in the system. The structure of liquid and amorphous models is analysed through the partial radial distribution functions, coordination number distributions, interatomic distances and bond angle distributions. The calculated data for the structure and atomization energy of GeO₂ system agree well with the experimental ones. Further, for the first time, diffusion in liquid GeO₂ has been studied. We found that the temperature dependence of the diffusion constant D shows an Arrhenius law at temperatures above the melting point and it shows a power law, , at higher temperatures. The evolution of the structure upon cooling has been observed and presented.

In columnar assemblies of helical bio-molecules the azimuthal degrees of freedom, i.e. rotations about the long axes of molecules, may be important in determining the structure of the assemblies, especially when the interaction energy between neighbouring molecules explicitly depends on their relative azimuthal orientations. For DNA this leads to a rich variety of mesophases for columnar assemblies, each categorized by a specific azimuthal ordering. In a preceding paper (Wynveen et al 2005 Eur. Phys. J. E 16 303) a statistical mechanical theory was developed for the assemblies of torsionally rigid molecules in order to determine how thermal fluctuations influence the structure of these mesophases. Here we extend this theory by including torsional fluctuations of the molecules, where a DNA molecule may twist about its long axis at the cost of torsional elastic energy. Comparing this with the previous study, we find that inclusion of torsional fluctuations further increases the density at which the transition between the hexagonal structure and the predicted rhombic phase occurs and reduces the level of distortion in the rhombic phase. As x-ray diffraction may probe the 2D lattice structure of such assemblies and provide information concerning the underlying interaction between molecules, we have also calculated correlation functions for the azimuthal ordering which are manifest in x-ray scattering intensity profiles.

We study the sedimentation equilibrium of low salt suspensions of binary mixtures of charged colloids, both by Monte Carlo simulations of an effective colloids-only system and by Poisson–Boltzmann theory of a colloid–ion mixture. We show that the theoretically predicted lifting and layering effect, which involves the entropy of the screening ions and a spontaneous macroscopic electric field (Zwanikken and van Roij 2005 Europhys. Lett. 71 480), can also be understood on the basis of an effective colloid-only system with pairwise screened-Coulomb interactions. We consider, by theory and by simulation, both repelling like-charged colloids and attracting oppositely charged colloids, and we find a re-entrant lifting and layering phenomenon when the charge ratio of the colloids varies from large positive through zero to large negative values.

We propose a three-order-parameter model to study the phase separations in a diblock copolymer–diblock copolymer mixture. The cell dynamical simulations provide rich information about the phase evolution and structural formation, especially the appearance of onion-rings. The parametric dependence and physical reason for the domain growth of onion-rings are discussed.

All living organisms rely upon networks of molecular interactions to carry out their vital processes. In order for a molecular system to display the properties of life, its constituent molecules must themselves be endowed with several features: stability, specificity, self-organization, functionality, sensitivity, robustness, diversity and adaptability. We argue that these are the emergent properties of a unique phase of matter, and we demonstrate that proteins, the functional molecules of terrestrial life, are perfectly suited to this phase. We explore, through an understanding of this phase of matter, the physical principles that govern the operation of living matter. Our work has implications for the design of functionally useful nanoscale devices and the ultimate development of physically based artificial life.

We describe an experimental study of the dynamic structure factor of liquid glycerol performed by complementary inelastic techniques such as Brillouin visible, ultraviolet and x-ray scattering. The spectra have been collected as a function of both temperature and momentum transfer. The relevant hypersonic parameters are evaluated from the spectral lineshape analysis modelling the data with a simple hydrodynamic profile. The study of their frequency dependence allows us to observe the occurrence of an active structural relaxation and to measure the related timescale. We also find signatures of further relaxation processes occurring below the accessible frequency window. As a result, the dynamic window traditionally probed in spectroscopic experiments is greatly extended and partially bridges the gap between MHz and THz techniques.

The structure and visco-elastic properties of K₂Te₄O₉ have been examined as a function of temperature, using neutron scattering and Brillouin light scattering, respectively. The neutron scattering data indicate that the coordination of tellurium by oxygen changes notably once the material is heated above the glass transition temperature. This and the associated decrease in elastic modulus are consistent with converting network building blocks from trigonal bipyramids to trigonal pyramids. The latter form chain-like structures that constitute a liquid characterized by a single relaxation mechanism. Structural relaxation in the liquid results in further decrease of its elastic storage capacity and in a maximum of dissipative losses due to viscous processes. The break-up of the glassy network, which is attributed to a frictionless transformation of building blocks, is distinct from the viscous relaxation of the liquid; their visco-elastic signatures can be observed in separate temperature intervals.

