Tesis/Trabajos de Grado

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Incluye documentos como: monografías, reportes, proyectos, prácticas, informes, entre otros; elaborados como requisito de grado para programas de pregrado y posgrado en la Universidad de los Andes.

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  • PublicaciónAcceso abierto
    Variable stars' light curve detection and classification using supervised machine learning
    (Universidad de los Andes, 2024-01-31) Elizabethson, Astaroth
    We present two applications of supervised machine learning aimed at addressing the light curve classification problem in stellar variability. Our main goal is to streamline the analysis of light curves obtained from large-scale photometric and multi-epoch astronomic surveys. In the first application, we conduct a variability and morphological classification study on TESS light curves for T Tauri star candidates in several regions, including Orion complex forming region, IC 348, gamma Velorum, Upper Scorpius, Corona Australis, and Perseus OB2. We introduce 11 morphological classes that link variations in brightness with potential physical or geometric phenomena in T Tauri stars. To automate the classification among these classes, we develop a supervised machine learning algorithm. Our algorithm optimizes and compares the true positive rate (recall) among k-nearest neighbors, classification trees, random forests, and support vector machines. We achieve this by characterizing light curves with features related to time, periodicity, and magnitude distribution. We train binary and multiclass classifiers and interpret the results in a way that allows our final algorithm to assign single or mixed classes. In the testing sample, the algorithm assigns mixed classes to 27% of the stars, with some stars receiving up to five simultaneous class assignments. We present a catalog of 3672 T Tauri star candidates, along with their possible period estimations, predicted morphological classes, and visually reviewed assignments. The cross validation estimated performance of the final classifiers is reported. Binary classifiers perform better than multiclass classifiers for classes with limited representation in the training sample. Support vector machines and random forest classifiers achieve better recalls. Furthermore, we provide another performance estimation of the final classifiers using the revised classes of our testing sample, indicating that this performance excels in single-classed stars, which account for approximately 75% of the testing sample. In the second application, we focus on the b278 and b279 fields of the VVV survey, conducted in the Ks infrared band. We analyze time-series data from over 60 epochs in each field to assess the performance of binary and multiclass classifiers. Our primary objective is to have these classifiers identify stellar variability and subsequently differentiate between various classes of variability, especially classical Cepheids, RR Lyrae, long-period variables, and Mira variables. Notably, the features used in this analysis are independent of a periodicity search process. This approach allows for the inclusion of variable stars that do not exhibit periodic changes in magnitude and saves the computational work of a priori period estimations over the whole initial data. We create the training dataset by extracting time-series data from the public catalog of the VVV template project. Additionally, we include time-series data from variable stars observed in the 2MASS-GC02 and Terzan10 globular clusters, and generate synthetic non-variable light curves that emulate the cadence and magnitude uncertainties of the VVV data. We conduct a comparative analysis of the F1 score of these classifiers. In the end, this research produces a catalogue of candidates for variable stars in the Galactic Bulge direction, including 266 candidates whose phased light curves are consistent with the morphology expected for their classes.
  • PublicaciónAcceso abierto
    Light-matter interactions and inhomogeneities in correlated systems: topological, superconducting, and magnetic properties
    (Universidad de los Andes, 2024-01-26) Méndez-Córdoba, Fabio Pablo Miguel
    This thesis delves into condensed-matter phenomenology, specifically addressing topological order, superconductivity, and magnetism in strongly correlated one-dimensional and quasi-one-dimensional systems. Utilizing both analytical and numerical approaches, we survey these properties in quantum models incorporating light-matter interactions and local control of parameters. We study the Kitaev and Hubbard models, both analytically and numerically, using techniques such as density matrix renormalization group. In particular, we study the topological properties of the Kitaev chain coupled to a QED cavity. This approach facilitates the addressing of the inherent challenges associated with the detection and manipulation of Majorana fermions. By studying the Kitaev chain wholly embedded in a cavity, we propose a technique to extract the system's bipartite entanglement from cavity observables. This technique presents an alternative to experimentally probing and controlling topological quantum phase transitions through the Rényi entropy. Later, by tailoring the coupling to the cavity, we demonstrate how local cavity couplings could create or displace Majorana fermion correlations depending on the geometry of the coupling. In the final part of the thesis, the focus shifts to the local control of parameters manifested as defects within the Hubbard model. Under this framework, our preliminary findings indicate that these defects can enhance superconducting correlations compared to lattices with homogeneous parameters. Additionally, we examine magnetic correlations and the total angular momentum of non-bipartite lattices. We establish that lattice structure defects can significantly change the total angular momentum depending on their lattice positioning. Our findings in this system scrutinize the close connection between magnetic correlations of the Heisenberg and Hubbard models, as the emerging ordering eludes the former. Collectively, the results of this thesis contribute to the understanding of the repercussions of light-matter interactions and defects in low-dimensional correlated systems. Particularly, it provides new insights into condensed-matter systems' topological, superconducting, and magnetic properties.
