Tesis Doctorals - Facultat - Física

URI permanent per a aquesta col·leccióhttps://hdl.handle.net/2445/41381

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B radiative decays at LHCb: measurement of B→ K∗γ isospin asymmetry and preparation of Run 3 analyses
(2025-09-08) Lobo Salvia, Aniol; Graugés Pous, Eugeni; Marin Benito, Carla; Universitat de Barcelona. Facultat de Física
[eng] The Standard Model (ME) of particle physics is the theory that describes fundamental particles and their interactions, except gravity. Despite its success and accuracy in predicting phenomena, it is known that it is not a complete theory. It does not describe dark matter or energy, the asymmetry between matter and antimatter in the Universe, or the origin of neutrino mass. Consequently, it is vitally important to investigate possible extensions of this model and measure processes that may reveal new physics. An experiment that contributes to this task is the LHCb, one of the four main detectors located in the LHC particle accelerator operated by CERN. His study focuses on high-precision measurements of particles involving heavy quarks b and c. Rare radiative decays of B mesons are an ideal platform to test EM and study its possible extensions. These decays cannot occur through tree diagrams within the description of the ME, but take place through loops in which unknown particles could participate, thus altering the observable quantities. This thesis presents three interconnected topics within the study of rare radiative decays of B mesons: the experiment's calorimeter monitoring tools, the radiative inclusive selections in the trigger system, and the measurement of the isospium asymmetry of the B→K*γ decay. The measurement of the isospium asymmetry of the B→K*γ decay has been done using data from Run 2, which covers the years 2015 to 2018. The other two tasks have been carried out within the framework of Run 3, starting in 2021. Between these two Runs, an ambitious improvement program, called Upgrade I, has made it possible to increase the capacity of the detector. Some of the subdetectors have been completely renewed, the data reading system has been improved to facilitate 40 MHz reading and the hardware phase of the trigger has been dismantled, becoming the first large detector with a completely software-based trigger. This set of improvements has made it possible to increase the instantaneous luminosity at which the detector operates by a factor of five, thus gaining sensitivity and its precision measurements. The monitoring of the calorimeter makes it possible to detect anomalies in the data collection and provides information on the origin of the problem in order to be able to solve it quickly. It has been a critical function during the commissioning period of Run 3, until the system has reached its stable operation. Trigger selections inclusive of radiative processes make it possible to capture events of this type and rule out those that constitute background noise, thus keeping the volume of data stored within the limits of the system. The selections for the new instantaneous high luminosity environment have been optimized and the BDT multivariable method has been used in order to obtain the best possible signal efficiency. Finally, the measurement of the isospy asymmetry of the B→K*γ decay is obtained for the first time with data from the LHCb and with an accuracy comparable to the measurements of other experiments, validating the ME and applying constraints on its possible extensions, in particular extensions of supersymmetry and extra Higgs sectors.
Artificial 3D Skeletal Muscle Tissues to Model and Investigate Disease Onset and Progression Mechanisms
(Universitat de Barcelona, 2025-07-22) Mughal, Sheeza; Ramón Azcón, Javier; Fernández-Costa, Juan Manuel; Universitat de Barcelona. Facultat de Física
[eng] This thesis investigates the innovation and application of three-dimensional (3D) in vitro skeletal muscle tissue models to study and model muscle pathophysiology, particularly in idiopathic and corticosteroid-induced myopathies. Based on the historical evolution of cell culture techniques, this work highlights the limitations of conventional two-dimensional systems in reproducing native muscle structure, bioenergetics, and functional features. In the first section of this work, 3D human skeletal muscle constructs were developed using human muscle progenitor cells and exposed these tissues to sera from patients with Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and long COVID (LC-19). Structural, functional and transcriptomic analyses post-treatment revealed disease-specific, time-dependent disturbances in mitochondrial function, protein homeostasis, and inflammatory signaling. The disease pathology in these models demonstrated mitochondrial stress, IL-6–mediated catabolism, and metabolic reprogramming toward glycolysis, with ME/CFS samples showing particularly severe contractile deficits. In the second part, a steroid myopathy model was produced by exposing 3D muscle tissues to dexamethasone. Taurine supplementation effectively restored contractile function by promoting protein synthesis and suppressing proteolysis by regulating the AKT/mTOR pathway modulation, even in the continued presence of dexamethasone. Together, these models provide powerful platforms for investigating disease mechanisms and testing therapeutic interventions, offering new mechanistic insights into muscle dysfunction. These insights are particularly important in conditions with unknown etiologies. Future prospects include developing an in vitro construct together with immune and neural components, increasing patient cohorts, and improving these models for clinical translation, particularly for testing patient stratification.
