Understanding of mechanical properties of materials and a possibility to predicting them from ab initio calculations have fundamental importance for solid state theory. In this work we establish a significant correlation between the product of the macroscopic parameters of localized plastic flow auto-waves in deforming alloys, their length and propagation rate and the product of the microscopic (lattice) parameters of these materials, the spacing between close-packed planes of the lattice and the rate of transverse elastic waves. Thus, these products can be regard as invariants of plastic and elastic deformation processes, respectively. Moreover, the established regularity suggests that the elastic and the plastic processes simultaneously involved in the deformation are closely related. Our work also demonstrates that ab initio simulations can be used for the prediction of parameters of localized plastic flow auto-waves in deforming alloys.
Single phase ceramics CaCu3Ti4.0O12 and CaCu3Ti3.9O12 have been prepared using the traditional solid-state reaction method. Compared with the stoichiometric ceramics CaCu3Ti4.0O12, Ti-deficient ceramics CaCu3Ti3.9O12 have the larger lattice parameter, the higher force constant, and smaller dielectric constant and the lower dissipation factor, although their fundamental characters of dielectric response are similar. Their characteristic relaxation frequencies are not well fitted with the Arrhenius Law but a tentatively supposed relation. With the Cole-Cole Law, the fitted broadened factors of dissipation peaks are 0.5433 and 0.8651 for CaCu3Ti3.9O12 and CaCu3Ti4.0O12, respectively. All facts mentioned above imply that mutually correlated motion of Ti ions or defects may be expected to be responsible for the giant dielectric constant and high dissipation factor of CaCu3T4.0O12. (c) 2006 Elsevier Ltd. All rights reserved.
An international symposium on polymer electronics and general nanotechnology was held in Linkoping, Sweden on August 17 August 19, 2006. The topics that were featured in the symposium include synthetic chemistry, electronic structure, and charge transport and devices. The developments in polymer electronics and nanotechnology that were described by invited speakers who are internationally recognized leaders in their respective areas are discussed. Commercial areas focused for include polymer based light emitting devices (P-LED) for laptop and TV-type display, and polymer-based transistor circuits. The use of modern synchrotron radiation techniques in nanotechnology in general and organic electronics was one of the main themes of the symposium. Polymer-based transistors comprise another area of importance in polymer electronics, where low cost consumer-oriented electronic applications are expected to dominate conventional inorganic electronics.
Due to possible technological applications in opto-electronic devices, the interest in characterizing porous silicon structure patterns has recently increased. From scanning force microscopy (SFM) we have obtained images of different samples of porous silicon and applied pattern characterization operators on these matrices. In this paper, asymmetric spatial fragmentation in amplitude envelopes of porous silicon samples are characterized by means of a parameter that quantifies the amount of spatial asymmetry in the gradient field. The results show that this method is well suited to characterize silicon porosity quantitatively.
The first calculation of the density of states of expanded crystalline Hg is reported for the f.c.c., b.c.c., and s.c. structures in the density range ∼ 4–9 g/cm3. The calculations are based on Animalu's local pseudopotential. It is found that a band gap opens up at 6.5 g/cm3 for f.c.c., 5.5 g/cm3 for b.c.c., and 4 g/cm3 for s.c.
We present experimental studies of crossings of spin-split one-dimensional subbands in ballistic quantum wires in an in-plane magnetic field B ?. At low electron densities, a spontaneous spin-splitting occurs as subbands cross, which gives rise to additional non-quantised conductance structures called 0.7 analogues. We analyse the data within a spin-density-functional model, which includes exchange interactions in a magnetic field. Focussing on the region of the crossings of spin-split subbands, it is found that the energy levels rearrange as they cross due to exchange interactions. © 2004 Elsevier Ltd. All rights reserved.
The phonon spectra and phonon density of states of the Ni3Al and NiAl intermetallic compounds are calculated from first principles using the linear response method in conjunction with ultrasoft pseudopotentials. The calculated phonon dispersion curves are in good agreement with available experimental results from inelastic neutron scattering. © 2003 Elsevier Ltd. All rights reserved.
