Browsing by Department "Department of Engineering Physics and Mathematics"

Now showing 1 - 20 of 174
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    Advanced mobile network monitoring and automated optimization methods
    (Helsinki University of Technology, 2006-03-23) Höglund, Albert; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Systems Analysis Laboratory; Systeemianalyysin laboratorio
    The operation of mobile networks is a complex task with the networks serving a large amount of subscribers with both voice and data services, containing extensive sets of elements, generating extensive amounts of measurement data and being controlled by a large amount of parameters. The objective of this thesis was to ease the operation of mobile networks by introducing advanced monitoring and automated optimization methods. In the monitoring domain the thesis introduced visualization and anomaly detection methods that were applied to detect intrusions, mal-functioning network elements and cluster network elements to do parameter optimization on network-element-cluster level. A key component in the monitoring methods was the Self-Organizing Map. In the automated optimization domain several rule-based Wideband CDMA radio access parameter optimization methods were introduced. The methods tackled automated optimization in areas such as admission control, handover control and mobile base station cell size setting. The results from test usage of the monitoring methods indicated good performance and simulations indicated that the automated optimization methods enable significant improvements in mobile network performance. The presented methods constitute promising feature candidates for the mobile network management system.
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    Aerosol production and crystallization of titanium dioxide from metal alkoxide droplets
    (VTT Technical Research Centre of Finland, 2001-09-14) Ahonen, Petri; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto
    In this experimental study, aerosol methods for producing titanium dioxide powders and increasing our knowledge of particle and crystal formation have been developed. Powders and ultrafine particles of titanium oxide were produced by an aerosol droplet decomposition route in tubular laminar flow reactors in air and nitrogen atmospheres. Reactor temperatures up to 1500°C were used with residence times in the range of 1.50 s. Novel methods were introduced for the production of micron sized powders, investigation of crystallization of anatase in the particles, and for studying the formation of crystal phase and morphology on ultrafine particles at different temperatures. High-resolution transmission electron microscopy, scanning electron microscopy, aerosol measurements by differential mobility analyzer and inertial impactor as well as materials characterization by diffraction and spectroscopic methods were performed. In addition, the production conditions in aerosol reactors were evaluated using computational fluid dynamics calculations. The results showed that titanium dioxide powders can be produced from ultrafine up to micron sized particles via droplet decomposition and in-droplet hydrolysis methods starting from a titanium alkoxide precursor. Crystal phase and crystallite size can be controlled by reactor conditions and by thermal post-annealing. Anatase formation in amorphous particles was observed near surfaces. Investigation of ultrafine particles revealed morphology development of rutile and anatase single crystals. The 60 and 120 nm diameter rutile crystal morphologies development was observed in mobility particle size measurements. The 20 nm diameter anatase particle morphology showed the development of crystallographic {011} and {001} surfaces.
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    Affine equations as dynamic variables to obtain economic equilibria
    (Helsinki University of Technology, 2006-02-10) Kitti, Mitri; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Systems Analysis Laboratory; Systeemianalyysin laboratorio
    This thesis studies how economic equilibria can be reached with limited information by adjusting affine equations, and how collusion in oligopolistic markets can be maintained with affine reaction strategies. The first question is considered for exchange economies, contracting problems, and a two-party negotiation support method. The main contributions are new ways to reach and maintain equilibria. For exchange economies the thesis shows that a modification of fixed-point iteration converges globally under conditions that are remarkably close to those required for the continuous time tâtonnement process presented by Arrow, Block, and Hurwicz. For the constraint proposal method for two-party negotiations this thesis provides answers for three major questions: does the method produce Pareto-optimal points, does it lead to a problem that has a solution, and can the solutions be found with fixed-point iteration. The thesis also shows that the complete information equilibrium of a contract design problem can be reached by adjusting the contract with fixed-point iteration. Finally, the thesis formulates a simple dynamic counterpart of a static reaction strategy by D. K. Osborne in a repeated oligopoly game with discounting. It is proven that these strategies lead to a subgame perfect equilibrium when the possible deviations are bounded and the proportional reaction strategies have sufficiently large slopes.
