Browsing by discipline "Theoretical Physics"
Now showing items 120 of 96

(Helsingin yliopistoHelsingfors universitetUniversity of Helsinki, 2008)The molecular level structure of mixtures of water and alcohols is very complicated and has been under intense research in the recent past. Both experimental and computational methods have been used in the studies. One method for studying the intra and intermolecular bindings in the mixtures is the use of the so called difference Compton profiles, which are a way to obtain information about changes in the electron wave functions. In the process of Compton scattering a photon scatters inelastically from an electron. The Compton profile that is obtained from the electron wave functions is directly proportional to the probability of photon scattering at a given energy to a given solid angle. In this work we develop a method to compute Compton profiles numerically for mixtures of liquids. In order to obtain the electronic wave functions necessary to calculate the Compton profiles we need some statistical information about atomic coordinates. Acquiring this using abinitio molecular dynamics is beyond our computational capabilities and therefore we use classical molecular dynamics to model the movement of atoms in the mixture. We discuss the validity of the chosen method in view of the results obtained from the simulations. There are some difficulties in using classical molecular dynamics for the quantum mechanical calculations, but these can possibly be overcome by parameter tuning. According to the calculations clear differences can be seen in the Compton profiles of different mixtures. This prediction needs to be tested in experiments in order to find out whether the approximations made are valid.

(2015)Even after 50 years, there is still no standard, analytically tractable way to treat Quantum Chromodynamics (QCD) nonnumerically besides perturbation theory. In the highenergy regime perturbation theory agrees with experimental results to a great accuracy. However, at low energies the theory becomes strongly coupled and therefore not computable by perturbative methods. Therefore, nonperturbative methods are needed, and one of the candidates is lightfront holography. In this thesis, the basics of lightfront quantisation and holography are discussed. Lightfront quantisation takes lightcone coordinates and the Hamiltonian quantisation scheme as its basis and the resulting field theory has many beneficial properties like frameindependence. Still, to extract meaningful results from the lightfront QCD, one needs to apply bottomup holographic methods. Last part of this work focuses on the applicability of lightfront holographic QCD in the area of dark matter. We find that one can build a secluded SU(3) sector consisting of a doublet of elementary particles, analogous to quarks and gluons. Due to a global symmetry, the lightest stable particle is analogous with ordinary neutron. It meets the basic requirements for being a WIMP candidate when its mass is higher than 5 TeV.

(2018)A new theoretical model for the structure of glasses is presented and used to study the boson peak found in glasses. The model is based on a simple lattice model familiar from crystals, which is disordered using techniques from noncommutative fluid models. First classical crystal models and concepts of lattice vibrations are reviewed, focusing on acoustic and optical waves, the density of vibrational states, heat capacity and the Debye model. Then noncommutative fluid theory and noncommutative geometry are shortly introduced to show the connection to fluids in our model. After these introductions, the glass model is formulated and used to calculate the dispersion relations, the density of vibrational states and the heat capacity. The density of states has a Van Hove singularity at low frequencies, which generates the boson peak seen in experiments. The glass is found to have both acoustic and optical waves, and the acoustic waves are located very close to the frequency of the Van Hove singularity, which hints that the boson peak should be related to acoustic waves.

(2018)We review basics of quantum field theories (QFT) and lattice field theories (LFT). We present, evaluate, and compare possible solutions for creating portable high performance LFT simulation programs. We choose one of the possible solutions, creating our own programming language, discuss its features the our prototype of it. Last we present improvement ideas to the implemented solution.

(2015)The SM, conceptually and phenomenologically fails to incorporate and explain few fundamental problems of particle physics and cosmology, such as a viable dark matter candidate, mechanism for inflation, neutrino masses, the hierarchy problem etc. In addition, the recent discovery of the 125 GeV Higgs boson and the top quark mass favor the metastablility of the electroweak vacuum, implying the Higgs boson is trapped in a false vacuum. In this thesis we propose the simplest extension of the SM by adding an extra degree of freedom, a scalar singlet. The singlet can mix with the Higgs field via the Higgs portal, and as a result we obtain two scalar mass eigenstates (Higgslike and singletlike). We identify the lighter mass eigenstate with the 125 GeV SM Higgs boson. Due to the mixing, the SM Higgs quartic coupling receives a finite tree level correction which can make the electroweak vacuum completely stable. We then study the stability bounds on the tree level parameters and determine the allowed mass region of the heavier mass eigenstate (or singletlike) for range of mixing angles where all the bounds are satisfied. We also obtain regions of parameter space for different signs of the Higgs portal coupling. In the allowed region, the singletlike state can decay into two Higgslike states. We find the corresponding decay rate to be substantial. Finally, we review various applications of the singlet extension, most notably, to the problem of dark matter and inflation.