We consider the theoretical model of an amphiphilic macromolecule with a complex structure of hydrophobic/hydrophilic monomer units. Each unit consists of a hydrophobic group (H) in the backbone and a side hydrophilic group (P). The units are able to orient in the density gradient at the surface layer of a globule. First, we use the density functional method to obtain the surface tension at a flat surface. We obtain that the effect of orientation decreases the surface tension of an amphiphilic globule in comparison with the surface tension of a homopolymer globule of the same density. Therefore, the amphiphilic globule is more stable with respect to the transition to a coil conformation. Then, macromolecules with strong orientational ability of amphiphilic units are considered. The free energy of spherical, bead-like, disc-like and torus-like globules is analysed for flexible and rigid macromolecules. For very long macromolecules in poor solvent, it is predicted that a disc-like globule for flexible chains should be formed. It is shown that the coil–globule transition in amphiphilic macromolecules is in most cases accompanied by a disintegration of the initially formed globule into several 'bead globules'. Upon further increase of the attraction of hydrophobic units, these beads merge with each other with the formation of a disc-like or torus-like globule, depending on the chain stiffness.

The reorientational dynamics of a stiff paramagnetic tracer dissolved in the glass former salol is investigated by means of electron spin resonance over a broad temperature range. The Debye–Stokes–Einstein law describing rotational diffusion in simple liquids is found to break down in the supercooled region where the diffusion is less temperature dependent than the viscosity. Over a large temperature interval a simple power law relates diffusion and viscosity, whereas at lower temperatures the decoupling is stronger and an activated dynamics is observed. These experiments are discussed together with previous data concerning other tracer/glass former couples. Starting from some observed common features, an attempt is made to obtain a unifying interpretation of the data in the framework of the energy landscape picture.

Single-crystal magnetic tunnel junctions employing bcc (100) Fe electrodes and MgO(100) insulating barrier are elaborated by molecular beam epitaxy. The magneto-transport properties are investigated in two extreme regimes. First, for extremely small MgO thickness, we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunnelling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effects in terms of symmetry of the electronic Bloch function and symmetry-dependent wavefunction attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–160%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the spin filtering. We point out imperfect filtering effects and a strong implication of the minority surface state of Fe on the low voltage variation of tunnel magnetoresistance. The insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance and activates a resonant tunnelling mechanism via the interfacial resonance state.

The spectral widths of Raman lines in nominally pure lithium niobate are measured as a function of the cationic molar ratio, in a wide range from stoichiometry (50 mol% Li) to sub-congruent compositions (below 48.6 down to 47 mol% Li). The broadening observed on the sub-congruent side follows the same slopes as on the congruent-to-stoichiometric side. It is shown that micro-Raman analysis permits one to characterize short-range heterogeneities at the micron scale, such as growth striations in bulk crystals or lithium out-diffusion in the surroundings of titanium-diffused waveguides, with a typical accuracy of 0.04 mol%.

We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe (a-GeTe) films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous to crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed which is compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

We investigate static and dynamical ground-state properties of the two-impurity Anderson model at half filling in the limit of vanishing impurity separation using the dynamical density-matrix renormalization group method. In the weak-coupling regime, we find a quantum phase transition as a function of inter-impurity hopping driven by the charge degrees of freedom. For large values of the local Coulomb repulsion, the transition is driven instead by a competition between local and non-local magnetic correlations. We find evidence that, in contrast to the usual phenomenological picture, it seems to be the bare effective exchange interactions which trigger the observed transition.

We report the structural and electronic characterization of the charge order phase in Bi_0.4Ca_0.6MnO₃ films, in which photoinduced resistivity changes have been observed at temperatures approaching room temperature. Lattice distortions associated with the charge order are observed in all films, and both the wavevectors and displacements of the distortions are in the plane of the film. Films under compressive and tensile strain are observed to have different resonant x-ray scattering characteristics—a result that may shed light on the mechanism responsible for the photosensitivity exhibited by this material.

We report a calculation of the ground state binding energies of the on- and off-axis neutral, D⁰, and negatively charged, D⁻, donors in an oblate lens-shaped InAlAs/GaAlAs quantum dot in the presence of a uniform magnetic field applied parallel to the dot axis. The calculations have been performed by using the adiabatic approximation for the electron, a Bastard-type trial function for the D⁰, and the Hylleraas-type trial function for the D⁻ ion. Novel curves and contour plots for the energies of donors confined in a quantum lens as functions of the distance from the axis for several dot heights and magnetic field strengths are presented. We show that under strong magnetic field the on-axis D⁻ ion binding energy becomes larger than the corresponding value of the D⁰ located in the peripheral region close to the barrier, making possible the formation of on-axis D⁻ centres in equilibrium via electron transfer from the peripheral donor to the on-axis D⁰.