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    Analysis and modeling of surface-acoustic wave resonators
    (Helsinki University of Technology, 2001-01-12) Koskela, Julius; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Materials Physics Laboratory; Materiaalifysiikan laboratorio
    This thesis discusses the analysis and simulation of surface-acoustic wave (SAW) resonators. SAW resonators constitute building blocks for radio-frequency SAW bandpass filters, which are widely employed in modern cordless and cellular telecommunication systems. Theoretical tools for the analysis and simulation of SAWs and SAW resonators are vital in the development of high-performance SAW devices. The carrying theoretical concept is the excitation of acoustic waves in periodic electrode arrays. The first part of the thesis is concerned with the characterization of periodic electrode arrays with rigorous simulations. In this work a structure simulator based on combined finite-element and boundary-element methods has been implemented, and it has been applied to analyze leaky surface-acoustic waves (LSAW) in rotated Y-cut LiNbO3 and LiTaO3 substrates, and surface transverse waves (STW) in langasite. The second part of the thesis consists of contributions to phenomenological modeling. Firstly, it is shown that the parameters used in the phenomenological coupling-of-modes theory, popular in device design, may be efficiently extracted from rigorous simulations of periodic electrode arrays using a novel phase-shift algorithm. Secondly, so-called Plessky's model for the STW dispersion in periodic structures is extended into a simulation model for finite, synchronous LSAW and STW resonators. The third part of the thesis concentrates on acoustic loss mechanisms in LSAW resonators on LiTaO3 substrate. The electric losses due to various mechanisms are estimated and discussed. Most importantly, a recently observed acoustic leakage is identified as parasitic excitation of LSAWs to the busbars of the resonator.
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    Analysis of reduced finite element schemes in parameter dependent elliptic problems
    (Helsinki University of Technology, 2001-12-14) Havu, Ville; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Institute of Mathematics; Matematiikan laitos
    This thesis presents an analysis of modified finite element schemes applied to parameter dependent elliptic problems prone to locking. Two different problems of similar type are considered: the problem of anisotropic heat conduction and the thin shell problem.
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    Analysis of the structure of time-frequency information in electromagnetic brain signals
    (Helsinki University of Technology, 2006-04-28) Mäkinen, Ville; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Biomedical Engineering; Lääketieteellisen tekniikan laboratorio
    This thesis encompasses methodological developments and experimental work aimed at revealing information contained in time, frequency, and time–frequency representations of electromagnetic, specifically magnetoencephalographic, brain signals. The work can be divided into six endeavors. First, it was shown that sound slopes increasing in intensity from undetectable to audible elicit event-related responses (ERRs) that predict behavioral sound detection. This provides an opportunity to use non-invasive brain measures in hearing assessment. Second, the actively debated generation mechanism of ERRs was examined using novel analysis techniques, which showed that auditory stimulation did not result in phase reorganization of ongoing neural oscillations, and that processes additive to the oscillations accounted for the generation of ERRs. Third, the prerequisites for the use of continuous wavelet transform in the interrogation of event-related brain processes were established. Subsequently, it was found that auditory stimulation resulted in an intermittent dampening of ongoing oscillations. Fourth, information on the time–frequency structure of ERRs was used to reveal that, depending on measurement condition, amplitude differences in averaged ERRs were due to changes in temporal alignment or in amplitudes of the single-trial ERRs. Fifth, a method that exploits mutual information of spectral estimates obtained with several window lengths was introduced. It allows the removal of frequency-dependent noise slopes and the accentuation of spectral peaks. Finally, a two-dimensional statistical data representation was developed, wherein all frequency components of a signal are made directly comparable according to spectral distribution of their envelope modulations by using the fractal property of the wavelet transform. This representation reveals noise buried processes and describes their envelope behavior. These examinations provide for two general conjectures. The stability of structures, or the level of stationarity, in a signal determines the appropriate analysis method and can be used as a measure to reveal processes that may not be observable with other available analysis approaches. The results also indicate that transient neural activity, reflected in ERRs, is a viable means of representing information in the human brain.