(2016)This thesis discusses various topics related to the study of strongly coupled quantum field theories at finite density or, equivalently, finite chemical potential. In particular, the focus is on the theory of strong interactions, quantum chromodynamics (QCD). Finitedensity QCD is important in the description of numerous physical systems such as neutron stars or heavyion collisions, a brief overview of which are given, alongside with the QCD phase diagram as motivational examples. After this, the general construction of a Lagrangian finitedensity quantum field theory is described. In contrast with the zerodensity setting, a finitedensity field theory does not admit a simple description on the lattice, rendering this standard approach to strongly coupled theories impractical due to the socalled sign problem. Various attempts of addressing the sign problem are reviewed, and the socalled Lefschetz thimble approach and the complex Langevin method are discussed in detail. Some mathematical details related to these approaches are elaborated in the appendices. Due to the impracticality of lattice methods, a perturbative description becomes more important at finite density. Perturbative finitedensity QCD and methods useful in practical calculations are discussed. Amongst them is a detailed proof of a set of socalled 'cutting rules' that apply to zerotemperature finitedensity quantum field theory, an example computation using these rules as well as a discussion on various divergences and their relation to zerodensity theory.

(Helsingin yliopistoHelsingfors universitetUniversity of Helsinki, 2007)Monissa viimeaikaisissa tutkimuksissa on tutkittu aurinkotuulen dynaamisen paineen vaikutusta revontulialueen hiukkaspresipitaatioon. Tutkimukset ovat kuitenkin perustuneet muutamaan yksittäiseen tapaukseen ja laskevan dynaamisen paineen vaikutuksia ei juuri ole tarkasteltu. Tämän opinnäytetyön tavoitteena oli selvittää suuremmasta statistiikasta aurinkotuulen dynaamisen paineen nousujen ja laskujen vaikutusta ionosfäärin dynamiikkaan. Paineen muutoksia etsittiin ACE:n mittausdatasta vuosilta 1998 2004 ja ionosfäärin vastetta näihin muutoksiin tutkittiin käyttäen IMAGEmagnetometriverkon tuottamaa magneettisen aktiivisuuden indeksiä (IEindeksi). Tutkimuksen kohteeksi valittiin 286 painepulssia, joita edelsi ja seurasi tasaisen paineen jakso, sekä 171 vastaavaa paineen laskua (negatiivista painepulssia). Näiden paineen muutosten ionosfäärivastetta tutkittaessa käytettiin tilastollista superposed epoch menetelmää. Tutkimuksen tulokseksi saatiin selvä positiivinen korrelaatio IEindeksin ja aurinkotuulen dynaamisen paineen välillä. Korrelaatio on vähemmän selkeää paineen laskujen kuin nousujen yhteydessä. Tälle on useita mahdollisia selityksiä: Tutkimusaineisto paineen laskuista oli suppeampi. Toisin kuin painepulsseihin, paineen laskuihin ei liittynyt aurinkotuulen nopeuden muutosta. Lisäksi IMF:n magnitudi kasvoi lähes kaikkien paineen laskujen aikana, joten magnitudin ja IE:n välinen positiivinen korrelaatio voisi peittää paineen laskun vaikutusta. Eteläisen IMF:n painepulssien aikana IE:n muutokset aiheutuivat enimmäkseen läntisen elektrojetin vahvistumisesta ja pohjoisen IMF:n aikana havaitut IE:n muutokset liittyivät enemmän itäiseen elektrojettiin. Aurinkotuulen dynaamisen paineen ja elektrojettien korrelaation selitykseksi tarjotaan ionosfääriin saapuvien kentänsuuntaisten virtojen välityksellä tapahtuvaa kytkentää aurinkotuuleen. IMF:n zkomponentin suunnalla oli odotetun merkittävä vaikutus IE:n yleiseen tasoon, mutta IE:n korrelaatio paineen muutosten kanssa oli samaa tasoa zkomponentin etumerkistä riippumatta. Myös IMF:n ykomponentti osoittautui merkittäväksi pohjoisen IMF:n aikana, sillä tällöin IE:n yleinen taso oli korkeampi ja paineen muutosten vaikutus paljon selkeämpi IMF:n ykomponentin ollessa positiivinen kuin negatiivinen.