  • PublicaciónAcceso abierto
    Kronecker states: a powerful source of multipartite maximally entangled states in quantum information
    (Universidad de los Andes, 2024-01-31) González Olaya, Walther Leónardo
    In quantum information theory, maximally entangled states are essential for well-known protocols like quantum teleportation or quantum key distribution. While many of these protocols focus on bipartite entanglement, other applications, such as quantum error correction or multiparty quantum secret sharing, are based on multipartite entanglement, precisely, on the so-called locally maximally entangled (LME) multipartite states, where each part is maximally entangled with their complement. Such LME states appear naturally in the invariant subspaces of tensor products of irreducible representations of the symmetric group Sn, which we term Kronecker subspaces, given that their dimensions are the so-called Kronecker coefficients. A Kronecker subspace is a vector space of LME multipartite states that we call Kronecker states, which entangle Hilbert spaces of large dimensions. Although such states can in principle be obtained from the Clebsch-Gordan coefficients of the symmetric group, the known methods to compute these coefficients tend to be inefficient even for small values of n. An alternative quantum-information-based approach is inspired by entanglement concentration protocols, where Kronecker subspaces appear naturally in the isotypic decomposition of tensor products of copies of multipartite entangled states. In this context, closed expressions have been obtained for a limited class of Kronecker states, associated with states in the so-called multiqubit Wclass. Our aim in this thesis is to extend this approach to build bases for Kronecker subspaces associated with any multiqubit system. For developing our method we first propose a graphical construction that we call “W-state Stitching”, where multiqubit entangled states are obtained as tensor networks built from W states. Analyzing the isotypic decomposition of copies of the graph state, an analogous set of graph Kronecker states, made from W-Kronecker states, can be obtained. In particular, the graph states of generic multiqubit states can generate any Kronecker subspace completely. Using this method, we show how to build any Kronecker subspace corresponding to systems of three and four qubits. Independently of the Kronecker state construction, the W-stitching technique has proven to be a powerful method for multiqubit entanglement classification. We hope the results of this work motivate the study of applications of Kronecker states in quantum information, and serve as a starting point for a resource theory of multipartite entanglement, with bipartite states and tripartite W states as building blocks, where the asymptotic analysis is based on Kronecker states.
  • PublicaciónAcceso abierto
    Probing heavy neutral resonances with preferential coupling to third generation fermions at the LHC
    (Universidad de los Andes, 2023-12-06) Barbosa Trujillo, Diego Andrés
    In this thesis a phenomenological feasibility of searching for heavy neutral mass resonances $Z'$ at the LHC is presented. In particular, this study considers a pair of two b-quarks as final states with non-universal fermion couplings in proton-proton collisions at $\sqrt{s}$=13 and 14 TeV. In the second part, we also present a search for heavy neutral mass resonances $Z'$ decaying into a hadronic tau and an electron with associated neutrinos. This search considers the neutral gauge boson with preferential coupling to third generation fermions using the full collected luminosity of 137 fb$^{-1}$ by the CMS detector of the LHC experiment.