Some considerations about the climate variability and change in the Mediterranean region
(Universitat de Barcelona, 2025-09-04) Cos i Espuña, Pep; Doblas Reyes, Francisco; Marcos Matamoros, Raúl; Universitat de Barcelona. Facultat de Física
[eng] Climate change has many effects, and while global warming caused by human activity is widely recognised, regional-scale changes are particularly important yet uncertain. To effectively communicate, plan protective measures, and adapt to these changes, it is crucial to improve the regional climate information and expand the knowledge of regional climate variability. This thesis focuses on the Mediterranean region, not only because it is where we are located, but also due to its high population density, distinct climate, and the expected intensification of climate change beyond the global average. The thesis begins with an analysis of temperature and precipitation projections from the latest phase of the Coupled Model Intercomparison Project (CMIP6). It considers multiple sources of uncertainty, including different climate models and emission scenarios, and compares the results with those from the previous CMIP5 multi-model set. Findings confirm that the Mediterranean is a climate-change hotspot, showing significant warming and drying trends, especially in summer. Despite differences in global warming projections, CMIP6 projects a warmer future than CMIP5, the pat-tern of amplified regional warming remains consistent. Uncertainty increases toward the end of the 21st Century, driven by the differences in emission scenarios, under-scoring the urgent need for mitigation efforts. Without emission reductions, summer mean temperatures (JJA) could rise by up to 8◦CZ by 2100. Precipitation projections vary widely, but drying trends become clearer under higher emission scenarios, highlighting the region’s future vulnerability to drought and water shortages. The study employs weighting methodologies on the multi-model ensembles to constrain their sampling and performance issues. The thesis then examines the near-term climate in the region, focusing on methods to estimate Mediterranean summer temperatures over the next 20 years from CMIP6 projections. At this timescale, internal climate variability plays a dominant role in uncertainty. The study evaluates different methods that incorporate internal variability by selecting simulations that match the recent climate state. This comparative assessment of constraining methods for short-term climate projections is novel and provides a framework for testing them against observational data. Results show that effectiveness varies depending on the method used and the area considered within the Mediterranean region. Selection approaches based on sea surface temperatures appear promising for improving the estimates but require further refinement for reliability. The research highlights the importance of evaluating the methods to estimate future climate against past observations. Given the complexity of the constraining methodologies that attempt to capture internal variability in uninitialized simulations, the study proposes a framework to improve the understanding of the results and guide future constraining techniques. The thesis concludes addressing another process that affects the Mediterranean climate and has received little attention: the Saharan warm air intrusions, which are not necessarily linked to dust storms and transport. These intrusions can impact extreme temperatures across large parts of the Euro-Mediterranean region throughout the year. The study identifies a rising trend in these events during summer, winter, and autumn over the historical period. It also identifies large-scale atmospheric circulation patterns associated with their occurrence. Understanding these mechanisms could improve climate estimates and serve as an additional metric for evaluating climate models. Initial results indicate that CMIP6 models perform in a variety of ways when reproducing the historical frequency, seasonality, and trends of the intrusion events. This research explores open questions about the Mediterranean climate, enhances understanding of its variability, and proposes ways to improve the quality of available climate information using climate simulations and observational products.