(2012)Magnetic resonance imaging (MRI) provides spatially accurate, three dimensional structural images of the human brain in a noninvasive way. This allows us to study the structure and function of the brain by analysing the shapes and sizes of different brain structures in an MRI image. Morphometric changes in different brain structures are associated with many neurological and psychiatric disorders, for example Alzheimer's disease. Tracking these changes automatically using automated segmentation methods would aid in diagnosing a particular brain disease and follow its progression. In this thesis we present a method for automatic segmentation of MRI brain scans using parametric generative models and Bayesian inference. Our method segments a given MRI scan to 41 different structures including for example hippocampus, thalamus and ventricles. In contrast to the current stateoftheart methods in wholebrain segmentation, our method does not pose any constraints on the MRI scanning protocol used to acquire the images. Our model is based on two parts: the first part is a labeling model that models the anatomy of the brain and the second part is an imaging model that relates the label images to intensity images. Using these models and Bayesian inference we can find the most probable segmentation of a given MRI scan. We show how to train the labeling model using manual segmentations performed by experts and how to find optimal imaging model parameters using expectationmaximization (EM) optimizer. We compare our automated segmentations against expert segmentations by means of Dice scores and point out places for improvement. We then extend the labeling and imaging models and show, using a database consisting of MRI scans of 30 subjects, that the new models improve the segmentations compared to the original models. Finally we compare our method against the current stateoftheart segmentation methods. The results show that the new models are an improvement over the old ones, and compare fairly well against other automated segmentation methods. This is encouraging, because there is still room for improvement in our models. The labeling model was trained using only nine expert segmentations, which is quite a small amount, and the automated segmentations should improve as the number of training samples grows. The upside of our method is that it is fast and generalizes straightforwardly to MRI images with varying contrast properties.

(2017)In a quantum computer, the information carriers, which are bits in ordinary computers, are implemented as devices that exhibit coherent superpositions of physical states and entanglement. Such components, known as quantum bits or qubits, can be realized with various different types of twostate quantum systems. Quantum computers will be built for computational speed, with hoped for applications especially in cryptography and in other tasks where classical computers remain inefficient. Circuit quantum electrodynamics (cQED) is a quantumcomputer architecture which employs superconducting electronic components and microwave photon fields as building blocks. Compared to cavity quantum electrodynamics (CQED), where atoms are trapped in physical cavities, cQED is more attractive in that its qubits are tunable and conveniently integrable with the electronics already in use. This architecture has shown some of the most promising qubit designs, despite their coherence times reaching tens of microseconds, are still below the state of the art with spin qubits, which reach milliseconds. Coherence times are historically the most relevant parameters describing the fitness of a qubit, although these days not necessarily the limiting factor. This thesis presents a comprehensive set of theoretical and experimental methods for measuring the characteristic parameters of superconducting qubits. We especially study transmissionlineshunted plasma oscillation qubits, or transmons, and presents experimental results for a single sample. Transmon capacitively couples a superconducting quantum interference device (SQUID) with a coplanar waveguide (CPW) resonator, often with added frequency tunability utilizing an external magnet. The number of superconducting charge carriers tunnelled through a junction in the SQUID are used as qubit degrees of freedom. Readout of the qubit state is carried out by measuring transmission through the CPW. A cryogenic setup is employed with measurement and driving pulses delivered from microwave sources. Steadystate spectroscopy is employed to determine the resonance frequencies of the qubit and the resonator, qubitresonator coupling constants, and the energy parameters of the qubit. Pulsemodulated measurements are employed to determine the coherence times of the qubit. The related analysis and simulation programs and scripts are collected togithub.com/patomaki.

