Two aspects of applied nuclear physics have been studied in this thesis; Monte Carlo simulations for high-sensitivity monitoring of radioactive xenon and high-accuracy neutron-proton scattering measurements for neutron physics applications and fundamental physics. The Monte Carlo simulations have been performed for two systems for detection of radioactive xenon, using the MCNP code. These systems, designed for monitoring of violations of the Comprehensive Nuclear-Test-Ban Treaty, are based on coincident detection of electrons and gamma rays, emitted in beta decay of xenon nuclides produced in nuclear weapons explosions. In general, the simulations describe test data well, and the deviations from experimental data are understood. The neutron-proton scattering measurements have been performed by measuring the differential np scattering cross section at 96 MeV in the angular range θc.m.= 20° – 76°. Together with an earlier data set at the same energy, covering the angles θc.m.= 74° – 180°, a new data set has been formed in the angular range θc.m.= 20° – 180°. This extended data set has been normalised to the experimental total np cross section, resulting in a renormalisation of the earlier data of 0.7 %, which is well within the stated normalisation uncertainty for that experiment. The results on forward np scattering are in reasonable agreement with theory models and partial wave analyses and have been compared with data from the literature.

We have studied the critical properties of three-dimensional U(1)-symmetric lattice gauge theories. The models apply to various physical systems such as insulating phases of strongly correlated electron systems as well as superconducting and superfluid states of liquid metallic hydrogen under extreme pressures. This thesis contains an introductory part and a collection of research papers of which seven are published works and one is submitted for publication. Paper I: Critical properties of the 2+1-dimensional compact abelian Higgs model with gauge charge q=2 are studied. We introduce a novel method of computing the third moment M3 of the action which allows us to extract correlation length and specific heat critical exponents ν and α without invoking hyperscaling. Finite-size scaling analysis of M3 yields the ratio (1+α)/ν and 1/ν separately. We find that α and ν vary along the critical line of the theory, which however exhibits a remarkable resilience of Z2 criticality. We conclude that the model is a fixed-line theory, which we propose to characterize the zero temperature quantum phase transition from a Mott-Hubbard insulator to a charge fractionalized insulator in two spatial dimensions. Paper II: Large scale Monte Carlo simulations are employed to study phase transitions in the three-dimensional compact abelian Higgs model in adjoint representations of the matter field, labeled by an integer q, for q=2,3,4,5. We also study various limiting cases of the model, such as the Zq lattice gauge theory, dual to the 3DZq spin model, and the 3D xy spin model which is dual to the Zq lattice gauge theory in the limit q → ∞. In addition, for benchmark purposes, we study the 2D square lattice 8-vertex model, which is exactly solvable and features non-universal critical exponents. The critical exponents α and ν are calculated from finite size scaling of the third moment of the action, and the method is tested thoroughly on models with known values for these exponents. We have found that for q=3, the three-dimensional compact abelian Higgs model exhibits a second order phase transition line which joins a first order phase transition line at a tricritical point. The results for q=2 in Paper I are reported with a higher lever of detail. Paper III: This paper is based on a talk by F. S. Nogueira in the Aachen HEP 2003 conference where a review of the results for the compact abelian Higgs model from Paper I and Paper II was presented, as well as the results for the q=1 case studied by F. S. Nogueira, H. Kleinert and A. Sudbø. Paper IV: We study the effects of a Chern-Simons (CS) term in the phase structure of two different abelian gauge theories in three dimensions. By duality transformations we show how the compact U(1) gauge theory with a CS term for certain values of the CS coupling can be written as a gas of vortex loops interacting through steric repulsion. This theory is known to exhibit a phase transition governed by proliferation of vortex loops. We also employ Monte Carlo simulations to study the non-compact U(1) abelian Higgs model with a CS term. Finite size scaling of the third moment of the action yields critical exponents α and ν that vary continuously with the strength of the CS term, and a comparison with available analytical results is made. Paper V: The critical properties of N-component Ginzburg-Landau theory are studied in d=2+1 dimensions. The model is dualized to a theory of N vortex fields interacting through a Coulomb and a screened potential. The model with N=2 shows two anomalies in the specific heat. From Monte Carlo simulations we calculate the critical exponents α and ν and the mass of the gauge field. We conclude that one anomaly corresponds to an inverted 3D xy fixed point, while the other corresponds to a 3D xy fixed point. There are N fixed points, namely one corresponding to an inverted 3D xy fixed point, and N-1corresponding to neutral 3D xy fixed