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We present results of detailed investigation of the crystal structure of Pb_1−3x/2La_x(Zr_1−yTi_y)O₃ solid solutions. In this letter our attention is concentrated on the series of solid solutions with x = 6% usually referred to as relaxor ferroelectrics. We have established the reasons for the non-cubic crystal structure of these solid solutions at the temperatures below T_C. It is demonstrated that the peculiarities of the properties of Pb_1−3x/2La_x(Zr_1−yTi_y)O₃ depend on the position of a particular solid solution with respect to the hysteresis ferroelectric–antiferroelectric region in the 'Ti-content–temperature' phase diagram.

We propose a solid-state quantum structure capable of generating Einstein–Podolsky–Rosen (EPR) electron pairs using a parametric electron pumping idea and the Coulomb blockade phenomenon. The quantum structure consists of two coupled quantum dots and four leads. Our scheme is easy to implement, and it does not impose special requirements on the leads. By employing the parametric pumping idea, harmful processes can be avoided and only two quantum dots are needed. Furthermore, the EPR electron pairs are spatially separated.

We investigate the signatures of dynamical tunnelling in open quantum dots, by implementing a soft-walled microwave cavity as a novel analogue system. We explore the evidence for dynamical tunnelling by studying the evolution of the wavefunction phase as a function of frequency and show evidence for evanescent coupling to isolated orbits, including the existence of 'dirty' states in the wavefunction that are generated from a degenerate pair of 'clean' states when they are degraded by their tunnelling interaction. Our investigations provide a useful analogue of quantum transport in open quantum dots, and demonstrate the importance of dynamical tunnelling that arises from the mixed classical dynamics that is inherent to these structures.

The crystalline state of a single polyethylene chain with N = 500 monomers is investigated by extensive MD simulations. The polymer is folded in a well defined lamella with ten stems of approximately equal length, arranged into a regular, hexagonal pattern. The study of the microscopic organization of the lamella, which is in an equilibrium condition, evidences that the two caps are rather flat, i.e. the loops connecting the stems are short. An analytic model of the global minimum of the free energy, based on the assumption that the entropic contribution is mainly due to the combinatorics of the stems and loops and neglecting any conformational contribution, is presented. It provides for the first time a quantitative explanation of the MD results on the equilibrium geometry of single-chain crystals.

The Rouse model is a well-established model for non-entangled polymer chains and also serves as a fundamental model for entangled chains. The dynamic behaviour of this model under strain-controlled conditions has been fully analysed in the literature. However, despite the importance of the Rouse model, no analysis has been made so far of the orientational anisotropy of the Rouse eigenmodes during the stress-controlled, creep and recovery processes.

For completeness of the analysis of the model, the Rouse equation of motion is solved to calculate this anisotropy for monodisperse chains and their binary blends during the creep/recovery processes. The calculation is simple and straightforward, but the result is intriguing in the sense that each Rouse eigenmode during these processes has a distribution in the retardation times. This behaviour, reflecting the interplay/correlation among the Rouse eigenmodes of different orders (and for different chains in the blends) under the constant stress condition, is quite different from the behaviour under rate-controlled flow (where each eigenmode exhibits retardation/relaxation associated with a single characteristic time). Furthermore, the calculation indicates that the Rouse chains exhibit affine deformation on sudden imposition/removal of the stress and the magnitude of this deformation is inversely proportional to the number of bond vectors per chain. In relation to these results, a difference between the creep and relaxation properties is also discussed for chains obeying multiple relaxation mechanisms (Rouse and reptation mechanisms).

We study a fluid of nematogenic molecules with centres of mass constrained to lie in a plane but with axes free to rotate in any direction. An external disorienting field perpendicular to the plane along with a second orienting field in the plane induce an in-plane order–disorder transition. We analyse the behaviour of this simple biaxial model using a well-established generalization of molecular integral equation methods built upon specially tailored basis functions that maintain orthogonality in the presence of anisotropy. Computer simulation and integral equation calculations predict an isotropic–nematic transition at low temperatures in zero field and an in-plane transition at somewhat higher temperatures in the presence of the disorienting field. The oriented states obtained in the presence of both fields can subsequently be used as input to uncover in detail first the transition in the absence of the in-plane orienting field and finally the spontaneous transition in the absence of any field. According to the simulation, the transition apparently belongs to the Berezinskii–Kosterlitz–Thouless defect-mediated type, whereas the theory reproduces a weak first-order transition.

We present a study of orientational relaxation dynamics in thin films of a low-molecular-weight glass former as a function of temperature and film thickness. The relaxation is probed by means of second-harmonic generation after release of a poling electric field. From the measured decays of the second-harmonic signal and their fitting with a stretched exponential, we can determine the distribution of relaxation times in the system. As temperature decreases from above the glass transition, we observe that the width of the distribution first increases under confinement, but that deeper in the glassy state, confinement has no effect any longer on the dynamics.