  • PublicaciónAcceso abierto
    Search for $Z'\rightarrow\tau_\mu \tau_h$ in full LHC-run II data with the CMS detector
    (Universidad de los Andes, 2024-01-24) Fraga Flores, Jorge Fernando
    Beyond standard model (BSM) theories have proposed additional neutral boson particles that could provide insights into the Standard Model's issues \cite{bsm}. For instance, searches have been performed for a neutral gauge boson called $Z'$, a particle with similar properties to the neutral $Z$ massive boson from the SM. There are many ways to generate additional neutral gauge bosons, like extending the SM symmetries or from other frameworks like Grand Unified Theories (GUT), string theories, and extensions of the SM. Finding a $Z'$ boson would have exciting implications because requiring additional gauge $U(1)'$ symmetries would generate an extended Higgs sector and, in the case of supersymmetric models, extended neutralino sectors \cite{zprime}. $Z'$ boson searches are well motivated by those BSM scenarios that predict these particles with masses at the TeV order \cite{cdf,dzero} since these would be produced at the current energies of the Large Hadron Collider (LHC) at CERN \cite{atlas,cms}. In the current $Z'$ boson searches, there is an interest in models that include extra neutral gauge bosons that decay into pairs of high-transverse momentum tau leptons, like for example the sequential standard model (SSM) \cite{ssm}, which predicts a neutral spin-1 boson, denoted $Z'_{SSM}$. Since the tau lepton has different decay modes, the search for $Z'$ bosons involves four possible final state experimental signatures: $\tau_h\tau_h$, $\tau_h\tau_e$, $\tau_h\tau_\mu$ and $\tau_e\tau_\mu$ where $\tau_h$ is a tau lepton that decays hadronically and $\tau_e$ ($\tau_\mu$) is a tau lepton that has an electron (muon) in the final state. In the present graduate thesis, we search for signatures of the production of a $Z'$ boson within the SSM framework in the final state channel $Z'\rightarrow \tau_h\tau_\mu$. We analyze proton-proton collisions at a center of mass energy of $\sqrt{s}=13$ TeV, corresponding to data collected by the CMS experiment during the full LHC-Run 2. The hypothetical experimental signature of a $Z'$ can be obtained, through its tau lepton decays, on events with oppositely charged and almost back-to-back tau pairs with high transverse momentum. Missing transverse energy (MET) is expected in the signal events since neutrinos are produced in the tau decay chain, therefore, we look for a broad enhancement in the reconstructed mass distribution from MET, the tau, and muon leptons. We have performed optimization studies in the topological kinematical selections to maximize the signal significance over backgrounds. We have proposed control regions with orthogonal requirements to the signal region, to verify the MC simulation for backgrounds, and in the case of QCD we have implemented data-driven methods. After unblinding the signal region, no excess above the SM backgrounds has been observed as possible evidence of a $Z'$ boson, therefore, we have expanded the exclusion limits of the $Z'\rightarrow \tau_h\tau_\mu$ channel up to masses of 3150 GeV.
  • PublicaciónAcceso abierto
    Variación de propiedades electrónicas del dicalcogenuro de metal de transición WSe2 a partir de dopaje químico
    (Universidad de los Andes, 2022-07-27) Vega Bustos, Karen Alejandra
    La búsqueda de nuevos y mejores materiales multiferroicos está motivada por las posibilidades que ofrecen la sintonización de propiedades magnéticas mediante la aplicación de un campo eléctrico o, viceversa, el control de la polarización eléctrica por campos magnéticos. En la actualidad, la familia de materiales vdW más investigada son los dicalcogenuros de metales de transición (TMD) por exhibir propiedades eléctricas, mecánicas y ópticas prometedoras. Dentro de los TMDs se tienen materiales que bajo ciertas condiciones se ha reportado ferroelectricidad o ferromagnetismo por separado (WTe2, MoS2, MoSe2, WS2, WSe2). En este trabajo, se estudia la variación de las propiedades electrónicas de monocristales de WSe2 a partir de dopaje con Telurio (Te). Como resultado de este dopaje químico, se encontró la presencia en simultáneo de estados ferromagnéticos y ferroeléctricos, es decir, multiferroicidad a temperatura ambiente en el bulk de este material (detectado por primera vez en los TMDs). Monocristales con diferentes niveles de dopaje de Te fueron caracterizados a partir de medidas de XRD, magnetización, microscopía PFM/AMF, y transporte eléctrico. A partir de estas últimas se detectaron propiedades multifuncionales de diodo, suicheo resistivo e histéresis capacitiva.
  • PublicaciónEmbargo
    Study of macroscopic quantum tunnel junctions at high voltages and magnetic fields
    (Universidad de los Andes, 2023-01-31) Ríos Echeverry, Leónardo
    This thesis describes the optimal fabrication processes to obtain large area and high-quality tunnel junctions with structure Al/Al2O3/Al(Metal/Insulator/Metal) and Al/Co/CoO/Al2O3/Al (Metal/Ferromagnet/Antiferromagnet/Insulator/Metal) grown on oxidized substrates of Si(100). It is found, these high-quality tunnel junctions can withstand large tunneling currents without suffering considerable damage. However, due to the difficulty of fabrication and the device's durability, a characterization method based on voltage pulses, that reduces dissipated power, is proposed to investigate the lifetime of tunnel junctions. Furthermore, it was possible to study the quantum tunneling effect at voltages close to the potential barrier height. At this voltage range, we show that the Simmons Model, the most widely used model to find the physical parameters of the potential barrier, is no longer accurate. We suggest a correction that includes energy dissipation during quantum tunneling processes that permits a more accurate data fitting at high voltages. Experiments also show the characteristic barrier height temperature dependence; where a reduction of around 1% is found as the temperature increases up to room temperature. Additionally, the dissipative model allowed the study of the dissipated energy in the junction as a function of the physical parameters of the barrier, the temperature, and the applied voltage. This analysis can be used to identify the relevant physical characteristics of the barrier for the tunnel junction to withstand larger currents reducing energy dissipation. Finally, in the search for greater control of the physical parameters of the potential barrier, a tunnel junction with a magnetic tunnel barrier was fabricated. With this experiment, we have found an apparent increase in barrier height of about 10% as the applied magnetic field increases. This percentual change is one order of magnitude greater than the characteristic increase found in barrier height due to temperature. Additionally, thanks to the magnetic response of the ferromagnet and antiferromagnet layers, it is possible to have asymmetric behavior for the tunneling currents. The polarity of the injected current determines different tunneling currents correlated with a variation of the physical parameters of the junction. This experimental evidence suggests magnetic barrier junctions deserve further study.
