Showing papers by "Helsinki University of Technology published in 1996"

Engineering applications of the self-organizing map

Abstract: The self-organizing map (SOM) method is a new, powerful software tool for the visualization of high-dimensional data. It converts complex, nonlinear statistical relationships between high-dimensional data into simple geometric relationships on a low-dimensional display. As it thereby compresses information while preserving the most important topological and metric relationships of the primary data elements on the display, it may also be thought to produce some kind of abstractions. The term self-organizing map signifies a class of mappings defined by error-theoretic considerations. In practice they result in certain unsupervised, competitive learning processes, computed by simple-looking SOM algorithms. Many industries have found the SOM-based software tools useful. The most important property of the SOM, orderliness of the input-output mapping, can be utilized for many tasks: reduction of the amount of training data, speeding up learning nonlinear interpolation and extrapolation, generalization, and effective compression of information for its transmission.

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation

Abstract: TOPOLOGICAL defects formed during a rapid symmetry-breaking phase transition in the early Universe1,2 could be responsible for seeding large-scale structure, for the anisotropy of the microwave background radiation, and for the predominance of matter over antimatter3,4. The theory describing this cosmological phase transition is formally analogous to that describing the transition to the superfluid state in liquid 3He, so that in principle the process of cosmological defect formation can be modelled in the laboratory. Here we report the results of an experiment in which the 'primordial fireball' is mimicked using a neutron-induced nuclear reaction (n + 3He → p + 3He + 0.76 MeV) to heat small regions of superfluid 3He above the superfluid transition temperature. These bubbles of normal liquid cool extremely rapidly, and we find that their transition back to the superfluid state is accompanied by the formation of a random network of vortices (the superfluid analogue of cosmic strings). We monitor the evolution of this defect state by rotating the superfluid sample, allowing vortices to escape from the network and thus be probed individually. Our results provide clear confirmation of the idea that topological defects form at a rapid second-order phase transition, and give quantitative support to the Kibble–Zurek mechanism5,6 of cosmological defect formation.

Impaired visual word processing in dyslexia revealed with magnetoencephalography.

Abstract: Dyslexia is most often attributed to phonological impairments, manifested in abnormal activation of the left temporal and temporoparietal cortex in response to auditorily presented language and possibly associated with anomalies in the cytoarchitecture and hemispheric symmetry of the plana temporale. The immediate cortical correlate of the severely impaired reading process has, however, remained obscure. Here we report on the distinct time courses of cortical activation in dyslexic and control subjects during passive viewing of single words, tracked with whole-head magneto-encephalography. A striking difference was found in the left inferior temporo-occipital region where intracranial recordings have recently identified word-specific responses within 200 msec after stimulus onset : controls showed a sharp activation at about 180 msec after word presentation, whereas dyslexics failed to activate this area entirely, or showed a slowly increasing late response. Perception of words as specific units thus seems to be impaired in dyslexics.

Topology preservation in self-organizing maps

Abstract: This paper concentrates on the following issues: (1) discussion on what kind of mapping is produced by the SOM algorithm; (2) introduction of a quantitative measure of continuity for the mapping produced by SOM; (3) introduction of a variant of SOM, called the AdSOM, with locally adapting neighborhood radii.

Motion control of a spherical mobile robot

Abstract: The paper deals with dynamics and control of a special type of mobile robot designed to act as a small platform to carry sensing devices or actuators in an environment where stability of the platform is critical, like in surveying unstructured hostile industrial environment exploring other planets, or simply being a part of a human place, like office or home, which has not been designed for mobile machines. The spherical construction offers extraordinary motion properties in cases where turning over or falling down are risks for the robot to continue its motion. Also it has full capability to recover from collisions with obstacles or another robots traveling in the environment.

Synthesis of oxide thin films and overlayers by atomic layer epitaxy for advanced applications

Abstract: Atomic layer epitaxy (ALE) as a self-limiting thin film growth process results in several practical advantages including accurate and simple thickness control, large area and large batch capability, good conformality and reproducibility. straightforward doping and scale-up, possibility to produce sharp and tailored interfaces as well as capability to prepare multilayer structures in a continuous process. In this paper the use of ALE in deposition of numerous conducting and insulating oxide thin films will be reviewed. The versatile ALE chemistry allows the use of different types of metal-containing precursors from simple inorganic compounds to organometallics. Commonly used oxygen sources include water, oxygen, ozone, hydrogen peroxide and alcohols. The recent advances in oxide ALE processes, i.e. deposition of ternary and quaternary oxides, controlled doping of transparent conductors, preparation of multilayer structures as well as conformai coating of porous silicon will also be discussed in respect to their practical applications.