Dielectric properties of water adsorbed in pure siliceous and aluminium containing mesoporous MCM-41 materials have been investigated in the frequency range from 20 Hz to 1 MHz. The dielectric spectra revealed three dispersion regions, liquid-like free water in the centre of the mesopores, an intermediate water layer with reduced mobility, and an interfacial water layer at the inner surface of the mesopores. The analysis of the relaxation time distribution by means of a double-well potential indicates a strong dependence of the barrier height of reorienting water molecules dipoles in the interfacial layer on the Si/Al ratio in the framework.

The longitudinal evolution of a low energy phonon pulse in superfluid helium ⁴He is analysed theoretically. The phonons are considered to be strongly interacting and are described by the exact local equilibrium distribution function. The theory describes how the longitudinal pulse shape develops. We find that the temperature change in the pulse is determined by a simple running wave which moves with a velocity very close that of the sound velocity. This results in only a small variation of the pulse length as the pulse propagates. This is in contrast to the case of a non-interacting phonon pulse, which shows considerable dispersion. The details of the phonon pulse deformation depend on whether the pulse contains phonons with relatively high energies or not. The higher energy phonons cause the single wave to eventually break at both the front and back of the pulse. Without these high energy phonons, the pulse only breaks at the front. We find that, in the first approximation with respect to the small parameters of the problem, the anisotropic nature of the phonon pulse is conserved as it propagates.

As-grown single crystals of Bi₂Se₃ are doped with varying percentages of tellurium. These crystals are irradiated and implanted with electrons of energy 8 MeV and H⁺ and He⁺ ions of energy 1.26 MeV for comparative studies on their properties. Effects on the thermoelectric properties of Bi₂Se₃ due to high-energy electron bombardment (8 MeV), H⁺ and He⁺ ion implantation and doping are studied at temperatures ranging from 150 to 380 K. Crystal homogeneity and surface dislocations are determined using EDAX and SEM. Hot-probe and Hall effect measurements show that as-grown, electron irradiated and ion implanted crystals are n-type. Thermal diffusivity measurements prove the effective scattering mechanism (phonons) in Bi₂Se₃ crystals and provide a valid reason for reduced thermo-power in doped crystals.

The morphological stability of a two-dimensional cylindrical crystal growing in a solution at the local growth rate proportional to the squared supersaturation has been analysed for the first time by the weakly nonlinear technique. A correction to the stability size, which was determined from the linear stability theory, was found and analysed. It was shown that in most cases the critical size of the crystal stability decreased with growing perturbation amplitude. This result has been discussed in terms of the nonequilibrium phase transition theory.

We treat the two-particle Green function in the Hubbard model using the recently developed τ-CPA, a hybrid treatment that applies the coherent potential approximation (CPA) up to a time τ related to the inverse of the bandwidth, after which the system is averaged using the virtual crystal approximation. This model, with suitable approximations, does predict magnetism for a modified Stoner criterion. The evaluation of the two-particle propagator in the τ-CPA requires the solution of the pure CPA, within whose formalism the vertex correction and the weighted Green functions are obtained. The dynamical susceptibility, including the vertex correction and the weighted scattering by the residual interaction, is calculated and shows a spin wave spectrum in the ferromagnetic regime.

The current–voltage relationship of organic diodes that are used as organic light emitting devices is reported. Charged molecules are created at the electrodes by an oxidation/reduction reaction. The charge moves across the material by an electron transfer mechanism from a charged molecule to a neutral molecule. The charges are described by a lattice gas because there is a low concentration of charges. The concentration of charged molecules at the electrode is governed by the Nernst equation and is coupled to the equations of transport. The concentration of charge is determined by a competition between charge creation and charge transport. The applied voltage creates the charge at the electrodes and causes charge transport. The graphical analysis results in the charge density and current density as a function of the applied voltage and the dependence of the current density on both temperature and device thickness.

The paramagnetic phase of heavy fermion systems is investigated, using a non-perturbative local moment approach to the asymmetric periodic Anderson model within the framework of dynamical mean-field theory. The natural focus is on the strong coupling Kondo lattice regime wherein single-particle spectra, scattering rates, dc transport and optics are found to exhibit (ω/ω_L,T/ω_L) scaling in terms of a single underlying low-energy coherence scale ω_L. Dynamics/transport on all relevant (ω,T) scales are encompassed, from the low-energy behaviour characteristic of the lattice coherent Fermi liquid, through incoherent effective single-impurity physics likewise found to arise in the universal scaling regime, to non-universal high-energy scales; and which description in turn enables viable quantitative comparison to experiment.