(2015)Flavour violating processes have never been observed for charged leptons, electron, muon and tau. The existence of charged lepton flavour violating (CLFV) processes is however expected, since flavour is violated by all the other fermions of the standard model (SM). In the standard model the neutrinos are massless, which forbids the mixing of neutrino flavour and also the violation of lepton flavour. The zero mass of the neutrinos in the SM is in conflict with the experimentally observed neutrino oscillations. The standard model has to be extended to include massive neutrinos. The easiest way to explain the neutrino mass is to assume that they acquire masses in the same way as the rest of the SM fermions: in the Higgs mechanism. This way however leads to problems with the naturality of the neutrino Yukawa coupling. One of the most popular methods of generating the neutrino mass is the so called seesaw mechanism (typeI). The standard model, extended with the neutrino masses, allows the charged lepton flavour to be violated. This leads to unobservably small transition rates however. Therefore an observation of charged lepton flavour violating process would be a clear evidence of the existence of new physics beyond the standard model and it's trivial extensions. To have hope of ever observing charged lepton flavour violating processes, there must be an extension of the standard model which produces observable, though small, rates for CLFV processes. One of the most popular extensions of the standard model is the so called minimal supersymmetric standard model (MSSM). The neutrinos are massless in the MSSM, as they are in SM, and therefore CLFV processes are forbidden in the MSSM. Luckily the neutrino masses can be generated via seesaw mechanism in the MSSM as well as in the SM. The MSSM contains more potential sources for CLFV processes than the SM. The extra sources are the soft mass parameters of the sleptons. In supersymmetric models the sleptons couple to the leptons through the sleptonleptongauginovertices. These generate the CLFV processes at the looplevel. Often the offdiagonal soft terms are assumed zero in the MSSM at the input scale, where the supersymmetry breaks. Experiments are done at much lower electroweak scale. The soft SUSYbreaking terms acquire large radiative corrections as they are run from the input scale down to the electroweak scale. Here the seesaw mechanism kicks in. The seesaw mechanism brings with it the offdiagonal neutrino Yukawa coupling matrices. This allows the offdiagonal slepton mass terms to evolve nonzero at the electroweak scale. In this thesis the charged lepton flavour violation is discussed first in the context of the standard model. Then the CLFV processes, l_i → (l_j γ), l_i → (l_j l_k l_l) and l_i ↔ l_j, are studied in the most general way: in the effective theories. Finally the charged lepton flavour violation is studied in the supersymmetric theories in general and more specifically in the minimal supersymmetric standard model extended with the seesaw mechanism (typeI).

(2015)The AdS/CFT correspondence is the first realization of the holographic principle. The holographic principle makes the bold statement that in a theory of quantum gravity all information in a region of spacetime can be completely described by information on its boundary. This would make the universe in certain sense a hologram, as our spacetime and everything in it could be described by some fundamental degrees of freedom living on the boundary of spacetime. Gauge/gravity dualities realize the holographic principle by stating that string theory in tendimensional spacetime and certain gauge field theories living on its boundary can be equivalent descriptions of the same physics. The AdS/CFT correspondence was the first one of these dualities to be discovered. The correspondence equates type IIB string theory on AdS_5× S^5 with \mathcal{N}=4 super YangMills theory living on fourdimensional Minkowski space, which is the boundary of fivedimensional antide Sitter space, AdS_5. In this thesis, we first briefly review the necessary theoretical components, which are combined in the correspondence. Then, the AdS/CFT correspondence is motivated by considering the low energy limit of string theory in a spacetime with a stack of coincident D3branes. The third part of this work is dedicated to the study of the properties and dynamics of the antide Sitter bulk. A black hole solution with an asymptotically AdS background is discussed along with its thermodynamics, and the connection with the dual field theory is emphasized. Then we present a model for a collapsing shell in AdS space and solve its dynamics. The black hole state, which the collapsing shell approaches corresponds to thermal equilibrium in the dual field theory. Lastly, we consider our shell model in the context of the AdS/CFT correspondence and present a method for computing twopoint correlation functions on the field theory side. This method is then used to compute retarded correlators in a twodimensional CFT in finite temperature. We are able to reproduce previous results obtained using different computational methods, following a seminal work of Son and Starinets.

(2018)Hybrid organicinorganic perovskites (HPs) are a novel materials class in photovoltaic (PV) power generation. The PV performance of HPs is impressive, although the microscopic origin of it is not well known due to the complex atomic structure of HPs. Specifically, the disordered mobile organic cations aggravate the use of conventional computational models. I have addressed this structural complexity by developing a multiscale model that applies quantum mechanical (QM) calculations of small HP supercell models in large coarsegrained structures. With a mixed QMclassical hopping model, I have studied the effects of cation disorder on charge mobility in HPs, which is a key feature to optimize their PV performance. My multiscale model parametrizes the interaction between neighboring methylammonium cations (MA) in the prototypical HP material, methylammonium lead triiodide (CH3NH3PbI3, or MAPbI3). For the charge mobility analysis with my hopping model, I solved the QM sitetosite hopping probabilities analytically and computed the nearestneighbor electronic coupling energies from the band structure of MAPbI3 with densityfunctional theory. I investigated the charge mobility in various MAPbI3 supercell models of ordered and disordered MA cations. My results indicate a structuredependent mobility, in range of 50–66 cm2/Vs, with the highest observed in the ordered tetragonal phase. My multiscale model enables the study of longrange atomistic processes in complex structures in an unprecedented scale with QM accuracy, with potential applications way beyond this study.