  • PublicaciónAcceso abierto
    Geometric and topological aspects of quasi-free states on self-dual algebras
    (Universidad de los Andes, 2022-01-28) Sequera Marín, Ling Alfonso
    We focus on physical and mathematical results about equilibrium states of gapped and gapless Hamiltonians of lattice fermion and lattice spin systems. We use the algebraic formulation of quantum mechanics to describe and characterize such states as functionals on a fermionic observable algebra. The mathematical framework of the work is Araki's self-dual formalism [1]. We study equivalence classes of quadratic Hamiltonians defined as elements of self-dual CAR algebra ASD (K, Gamma). The self-dual algebra ASD (K, Gamma), defined in terms of a Hilbert space K (the one particle self-dual space) and a conjugation Gamma on K, can be used to describe fermion systems. A special class of states on ASD (K, Gamma) are the so-called quasi-free states, which can be completely determined by their 2-point functions. Among these states, we distinguish ground states and equilibrium states, so the theory is robust and convenient enough to describe a wide range of physical quantum systems. We explore two cases: 1. The zero temperature case, where the fluctuations are due only to quantum effects, and ground states display many of the relevant properties of the system. 2. The finite temperature case, where thermal fluctuations prevail and equilibrium states become more relevant. In the first case, the states are determined by special projections called basis projections. The topology of the space of all the basis projections is well known. There is a Z2-index (introduced by Araki, see [2]) that classifies these projections and this classification is lifted to the space of the states. In this work, we extend these results for lattices in any finite dimension and in the thermodynamic limit for systems that can even include disorder [3]. We also show that the index remains invariant along curves of Hamiltonians for which the gap remains open. Hence, any change in the value of the index is related to a gap closing. In the second case, the states are equilibrium states associated with systems with inverse finite temperature beta. These states are Gibbs states, which, as is well-known, satisfy the KMS condition. On the other hand, and following the self-dual formalism, given the uniqueness condition of the KMS states, there exist symbols S_beta in B(K), positive bounded and self-adjoint operators on the one-particle self-dual Hilbert space K, that determine the Gibbs states. The symbols S_beta do not always turn out to be basis projections, so unfortunately we do not have a topological characterization of the state space a priori (as in case 1). However, we have been able to study geometric aspects of the space of equilibrium states through the study of holonomies. In the literature there are several proposals to calculate geometric phases associated with states given by density operators, among them, we highlight the Uhlmann phase [4], [5]. There is a way to purify these equilibrium states and find basis projections in a larger space K^ = K + K (direct sum), in the same spirit as what is known as thermo-field dynamics. Using results from Kato [6], the calculation of holonomy and geometric phase in this larger space K^ is reduced to the study of these geometric aspects in the case of zero temperature. Applying the pertinent restrictions to the initial space, we recover the results of Uhlmann's proposal. In the concrete example of the Ising XY model, we also check that this geometric phase models a regime change given by the longitudinal magnetization of the system [7].