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    Applications of decision analysis in the assessment of energy technologies for buildings
    (Helsinki University of Technology, 2007-02-23) Alanne, Kari; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Systems Analysis Laboratory; Systeemianalyysin laboratorio
    The transition to sustainable energy system calls for changes in both the production and consumption of energy, including issues such as the implementation of sustainable technologies and practices for energy conversion and the improvement of energy efficiency at the demand side. This Thesis i) identifies the need of decision support in the commercialization of sustainable energy technologies in buildings, ii) characterizes decision-making problems related to the above context, iii) develops and implements a methodology to assess energy technologies for buildings, and iv) presents two fields of application where the above assessment is essential. The decision-making problem is characterized by i) multiple objectives, ii) several interest groups with different preferences, iii) new alternatives with the lack of operational experiences and thus plenty of uncertainties, and iv) a broad portfolio of applicable technologies that have to be combined into a workable entity. Hence, an interdisciplinary decision support framework is required that combines basic theories of life cycle and decision analyses including sensitivity assessments. In this Thesis, the above methodological framework is implemented in terms of two applications: i) the assessment of heating systems for a single-family house and ii) the selection of technology portfolio in a retrofit project that results in improved energy efficiency and thermal comfort, and reduced environmental burdens. Specifically, the competitiveness of a natural gas heating system containing a solid-oxide fuel cell (SOFC) is examined with respect to residential heating systems containing no electricity generation. Moreover, a multi-criteria portfolio model is applied to determine the most preferred retrofit measures in an apartment building. The above examples are selected because i) they represent a new field of research and ii) they are interesting due to the challenges they provide in decision-making. In the assessment of heating systems that incorporate new technologies, the mutual ranking of alternatives often must be established on the basis of incomplete information. Here, the extensive framework of decision-making was useful. In the second application, the multi-criteria portfolio model was suitable in the search of optimal technological solutions in retrofit projects. According to computational studies, a small (1 kWe) SOFC heating system is an attractive alternative to traditional heating systems and simple, inexpensive measures with good price-quality ratio were preferred as retrofit actions. While the methodological framework is generally applicable, the computational examples are mainly indicative and illustrative.
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    Applying mathematical finance tools to the competitive Nordic electricity market
    (Helsinki University of Technology, 2004-12-03) Vehviläinen, Iivo; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Institute of Mathematics; Matematiikan laitos
    This thesis models competitive electricity markets using the methods of mathematical finance. Fundamental problems of finance are market price modelling, derivative pricing, and optimal portfolio selection. The same questions arise in competitive electricity markets. The thesis presents an electricity spot price model based on the fundamental stochastic factors that affect electricity prices. The resulting price model has sound economic foundations, is able to explain spot market price movements, and offers a computationally efficient way of simulating spot prices. The thesis shows that the connection between spot prices and electricity forward prices is nontrivial because electricity is a commodity that must be consumed immediately. Consequently, forward prices of different times are based on the supply-demand conditions at those times. This thesis introduces a statistical model that captures the main characteristics of observed forward price movements. The thesis presents the pricing problems relating to the common Nordic electricity derivatives, as well as the pricing relations between electricity derivatives. The special characteristics of electricity make spot electricity market incomplete. The thesis assumes the existence of a risk-neutral martingale measure so that formal pricing results can be obtained. Some concepts introduced in financial markets are directly usable in the electricity markets. The risk management application in this thesis uses a static optimal portfolio selection framework where Monte Carlo simulation provides quantitative results. The application of mathematical finance requires careful consideration of the special characteristics of the electricity markets. Economic theory and reasoning have to be taken into account when constructing financial models in competitive electricity markets.
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    Approaches for modelling and reconstruction in optical tomography in the presence of anisotropies
    (Helsinki University of Technology, 2005-05-20) Heino, Jenni; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Biomedical Engineering; Lääketieteellisen tekniikan laboratorio
    In this thesis, models for light propagation and solution methods for the inverse problem in medical optical tomography (OT) in the presence of anisotropies are developed. Light propagation is modelled using the anisotropic diffusion equation (DE) in the frequency domain. The derivation of the diffusion equation in an anisotropic case is sketched, and the relevant boundary and source conditions presented. The numerical solution is obtained using the finite element (FE) method. The numerical work is done in two dimensional space in order to facilitate the testing of the novel methods for solving the inverse problem. To verify the light propagation model, the 2D FE solution is compared to the boundary element method solution to the DE and to a Monte Carlo simulation. The main emphasis is on the solution of the inverse problem in OT in the presence of anisotropies. The anisotropic inverse problem is non-unique, and hence simultaneous reconstruction of both the anisotropic diffusion tensor and the absorption coefficient is not feasible without substantial prior knowledge. The goal in this work is to reconstruct the spatial distribution of the optical absorption coefficient and overcome the disturbing effect of the background anisotropies. At the same time, the prior knowledge available on the anisotropies is assumed to be rather limited. A few different approaches for estimating the absorption coefficient is presented. Firstly, an attempt to use a conventional isotropic reconstruction scheme is considered. In this case, the obtained estimates suffer from relative large artefacts. In the second approach the structure of the anisotropy is assumed known, and the spatially constant strength is reconstructed simultaneously with the absorption. The quality of the absorption estimate degrades quickly with the accuracy of the underlying anisotropy structure. To help this, statistical inversion methods are employed. Statistical methods provide means to model the prior information on unknown parameters through probability distributions. The anisotropy parameters are modelled using relatively loose Gaussian priors. By using Gaussian priors the posterior probability distribution of the absorption on condition of the measurements also assumes a Gaussian form. Hence the conditional mean estimates and covariances can be derived in a closed form without having to resort to numerical integration. The statistical approach is used to derive a modelling error approach, where the effect of anisotropies is treated as a modelling error and included into estimation. Implementing the statistical inversion methods enables recovering the main features in the absorption distribution. The statistical treatment of anisotropies is also applied to help the inverse problem when the boundary shape and source/measurement locations are modelled inaccurately.