points. Applications are briefly discussed. Paper VI: The phase diagram and critical properties of the N-component London superconductor are studied both analytically and through large-scale Monte-Carlo simulations in d=2+1 dimensions. The model with different bare phase stiffnesses for each flavor is a model of superconductivity which should arise out of metallic phases of light atoms under extreme pressure. A projected mixture of electronic and protonic condensates in liquid metallic hydrogen under extreme pressure is the simplest example, corresponding to N=2 with individually conserved matter fields. We compute critical exponents α and ν for N=2 and N=3. The results from Paper V are presented at a higher level of detail. For the arbitrary N case, there are N fixed points,namely one charged inverted 3D xy fixed point, and N-1 neutral 3D xy fixed points. We explicitly identify one charged vortex mode and N-1 neutral vortex modes. The model for N=2 and equal bare phase stiffnesses corresponds to a field theoretical description of an easy-plane quantum antiferromagnet. In this case, the critical exponents are computed and found to be non 3D xy values. Furthermore, we study the model in an external magnetic field, and find a novel feature, namely N-1 superfluid phases arising out of N charged condensates. In particular, for N=2 we point out the possibility of two novel types of field-induced phase transitions in ordered quantum fluids: i) A phase transition from a superconductor to a superfluid or vice versa, driven by tuning an external magnetic field. This identifies the superconducting phase of liquid metallic hydrogen as a novel quantum fluid. ii) A phase transition corresponding to a quantum fluid analogue of sublattice melting, where a composite field-induced Abrikosov vortex lattice is decomposed and disorders the phases of the constituent condensate with lowest bare phase stiffness. Both transitions belong to the 3D xy universality class. Paper VII: We consider the vortex superconductor with two individually conserved condensates in a finite magnetic field. The ground state is a lattice of cocentered vortices in both order parameters. We find two novel phase transitions when temperature is increased at fixed magnetic field. i) A "vortex sublattice melting" transition where vortices in the field with lowest phase stiffness ("light vortices") loose cocentricity with the vortices with large phase stiffness ("heavy vortices"), entering a liquid state (the structure factor of the light vortex sublattice vanishes continuously.) This transition is in the 3D xy universality class. ii) A first order melting transition of the lattice of heavy vortices in a liquid of light vortices. Paper VIII: We report on large-scale Monte Carlo simulations of a novel type of a vortex matter phase transition which should take place in a three dimensional two-component superconductor. We identify the regime where first, at a certain temperature a field-induced lattice of co-centered vortices of both order parameters melts, causing the system to loose superconductivity. In this state the two-gap system retains a broken composite symmetry and we observe that at a higher temperature it undergoes an extra phase transition where the disordered composite one-flux-quantum vortex lines are "ionized" into a "plasma" of constituent fractional flux vortex lines in individual order parameters. This is the hallmark of the superconductor-to-superfluid-to-normal fluid phase transitions projected to occur in e.g. liquid metallic hydrogen.

Dans cette thèse, nous abordons le sujet d'optimisation des méthodes utlisées pour l'évaluation de performance des codes correcteurs d'erreurs. La durée d'une simula- tion Monte Carlo pour estimer le taux d'erreurs dans un système de communication augmente exponentiellement avec l'accroissement du Rapport Signal sur Bruit (RSB). Importance Sampling (IS) est une des techniques qui permettent à réduire le temps de ces simulations. Dans ce travail, on a étudié et mis en oeuvre une version avancée d'IS, appelé Adaptive Importance Sampling (AIS), pour l'évaluation efficace des codes cor- recteurs d'erreurs aux taux d'erreur très bas. D'abord, nous présentons les inspirations et motivations en analysant différentes approches actuellement mises en pratique. On s'intéresse plus particulièrement aux méthodes inspirées de la physique statistique. Ensuite, basé sur notre analyse qualita- tive, nous présentons une méthode optimisée, appelé la méthode de Fast Flat Histogram (FFH) qui est intrinsèquement très générique. La méthode emploie l'algorithme de Wang-Landau, l'algorithme de Metropolis-Hastings et les chaines de Markov. Elle fonctionne dans le cadre de l'AIS et nous donne un gain de simulation satisfaisant. Différents paramètres sont utilisés pour assurer une précision statistique suffisante. L'extension vers d'autres types de codes correcteurs d'erreurs est directe. Nous présentons les résultats pour les codes LDPC et turbocodes ayant dif- férentes tailles et différents rendements. Par conséquent, nous montrons que la méthode FFH est générique et valable pour une large gamme des rendements, tailles et structures. De plus, nous montrons que la méthode FFH est un outil puissant pour trouver des pseudocodewords dans la région de RSB élévé en appliquant l'algorithme de décodage Belief Propagation aux codes LDPC.