  • PublicaciónAcceso abierto
    Ground- and excited-state calculations of cluster radioactivity and alpha decay
    (Universidad de los Andes, 2022-12-05) Rojas Gamboa, Diego Ferney
    The present work presents calculations for different aspects of alpha and cluster decay within a preformed cluster model where the light cluster is considered to tunnel the Coulomb barrier formed by its interaction with the heavy daughter nucleus. In the models used to study such decay, the effective interaction between the daughter nucleus and alpha is an essential ingredient for the proper determination of the nuclear potential used for evaluating half-lives. A good interaction potential must include effects due to nonlocal interactions and deformations in the shapes of the nuclei if necessary. For nucleon-nucleus scattering processes, the nonlocality has been characterized in different ways such as by introducing energy or velocity-dependent local equivalent potentials, a coordinate-dependent mass, or by modifying the nuclear density distribution. In this work, a comparison of the alpha and cluster decay half-lives using different nonlocal approaches are performed for deformed and spherical nuclei. The sensitivity of the results can provide a complementary tool for improving the models of the nonlocal interaction which are usually constrained by reaction data. Though the measured branching ratios of cluster radioactivity as compared to alpha decay are very small, the former rare decay may become important in hot environments such as those encountered in the nucleosynthesis of heavy elements. The r-process nucleosynthesis path is along highly unstable, exotic, and neutron-rich nuclei. Thus, the decay rates and fission, as well as the photo-dissociation and neutron capture cross sections, are important for the abundance evolution. The explosive conditions in supernovae and neutron star mergers leading to considerably high temperatures could result in nuclei existing in excited states. Though the possible influence of these nuclear thermal excitations is taken into account in the production reactions as well as in their reverse reactions, with libraries publicly available for the scientific community, the same is not true in the case of alpha and cluster decay. In order to include these effects, a model for the alpha and cluster decay of excited nuclei is developed in this work. Empirical formulae or universal decay laws obtained by performing fits to available data of excited nuclei are also provided. The latter can eventually be useful for nucleosynthesis calculations. Finally, an important aspect of such studies is the knowledge of cluster preformation probability. Phenomenological calculations of the preformation factors in several known alpha and cluster decays are presented.
  • PublicaciónAcceso abierto
    Tuning the magnetic properties of multiferroic BiFeO3 : From bulk to nanoscale.
    (Universidad de los Andes, 2022-08-22) Cardona Rodríguez, Alexander
    This multiferroic materials exhibit simultaneously magnetic and ferroelectric ordering. The archetypical multiferroic material, BiFeO3 (BFO), is a unique material with both properties present at room temperature. The BFO has attracted much attention due to its high ferroelectric Curie temperature (1103K) and high antiferromagnetic Neel temperature (643K) in bulk form. The antiferromagnetic ordering instead of a ferromagnetic one has limited the technological applica tions exploiting the ferroic order with both, voltages, and magnetic fields. In this thesis, we explore new routes of magnetic control via nano-structuration in the form of nanoparticles (NPs). The confinement at the nanoscale allows tuning the antisymmetric anisotropy (also called Dzyaloshinskii-Moriya interaction) that causes a distortion of the antiferromagnetic-coupled Fe spins along the [111]h direction and giving rise to a spin cycloid (Lambda). Therefore, NPs with sizes close to the (Lambda). may exhibit interesting magnetic phenomena. We fabricated the BFO NPs by the sol-gel method . We tune the nanoparticle size by varying the calcination temperature which allowed us to go from a few nm up to values close to bulk. All fab ricated BFO NPs show an R3c rhombohedral structure with a residual strain that is a function of the NP size. We found that the magnetic ordering of the BFO NPs is strongly affected by the structural disorder which inevitably arises when the nanoparticle size is decreased to a nanometer scale. Using HRTEM images, we identified that the planes at the surface are better defined in relation to those that are at the core of the particle, we can think that the degree of structural ordering between the surface and core is different due to presence of strain . We found a mixture of different magnetic contributions from superparamagnetism up to weak-ferromagnetis . Furthermore, the complex magnetic structure of the the NPs gives rise to different magnetic transitions at low temperature and high temperature . These transitions are fingerprints of a disorder-driven magnetism present in our BFO NPs. This is confirmed by models based on an atomic vibration instability approach. As a result, a magnetic glassy state can be identified in the smallest particles together with a magnetic core-shell structure in the bigger ones. We employed several characterization techniques to deconvolute the magnetic contributions as a function of size and strain, from in-house magnetometry measurements up to synchrotron-based X-ray magnetic dichroism measurements. In addition to the magnetic measurements, we investigated the optical properties of BFO using Ra man and UV-vis spectroscopy. The results showed a high coincidence between peaks as a consequence of the high crystallinity of our nanoparticles. Using the UV-vis spectroscopy measurements, the bandgap can be deduced by the well-established Tauc plot method. We find that the optical band gap is reduced with decreasing nanoparticle size. These results point to a novel route to control the optical properties in addition to the multiferroic properties of BFO NPs. We employed Density Functional Theory with input from the experimental crystal structures to link the crystallographic and strain contributions to observed magnetic moment . Interestingly, we find that due to the strong phonon-magnon coupling the strain effects alone can be responsible for the observed magnetic tunning. As a result of this thesis, we identify great opportunities for BFO NPs for spintronic applications
  • PublicaciónAcceso abierto
    Euler-heisenberg non-linear electrodynamics
    (Universidad de los Andes, 2017) Bermúdez Manjarres, Andrés Darío
    "This thesis is dedicated to the study of several aspects of the nonlinear theory that arises when the quantum corrections are added to the Maxwell's Lagrangian of classical electrodynamics. The areas of new research includes traveling waves in the non-linear theory, corrections to the field of electric charges and electrodynamic at finite temperature." -- Tomado del Formato de Documento de Grado.