(2013)This thesis aims to cover the central aspects of the current research and advancements in cosmic topology from a topological and observational perspective. Beginning with an overview of the basic concepts of cosmology, it is observed that though a determinant of local curvature, Einstein's equations of relativity do not constrain the global properties of spacetime. The topological requirements of a universal space time manifold are discussed, including requirements of spacetime orientability and causality. The basic topological concepts used in classification of spaces, i.e. the concept of the Fundamental Domain and Universal covering spaces are discussed briefly. The manifold properties and symmetry groups for three dimensional manifolds of constant curvature for negative, positive and zero curvature manifolds are laid out. Multiconnectedness is explored as a possible explanation for the detected anomalies in the quadrupole and octopole regions of the power spectrum, pointing at a possible compactness along one or more directions in space. The statistical significance of the evidence, however, is also scrutinized and I discuss briefly the bayesian and frequentist interpretation of the posterior probabilities of observing the anomalies in a ΛCDM universe. Some of the major topologies that have been proposed and investigated as possible candidates of a universal manifold are the Poincare Dodecahedron and Bianchi Universes, which are studied in detail. Lastly, the methods that have been proposed for detecting a multiconnected signature are discussed. These include ingenious observational methods like the circles in the sky method, cosmic crystallography and theoretical methods which have the additional advantage of being free from measurement errors and use the posterior likelihoods of models. As of the recent Planck mission, no pressing evidence of a multi connected topology has been detected.

(2016)In this thesis, we study the decoherence of cosmological scalar perturbations during inflation. We first discuss the FRW model and cosmic inflation. Inflation is a period of accelerated expansion in the early universe, in typical models caused by a scalar field called inflaton. We review cosmological perturbation theory, where perturbations of the inflaton field and scalar degrees of freedom of the metric tensor are combined into the gaugeinvariant SasakiMukhanov variable. We quantize this variable using canonical quantization. Then, we discuss how interactions between the perturbations and their environment can lead to decoherence. In decoherence, the reduced density operator of the perturbations becomes diagonal with respect to a particular pointer basis. We argue that the pointer basis for the cosmological scalar perturbations consists of approximate eigenstates of the field value operator. Finally, we discuss how decoherence can help understand the transition from quantum theory to classical perturbation theory, and justify the standard treatment of perturbations and their initial conditions in cosmology. We conclude that since decoherence should not spoil the observationally successful predictions of this standard treatment, it is unlikely that the actual amount of decoherence could be observed in, say, the CMB radiation.

(2018)Remote sensing of soil permittivity and soil freezing was investigated using two different satellite based microwave radars: ASCAT and ASAR. ASCAT is a scatterometer with a good temporal resolution but coarse spatial resolution. ASAR is a synthetic aperture radar and has fine spatial resolution, but lacks good temporal coverage. Soil permittivity is related to soil moisture, which is considered an essential climate vari able since it has an effect on both weather and climate. Soil freezing affects hydrological and carbon cycles, surface energy balance, photosynthesis of vegetation and the activity of soil microbes. A semiempirical model for backscattering of forested land was used to acquire soil permittivity retrievals from satellite measurements using the method of least squares. The onset of soil freezing was determined from the permittivity retrievals using a simple threshold method. A five year time series of satellite observations from July 2007 to June 2012 (April 2012 for ASAR) was investigated in Sodankylä in Northern Finland. The satellite based retrievals were compared against in situ measurements of soil permittivity, soil temperature, soil frost and snow depth. According to the results the satellite permittivity retrievals correlate with each other, but not with in situ permittivity measurements. ASCAT retrieval shows some correlation with in situ temperature measurements, which could impair its correlation with in situ permittivity. The explanation for this phenomenon needs further research. Comparison of soil freezing onset dates from satellite retrievals with in situ soil temperature and soil frost measurements showed quite good agreement for most years, and did not seem to be affected by first snowfall, even though the permittivity retrievals appeared to react in a similar way to snow cover and soil freezing. This indicates that with better calibration of the permittivity threshold limit this method could be used for soil freeze detection. Auxiliary information about air temperature and snow cover could also be used to filter out possible false estimates before freezing and after the snow cover starts to affect the satellite retrievals.