Nanostructured materials are the next generation of high-performance materials, harnessing the novel properties of their nanosized constituents. The controlled assembly of nanosized particles and the design of the optimal nanostructure require a detailed understanding of particle interactions and robust methods to tune them. This thesis describes innovative approaches to these challenges, relating to the determination of Hamaker constants for iron oxide nanoparticles, the packaging of nanopowders into redispersible granules, the tuning of the wetting behavior of nanocrystals and the simulation of collective magnetic properties in arrays of superparamagnetic nanoparticles. The non-retarded Hamaker constants for iron oxides have been calculated from their optical properties based on Lifshitz theory. The results show that the magnitude of vdW interactions in non-polar solvents has previously been overestimated up to 10 times. Our calculations support the experimental observations that oleate-capped nanoparticles smaller than 15 nm in diameter can indeed form colloidally-stable dispersions in hydrocarbons. In addition, a simple procedure has been devised to remove the oleate-capping on the iron oxide nanoparticles, enabling their use in fluorometric assays for water remediation, with a sensitivity more than 100 times below the critical micelle concentration for non-ionic surfactants. Nanosized particles are inherently more difficult to handle in the dry state than larger micron-sized powders, e.g. because of poor flowability, agglomeration and potential toxicity. The rheology of concentrated slurries of TiO2 powder was optimized by the addition of sodium polyacrylate, and spray-dried into fully redispersible micron-sized granules. The polymer was embedded into the granules, where it could serve as a re-dispersing aid. Monte Carlo (MC) simulations have been applied to the collective magnetic behavior of nanoparticle arrays of various thicknesses. The decrease in magnetic susceptibility with the thickness observed experimentally was reproduced by the simulations. Ferromagnetic couplings in the arrays are enhanced by the finite thickness, and decrease in strength with increasing thickness. The simulations indicate the formation of vortex states with increasing thickness, along with a change in their orientation, which becomes more and more isotropic as the thickness increases. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.</p>

To estimate the propagation of light in tissue-like optical phantoms (fat emulsions), this thesis utilized the diffuse reflectance spectroscopy in combination with Monte Carlo simulations. A method for determining the two-parametric Gegenbauer-kernal phase function was utilized in order to accurately describe the diffuse reflectance from poly-dispersive scattering optical phantoms with small source-detector separations. The method includes the spectral collimated transmission, spatially resolved diffuse reflectance spectra (SRDR) and the inverse technique of matching spectra from Monte Carlo simulations to those measured. An absolute calibration method using polystyrene micro-spheres was utilized to estimate the relation between simulated and measured SRDR intensities. The phase function parameters were comparable with previous studies and were able to model measured spectra with good accuracy. Significant differences between the phase functions for homogenized milk and the nutritive fat emulsions were found.

The impedance response of silicon carbide whisker-alumina composites is investigated utilizing novel stereological techniques along with a microstructural simulation. The stereological techniques developed allow for a measurement of the trivariate length, radius and orientation distribution of whiskers in the composite from measurements made on two-dimensional sectioning planes. The measured distributions are then utilized in a Monte Carlo simulation that predicts connectivity in the composite for a given volume fraction. It is assumed in the simulation that connectivity factors dominate the electrical response, not interfacial phenomena. The results of the simulation are compared with impedance spectra taken from real samples, and conclusions are drawn regarding the nature of the impedance response.