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    Aspects of fracture processes in paper
    (Helsinki University of Technology, 2003-06-05) Salminen, Lauri; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Physics; Fysiikan laboratorio
    The strength properties and fracture processes are studied in paper. This thesis deals with the fundamental structure and physical phenomenon of fracture. The methods employed are Monte Carlo simulations with a finite element model and experiments in fractography and acoustic emission. It is still unclear how the mechanical properties of paper, particularly strength depend on the disordered geometry of the fiber network. The shrinkage during manufacturing process induces internal stresses, which are crucial to macroscopic properties. The limiting strength in paper depends on crack pinning effects and obeys extremal statistics. The local stress variations introduce crack pinning and affect the fracture line topography. In the thesis geometrical effects of fiber network shrinkage are simulated and observed to follow a simple analytic expression. The shrinkage of fiber segments agrees qualitatively with microscopic measurements in literature. Extensive tensile strength distributions are obtained and compared with theoretical strength models. The strength of paper is found to be close to the Weibull and Duxbury distributions. Crack localization in tensile mode I loading is studied with initially notched strips. The resulting pinning probability agree with simple simulations and demonstrates that paper tolerates short order of fiber length notches. The fractal nature of paper crack line is analyzed in large samples. The geometry in fast crack propagation is found to be self-affine with a roughness exponent close to 0.6. The value is not in agreement with any fracture models. In addition systematic deviations from pure power law dependence is observed in the length-scale 5-20 mm. Acoustic emission spectroscopy is employed to study paper fracture in tensile and peel tests. By acoustic emission the energy released in micro fracturing is measured. The energy statistics are observed to obey power law analogously to Gutenberg-Richter's law for eartchquakes. In the tensile test the exponent characterizing the energy distribution is 1.2 and in the peel test 2.0. In the tensile tests the inter-arrival time between events obey a power law (Omori's law) with an exponent close to 1.0. In the peel tests deviations from Omori's power law are found. These observations suggest that the two often simultaneously witnessed power laws do no have a common origin. The acoustic emission results give new insight to fracture processes in the presence of disorder.
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    Atom traps on an evanescent-wave mirror
    (Helsinki University of Technology, 2004-10-08) Shevchenko, Andriy; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Optics and Molecular Materials; Optiikka ja molekyylimateriaalit
    During the past two decades the dramatic developments in cooling and trapping of gaseous atomic samples has produced a variety of techniques to manipulate the external and internal atomic states with electromagnetic fields. The research efforts so far have mainly focused on studying the fundamental aspects of modern quantum physics, although numerous practical applications, e.g., in integrated matter-wave optics and quantum information processing, are expected to soon be added to the already established atom-optics products, such as atomic clocks and acceleration sensors. One of the most promising foundations for the practical applications of Bose-Einstein condensates (BECs) and coherent matter waves is the technology based on the creation of microscopic atom traps on the surface of a solid substrate. This trapping technique has recently been used to successfully control the motion of microscopic atomic samples and to significantly simplify the creation of BEC. The main research topic of this thesis is the development and design of surface-mounted atom traps on transparent dielectric substrates. Such traps could provide an extra flexibility and stability for the experiments, since they would allow unimpeded control of atoms with laser light and provide reduction of magnetic-field fluctuations associated with the thermal motion of free electrons in the substrate material. The thesis describes several novel approaches to the creation of such surface traps by superimposing repulsive evanescent optical waves with strongly localized magnetic or electric fields. These fields can be produced by either conductive or permanently magnetized, optically transparent patterns imprinted in a thin layer on a transparent dielectric substrate. The evanescent wave can also be used to cool the atoms in a gravitational field before loading them into the microtrap. The lateral confinement of the atoms on the evanescent wave can be realized with a thin-walled hollow laser beam, the creation of which is demonstrated in the thesis. The thesis also describes certain general aspects concerning evanescent-wave cooling. In particular, the influence that multiple reabsorption of resonance-frequency photons in a cloud of evanescent-wave cooled atoms has on the cooling efficiency is investigated. Also, a theoretical model based on classical statistical mechanics and thermodynamics is introduced to show how a microtrap on an evanescent-wave mirror can be used to decrease the temperature, increase the phase-space density, and provide temperature conserving spin-polarization of the atoms.
