The first signs of language: Phonological development in British sign language

A total of 1018 signs in one deaf child’s naturalistic interaction with her deaf mother, between the ages 19-24 months were analysed. This study summarises regular modification processes in the phonology of the child sign’s handshape, location, movement and prosody. Firstly changes to signs were explained by the notion of phonological markedness. Secondly, the child managed her production of first signs through two universal processes: structural change and substitution. Constraints unique to the visual modality also caused sign language specific acquisition patterns, namely: more errors for handshape articulation in locations in peripheral vision, a high frequency of whole sign repetitions and feature group rather than one-to-one phoneme substitutions as in spoken language development

GEOMETRY AND CHARGE-DISTRIBUTION OF H-CENTERS IN THE FLUORITE STRUCTURE

The analysis of experimental optical and spin-resonance data for the H centre gives a consistent picture of the local geometry and one-electron wavefunctions. One of the two ions in the F2- molecular ion remains very close to the perfect lattice site the other is at a distance close to that found in other F2- centres. This analysis is confirmed by atomistic calculations using the HADES code. The results are used to give a preliminary analysis of the self-trapped exciton data

Making tracks: electronic excitation roles in forming swift heavy ion tracks

Swift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10 keV nm(-1), includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high T-c oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self- trapped exciton model of Itoh and Stoneham ( 1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self- trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser- induced time- resolved x- ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials

First principles simulations of 2D Cu superlattices on the MgO(001) surface

First principles slab simulations of copper 2D superlattices of different densities on the perfect MgO(0 0 1) surface are performed using the DFT method as implemented into the CRYSTAL98 computer code. In order to clarify the nature of interfacial bonding, we consider regular 1/4, 1/2 and I monolayer (ML) coverages and compare results of our calculations with various experimental and theoretical data. Our general conclusion is that the physical adhesion associated with a Cu polarization and charge redistribution gives the predominant contribution to the bonding of the regular Cu 2D layer on the MgO(0 0 1) surface. (C) 2003 Elsevier B.V. All rights reserved

Optical response of ferromagnetic YTiO_3 studied by spectral ellipsometry

We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric YTiO_3 single crystal with ferromagnetic Curie temperature T_C = 30 K. The optical response exhibits a weak but noticeable anisotropy. Using a classical dispersion analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV. Although the optical conductivity spectra are only weakly temperature dependent below 300 K, we are able to distinguish high- and low-temperature regimes with a distinct crossover point around 100 K. The low-temperature regime in the optical response coincides with the temperature range in which significant deviations from Curie-Weiss mean field behavior are observed in the magnetization. Using an analysis based on a simple superexchange model, the spectral weight rearrangement can be attributed to intersite d_i^1d_j^1 \longrightarrow d_i^2d_j^0 optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around 2.9 eV can be associated with the high-spin-state (^3T_1) optical transition. This indicates that other mechanisms, such as weakly dipole-allowed p-d transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at 2 eV. The recorded optical spectral weight gain of 2.9 eV optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbit-lattice phenomena near ferromagnetic ordering temperature in YTiO_3

The kinetic MC modelling of reversible pattern formation in initial stages of thin metallic film growth on crystalline substrates

The results of kinetic MC simulations of the reversible pattern formation during the adsorption of mobile metal atoms on crystalline substrates are discussed. Pattern formation, simulated for submonolayer metal coverage, is characterized in terms of the joint correlation functions for a spatial distribution of adsorbed atoms. A wide range of situations, from the almost irreversible to strongly reversible regimes, is simulated. We demonstrate that the patterns obtained are defined by a key dimensionless parameter: the ratio of the mutual attraction energy between atoms to the substrate temperature. Our ab initio calculations for the nearest Ag-Ag adsorbate atom interaction on an MgO substrate give an attraction energy as large as 1.6 eV, close to that in a free molecule. This is in contrast to the small Ag adhesion and migration energies (0.23 and 0.05 eV, respectively) on a defect-free MgO substrate. (C) 2003 Elsevier Science Ltd. All rights reserved

