Jacek C. Wojdeł

Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.610(-10) to 1.810(-9) m(2)/s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.

Comparison between different feature extraction techniques for audio-visual speech recognition

The current audio-only speech recognition still lacks the expected robustness when the Signal to Noise Ratio (SNR) decreases. The video information is not affected by noise which makes it an ideal candidate for data fusion for speech recognition benefit. In the paper [1] the authors have shown that most of the techniques used for extraction of static visual features result in equivalent features or at least the most informative features exhibit this property. We argue that one of the main problems of existing methods is that the resulting features contain no information about the motion of the speaker’s lips. Therefore, in this paper we will analyze the importance of motion detection for speech recognition. For this we will first present the Lip Geometry Estimation(LGE) method for static feature extraction. This method combines an appearance based approach with a statistical based approach for extracting the shape of the mouth. The method was introduced in [2] and explored in detail in [3]. Further more, we introduce a second method based on a novel approach that captures the relevant motion information with respect to speech recognition by performing optical flow analysis on the contour of the speaker’s mouth. For completion, a middle way approach is also analyzed. This third method considers recovering the motion information by computing the first derivatives of the static visual features. All methods were tested and compared on a continuous speech recognizer for Dutch. The evaluation of these methods is done under different noise conditions. We show that the audio-video recognition based on the true motion features, namely obtained by performing optical flow analysis, outperforms the other settings in low SNR conditions.

On the prediction of the crystal and electronic structure of mixed-valence materials by periodic density functional calculations: The case of Prussian Blue

The consistency of periodic density functional approaches to properly describe the crystal and electronic structure of mixed-valence materials is investigated by taking Prussian Blue as prototypical example. Hybrid B3LYP, GGA, and, GGA+U exchange-correlation potentials have been explored. Localized Gaussian-type orbitals or plane waves have been chosen to expand the valence electron density, and the effect of the core electrons on the electronic structure was accounted for either (i) explicitly by including all electrons in the calculations, (ii) by making use of ultrasoft pseudopotentials, or (iii) by the use of the projected augmented wave method. Comparison to available experimental data shows that all-electron calculations within the hybrid exchange-correlation potential can be taken as appropriate benchmarks. It is also concluded that a proper description of the complex magnetic ground state of Prussian Blue can be reached by using a plane-wave basis set and nonhybrid density functional potentials but only if the electronically distinct iron centers in the material are treated in an independent manner. Physical reasons for such rather unexpected results are given and implications for the description of mixed-valence materials by means of density functional approaches are discussed.

Prediction of half-metallic conductivity in Prussian Blue derivatives

Half-metallicity and spin-crossover behavior are predicted for two well-defined Prussian Blue (PB) analogues using periodic density functional calculations with GGA + U and hybrid exhange-correlation potentials. Fully oxidized PB possesses a narrow spin-polarized band through which it should be possible to control the spin conduction by oxidative/reductive doping. Fully reduced PB is also a half-metal in its high-spin phase and exhibits an Fe2+ spin-crossover. A simple model is used to rationalize the electronic properties of these systems which may have potential applications in spintronics.

Periodic density functional theory study of spin crossover in the cesium iron hexacyanochromate prussian blue analog

This paper presents a detailed theoretical analysis of the electronic structure of the CsFe[Cr(CN)(6)] prussian blue analog with emphasis on the structural origin of the experimentally observed spin crossover transition in this material. Periodic density functional calculations using generalized gradient approximation (GGA)+U and nonlocal hybrid exchange-correlation potentials show that, for the experimental low temperature crystal structure, the t(2g)(6) e(g)(0) low spin configuration of Fe(II) is the most stable and Cr(III) (S = 3/2, t(2g)(3) e(g)(0)) remains the same in all cases. This is also found to be the case for the low spin GGA+U fully relaxed structure with the optimized unit cell. A completely different situation emerges when calculations are carried out using the experimental high temperature structure. Here, GGA+U and hybrid density functional theory calculations consistently predict that the t(2g)(4) e(g)(2) Fe(II) high spin configuration is the ground state. However, the two spin configurations appear to be nearly degenerate when calculations are carried out for the geometries arising from a GGA+U full relaxation of the atomic structure carried out at experimental high temperature lattice constant. A detailed analysis of the energy difference between the two spin configurations as a function of the lattice constant strongly suggests that the observed spin crossover transition has a structural origin with non-negligible entropic contributions of the high spin state.