Employing a local moment approach to the periodic Anderson model within the framework of dynamical mean-field theory, direct comparison is made between theory and experiment for the dc transport and optical conductivities of paramagnetic heavy fermion and intermediate valence metals. Four materials, exhibiting a diverse range of behaviour in their transport/optics, are analysed in detail: CeB₆, Y bAl₃, CeAl₃ and CeCoIn₅. Good agreement between theory and experiment is in general found, even quantitatively, and a mutually consistent picture of transport and optics results.

An anyon wavefunction (characterized by the statistical factor n) projected onto the lowest Landau level is derived for the fractional quantum Hall effect states at filling factor ν = n/(2pn+1) (p and n are integers). We study the properties of the anyon wavefunction by using detailed Monte Carlo simulations in disc geometry and show that the anyon ground-state energy is a lower bound to the composite fermion one. Our results suggest that the composite fermions can be viewed as a combination of anyons and a fluid of charge–neutral dipoles.

Magnetic properties of a nanocrystalline σ-Fe_55.4Cr_44.6 alloy were investigated by means of Mössbauer spectroscopy and vibrating sample magnetometry both as a function of temperature (4 K ≤T≤300 K) and, for the latter, also as a function of external magnetic field (B_a≤15 T). The methods used enabled us to determine the mean Curie temperature, , and the average magnetic moment per Fe atom, , as well as to find that the sample behaves like an ensemble of interacting superparamagnetic particles. Comparison of these results with our recent corresponding investigations on microcrystalline σ-Fe_100−xCr_x alloys with 45≤x≤50 shows that the ratio between the average hyperfine field, , and for the nanocrystalline sample fits well to that of the microcrystalline ones of similar composition. The non-linear relationship points to composition-dependent valence hyperfine field contributions.

We have studied the charge carrier doping dependent changes in the valence band electronic structure of the Pr_1−xSr_xMnO₃ system across the orbital ordered compositions using ultraviolet photoelectron spectroscopy. The d_x²−y² orbital ordering is found to be causing an enhancement of the Mn 3d–O 2p hybridization strength and thereby the Mn 3d contribution to the subbands in the valence region. Our photoemission studies using different photon energies help in elucidating the nature and composition of the valence band features.

The effects of spin–phonon interaction on the temperature dependence of the spin-wave and phonon spectrum in thin ferroelectric transverse Ising films are studied using a Green function formalism beyond the random phase approximation. It is shown that due to the surface modes and the spin–phonon interaction the spin-wave damping effects in thin ferroelectric films are enhanced in comparison to the bulk. The phonon spectrum is discussed, too. Additional phonon damping and phonon frequency shift arise when the spin–phonon interaction is properly included.

We report on the femtosecond time-resolved detection of coherent phonons in single-crystal tellurium. For different crystallographic faces a detection scheme is employed which is sensitive to the anisotropic part of the Raman tensor. In this scheme we observe all three Raman allowed phonons, i.e. one of A₁ and two of E symmetry. Furthermore, for both doubly degenerate E-symmetry modes obtained from different crystallographic faces, longitudinal optical–transverse optical splitting is observed. In addition, we show that even in the low fluence regime the frequency of the fully symmetric phonon in Te is chirped and that it demonstrates an anomalous dependence on the pump fluence.

We investigate the pressure-induced structural transformation in liquid Al₂O₃ by a molecular dynamics (MD) method. Simulations were done in the basic cube, under periodic boundary conditions, containing 3000 ions with Born–Mayer-type pair potentials. The structure of the liquid Al₂O₃ model with a real density at ambient pressure is in good agreement with Landron's experiment. In order to study the structural transformation, seven models of liquid alumina at temperature 2500 K and at densities in the range 2.80–4.5 g cm⁻³ have been built. The microstructure of Al₂O₃ systems has been analysed through the pair radial distribution functions, coordination number distributions, interatomic distances and bond-angle distributions. And we found clear evidence of a structural transition in liquid alumina from a tetrahedral to an octahedral network. According to our results, this transformation occurred at densities in the range 3.6–4.5 g cm⁻³. We also obtained an anomalous density dependence of the self-diffusion constant in the region of the structural transformation.

Low dimensional (nanowall) ZnO structures were prepared by a two-step growth method with oxygen-plasma-assisted molecular beam epitaxy, where the as-grown film was first engraved on a porous template using the oxygen plasma and then the ZnO nanowalls were grown on the template. The resonance Raman spectra showed the surface mode. A morphology model was proposed on the basis of the scanning electron microscopy patterns and this mode. The room and low temperature photoluminescence showed that the nanowalls had intense ultraviolet emission properties, which benefited from the low dimensional structure with few defects.