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    Atomistic modelling of anisotropic etching of crystalline silicon
    (Helsinki University of Technology, 2003-09-19) Gosálvez, Miguel A.; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Physics; Fysiikan laboratorio
    An atomistic model for the simulation of anisotropic wet chemical etching of crystalline silicon is developed. Special attention is paid to the relation between the atomistic processes, the mesoscopic features of the surface morphology and the macroscopic anisotropy of the process, bridging the different length scales. The development of the atomistic model is made by direct comparison of atomistic kinetic Monte Carlo and Cellular Automaton simulations with experimental results, guided by first-principles calculations. The model explains the anisotropy of the etching process and the orientation-dependent surface morphology as two different manifestations of the same atomistic mechanisms, namely, the weakening of backbonds following OH termination of surface atoms and the existence of significant interaction between the terminating species (H / OH). The versatility of the atomistic model is demonstrated by the concentration and time dependence of the simulated under-etched structures and surface morphology. A substantial effort has been made to develop an efficient program in order to simulate the etching process in arbitrarily oriented, large, micrometer-scale systems in the presence (or absence) of masking patterns and considering the effects of temperature and etchant concentration. The program has a great potential for use in the optimization of the processing parameters in industrial applications.
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    Atomistic simulations of magnetic shape memory alloys
    (Helsinki University of Technology, 2003-02-21) Enkovaara, Jussi; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Physics; Fysiikan laboratorio
    Magnetic shape memory (MSM) alloys are novel smart materials which exhibit magnetic field induced strains of up to 10 %. As such they have potential for many technological applications. Also, the strong magneto-structural couplings of the MSM effect make the phenomenon very interesting from a scientific point of view. In this thesis, materials and properties related to the MSM effect are studied with atomistic simulations. Main interest is in the known MSM alloy Ni-Mn-Ga around the Ni2MnGa stoichiometry. One pre-requisite for the MSM effect is the existence of a structural transformation in a magnetic material, and therefore some candidate materials are investigated from this perpective. Here, Ni2MnAl is found to have potential for further studies. The magnetic moment is seen to originate mainly from Mn in the Mn-containing alloys and the existence of different structural phases is ascribed to a band Jahn-Teller effect in the Ni band. This picture is confirmed by comparisons between theoretical and experimental neutron diffraction results. In Ni2MnGa the structural phase transformations are found to be driven by vibrational entropy at finite temperatures. The magnetic key property in the MSM effect is the magnetic anisotropy energy which is studied in Ni2MnGa. The tetragonal structure with c/a = 0.94 is magnetically uniaxial characterized by the first anisotropy constant, but in the presence of several twin variants only the second anisotropy constant may be observed in the measurements. Analysis of the microscopic origins of the magnetic anisotropy shows that Ni has the largest contribution to the magnetic anisotropy energy. Investigations of other structures show that in Ni2MnGa the shortest crystal axis is always the easy axis of magnetization. From other magnetic properties, the Curie temperatures of Ni2MnGa and Ni2MnAl are estimated on the basis of total energy calculations of spin spirals. Ni is found to have an important effect also on the Curie temperatures despite its smaller magnetic moment when compared to Mn. Non-stoichiometric compositions of Ni-Mn-Ga are studied within the rigid band approximation and with supercell calculations. In some cases the rigid band approximation describes the correct trends, but more insight into the alloying effects can be obtained from the supercell calculations. The most important result of these investigations is that in Mn-rich compositions the extra Mn atoms couple antiferromagnetically to the neighbouring Mn atoms. This result implies a decrease of the total magnetic moment with Mn-doping. Also, all the experimentally observed martensite phases are explained theoretically when the extra Mn is explicitly included.