  • PublicaciónAcceso abierto
    Physiology, evolution and single cell gene expression : an experimental approach using a microfluidics technique
    (Universidad de los Andes, 2017) Arias Castro, Juan Carlos
    "Interest in studies about variability in gene expression has been increasing due to its applications in pharmaceutical, medical and biological sciences. Genetically identical cells can have different phenotypes even when they are growing in the same environmental conditions; this is just one of many examples of the consequences of stochasticity in the chemical reactions at a molecular level. In our work we try to find insights on the sources of physiology and gene expression variability and their interaction with the environment, such as the case of nutrient limitation." -- Tomado del Formato de Documento de Grado.
  • PublicaciónAcceso abierto
    Activity of an alfa-helical antimicrobial peptide on different model membranes studied with biophysical techniques
    (Universidad de los Andes, 2017) Marín Medina, Nathaly Melina
    La resistencia bacteriana a los antibióticos ha sido declarada problema de salud pública a nivel mundial ya que en las últimas décadas las bacterias han adquirido resistencia a los antibióticos que salen al mercado en períodos de tiempo cada vez más cortos. Los péptidos antimicrobianos (PAMs), considerados como "antibióticos naturales", son moléculas anfipáticas que se encuentran en todos los organismos vivos complejos como componentes esenciales del sistema inmune, y cuya función es atacar las infecciones bacterianas afectando, entre otros, la estructura de la membrana de las bacterias. En las últimas tres décadas los PAMs han sido investigados de forma extensiva ya que entender sus mecanismos de acción facilitaría el diseño y desarrollo de alternativas a los antibióticos tradicionales. La interacción entre PAMs y membranas lipídicas (MLs) se ha estudiado con diversas técnicas experimentales, conllevando a diferentes modelos de actividad bactericida. Sin embargo, la microscopía de fuerza atómica (MFA) ha sido poco usada para este tipo de estudios. Una de las hipótesis en las que ha trabajado el grupo de Biofísica en Uniandes es que existe una relación directa entre la potencia de un PAM dado y las propiedades mecánicas de la ML. En nuestro proyecto de investigación estudiamos la interacción entre un PAM catiónico corto y diferentes tipos de membranas modelo. Las técnicas experimentales usadas fueron espectroscopía de fluorescencia, MFA y aspiración con micropipeta, explorando vesículas unilamelares grandes, MLs apoyadas y vesículas unilamelares gigantes, respectivamente. Magainina-H2, el péptido que estudiamos, adquiere una estructura de alfa-hélice al adherirse a la membrana y forma poros multiméricos después de cierta proporción púptido/lípido.--Tomado del Formato de Documento de Grado.
  • PublicaciónAcceso abierto
    Large-N Pion-Pion scattering, finite temperature effects and chiral symmetry restoration
    (Universidad de los Andes, 2017) Cortés González, Juan Santiago
    The main focus of this thesis is to study QCD vacuum phenomena such as Chiral Symmetry Restoration (CSR) and generation of Bound States and Resonances; we attain this by analyzing two large N QCD-like models such as the nonlinear sigma model for a large number of flavors, and AdS/QCD nonconformal approaches where a large number of color is considered. We use the first model to study CSR after breaking it both dynamically and explicitly so that finite-temperature effects are easily introduced. We find some interesting properties regarding critical phenomena and universality classes. The second model we use allows to find the masses of light meson resonances in a gravitational-like approach defined at zero temperature (without Chiral Symmetry Breaking effects). We find here some interesting results since most of these masses are in great concordance with both experimental and theoretical/phenomenological results.--Tomado del Formato de Documento de Grado.