A photoluminescence study was performed at different temperatures on bulk ZnO samples annealed in zinc- and oxygen-rich atmospheres. The different annealing conditions create oxygen and zinc vacancies in a controlled way in the ZnO samples. These defects are both involved in the deep band emission (DBE) that is often observed in ZnO but exhibit different optical characteristics promoting defect identification. In particular, when decreasing the PL measurement temperature the energy peak position of the -related band decreases while that of increases. Secondly, phonon replicas are clearly observed in the DBE spectra in the sample containing . Finally, the characteristics of the DBE decay time for - and -enriched samples are also different. Specifically, for the -enriched sample the decay curves show strong wavelength dependence and generally slower decay components as compared to the sample enriched with .
By doing quantum Monte Carlo ab initio simulations we show that dipolar excitons, which are now under experimental study, actually are strongly correlated systems. Strong correlations manifest in significant deviations of excitation spectra from the Bogoliubov one, large Bose condensate depletion, short-range order in the pair correlation function, and peak(s) in the structure factor. © 2007 Elsevier Ltd. All rights reserved.
Spherical quantum dots with a few charged Fermi particles (electrons or holes) are studied for different total spins. Simulation by quantum path integral Monte Carlo method is performed. The dependence of the electron correlations in the quantum dot is studied at different mean interelectron separation controlled by number of electrons in the quantum dot and by steepness of electron confinement (the latter parameter can be changed by the gate voltage). The cold melting-quantum transition from Wigner crystal-like state (i.e. from regime of strongly correlated electrons) to a Ferad liquid-like state-driven by the steepness of electron confinement is studied. The pair correlation function and radial function characterizing electron quantum delocalization are analyzed.
The concept of Wannier-Stark (WS) quantization literally refers to infinite crystals or bulk electron states. In real finite crystals, the bulk and edge or surface states always coexist. Moreover, when the surface states are already considerably localized due to the presence of a constant electric field, the WS ladder (WSL) and hence the WS localization may not yet come into play, at least, in the canonical form of WS quantization Ej = const±je, j = 0, 1, 2, ... (Ej is the one-electron energy, and the parameter e is associated with the electric field strength). We show that at certain voltages Vm, m = 3, 4, ..., which are lower than V8 needed for the WS band opening (the sub-WS regime), the mid-spectrum levels can form triple-, double-, and fractional-spaced WSLs, where El = ±l[1+2/(m-2)]em. It is also found that in the WS regime, the quantization of surface localized states (sls) smoothly changes from the Airy type (at the spectrum edges) to the WS type with a pronounced energy interval in between, where the level spacing doubles that of canonical WSL. Possible experimental manifestations of predicted effects are also outlined.
Transport properties of single-layer graphene with correlated one-dimensional defects are studied theoretically using the computational model within the time-dependent real-space Kubo-Greenwood formalism. Such defects are present in epitaxial graphene, comprising atomic terraces and steps due to the substrate morphology, and in polycrystalline chemically vapor-deposited (CVD) graphene due to the grain boundaries, composed of a periodic array of dislocations, or quasi-periodic nanoripples originated from the metal substrate. The extended line defects are described by the long-range Lorentzian-type scattering potential. The dc conductivity is calculated numerically for different cases of distribution of line defects. This includes a random (uncorrelated) and a correlated distribution with a prevailing direction in the orientation of lines. The anisotropy of the conductivity along and across the line defects is revealed, which agrees with experimental measurements for epitaxial graphene grown on SiC. We performed a detailed study of the conductivity for different defect correlations, introducing the correlation angle alpha(max)-the maximum possible angle between any two lines. We find that for a given electron density, the relative enhancement of the conductivity for the case of fully correlated line defects in comparison to the case of uncorrelatecl ones is larger for a higher defect density. Finally, we, for the first time, study the conductivity of realistic samples where both extended line defects and point-like scatterers such as adatoms and charged impurities are presented.