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    The Blagoveščenskiĭ identity and the inverse scattering problem
    (Helsinki University of Technology, 2005-06-10) Bingham, Kenrick; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Institute of Mathematics; Matematiikan laitos
    The inverse scattering problem for the plasma wave equation [∂2t − ∆ + q(x)] u(x,t) = 0 in three space dimensions is considered in this thesis. It is shown that, under certain assumptions about the potential, the time domain scattering problem can be formulated equivalently in the frequency domain. Time and frequency domain techniques are combined in the subsequent analysis. The Blagoveščenskiĭ identity is generalised to the case of scattering data, assuming an inverse polynomial decay of the potential. This identity makes it possible to calculate the inner product of certain solutions of the plasma wave equation at a given time, if the corresponding incident waves and the scattering amplitude are known. In the case of a compactly supported potential, these inner products can be calculated for the time derivatives of all solutions. In the remaining part of the work, the potential is assumed to be compactly supported. A variant of the boundary control method is used to show that using appropriate superpositions of plane waves as incident waves, it is possible to excite a wave basis over a compact set. Letting this set shrink to a point, the Blagoveščenskiĭ identity provides pointwise information about the solutions. When substituted into the plasma wave equation, this yields a method for solving the inverse problem.
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    Brain mechanisms of audiotactile and audiomotor interactions
    (Helsinki University of Technology, 2007-12-21) Caetano, Gina; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Low Temperature Laboratory, Brain Research Unit; Kylmälaboratorio, Aivotutkimusyksikkö
    This thesis focuses on audiotactile integration, brain areas activated by vibrotactile stimulation, transfer of vibrotactile information to motor output, and reactivity of the human primary motor and somatosensory cortices in action observation. Human experience of the outside world results from integration of information obtained simultaneously via multiple senses. Accumulating evidence, from studies in both primates and humans, suggests that integration between different sensory modalities also occurs at early stages of cortical processing, in areas classically considered as purely unisensory. In Study I we studied integration between auditory and somatosensory systems. We showed, in a loudness-matching task, that subjects chose lower intensities for the probe than for the reference tone, when auditory and vibrotactile stimuli were presented simultaneously. In Studies II and III we explored brain areas involved in processing vibrotactile and tactile information, respectively. We showed that, besides primary and secondary somatosensory areas, auditory areas are also activated. In Study II we characterized the time course of brain activations and showed convergence of vibrotactile information to auditory areas. On the other hand, in Study III we identified, with good spatial accuracy, common neural substrates that process auditory and tactile information in auditory belt areas. In Study IV we assessed whether frequency information transfers from touch to vocal utterance in normal-hearing female adults. We demonstrated that such information transfer occurs clearly between 150–400 Hz. Based on findings in Studies II and III, we hypothesized that this transfer may involve at least primary and secondary somatosensory and auditory areas. Our social skills rely on the capability to understand others. In the human brain, the mirror-neuron system matches observation and execution of actions. This system comprises at least the inferior frontal gyrus, premotor areas, primary motor cortex, and the inferior parietal lobule. In Study V we investigated similarities in sensorimotor oscillatory activity between own, observed, and heard actions. We demonstrated that the primary motor cortex is activated before own and observed actions and stabilizes similarly. We also showed that rhythmic activity in the primary somatosensory cortex recovers later during own actions, which may be related to proprioceptive input and contribute to maintaining the sense of agency.
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    Carbon nanotube single-electron devices at audio and radio frequencies
    (Helsinki University of Technology, 2004-06-01) Roschier, Leif; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Low Temperature Laboratory; Kylmälaboratorio
    A single-electron transistor is the most sensitive charge detector known today. It is formed by a small piece of a conductor coupled to electrodes by tunnel junctions. At low frequencies, the charge sensitivity is limited by the 1/f-noise. The use of a radio-frequency modulation technique allows a wide operational bandwidth with negligible 1/f-noise contribution. In this Thesis, a multiwalled carbon nanotube brought to contact with metal electrodes was demonstrated to work as a single-electron transistor. A scanning probe manipulation scheme was developed and it was used to fabricate the sample. The manipulation scheme was also employed to construct more complicated electronic carbon nanotube devices. It was shown that it is possible to construct a multiwalled carbon nanotube single-electron transistor having an equal to, or even higher charge sensitivity than a typical metallic device. The transmission-line parameters of the multiwalled carbon nanotube were estimated by using the environment-quantum-fluctuation theory. The radio-frequency single-electron transistor setup was analyzed in depth and a simplified engineering formula for the charge sensitivity was derived. A radio-frequency single-electron transistor setup using a multiwalled carbon nanotube single-electron transistor was demonstrated in the built cryogenic high-frequency measurement system. A low-temperature high-electron-mobility-transistor amplifier was designed and built for the system. Measurements of the amplifier indicated a noise temperature of three Kelvins.