  • PublicaciónAcceso abierto
    Resultados exactos y mecanismos de fusión en sistemas bidimensionales = [Exact results and melting theories in two-dimensional systems = Résultats exacts et mécanismes de fusion pour les systáemes bidimensionnels]
    (Universidad de los Andes, 2017) Salazar Romero, Robert Paul
    "Les systáemes de nombreuses particules peuvent présenter des comportements variés en fonction du type d'interaction entre ses composants. Dans certaines situations, des structures macroscopiques hautement ordonnées peuvent émerger de telles interactions. Le probláeme de l'identification des mécanismes qui activent l'ordre microscopique dans les systáemes bidimensionnels a fait l'objet d'études théoriques et expérimentales. Il y a plusieurs décennies, il a été montré que les systáemes bidimensionnels avec des interactions de paramáetres d?ordre suffisamment court et d'ordre continu n'ont pas d'ordre áa longue portée (ils n'ont pas de phase solide). D'autre part, des études numériques sur des systáemes sans ordre positionnel ont montré que de tels systáemes pourraient présenter des transitions de phase. Cette contradiction apparente dans les systáemes bidimensionnels a été expliquée dans la transition KT (Kosterlitz-Thouless) proposée pour le modáele XY. Depuis lors, on a commencé áa observer que les systáemes sans ordre positionnel pouvaient montrer des transitions de phase quand ils avaient un ordre de demi-longue portée (ODLP). Ce type d'ordre est associé áa l'ordre d?orientation du systáeme qui est perdu lorsque les défauts topologiques activés par les fluctuations thermiques sont divisés en paires produisant une transition. D'autre part, les systáemes bidimensionnels avec ordre de position áa la température T = 0 peuvent ãetre fusionnés dans un scénario comprenant trois phases : solide / hexatique / liquide dont les transitions sont dues áa la division en deux étapes de défauts topologiques áa deux températures différentes (Théorie de Kosterlitz-Thouless-Halperin-Nelson-Young KTHNY). Ce travail se concentre sur l'étude du plasma d'un composant bidimensionnel (PUC2d), un systáeme classique de N charges ponctuelles identiques qui interagissent áa travers un potentiel électrique et immergées dans une surface bidimensionnelle avec densité de charge opposée. Le systáeme est un cristal áa T = 0 qui commence áa fondre si T est suffisamment élevé. Si le potentiel d?interaction entre les particules est logarithmique, alors le systáeme dans le plan et la spháere a une solution exacte pour une valeur spéciale de T située dans la phase fluide. Dans cette étude, un formalisme analytique est utilisé pour déterminer exactement les propriétés thermodynamiques et structurelles qui permettent de suivre le comportement du PUC2d en la phase désordonnée jusqu'áa ce que celui-ci cristallise avec la restriction de N pas tráes grand. Par le formalisme, nous trouvons des connexions intéressantes avec l'ensemble de Ginibre défini dans la théorie des matrices aléatoires. Nous avons effectué des simulations de Monte Carlo pour modéliser le PUC2d avec des interactions potentiel en inverse de distance et des conditions aux limites périodiques dans le plan. Trois phases sont identifiées incluant la phase hexatique pour des systáemes suffisamment grands. Nous avons déterminé par l'analyse de taille finie et la méthode multi-histogramme que la transition hexatique/ liquide est de premier ordre faible. Finalement, une étude statistique sur les arrangements de défauts (clusters) lors de la fusion cristalline est effectuée, confirmant en détail la théorie de KTHNY et décrivant des alternatives pour la détection de transitions en deux dimensions."--Tomado del portal SUDOC (Francia).
  • PublicaciónAcceso abierto
    Exploring the physics of the sorkin-johnston state : renormalized stress-energy tensor, hadamard and energy conditions
    (Universidad de los Andes, 2016) Avilán Vargas, Nicolás Guillermo
    The non-uniqueness of the vacuum state is one of the most characteristic features of the theory of quantum fields in curved backgrounds. Whereas in Minkowski spacetime the invariance with respect to Poincaré symmetry singles out the vacuum state, in general backgrounds there is no a priori given physical criterion on which the choice of a unique vacuum can rely. Recently, a construction for a new state has been proposed, which can be applied to a wide class of globally hyperbolic spacetimes. In view of potential applications to problems related to cosmology and black hole entropy, it is necessary to explore the physical and mathematical properties of this state, the Sorkin-Johnston state. This work focuses on the construction of a renormalized stress-energy tensor for the Sorkin-Johnston state on finite regions in 1+1 and 2+1 dimensional spacetimes. We consider cases with pseudo-periodic and Dirichlet boundary conditions in the spatial region. In each...
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    Método de renormalización por sustracción de Taylor en el modelo DFR
    (Universidad de los Andes, 2016) Salazar Montenegro, Juan Carlos
    The noncommutative space-time model introduced by Doplicher, Fredenhagen and Roberts (DFR), is based on commutation relations of the form [qu, qv] = Lp Quv for the quantum coordinate operators qu, where Lp represents a length scale and Quv is a tensor operator. In this thesis, the DFR model, and in particular the commutative limit (when Lp tends to zero) and the ensuing divergencies for field theories thereon are studied. As a method of renormalization to remove the divergences in question, the application of a Taylor subtraction (as in the BPHZ method for quantum field theory on commutative space-time) is proposed. Specifically, through the calculation of the contributions of fish and sunrise diagrams, we show that the subtraction operator removes divergencies that would otherwise appear in the commutative limit. This way of studying the commutative limit is also helpful in setting bounds on the nocommutativity parameter, for which its effects could be observable.