Nonequilibrium Generalised Langevin Equation for the calculation of heat transport properties in model 1D atomic chains coupled to two 3D thermal baths

We use a Generalised Langevin Equation (GLE) scheme to study the thermal transport of low dimensional systems. In this approach, the central classical region is connected to two realistic thermal baths kept at two different temperatures [H. Ness et al., Phys. Rev. B {\bf 93}, 174303 (2016)]. We consider model Al systems, i.e. one-dimensional atomic chains connected to three-dimensional baths. The thermal transport properties are studied as a function of the chain length $N$ and the temperature difference $\Delta T$ between the baths. We calculate the transport properties both in the linear response regime and in the non-linear regime. Two different laws are obtained for the linear conductance versus the length of the chains. For large temperatures ($T \gtrsim 500$ K) and temperature differences ($\Delta T \gtrsim 500$ K), the chains, with $N > 18$ atoms, present a diffusive transport regime with the presence of a temperature gradient across the system. For lower temperatures($T \lesssim 500$ K) and temperature differences ($\Delta T \lesssim 400$ K), a regime similar to the ballistic regime is observed. Such a ballistic-like regime is also obtained for shorter chains ($N \le 15$). Our detailed analysis suggests that the behaviour at higher temperatures and temperature differences is mainly due to anharmonic effects within the long chains.Comment: Accepted for publication in J. Chem. Phy

Mesoscopic modelling of conducting and semiconducting polymers

We present generalized Monte Carlo calculations to assess the effects of texture and related key factors on the properties of polymer-based light emitting diodes. We, describe one class of mesoscopic model giving specific realizations of the polymer network. The model, with simple physically based rules, shows the effects of polymer structural order on current flow, trapping and radiative and non-radiative charge recombination within the polymer layer. Interactions between charges are included explicitly, as are image interactions with the electrodes. It is important that these Coulomb interactions are not simplified to an averaged space charge, since the local interactions can lead to effective trapping of charge, even in the absence of defective chains or impurity trapping. There proves to be an important role for trapping, in which charges are localized for times long compared with transit times. The competition between current flow, trapping and radiative and non-radiative charge recombination means that some of the trends are not intuitively obvious. For example, if radiative recombination occurs only on short polymer chains, as is the case for certain polymer systems, the internal efficiency appears to saturate for a concentration of these shorter luminescent chains of about 20-30%. As the proportion of shorter chains increases, trapping increases, whereas current efficiency decreases. Our approach provides a natural link between atomistic models of individual polymer molecules and the macroscopic descriptions of device modelling. Such mesoscopic models provide a means to design better film structures, and hence to optimize the effectiveness of new organic materials in a range of applications

Photoluminescence dispersion as a probe of structural inhomogeneity in silica

We report time-resolved photoluminescence spectra of point defects in amorphous silicon dioxide (silica), in particular the decay kinetics of the emission signals of extrinsic Oxygen Deficient Centres of the second type from singlet and directly-excited triplet states are measured and used as a probe of structural inhomogeneity. Luminescence activity in sapphire ($\alpha$-Al$_2$O$_3$) is studied as well and used as a model system to compare the optical properties of defects in silica with those of defects embedded in a crystalline matrix. Only for defects in silica, we observe a variation of the decay lifetimes with emission energy and a time dependence of the first moment of the emission bands. These features are analyzed within a theoretical model with explicit hypothesis about the effect introduced by the disorder of vitreous systems. Separate estimations of the homogenous and inhomogeneous contributions to the measured emission linewidth are obtained: it is found that inhomogeneous effects strongly condition both the triplet and singlet luminescence activities of oxygen deficient centres in silica, although the degree of inhomogeneity of the triplet emission turns out to be lower than that of the singlet emission. Inhomogeneous effects appear to be negligible in sapphire