The structure factor, pair distribution function, screened impurity potential, density of screening charge, and exchange and screened exchange energies have been theoretically investigated for a semiconductor quantum wire using an improved random phase approximation that takes into account the local field corrections within the Hubbard approximation. Our approach enabled us to obtain approximate analytical results on some of the aspects and to greatly simplify the computation task on others. However, computed results from our simple approach show very good agreement with those obtained by performing cumbersome numerical solutions for the structure factor, density–density response function and the static local field corrections, within the Singwi–Tosi–Land–Sjölander approximation. Our investigations suggest that: (i) the magnitude of the screened impurity potential and the average distribution of electrons about an electron at larger distances are enhanced on reducing the width of quantum wire, and (ii) the exchange interactions strengthen on narrowing the quantum wire and on increasing the carrier density. Friedel oscillations are seen in both our computed screened potential and the density of screening charge.

Electron paramagnetic resonance (EPR) is used to identify the different substitution sites of Yb³⁺ ions in the LuVO₄ host. Three different types of sites are observed. One site, referred to as Yb_I, with tetragonal D_2d symmetry characterized by g-values of and , corresponds to 80% (50%) of the total number of Yb³⁺ ions for the 1% (5%) doped compound. Two other tetragonal sites, referred to as Yb_{II a,II b}, with the same D_2d symmetry and characterized by g-values of , and , , represent 20% (50%) of the total number of ytterbium ions for the 1% (5%) compound. One minor site, referred to as Yb_III, corresponding to less than 1% of the Yb³⁺ ions, with a lower C_2v or D₂ symmetry, is also seen in the EPR spectra. The temperature dependence of the EPR linewidth is studied and shows for all the sites a dominant Orbach process for the spin–lattice relaxation time T₁ for T>12 K.

The adsorption of H₂O on Pd deposited upon MgO(100) thin films prepared on Mo(100) was studied using low-energy electron diffraction, ultraviolet photoelectron spectroscopy and high-resolution electron energy loss spectroscopy. A non-metallic feature of Pd has been observed at the coverage of Pd less than 1.5 monolayer equivalents (MLE) on the MgO(100) films. Partially dissociated H₂O was detected on Pd/MgO(100)/Mo(100), and H₂O dissociates most readily at ∼0.5 MLE Pd coverage. This coverage-dependent dissociation has arisen from a combination of the size effect of Pd particles and a vacancy-induced modification of surface reactivity.

We report a theoretical study of modulational instability of extended nonlinear spin waves in a one-dimensional ferromagnetic chain. The investigation is made both analytically within the framework of the linear stability analysis and also numerically by means of molecular dynamics simulations. Using a Holstein–Primakoff transformation for the spin operators, the Hamiltonian, which is constituted by a Heisenberg exchange term, a biquadratic exchange energy, an anisotropic energy and a Zeeman term, is bosonized. Then we derive a discrete nonlinear Schrödinger-like equation for the spin-wave motion. Using a linear stability analysis, we establish the stability criteria of the spin waves in such a ferromagnetic chain. From our numerical simulations of the discrete spin chain for the onset of instability, it emerges that the analytical predictions are correctly verified. For a long timescale, depending on the strength of the biquadratic exchange interaction relative to the exchange energy and the anisotropy energy, on the one hand an intrinsic localized wave train can be created displaying properties of the breather motion. On the other hand, due to the increasing size of the instability domain, with increase of the biquadratic parameter, the instability can fully develop and the linear stability fails; consequently, the time evolution of the modulated spin waves can show both regular and chaotic behaviour.

Certain experimental results pertaining to the low temperature magnetic state of UCu₂Ge₂, which are not covered in a recent 'Commented review' (Kuznietz 2003 J. Phys.: Condens. Matter 15 8957), are discussed. These results need to be understood and explained within an accepted model of the magnetic state of this interesting compound.

In a reply to the preceding Comment by Roy et al (2005 J. Phys.: Condens. Matter17 3113) on 'Commented review: UCu₂Ge₂ and UCu₂Si₂—compounds with only ferromagnetic ordering', published recently by the present author (Kuznietz 2003 J. Phys.: Condens. Matter15 8957), the assertion that only ferromagnetic ordering occurs in UCu₂Ge₂, as observed by means of neutron diffraction and other methods, is stated and documented. None of the variety of experimental results on UCu₂Ge₂ produced by Roy et al and summarized briefly, but without any new neutron diffraction data, can contradict or serve as a basis for disputing that there is only ferromagnetic ordering in UCu₂Ge₂ in zero and low applied magnetic fields, as observed by means of neutron diffraction and ac susceptibility, respectively. The comparison between UCu₂Ge₂ and some Ce(Fe,M)₂ solid solutions made by Roy et al, and the similarities of some of their magnetic properties, are claimed to be coincidental, and not to lead to conclusions regarding UCu₂Ge₂ magnetism. Only new neutron diffraction data could truly justify such a Comment on the 'Commented review'.