Formation of Ion Irradiation-Induced Small-Scale Defects on Graphite Surfaces.

Abstract: We use molecular dynamics simulations and ab initio calculations to study the structures and formation probabilities of isolated surface defects produced by ion irradiation of (1000) graphite. We improve the conventionally used Tersoff potential [J. Tersoff, Phys. Rev. Lett. 61, 2879 (1988)] to realistically describe interlayer forces in graphite and high-energy processes in carbon. We identify three defect structures which correspond to experimentally observed hillocks on graphite surfaces, and examine their formation at different implantation energies.

Ab initio study of oxygen point defects in GaAs, GaN, and AlN

Abstract: We have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. ${\mathrm{O}}_{\mathrm{As}}$ in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. ${\mathrm{O}}_{\mathrm{N}}$ in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, ${\mathrm{O}}_{\mathrm{N}}$ in AlN is shown to be a deep center due to the wide band gap, in contrast with ${\mathrm{O}}_{\mathrm{N}}$ in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$N alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented. \textcopyright{} 1996 The American Physical Society.

Resonant tunneling through ultrasmall quantum dots: Zero-bias anomalies, magnetic-field dependence, and boson-assisted transport.

Abstract: We study resonant tunneling through a single-level quantum dot in the presence of strong Coulomb repulsion beyond the perturbative regime. The level is either spin degenerate or can be split by a magnetic field. Furthermore we discuss the influence of a bosonic environment. Using a real-time diagrammatic formulation, we calculate transition rates, the spectral density, and the nonlinear I-V characteristic. The spectral density shows a multiplet of Kondo peaks split by the transport voltage and the boson frequencies and shifted by the magnetic field. This leads to zero-bias anomalies in the differential conductance, which agree well with recent experimental results for the electron transport through single-charge traps. Furthermore, we predict that the sign of the zero-bias anomaly depends on the level position relative to the Fermi level of the leads. \textcopyright{} 1996 The American Physical Society.

Biodegradable lactone copolymers. I: Characterization and mechanical behavior of ε-caprolactone and lactide copolymers

Abstract: Copolymers of e-caprolactone and L-lactide (e-CL/L-LA) and e-caprolactone and DL-lactide (e-CL/DL-LA) were synthesized with compositions 80/20, 60/40, and 40/60 (wt % in feed). The polymerization temperature was 140°C and Sn(II)octoate was used as a catalyst. The effect of the comonomer ratio on the thermal and mechanical properties of the copolymers was investigated by size-exclusion chromatography (SEC), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) spectrometry, and tensile testing. The copolymers differed widely in their physical characteristics, ranging from weak elastomers to tougher thermoplastics according to the ratio of e-CL and LA in the copolymerization. Poly(L-lactide) (PLLA), poly(DL-lactide) (PDLLA), and poly(e-caprolactone) (PCL) homopolymers were studied as references. The tensile modulus and tensile strength were much higher for PLLA, PDLLA, and PCL homopolymers than for the copolymers. The maximum strain was very low for PLLA and PDLLA, whereas the copolymers and PCL exhibited large elongation. © 1996 John Wiley & Sons, Inc.

Modification of poly(L-lactides) by blending: Mechanical and hydrolytic behavior

Abstract: Two different poly(L-lactides) (PLLAb and PLLAa) were blended with poly(e-caprolactone) (PCL) and an elastic poly(e-caprolactone/L-lactide) P(CL/L-LA) copolymer to modify the mechanical properties of PLLA. Blends of both PLLAs with 20 wt.-% of PCL were prepared. PLLAa was blended with 20 wt.-% of P(CL/L-LA) copolymer, and blends of PLLAb and P(CL/L-LA) copolymer were made with copolymer contents of 5, 10, 20 and 30 wt.-%. The tensile properties and impact and shear strengths were determined as a measure of the mechanical properties. The hydrolytic behavior of the blends was investigated, and the changes in shear strength as a function of hydrolysis were followed. The structure of the hydrolyzed samples was studied by means of scanning electron microscopy (SEM). The properties of PLLA changed considerably when blended with PCL or P(CL/L-LA) copolymer. An increasing amount of copolymer in PLLAb decreased tensile modulus and shear strength, but improved the strain and impact strength. In hydrolysis, blends containing PLLAa degraded slower than blends of PLLAb because of the higher initial molar mass and purity of PLLAa. The molar masses of PLLAb blends decreased dramatically, and less than 10% of the initial molar masses and shear strengths were left after 12 weeks in vitro. The morphology of the blends affected the degradation. The blends containing P(CL/L-LA) copolymer had a porous structure which facilitated water absorption into the blend.