(2016)Magnetic field has a central role in many dynamical phenomena in the solar corona, and the accurate determination of the coronal magnetic field holds the key to solving a whole range of open research problems in solar physics. In particular, realistic estimates for the magnetic structure of Coronal Mass Ejections (CMEs) enable better understanding of the initiation mechanisms of these eruptions as well as more accurate forecasts of their space weather effects. Due to the lack of direct measurements of the coronal magnetic field the best way to study the field evolution is to use datadriven modelling, in which routinely available photospheric remote sensing measurements are used as a boundary condition. Magnetofrictional method (MFM) stands out from the variety of existing modelling approaches as a particularly promising method. The approach is computationally inexpensive but still has sufficient physical accuracy. The databased input to the MFM is the photospheric electric field as the photospheric boundary condition. The determination of the photospheric electric field is a challenging inversion problem, in which the electric field is deduced from the available photospheric magnetic field and plasma velocity measurements. This thesis presents and discusses the stateoftheart electric field inversion methods and the properties of the currently available photospheric measurements. The central outcome of the thesis project is the development and testing of a novel ELECTRICIT software toolkit that processes the photospheric magnetic field data and uses it to invert the photospheric electric field. The main motivation for the toolkit is the coronal modelling using MFM, but the processed magnetic field and electric field data products of the toolkit are usable also in other applications such as forcefree extrapolations or highresolution studies of photospheric evolution. This thesis presents the current state of the ELECTRICIT toolkit as well as the optimization and first tests of its functionality. The tests show that the toolkit can already in its current state produce photospheric electric field estimates to a reasonable accuracy, despite the fact that some of the stateoftheart electric field inversion methods are yet to be implemented in the toolkit. Moreover, the optimal values of the free parameters in the currently implemented inversion methods are shown to be physically justifiable. The electric field inversions of the toolkit are also used to study other questions. It is shown that the large noise levels of the vector magnetograms in the quiet Sun cause the inverted electric field to be noisedominated, and thus the magnetic field data from this region should not be considered in the inversion. Another aspect that is studied is the electric field inversion based only on lineofsight (LOS) magnetograms, which is a considerable option due to much shorter cadence and better availability of the LOS data. The tests show that the inversions based on the LOS data have large errors when compared to the vector data based inversions. However, the results are shown to have reasonable consistency in the horizontal components of the electric field, when the region of interest is near the centre of the solar disk.

(2018)Firstorder phase transitions in the electroweak sector are an active subject of research as they contain ingredients for baryon number violation and gravitationalwave production. The electroweak phase transition in the Standard Model (SM) is of a crossover type, but firstorder transitions are possible in scalar extensions of the SM, provided that interactions of the Higgs boson with the new particles are sufficiently strong. If such particles exist, they are expected to have observable signatures in future collider experiments. Conversely, studying the electroweak transition in theories beyond the SM can bring new insight on the cosmological implications of these models. Reliable estimates of the properties of the transition require nonperturbative approaches to quantum field theory due to infrared problems plaguing perturbative calculations at high temperatures. We discuss threedimensional effective theories that are suitable for lattice simulations of the transition. These theories are constructed perturbatively by factorizing correlation functions so that contributions from light field modes driving the phase transition can be identified. Resummation of infrared divergences is naturally carried out in the construction procedure, and simulating the resulting effective theory on the lattice allows for a nonperturbative phasetransition study that is also free of infrared problems. Dimensionallyreduced theories can thus be used to probe the conditions under which perturbative treatments of the electroweak phase transition are valid. We apply the method to the SM augmented with a real $\text{SU}(2)$ triplet scalar and provide a detailed description of dimensional reduction of this model. Regions of a firstorder transition in the parameter space are identified in the heavy triplet limit by the use of an effective theory for which lattice results are known. We provide a rough estimate for the accuracy of our results by considering higherorder operators that have been omitted from the effective theory and discuss future prospects for the threedimensional approach.