The electronic properties of strained Si_1−xGe_x alloys epitaxially grown on (001) Si_1−yGe_y relaxed substrates for any x and y Ge concentrations are presented here. Our calculations are based on an sp³d⁵s^* nearest-neighbour tight-binding Hamiltonian and exploit appropriate scaling laws of the Hamiltonian interactions to account for strain effects. Spin–orbit interaction is also included in the Hamiltonian. We first provide the valence and conduction band offsets at the heterointerfaces between Si_1−xGe_x and Si_1−yGe_y, as well as the fundamental energy gap for Si_1−xGe_x strained alloys. We are thus able to distinguish the region in the (x,y) plane where robust type-I alignment is achieved. Then this information on band alignment is exploited to propose a heterostructure which is both type I in -space and direct in -space. With this aim we adopt the decimation–renormalization method for the determination of the electronic properties of the multilayer structure; from the Green's function the energy spectrum and the partial and the total densities of states projected on each layer of the system are obtained. Our conclusion is that by suitable control of alloying, stress, band offsets and folding, truly direct (both in - and in -space) semiconducting heterostructures based on silicon and germanium can be realized. As an example, the case of pure Ge sandwiched between Si_0.25Ge_0.75 alloys, grown on a Si_0.2Ge_0.8 substrate, is fully discussed.

We present a comparative study of the in situ modifications induced by a 10 keV electron beam in gallium nitride (GaN) quantum wells and quantum dots as well as in ELOG (epitaxial laterally overgrown GaN) and AlGaN epilayers. Cathodoluminescence (CL) experiments were performed to investigate the room temperature evolution of the optical properties as a function of the beam current density. Higher dot resistance is much more apparent when the beam current density is larger than about 6 A cm⁻². Recombination enhanced diffusion of defects is involved in the degradation of the CL signal. The two serial mechanisms inferred to be present in the degradation process are described.

Heat capacity and magnetocaloric studies on the compounds RPd₂Si (R = Dy,Tb and Gd) are presented. The zero-field heat capacity shows a clear peak around the magnetic transition temperature. The magnetic entropy change across the transition indicates a doublet ground state for TbPd₂Si and a quartet ground state for DyPd₂Si. For GdPd₂Si an excess contribution to heat capacity well above the magnetic ordering temperature (for T>2T_N) has been observed, which might be attributable to Pd ions. The magnetocaloric effect (MCE) in the compound DyPd₂Si around 10 K is comparable to other large MCE materials in this temperature range, whereas in the case of TbPd₂Si the MCE is small due to the presence of strong crystalline electric field and two well separated magnetic transitions.

A shallow-to-deep instability of hydrogen defect centres in narrow-gap oxide semiconductors is revealed by a study of the electronic structure and electrical activity of their muonium counterparts, a methodology that we term 'muonics'. In CdO, Ag₂O and Cu₂O, paramagnetic muonium centres show varying degrees of delocalization of the singly occupied orbital, their hyperfine constants spanning 4 orders of magnitude. PbO and RuO₂, on the other hand, show only electronically diamagnetic muon states, mimicking those of interstitial protons. Muonium in CdO shows shallow-donor behaviour, dissociating between 50 and 150 K; the effective ionization energy of 0.1 eV is at some variance with the effective-mass model but illustrates the possibility of hydrogen doping, inducing n-type conductivity as in the wider-gap oxide, ZnO. For Ag₂O, the principal donor level is deeper (0.25 eV) but ionization is nonetheless complete by room temperature. Striking examples of level-crossing and RF resonance spectroscopy reveal a more complex interplay of several metastable states in this case. In Cu₂O, muonium has quasi-atomic character and is stable to 600 K, although the electron orbital is substantially more delocalized than in the trapped-atom states known in certain wide-gap dielectric oxides. Its eventual disappearance towards 900 K, with an effective ionization energy of 1 eV, defines an electrically active level near mid-gap in this material.

Following the prediction and confirmation that interstitial hydrogen forms shallow donors in zinc oxide, inducing electronic conductivity, the question arises as to whether it could do so in other oxides, not least in those under consideration as thin-film insulators or high-permittivity gate dielectrics. We have screened a wide selection of binary oxides for this behaviour, therefore, using muonium as an accessible experimental model for hydrogen. New examples of the shallow-donor states that are required for n-type doping are inferred from hyperfine broadening or splitting of the muon spin rotation spectra. Electron effective masses are estimated (for several materials where they are not previously reported) although polaronic rather than hydrogenic models appear in some cases to be appropriate. Deep states are characterized by hyperfine decoupling methods, with new examples found of the neutral interstitial atom even in materials where hydrogen is predicted to have negative-U character, as well as a highly anisotropic deep-donor state assigned to a muonium–vacancy complex. Comprehensive data on the thermal stability of the various neutral states are given, with effective ionization temperatures ranging from 10 K for the shallow to over 1000 K for the deep states, and corresponding activation energies between tens of meV and several eV. A striking feature of the systematics, rationalized in a new model, is the preponderance of shallow states in materials with band-gaps less below 5 eV, atomic states above 7 eV, and their coexistence in the intervening threshold range, 5–7 eV.