Deviant Auditory Stimuli Activate Human Left and Right Auditory Cortex Differently

Abstract: Infrequent "deviant' auditory stimuli embedded in a homogeneous sequence of "standard' sounds evoke a neuromagnetic mismatch field (MMF), which is assumed to reflect automatic change detection in the brain. We investigated whether MMFs would reveal hemispheric differences in cortical auditory processing. Seven healthy adults were studied with a whole-scalp neuromagnetometer. The sound sequence, delivered to one ear at time, contained three infrequent deviants (differing from standards in duration, frequency, or interstimulus interval) intermixed with standard tones. MMFs peaked 9-34 msec earlier in the right than in the left hemisphere, irrespective of the stimulated ear. Whereas deviants activated only one MMF source in the left hemisphere, two temporally overlapping but spatially separate sources, one in the temporal lobe and another in the inferior parietal cortex, were necessary to explain the right-hemisphere MMFs. We suggest that the bilateral MMF components originating in the supratemporal cortex are feature specific whereas the right-hemisphere parietal component reflects more global auditory change detection. The results imply hemispheric differences in sound processing and suggest stronger involvement of the right than the left hemisphere in change detection.

Sensitivity to Doppler shift and carrier frequency errors in OFDM systems-the consequences and solutions

Abstract: Due to the intercarrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) mobile radio communication systems, the bit error rate (BER) of the received signals are extremely sensitive to Doppler frequency shifts and carrier synchronization errors. This paper studies the mechanism of the ICI signals, and then proposes a new method to overcome such problems without any equalization procedures. The results show that by using the self ICI cancellation scheme proposed, the ICI can be reduced significantly. Therefore this OFDM system suffers much less from performance degradation caused by frequency errors than the normal OFDM systems.

Theoretical and experimental study of positron annihilation with core electrons in solids

Abstract: A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively. @S01631829~96!04327-5#

Local density of states in a dirty normal metal connected to a superconductor

Abstract: A superconductor in contact with a normal metal not only induces superconducting correlations, known as the proximity effect, but also modifies the density of states at some distance from the interface. These modifications can be resolved experimentally in microstructured systems. We therefore study the local density of states N(E,x) of a superconductor--normal-metal heterostructure. We find a suppression of N(E,x) at small energies, which persists to large distances. If the normal metal forms a thin layer of thickness ${\mathit{L}}_{\mathit{n}}$, a minigap in the density of states appears which is of the order of the Thouless energy \ensuremath{\sim}\ensuremath{\Elzxh}D/${\mathit{L}}_{\mathit{n}}^{2}$. A magnetic field suppresses the features. We find good agreement with recent experiments of Gu\'eron et al. \textcopyright{} 1996 The American Physical Society.

Self-organizing maps of document collections: A new approach to interactive exploration

Abstract: Powerful methods for interactive exploration and search from collections of free-form textual documents are needed to manage the ever-increasing flood of digital information. In this article we present a method, WEBSOM, for automatic organization of full-text document collections using the self-organizing map (SOM) algorithm. The document collection is ordered onto a map in an unsupervised manner utilizing statistical information of short word contexts. The resulting ordered map where similar documents lie near each other thus presents a general view of the document space. With the aid of a suitable (WWW-based) interface, documents in interesting areas of the map can be browsed. The browsing can also be interactively extended to related topics, which appear in nearby areas on the map. Along with the method we present a case study of its use.

Comparing Self-Organizing Maps

Abstract: In exploratory analysis of high-dimensional data the self-organizing map can be used to illustrate relations between the data items. We have developed two measures for comparing how different maps represent these relations. The other combines an index of discontinuities in the mapping from the input data set to the map grid with an index of the accuracy with which the map represents the data set. This measure can be used for determining the goodness of single maps. The other measure has been used to directly compare how similarly two maps represent relations between data items. Such a measure of the dissimilarity of maps is useful, e.g., for analyzing the sensitivity of maps to variations in their inputs or in the learning process. Also the similarity of two data sets can be compared indirectly by comparing the maps that represent them.

Self-organizing maps of document collections: a new approach to interactive exploration

Abstract: Powerful methods for interactive exploration and search from collections of free-form textual documents are needed to manage the ever-increasing flood of digital information. In this article we present a method, WEBSOM, for automatic organization of full-text document collections using the self-organizing map (SOM) algorithm. The document collection is ordered onto a map in an unsupervised manner utilizing statistical information of short word contexts. The resulting ordered map where similar documents lie near each other thus presents a general view of the document space. With the aid of a suitable (WWW-based) interface, documents in interesting areas of the map can be browsed. The browsing can also be interactively extended to related topics, which appear in nearby areas on the map. Along with the method we present a case study of its use.