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    Cardiac exercise studies with bioelectromagnetic mapping
    (Helsinki University of Technology, 2001-11-17) Takala, Panu; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Biomedical Engineering; Lääketieteellisen tekniikan laboratorio
    Bioelectric currents in the heart give rise to differences in electric potential in the body and on its surface. The currents also induce a magnetic field within and outside the thorax. Recording of the electric potential on the surface of the body, electrocardiography (ECG), is a well established clinical tool for detecting insufficient perfusion of blood, i.e., ischemia during exercise testing. In a more recent technique, magnetocardiography (MCG), the cardiac magnetic field is recorded in the vicinity of the chest. Despite the clinical significance of the exercise ECG recordings in patients with suspected coronary artery disease (CAD), little is known about the effect of stress in the MCG of healthy subjects and patients with CAD. Methods for analysing multichannel MCG signals, recorded during physical exercise testing, were developed in this thesis. They were applied to data recorded in healthy subjects to clarify the normal response to exercise in the MCG, and to data of patients with CAD to detect exercise-induced myocardial ischemia. Together with the MCG, spatially extensive ECG, i.e., body surface potential mapping (BSPM) was studied and the exercise-induced alterations in the two mappings were compared. In healthy volunteers, exercise was found to induce more extensive alterations in the MCG than in the BSPM during the ventricular repolarisation. In patients with CAD, when optimal recording locations were found and evaluated, alterations of the ST segment in the MCG could be used as indicators of ischemia. Also, ischemia was found to induce a rotation of magnetic field maps (MFMs) which illustrate the spatial MCG signal distribution. The MFM orientation could successfully be used as a parameter for ischemia detection. In the BSPM, regions sensitive to ischemia-induced ST segment depression, ST segment elevation, and ST segment slope decrease were identified. An analysis method was also developed for monitoring the development of the MCG and the BSPM distributions. It enables examination of different features of the MCG and the BSPM signals as a function of time or the heart rate. In this thesis, the method was used for quantifying exercise-induced change in the orientation of MFMs. Adjustment of the orientation change with the corresponding alteration of the heart rate was found to improve ischemia detection by the exercise MCG. When data recorded during the recovery period of exercise testing were evaluated with similar type of analysis methods, the MCG showed better performance in ischemia detection than the simultanously recorded 12-lead ECG.
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    Cardiomagnetic source imaging
    (Helsinki University of Technology, 2000-05-19) Pesola, Katja; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Biomedical Engineering; Lääketieteellisen tekniikan instituutti
    Magnetocardiographic (MCG) source imaging has received increasing interest in recent years. With a high enough localization accuracy of the current sources in the heart, valuable information can be provided, e.g., for the pre-ablative evaluation of arrhythmia patients. Furthermore, preliminary studies indicate that ischemic areas, i.e. areas which are suffering from lack of oxygen, and infarcted regions could be localized from multichannel MCG recordings. In this thesis, the accuracy of cardiomagnetic source imaging results, obtained by using different current source models, was investigated. In addition, the effect of the torso model on the localization accuracy was examined. In some studies, also body surface potential mapping (BSPM) data were used for comparison purposes. A high impact was given to clinical validation, i.e. how the calculation methods would work in patients. The equivalent current dipole (ECD) source model was found to produce accurate (within 3-11 mm) localizations of focal current sources in a thorax phantom and in 15 patients with a non-magnetic stimulation catheter in the heart. The accuracy was found to depend on the signal-to-noise ratio and on the goodness of fit of the localizations. The corresponding accuracy determined from simultaneous multichannel BSPM recordings in 10 patients was 25 mm. In order to localize wider source regions in the heart, distributed source models were also investigated in the thesis. Current density estimates (CDEs) were calculated in the catheter patients and in 13 patients with coronary artery disease (CAD). Promising results were obtained by using second-order Tikhonov regularization in the calculations. CDEs were found to localize both myocardial ischemia in single-vessel CAD patients, as well as more complex chronic ischemia in three-vessel CAD patients. In addition to the ECD and CDE source models, the uniform double layer (UDL) model was used in the source imaging studies. With the UDL model, the whole depolarization of the ventricles can be represented with a single inverse solution. In the validation of the activation time maps calculated from MCG and BSPM recordings, invasively measured epicardial electrograms were used to construct the reference epicardial activation times. The overall patterns of activation in the reference data were reproduced relatively well in the calculated activation time maps. The high quality of the inverse solutions obtained in this thesis prompts the use of cardiomagnetic source imaging in several clinical applications, such as in electrophysiological studies and in the estimation of myocardial viability.