Product tolerancing decisions made in product design have a significant effect on manufacturing environmental and cost performances by strongly influencing both the selection and operation of processing machinery. These decisions however are typically made without quantitative knowledge of their effects in manufacturing. With estimates of environmental and cost performances of manufacturing processes required to achieve specific part designs earlier in the product design cycle, designers may make more informed, and potentially better, design decisions with respect to manufacturing environmental and cost performance goals. In this thesis a method for quantifiably relating product tolerancing decisions to environmental and cost performances in manufacturing in order to provide decision support for cost and environmentally conscious design for manufacturing is developed. The method is instantiated as an Excel-based tool and exercised by two illustrative examples of increasing complexity, as well as a study of the manufacture of automotive transmission pinion gears with differing tolerance requirements. Uncertainty analysis is performed through the use of @RISK software; the uncertainty of parameters associated with manufacturing operations and machinery is captured through the use of probability density functions and Monte Carlo simulation is performed. Simulation results provide insight into the uncertainty of performance estimates and the risks associated with ensuing decision making. This method may be useful to product designers, as well as process planners, to support decision making efforts related to cost and environmental consciousness in the manufacturing phase of the product life cycle.

Frequency domain photon migration (FDPM) measurements are used to study the properties of dense colloidal dispersions with hard sphere and electrostatic interactions, which are otherwise difficult to analyze due to multiple scattering effects. Hard sphere interactions were studied using a theoretical model based upon a polydisperse mixture of particles using the hard sphere Percus Yevick theory. The particle size distribution and volume fraction were recovered by solving a non linear inverse problem using genetic algorithms. The mean sizes of the particles of 144 and 223 nm diameter were recovered within an error range of 0-15.53% of the mean diameters determined from dynamic light scattering measurements. The volume fraction was recovered within an error range of 0-24% of the experimentally determined volume fractions. At ionic strengths varying between 0.5 and 4 mM, multiple wavelength (660, 685, 785 and 828 nm) FDPM measurements of isotropic scattering coefficients were made of 144 and 223 nm diameter, monodisperse dispersions varying between 15% - 22% volume fraction, as well as of bidisperse mixtures of 144 and 223 nm diameter latex particles in 1:3, 1:1 and 3:1 mixtures varying between volume fractions of 15% - 24%. Structure factor models with Yukawa potential were computed by Monte Carlo (MC) simulations and numerical solution of the coupled Ornstein Zernike equations. In monodisperse dispersions of particle diameter 144 nm the isotropic scattering coefficient versus ionic strength show an increase with increasing ionic strength consistent with model predictions, whereas there was a reversal of trends and fluctuations for the particle diameter of 223 nm. In bidisperse mixtures for the case of maximum number of smaller particles, the isotropic scattering coefficient increased with increasing ionic strength and the trends were in conformity with MC simulations of binary Yukawa potential models. As the number of larger diameter particles increased in the dispersions, the isotropic scattering coefficients depicted fluctuations, and no match was found between the models and measurements for a number ratio of 1:3. The research lays the foundation for the determination of particle size distribution, volume fractions and an estimate of effective charge for high density of particles.