Effect of temperature and various nitrogen sources on L (+)-lactic acid production by Lactobacillus casei

Abstract: Two homofermentative strains, Lactobacillus casei NRRL B-441 and Lactobacillus casei subsp. rhamnosus NRRL B-445 were selected for further study from 17 lactic acid bacterial strains screened for lactic acid production. The effect of temperature on lactic acid production with the selected strains was investigated by adapting both strains to four different temperatures. The production of L(+)-lactic acid by both strains was most efficient at 37°C, although with L. casei the highest lactic acid concentration was obtained at 41°C. The maximal volumetric productivity with L. casei was 4.1 g l-1 h-1 and with L. casei subsp. rhamnosus 3.5 g l-1 h-1. The composition of the medium was studied in order to replace the costly yeast extract with less expensive sources of nitrogen and amino acids. From 11 different nitrogen sources investigated at 37°C, barley malt sprouts (88 g l-1 lactic acid in 66 h) and grass extract (74 g l-1 lactic acid in 73 h) were the best economic alternatives. The effect of different combinations of yeast extract, peptone and malt sprouts was further studied by using statistical experimental design, and an empirical second-order polynomial model was constructed on the basis of the results. With the right combination most of the yeast extract could be substituted by barley malt sprouts for efficient lactic acid production. A method for extraction of nutrients and growth factors from malt sprouts is also described.

Calculation of positron states and annihilation in solids: A density-gradient-correction scheme

Abstract: The generalized gradient correction method for positron-electron correlation effects in solids [B. Barbiellini et al., Phys. Rev. B 51, 7341 (1995)] is applied in several test cases. The positron lifetime, energetics, and momentum distribution of the annihilating electron-positron pairs are considered. The comparison with experiments shows systematic improvement in the predictive power of the theory compared to the local-density approximation results for positron states and annihilation characteristics. \textcopyright{} 1996 The American Physical Society.

Synthesis and Characterization of Lactic Acid Based Telechelic Prepolymers

Abstract: The synthesis of low molecular weight (Mn (NMR) < 7000 g/mol) lactic acid prepolymers by condensation polymerization of l-lactic acid was investigated. Besides the l-lactic acid polymer, hydroxyl- and carboxyl-terminated telechelic prepolymers were also prepared by the addition of small amounts of 1,4-butanediol and adipic acid, respectively. All polymerizations were carried out in a melt with tin octoate as the catalyst. The products were characterized by differential scanning calorimetry, gel permeation chromatography (GPC), IR, 1H-NMR, and 13C-NMR. According to NMR, the resulting prepolymers contained less than 1 mol % of lactic acid monomer and less than 4.1 mol % of lactide. End group analysis of the polymers was carried out by comparing the NMR spectra of different polymers. According to NMR, the lactic acid can be copolymerized so that the resulting prepolymer chains have only one kind of end group, hydroxyl or carbonyl. The integrated area of the identified end group peak (hydroxyl or acid) was th...

Processing of complex sounds in the human auditory cortex as revealed by magnetic brain responses.

Abstract: Processing of simple and complex sounds in the human brain was compared by recording extracranial magnetic mismatch responses (MMNm; the magnetic counterpart of the mismatch negativity, or MMN) to frequency changes in these sounds. Generator sources, modeled as equivalent current dipoles (ECDs), of MMNm responses to a change in one frequency element of complex sounds (a chord and a serial tone pattern) were located in supratemporal auditory cortex, on average, 10 mm medially to the source of an MMNm elicited by an identical frequency change in a simple tone. These results suggest that at least partially different supratemporal neuron populations are involved in processing changes in simple and complex sounds and that sensory-memory representations for these sounds may be located in different fields of the auditory cortex.

Mechanism of High Pressure Denaturation of Proteins

Abstract: High pressure denaturation of ovalbumin, bovine serum albumin (BSA) and β-lactoglobulin was assessed by spectrofluorometry, specific rotation analysis and differential scanning calorimetry (DSC), and compared with heat and chemical denaturation. In all cases the denaturation caused by high pressure was similar to that caused by the cleavage of hydrogen bonds with urea or guanidine hydrochloride.

Emergence of invariant-feature detectors in the adaptive-subspace self-organizing map

Abstract: A new self-organizing map (SOM) architecture called the ASSOM (adaptive-subspace SOM) is shown to create sets of translation-invariant filters when randomly displaced or moving input patterns are used as training data. No analytical functional forms for these filters are thereby postulated. Different kinds of filters are formed by the ASSOM when pictures are rotated during learning, or when they are zoomed. The ASSOM can thus act as a learning feature-extraction stage for pattern recognizers, being able to adapt to many sensory environments and to many different transformation groups of patterns.