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    Coulomb systems in one and two dimensions: Exact results
    (Universidad de los Andes, 2021) Varela Álvarez, Lucas
    Soft matter often features charged units. These charges interact through Coulomb forces, the treatment of which is difficult due to their long-range nature. This thesis gives results for three many- body systems where every long-range interaction is included, without approximations. The document includes a longer summary in English
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    Modular theory and algebraic quantum physics
    (Universidad de los Andes, 2022-02-26) Tabban Sabbagh, Souad María
    The description of a quantum physical system, in the algebraic approach, is given through a von Neumann algebra of observables A and a state w on it. In this context, the study of entanglement of quantum systems is relevant. That requires an appropriate assignment of an entropy to the algebraic states. This entropy can be obtained through the Gelfand-Naimark-Segal (GNS) construction, which leads to a density operator associated to the state. Recently, Balachandran et al. (2013) used the algebraic approach to deal with entanglement in systems of identical particles. As is well known, the standard approach fails in these systems due to the fact that partial trace loses its intended meaning, since the Hilbert space is not a simple tensor product. Instead of partial trace, they considered the restriction of a state to a subsystem, which in the algebraic formulation becomes particularly clear. By means of the GNS construction, they construct density operators such that their restriction to the algebra A coincide with w. Then, the von Neumann entropy of these density operators can be regarded as the entropy of the algebraic state. However, this approach is ambiguous and assigns multiple density operators to the same state. This occurs whenever the irreducible components of the representation appear in the GNS Hilbert space H with multiplicities different from one. In this dissertation, we used Tomita-Takesaki (modular) Theory (TTT) to develop an interpretation of this phenomenon as an emergent gauge symmetry, in the sense of Doplicher, Haag, and Roberts. The gauge group arises from the action of unitaries in the commutant of the representation via TTT. We characterize the ambiguity in the entropy through the modular objects, in particular the modular conjugation. Moreover, we provide a quantum operation which implements the gauge group and increases entropy. In this way, we relate the realm of quantum information theory to that of the theory of gauge fields. We apply the above for two fundamental cases. In the first case, we consider general finite dimensional algebras and we give a physical interpretation in terms of an equivalent description of the system as a bipartite system. In the second case, we consider quantum systems whose classical configuration spaces are homogeneous spaces of the form Q=G/H, where G is a compact Lie group. In this case, the von Neumann algebra is obtained by quantizing Q through an approach based on the use of the transformation group C*-algebras. We prove that the emergent gauge group contains the classical gauge group of Q. Additionally, we also study other applications of TTT to spin chains in the context of Araki's self-dual formalism. In this case, for a given thermal KMS state, the modular theory (through the GNS construction) provides a purification of the KMS state. This construction preserves the canonical anticommutation relations of the fermionic algebra, unlike the one obtained using thermofield dynamics. Possible applications of this approach to the study of quantum phases of matter are also discussed.
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    Quantum non-equilibrium many-body spin-photon systems
    (Uniandes, 2019) Gómez Ruiz, Fernando Javier
    Esta tesis de doctorado se estudia la dinámica de sistemas cuánticos fuertemente correlacionados fuera del equilibrio. La investigación no se limita a las propiedades estáticas o tiempos largos de evolución/relajación, ni tampoco se descuida los efectos en los subsistema, como se hace con frecuencia con los diferentes enfoques de mecánica cuántica como por ejemplo: Ecuación Maestra. El objetivo de este trabajo es explorar diferentes sistemas cuánticos durante varios regímenes de operaciones, luego descubrir resultados que puedan ser de interés para el control cuántico y, por lo tanto, computación cuántica y el procesamiento de información. Nuestros principales resultados se pueden resumir de la siguiente manera en tres partes: Características emergentes de una dinámica crítica, Modelo de Dicke pulsado como un banco de pruebas de acoplamiento luz y materia ultra fuerte. Finalmente, más allá del mecanismo Kibble-Zurek. Nuestros hallazgos muestran robustez a la decoherencia y al ruido. Adicionalmente, tienen implicaciones experimentales potenciales a nivel de sistemas cuánticos para una variedad variedad, incluidas colecciones de átomos, moléculas, espines, qubits superconductores en cavidades, y posiblemente incluso procesos de recolección de luz mejorados por vibración en macromoléculas