Αντικείμενο αυτής της διδακτορικής διατριβής είναι η μελέτη της δυναμικής σύμμορφης ακτινοθεραπείας με χρήση φυσικών διαμορφωτών δέσμης. Η δυναμική διαμόρφωση δέσμης επιτυγχάνεται με δύο ειδών φυσικούς διαμορφωτές δέσμης: τους προστατευτές, που είναι αντίγραφα σε σμίκρυνση του υπό προστασία οργάνου, κατασκευασμένα από υλικό μεγάλου ατομικού αριθμού, και διατηρούν διεύθυνση παράλληλη σε αυτό κατά την περιστροφή, και τους διαμορφωτές, που τοποθετούνται και στις δύο πλευρές του προστατευτή εξασφαλίζοντας ομοιόμορφη δόση στην περιοχή του όγκου. Για τον σκοπό αυτό αναπτύχθηκε μαθηματικό μοντέλο που περιγράφει τις βέλτιστες διαστάσεις και τις ακριβείς θέσεις των διαμορφωτών κατά τη διάρκεια της περιστροφής της ακτινογραφικής κεφαλής με βάση τις αρχές που διέπουν την κίνησή τους. Παράλληλα αναπτύχθηκε ένα εργαλείο λογισμικού, που ενσωματώνει το μαθηματικό υπόβαθρο και διευκολύνει την εισαγωγή των παραμέτρων θέσης και σύστασης των διαμορφωτών δέσμης. Το εργαλείο λογισμικού ενσωματώθηκε στον προσομοιωτή θεραπείας ακτινοβολίας Monte Carlo (MCRTS), που χρησιμοποιήθηκε για την προσομοίωση της διάδοσης της ακτινοβολίας στη γεωμετρία του σχεδιασμένου συστήματος. Πραγματοποιήθηκαν μελέτες προσομοίωσης για την αποτελεσματικότητα της φυσικής διαμόρφωσης δέσμης στην περίπτωση όγκων στη περιοχή της κεφαλής και του αυχένα, βασισμένες σε γεωμετρική περιγραφή είτε με αναλυτικά αντικείμενα, είτε με στοιχειώδεις όγκους. Μελετήθηκε επίσης η επίδραση των διαμορφωτών στην τροποποίηση της κατανομής δόσης στις περιοχές που περιβάλλουν την προστατευμένη περιοχή. Οι μελέτες αυτές έδειξαν ότι η περιστροφική θεραπεία με διαμόρφωση δέσμης προσφέρει την επαρκή προστασία και ομοιόμορφη κατανομή δόσεων έξω από την προστατευμένη περιοχή. Προσομοιώσεις που χρησιμοποίησαν διαμορφωτές διαφορετικών υλικών οδήγησαν σε παρόμοιες κατανομές δόσεων. Επιπλέον μελετήθηκε η επίδραση των παραμέτρων του προστατευτή στη κατανομή δόσης στην περιστροφική θεραπεία. Εξετάστηκε η επίδραση μιας σειράς υλικών, που συνήθως χρησιμοποιούνται για προστασία στην ακτινοθεραπεία, καθώς επίσης και η επίδραση μερικών νέων μέταλλο-πολυμερών σύνθετων υλικών. Τα μέταλλο-πολυμερή σύνθετα παρέχουν προστασία στα ζωτικής σημασίας όργανα, συγκρίσιμη με αυτή του μολύβδου, εάν η πυκνότητα τους είναι υψηλή. Η τεχνική που μελετήθηκε έδωσε πολύ ικανοποιητικά αποτελέσματα από άποψη κατανομής δόσης και σχέσης κόστους-αποτελέσματος. / This doctoral thesis addresses dynamic intensity modulation by means of physical beam modifiers. The principles of gravity-oriented devices were generalized and extended, preserving principles of a beam shaping, but introducing motor-driven “patient-oriented” beam modifying devices. Beam modifying devices were divided in two categories: protectors and shapers. The protectors are diminished copies of the Organs At Risk (OARs) and stay parallel to them during gantry rotation, keeping them in the attenuated field for every gantry angle. Shapers are placed at the both sides of the protector to ensure uniform dose in the Planning Target Volume (PTV). Mathematical formalism for calculations of the dimensions and the initial coordinates of the beam modifying devices was developed as well as the laws of their motion during gantry rotation were derived. A software tool, incorporating the mathematical background, with user interface to facilitate the introduction of the input parameters was created. The software module was subsequently integrated into a Monte Carlo Radiation Therapy Simulator (MCRTS), used to simulate particle transport through the designed system. Simulation studies of field shaping in rotational therapy by means of beam modifying devices were carried out. Dose distributions in solid-geometry and voxel-based neck models were evaluated. Furthermore, the effectiveness of the shapers to modify the dose distribution outside the protected area was studied. The results of simulation studies showed that rotational therapy with beam modifying devices offers adequate protection of the OAR and a uniform dose distribution outside the protected region. Studies using shapers of different materials were also carried out and resulted in similar dose distributions. Additionally, the effect of protector’s parameters on the dose distribution in rotational therapy was studied in the thesis. A range of materials, consisting of commonly used for protection in radiotherapy, as well as by some new metal-polymer composites was under investigation. The metal-polymer composites can rival the lead in the protection of vital organs if the density provided is high. The presented technique has showed promising results in terms of conformal dose delivery and can be a preferred choice in radiotherapy departments due to comprehensive and adequate protection of the OAR and uniform dose in PTV ensured as well as of its cost effectiveness.