(2018)All aerobic organisms require oxygen, which is taken into the lungs from the outside air during inhalation. From the lungs it travels all the way to the alveoli. The lung surfactant inside the alveoli consists of roughly 90% lipids and 10% proteins. Its primary functions are the reduction of the surface tension of the fluid inside the alveoli and its role as a part of the innate immune defense. The four most abundant proteins in the lung surfactant are called SPA, SPB, SPC, and SPD. The hydrophobic surfactant protein C (or SPC) is the smallest of the four. It has a primarily αhelical structure with two palmitoylated cysteines attached to the Nterminal, helping SPC to be bound to the surfactant membranes more tightly. The primary functions of SPC include the transfer of lipids from lipid monolayers to multilayered structures, the enhancement of the adsorption of surface active molecules into the airliquid interface, and the maintenance of the integrity of the multilayered structure. Lack of SPC is known to lead to severe chronic respiratory pathologies. A potential dimerization motif has been suggested to be located near the Cterminus of SPC. The purpose of this project was to study the possible dimerization of SPC using the tools of molecular dynamics simulations. In this method Newton's second law is used to calculate the time evolution of the system. The resulted trajectory describes how the positions and velocities of the particles in the system change with time. Both coarsegrained (Martini force field) and atomistic (OPLS force field) models were used in the project. Dimerization was found to occur in coarsegrained simulations of 20 SPCs embedded into a bilayer: both aggregation and dissociation of the proteins were observed during a period of 1μs. Excessive aggregation of membrane proteins is known to be a problem when using the Martini force field. However, the dimers in the simulations were not irreversible, which indicates that the usage of the Martini force field was rather well justified. The dimerization motif found in the simulations is largely consistent with the one suggested by experiment. The dimers were also studied through atomistic simulations based on the finegrained structures of coarsegrained simulations, and the atomistic simulations indicated the dimers to be stable. Altogether, the simulation results are in favor of the view that SPC exists in a dimeric form. The function of the dimer structure remains to be clarified in future studies.

(2013)Porous structures, such as foams make excellent thermal insulators. This happens because thermal transfer by conduction is hindered by the voids in the material. However, heat can still radiate through the material or just past the voids. Due to StefanBoltzmann law, heat transfer by radiation can be especially significant for large temperatures, and it follows that thermal transfer models that account for radiation may be necessary in some cases. Several existing models for radiative thermal transfer in porous materials, such as continuum models and Monte Carlo, have been used in the past. What many of these models tend to have in common, is that they are highly specific to the systems they were originally made for and require some rather limiting approximations. A more general method which would only require knowing the material and the geometry of the system would be useful. One candidate for such a method, discrete dipole approximation for periodic structures, was tested. In the discrete dipole approximation a structure is discretized into a collection of polarizable points, or dipoles and an incoming electromagnetic planewave is set to polarize it. This has the benefits that it accurately takes into account the target geometry and possible near field effects. It was found that this method is limited for high wavelength, by computation time and for small wavelengths by errors. The cause of the errors for small wavelengths was not entirely caused by the discretization and remains not fully understood.

(2015)The presence of dislocations in metal crystals accounts for the plasticity of metals. These dislocations do not nucleate spontaneously, but require favorable conditions. These conditions include, but are not limited to, a high temperature, external stress, and an interface such as a grain boundary or a surface. The slip of dislocations leads to steps forming on the surface, as atomic planes are displaced along a line. If a void is placed very near a surface, the possibility of forming a dislocation platelet exists. The skip of the dislocation platelet would displace the surface atoms within a closed line. Repeating such a process may form a small protrusion on the surface. In this thesis, the mechanism with which a dislocations displace the surface atoms within a closed loop is studied by using molecular dynamics (MD) simulations of copper. A spherical void is placed within the lattice, and the lattice is then subjected to an external stress. The dislocation reactions which lead to the formation of the dislocation platelet after the initial dislocation nucleation on the void is studied by running MD simulations of a void with the radius of 3 nm under tensile stress. Since the dislocations are thermally activated, the simulation proceeded differently for each run. We describe the different ways the dislocations nucleate, and the dislocation reactions that occur when they intersect to form the platelet. The activation energy of this process was studied by simulating half of a much larger void, with a radius of 8 nm, in order to obtain a more realistic nucleation environment. Formulas connecting the observable and controllable simulation variables with the energies of the nucleation are derived. The activation energies are then calculated and compared with values from literature.
Now showing items 120 of 96