Emergent chirality in the electric polarization texture of titanate superlattices

Significance Many natural structures exhibit chirality that is essential to their functional interactions, yet the chiral electronic structures found in condensed matter systems have been primarily limited to magnetic materials. Notably, the electric dipole equivalent of magnetic skyrmions has remained conspicuously elusive. However, recent theoretical predictions and experimental observations of the continuous rotation of electric polarization in titanate superlattices suggests that such complex oxide nanocomposites are ideal candidates for realizing chiral electric dipole configurations. Here we present the results from superlattices of PbTiO3 and SrTiO3 using a combination of resonant soft X-ray diffraction and second-principles simulations. We observe chiral arrays of polar line defects, spontaneously formed by the complex interactions in these artificial superlattices constructed from two nonchiral lattices. Chirality is a geometrical property by which an object is not superimposable onto its mirror image, thereby imparting a handedness. Chirality determines many important properties in nature—from the strength of the weak interactions according to the electroweak theory in particle physics to the binding of enzymes with naturally occurring amino acids or sugars, reactions that are fundamental for life. In condensed matter physics, the prediction of topologically protected magnetic skyrmions and related spin textures in chiral magnets has stimulated significant research. If the magnetic dipoles were replaced by their electrical counterparts, then electrically controllable chiral devices could be designed. Complex oxide BaTiO3/SrTiO3 nanocomposites and PbTiO3/SrTiO3 superlattices are perfect candidates, since “polar vortices,” in which a continuous rotation of ferroelectric polarization spontaneously forms, have been recently discovered. Using resonant soft X-ray diffraction, we report the observation of a strong circular dichroism from the interaction between circularly polarized light and the chiral electric polarization texture that emerges in PbTiO3/SrTiO3 superlattices. This hallmark of chirality is explained by a helical rotation of electric polarization that second-principles simulations predict to reside within complex 3D polarization textures comprising ordered topological line defects. The handedness of the texture can be topologically characterized by the sign of the helicity number of the chiral line defects. This coupling between the optical and novel polar properties could be exploited to encode chiral signatures into photon or electron beams for information processing.

Chemical Bonding and Electronic and Magnetic Structure in LaOFeAs

Periodic density functional calculations using hybrid exchange-correlation functionals predict that LaOFeAs is a strongly frustrated antiferromagnetic insulator with important covalence between Fe and As, with evident similarities with cuprates.

First principles calculations on the influence of water-filled cavities on the electronic structure of Prussian Blue

AbstractPrussian Blue is a paradigmatic mixed valence material and a parent compound to a broad family of electronically, optically, and magnetically active materials. Its exact composition varies greatly depending on the preparation route, leading to large variations in its electronic properties. The influence of water molecules on the structural and electronic properties of Prussian Blue were studied using state-of-the-art first principles calculations. Water-filled cavities were found to have a profound influence on the band gap and density of states of this material while simultaneously leaving many of its properties largely unchanged. The resulting model of an almost independent superimposition of dehydrated material and hydrated sites is briefly discussed. Unit cell of “insoluble” Prussian Blue used in calculations

Fusing data streams in continuous audio-visual speech recognition

Speech recognition still lacks robustness when faced with changing noise characteristics. Automatic lip reading on the other hand is not affected by acoustic noise and can therefore provide the speech recognizer with valuable additional information, especially since the visual modality contains information that is complementary to information in the audio channel. In this paper we present a novel way of processing the video signal for lip reading and a post-processing data transformation that can be used alongside it. The presented Lip Geometry Estimation (LGE) is compared with other geometry- and image intensity-based techniques typically deployed for this task. A large vocabulary continuous audio-visual speech recognizer for Dutch using this method has been implemented. We show that a combined system improves upon audio-only recognition in the presence of noise.

A phononic switch based on ferroelectric domain walls

The ease with which domain walls (DWs) in ferroelectric materials can be written and erased provides a versatile way to dynamically modulate heat fluxes. In this work we evaluate the thermal boundary resistance (TBR) of 180