Τα αυτό -οργανούμενα μονοστρωματικά συστήματα (self-assembled monolayers, SAMs) παρουσιάζουν μεγάλο τεχνολογικό και βιομηχανικό ενδιαφέρον καθώς προσφέρουν μοναδική ευκαιρία για την κατανόηση των διεπιφανειακών φαινομένων και των διεργασιών που σχετίζονται με αυτά. Ο έλεγχος των ιδιοτήτων διαβροχής και λίπανσης της επιφάνειας , η επιλεκτική ρόφηση διαφόρων ειδών μορίων (π .χ ., μεγάλων βιολογικών μορίων) για το σχηματισμό επιπρόσθετου μονοστρώματος προς μία προεπιλεγμένη δομή (π.χ ., με συγκεκριμένο μοριακό προσανατολισμό), ο σχεδιασμός βιοαισθητήρων αλλά και άλλα παραδείγματα αποτελούν μερικές μόνο από τις πιο διαδεδομένες εφαρμογές των SAMs. Στην παρούσα εργασία εστιάσαμε στο πιο διαδεδομένο σύστημα SAM, αυτό που δημιουργείται κατά τη ρόφηση μορίων αλκανοθειολών σε επιφάνεια χρυσού (R-SH/ Au(111)). Πιο συγκεκριμένα διερευνήσαμε τις δομικές ιδιότητες καθώς και τις ιδιότητες διαμόρφωσης του σχηματιζόμενου μονοστρώματος με τη βοήθεια ενός καινούργιου αλγορίθμου Monte Carlo (MC) που σχεδιάσαμε στο εργαστήριο, βασισμένου σ’ ένα ιδιαίτερα αποδοτικό μίγμα τόσο απλών όσο και πιο σύνθετων (συχνά μη φυσικών) κινήσεων για τη δειγματοληψία απεικονίσεων του συστήματος. Η καινοτομία του αλγόριθμου MC συνίσταται στο ότι, ανεξάρτητα από την αρχική απεικόνιση του συστήματος, έχει την ικανότητα να οδηγεί αποτελεσματικά όλα τα μόρια της αλκανοθειόλης επάνω στο υπόστρωμα του χρυσού με αποτέλεσμα στο τέλος της προσομοίωσης αυτό να χαρακτηρίζεται από 100% επιφανειακή κάλυψη. Κατά τον τρόπο αυτό παρακάμπτεται ένας σημαντικός περιορισμός των προηγούμενων μεθόδων , οι οποίες ουσιαστικά προ-υπέθεταν την αρχική απεικόνιση του συστήματος (στη βάση πειραματικών δεδομένων). Επιπλέον, λαμβάνοντας υπόψη ένα εκτεταμένο σύνολο αντιγράφων του συστήματος καθένα από τα οποία προσομοιώνεται σε μία διαφορετική τιμή της διαμέτρου van der Waals των ατόμων θείου, σss, και επιχειρώντας ανταλλαγές απεικονίσεων μεταξύ συστημάτων με παρακείμενες τιμές σss, ο νέος αλγόριθμος μας επέτρεψε να προσομοιώσουμε αποτελεσματικά πρότυπα συστήματα R-SH/Au(111) για ένα φάσμα τιμών της παραμέτρου σss από 4.25 Å που αντιστοιχεί στο μοριακό μοντέλο των Hautman-Klein [J. Chem. Phys., 1988; 1989] έως 4.97 Å που αντιστοιχεί στο μοριακό μοντέλο των Siepmann-McDonald [Langmuir, 1993]. Η εφαρμογή του αλγορίθμου MC επεκτάθηκε ακολούθως σε συστήματα αλκανοθειολών ροφημένων σε διαφορετικά μεταλλικά υποστρώματα, με σκοπό τη μελέτη της επίδρασης του είδους της μεταλλικής επιφάνειας στις δομικές ιδιότητες των συστημάτων SAMs. Προς την κατεύθυνση αυτή, αρχικά εκτελέστηκαν κβαντομηχανικοί υπολογισμοί ( ab initio calculations) για ένα μόριο μεθανοθειόλης ροφημένου σε επιφάνεια χρυσού, αργύρου ή πλατίνας και τα αποτελέσματα χρησιμοποιήθηκαν για την εξαγωγή ενός κλασσικού δυναμικού για την περιγραφή των αλληλεπιδράσεων μεταξύ θείου -μετάλλου. Με το δυναμικό αυτό διεξήχθησαν στη συνέχεια ατομιστικές προσομοιώσεις MC για διάφορα μοριακά μήκη συστημάτων SAMs R-SH και στη συνέχεια έγινε ανάλυση των δεδομένων, με έμφαση στην εξάρτηση των δομικών ιδιοτήτων του σχηματιζόμενου φιλμ (μοριακός προσανατολισμός, διαμόρφωση αλυσίδων και στατιστική των ατελειών gauche) από τη φύση του μεταλλικού υποστρώματος. Στο τελευταίο στάδιο της διατριβής εστιάσαμε στη μελέτη της θερμοκρασίας υαλώδους μετάπτωσης Tg ισότροπων συστημάτων αλλά και λεπτών υμενίων πολυαιθυλενίου (PE) με τις αλυσίδες εμφυτευμένες σε σκληρή, ενθαλπικά ουδέτερη επιφάνεια και σχετικά μεγάλη πυκνότητα εμφύτευσης. Για το λόγο αυτό επεκτάθηκε η μεθοδολογία προσομοίωσης MC σε χαμηλές θερμοκρασίες (κοντά στο ή ακόμα και χαμηλότερα από το σημείο Tg) χρησιμοποιώντας την πολύ δραστική κίνηση MC αναγεφύρωσης άκρων (end-bridging, EB). Τα δεδομένα της προσομοίωσης για την εξάρτηση της πυκνότητας και της ενθαλπίας από την θερμοκρασία χρησιμοποιήθηκαν για τον υπολογισμό της θερμοκρασίας υαλώδους μετάπτωσης, με το αποτέλεσμα να συμφωνεί σχεδόν επακριβώς με την αντίστοιχη πειραματική τιμή για ημικρυσταλλικό πολυαιθυλένιο στο όριο μηδενικού