Surface processes on alpha-Al2O3 (0001) have been investigated theoretically using density functional theory. Ion-surface interactions prior to collision were investigated by means of ab initio molecular dynamics simulations, showing an adsorbate trajectory towards a preferred adsorption site. Furthermore, the adsorption process at different surface sites was investigated, together with calculations of barriers for migration between these sites. Although the equilibrium (bulk) adsorption site was energetically favourable, the presence and magnitude of the migration barriers indicate that activation energy is necessary to form the equilibrium surface structure. These results are of importance for a fundamental understanding of ion-surface interactions.
Ion-surface collisions have been investigated theoretically using ab initio molecular dynamics within density functional theory. The temporal evolution of the position of the bombarding ion, as well as its nearest neighbors, was studied for initial kinetic energies of 0 and 3.5 eV (0 and 5 km/s, respectively). Also investigated was the ion-surface interaction prior to collision and the following energy transfer, as indicated by changes in ion velocity. At 3.5 eV collision energy, the calculation results suggest the formation of local structural disorder within the simulation time frame studied. These results are of fundamental importance for an increased understanding of the ion-surface interaction during a collision event, with resulting changes in atomic level structure.
We have investigated the anisotropy in electronic transport of the layered ternary Ti2GeC by comparing the results of measurements on c-axis oriented epitaxial thin-film and polycrystalline bulk samples. The electrical conductivities, Hall coefficients, and magnetoresistances were analyzed within a multi-band framework. An adequate description of the magnetotransport data on the film with the highest mobility required the use of the explicit field-dependent conductivity tensor with three conduction bands. The analysis indicated that n ˜ p, although with n ˜ 3.5 × 1027 m- 3. The ratio of the a- to c-axis conductivities is small and contrary to theoretical predictions. © 2008 Elsevier Ltd. All rights reserved.
The multiferruic (Bi0.95RE0.05)(Fe0.95Mn0.05)O-3 (where RE is Gd (BGFM) and Dy (BDFM)) has been synthesized by using the solid state reaction (SSR) technique. Effects of Gd and Dy substitutions on the structure, electrical and ferroelectric properties of (Bi0.95RE0.05)(Fe0.95Mn0.05)O-3 samples have been studied by performing X-ray diffraction, dielectric measurements and magnetic measurements. The crystal structure of the ceramic samples shows a monoclinic phase. Studies of dielectric properties (dielectric constant (epsilon) and tangent loss (tan delta)) both as a function of frequency (10 and 100 kHz) and temperatures (20-300 degrees C) exhibit dielectric anomaly in the range of (225-245 degrees C) suggesting a possible ferroelectric-paraelectric phase transition in the compounds. The vibrating sample magnetometer (VSM) measurement shows a significant change in the magnetic properties of Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O-3. It is seen that the coercive field (H-C) and remanent magnetization (M-R) increase for Gd.
ere we review the concepts and technologies, in particular photochemical gating, which contributed to the recent progress in quantum Hall resistance metrology based on large scale epitaxial graphene on silicon carbide.
The results of fluorescence measurements of magnetic circular dichroism (MCD) in Mn L2,3 X-ray emission and absorption for Heusler alloys NiMnSb and Co2MnSb are presented. Very intense resonance Mn L3 emission is found at the Mn 2p3/2 threshold and attributed to a peculiarity of the threshold excitation in materials with a half-metallic character of the electronic structure. The excitation energy dependence of Mn L2,3 X-ray emission spectra (XES) was measured at beamline ID12B at the European Synchrotron Radiation Facility (ESRF, Grenoble) using 83% circularly polarized X-rays. A very large MCD effect found in XES is attributed to strong exchange splitting of spin-up and spin-down Mn 3d states. The anomalously high ratio of L2 emission intensity to L3 emission intensity is found.