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    Catalytic and surface oxidation processes on transition metal surfaces
    (Helsinki University of Technology, 2007-03-23) Jaatinen, Sampsa; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Laboratory of Physics; Fysiikan laboratorio
    Transition metals are technologically important catalytic materials. The transition metal catalysts are used for example in petroleum and fertilizer industry. In the car industry the catalytic materials are used in the catalytic converters. Because of the industrial importance the catalytic metals have been widely studied throughout the past decades. Nonetheless, the oxidation mechanisms of small molecules and the effect of alloying to catalytic properties of metals are not fully understood. In this thesis the catalytic effect of a palladium-silver alloy on the oxidation of a carbon monoxide molecule is studied with a computational approach. With ab initio methods we have been able to investigate the underlying microscopic processes of the oxidation. We have studied how the Pd atoms introduced to an Ag surface change the catalytic properties of the material. It has been seen that the Pd atoms enhance the dissociation of the O2 molecules, strengthen the binding of the CO molecules and lower the reaction barrier of the oxidation process. In addition to the oxidation of the carbon monoxide, the oxidation of copper surfaces has been studied. Copper is widely used for example in construction, water pipes and other large scale facilities. On the other hand, it is used in a microscopic level in electronics as s conductor and the copper oxide as an insulator. In all of these examples the oxidation of copper plays a crucial role and it also possesses a great commercial value. In some applications the oxidation is an unwanted phenomenon, whereas in some other applications the properties of grown oxide are exploited. The oxidation of copper surfaces is also scientifically interesting since the oxygen is known to induce a reconstruction of the surface. The reconstruction structure has intensively been studied, but still the initial steps of the oxidation and reconstruction are not completely explained. In this thesis, the reconstruction and the growth of the oxide layer on Cu(100) are studied with a multi-scale modeling involving both the atomistic electronic structure calculations and the larger scale statistical simulations. With the combined modeling we have been able to reproduce and further explain the structures observed in the experiments. We have shown that the dissociation and diffusion of oxygen is more rapid on the clean Cu(100) surface than on the reconstructed parts of the surface. The Cu adatoms form rapidly stable (100) islands and O adatoms form separate c(2 × 2) domains.
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    Chaos in high-power high-frequency gyrotrons
    (Helsinki University of Technology, 2004-02-06) Airila, Markus; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto; Advanced Energy Systems; Energiateknologiat
    Gyrotron interaction is a complex nonlinear dynamical process, which may turn chaotic in certain circumstances. The emergence of chaos renders dynamical systems unpredictable and causes bandwidth broadening of signals. Such effects would jeopardize the prospect of advanced gyrotrons in fusion. Therefore, it is important to be aware of the possibility of chaos in gyrotrons. There are three different chaos scenarios closely related to the development of high-power gyrotrons: First, the onset of chaos in electron trajectories would lead to difficulties in the design and efficient operation of depressed potential collectors, which are used for efficiency enhancement. Second, the radio-frequency signal could turn chaotic, decreasing the output power and the spectral purity of the output signal. As a result, mode conversion, transmission, and absorption efficiencies would be reduced. Third, spatio-temporal chaos in the resonator field structure can set a limit for the use of large-diameter interaction cavities and high-order TE modes (large azimuthal index) allowing higher generated power. In this thesis, the issues above are addressed with numerical modeling. It is found that chaos in electron residual energies is practically absent in the parameter region corresponding to high efficiency. Accordingly, depressed collectors are a feasible solution also in advanced high-power gyrotrons. A new method is presented for straightforward numerical solution of the one-dimensional self-consistent time-dependent gyrotron equations, and the method is generalized to two dimensions. In 1D, a chart of gyrotron oscillations is calculated. It is shown that the regions of stationary oscillations, automodulation, and chaos have a complicated topology in the plane of generalized gyrotron variables. The threshold current for chaotic oscillations exceeds typical operating currents by a factor of ten. However, reflection of the output signal may significantly lower the threshold. 2D computations indicate that stationary single-mode operation of gyrotrons would be impossible if the azimuthal index is about 46 or larger, which is rather close to the presently used values. Moreover, electron beam misalignment can lower this critical value. Above the critical value, less favorable modes suppress the operating mode.