βαθμού κρυσταλλικότητας (προβλεπόμενη τιμή Tg μεταξύ 220 και 240 Κ). / Self-assembled monolayers (SAMs) find numerous applications in a variety of fields: in the production of thin films from organic materials, in optics and electronics, as means for controlling the hydrophobic or hydrophilic behavior of a surface, as coatings for the protection of surfaces from corrosion, in molecular recognition, and more recently even as biosensors. In an effort to understand the mechanisms and interactions controlling chain organization and packing in these systems and how these affect their macroscopic properties, the present thesis has focused on the development of a Monte Carlo (MC) algorithm, built around a set of simpler but also more complex (sometimes non-physical) moves, for the atomistic simulation of the SAM structures formed by the adsorption of short alkanethiol molecules on a Au(111) surface. The innovation of the MC algorithm is that it is capable of efficiently driving all alkanethiol molecules to the Au(111), thereby leading to full surface coverage, irrespective of the initial setup of the system. This circumvents a significant limitation of previous methods in which the simulation typically starts from optimally packed structures on the substrate that are close to thermal equilibrium. Further, by considering an extended ensemble of configurations each one of which corresponds to a different value of the sulphur-sulphur repulsive core potential, σ ss , and by allowing for configurations to swap between different σ ss values, the new algorithm can adequately simulate model R-SH/ Au(111) systems for values of σ ss ranging from 4.25 Å corresponding to the Hautman-Klein molecular model [J. Chem. Phys., 1988; 1989] to 4.97 Å corresponding to the Siepmann-McDonald model [Langmuir, 1993]. A thorough investigation of the variation of molecular organization and ordering on the Au(111)substrate with chain length is presented. In a parallel study, the MC method was extended to alkanethiol SAM systems on different metal surfaces. This has allowed us to perform a detailed investigation of the substrate’s effect on the structure and conformation of the above systems through atomistic MC simulations based on a first-principles density functional modeling of the sulphur-metal interaction. Ab initio calculations on a methanethiol molecule adsorbed on gold, silver and platinum surfaces were conducted and the data obtained were used to develop an accurate classical force field which served as an input to the new MC algorithm. Emphasis was given primarily to the study of the effect of the substrate on the structural properties of the simulated R-SH SAM systems, like molecular orientation, molecular conformation, and statistics of gauche defects. In the last part of this thesis, and in an attempt to investigate the phenomenon of glass transition ( Tg ), the MC algorithm was employed in simulations with a less complex, than the SAM structures, system, that of amorphous polyethylene (PE). Two sets of simulations were executed: one with a bulk, isotropic sample, and the other with a thin film in which all PE chains were grafted on a hard surface on the one side and exposed to vacuum on the other. In all cases, the simulations were carried out for very long times in order for the autocorrelation function of the chain end-to-end vector to drop practically to zero. For both systems, the value of the glass transition temperature Tg was extracted using volumetric and enhtalpic simulation data and it was found to be between 220 and 240K, i.e., in remarkable agreement with measured data for semicrystalline PE in the limit of zero crystallinity. Additional results about the temperature dependence of the conformational (e.g., the equilibrium mean-square chain end-to-end distance) and structural (e.g., the intermolecular pair distribution function) properties in the two PE systems were also obtained and discussed in detail.