Toroidal nematics: Point defects, disclination lines and walls
(Universitat de Barcelona, 2025-05-29) Rojo-González, Javier; Fernández de las Nieves, Alberto; Universitat de Barcelona. Facultat de Física
[eng] Confining partially order fluids can lead to frustration if the locally preferred state can not be achieved everywhere simultaneously. When this happens, the configuration of the material can not be given by the local minimum, and a plethora of compatible frustrated configurations become possible. In this case, the ground state configuration results from the minimization of the total energy. This is specially exemplified by nematic liquid crystals where the anisotropic building blocks reduce their free energy by aligning parallel to each other. Perfect parallel alignment can become impossible everywhere when the nematic is confined within different shapes while fixing the orientation at the boundaries. How this frustration is resolved and which configuration the liquid crystal acquires depends on the geometry and topology of the boundary and might require the presence of regions with no defined alignment, called defects. In this thesis we mainly focus on experimentally studying the configurations acquired by a nematic liquid crystal confined to a stable toroidal droplet with different boundary conditions. We first study the case with tangential alignment at the surface. In this case, we find stable defect populated configurations when the torus is heated to the isotropic phase and cooled back to the nematic phase. They consist mainly on defect pairs whose number and location is random and that turn out to be metastable with an energetic barrier that is high enough for them to not spontaneously annihilate. We also report the appearance of a configuration not fully elucidated that we call ”X” pattern and that can have a solitonic character. Note that all these features are observed even when there is no topological requirement for the torus to have defects. We then study the effect of applying magnetic fields on planar nematic tori with and without defect pairs and find that stable inversion walls form. In the defect free case, two splay-bend walls form, while when a defect pair is present, either one of the walls does not appear or an extra wall is formed depending on the initial location of the defect pair. Interestingly, we find that the mobility of the defects allows for the defect pair wall to reorganize into different types of walls, becoming a wall of the twist type to reduce energy. We also show how, by heating and cooling our nematic droplets in an external magnetic field, the number of defect pairs generated can be controlled. We finish our work on nematic tori by studying the case with perpendicular anchoring at the surface. We recover previous experimental results and find that the anchoring strength plays a significant role in the appearance of a configuration characterized by four dark brushes forming a spiral. Moreover, for certain conditions, stable configurations filled with disclination lines are found and experimentally studied using epifluorescence microscopy. Aside from our experimental work, from a theoretical perspective, we use the decomposition of a director field distortion into four normal modes to derive the compatibility conditions for a 2D and 3D nematic. For that, we use a simple approach based on imposing that the director field variation has to be the same when going through two different paths up to second order. This approach gives an intuitive interpretation of the different equations and provides a simple way to relate them to their counterpart in curved space through the concept of holonomy. Finally, we also discuss some side projects collaterally related to the main thesis work. We first discuss our studies on water evaporation of chitosan hydrogels. We describe the constructed setup to generate the gels and our experiments that track the mass evolution of the gels in time. Our results suggest that the evaporation rate for our hydrogels is slower than pure water due to an increase in the energy barrier for the water molecules to escape the gel, thus bringing about a kinetic character to the process. We also show the design, construction and first tests on an upscaled prototype for the production of these chitosan hydrogels. We end by briefly describing our work with thermorresponsive hydrogels that present a constant volume phase transition leading to phase coexistence between a swollen and shrunken phases when the temperature is increased rapidly. We mainly show how cylindrical gels suddenly bend due to a spontaneous polarization of the region that shrinks, which is in accordance with the current theory for this process.
Mathematical Ideas in Quantum Computing
(2025-05-15) Lin, Ruge; Latorre, José Ignacio; Universitat de Barcelona. Facultat de Física
[eng] This thesis can be classified into the research field of quantum algorithms. A typical paper in this field usually focuses on what kind of things we can do with a quantum computer, whether they are useful or advantageous compared to classical computers, and how we can do it more efficiently. Since the ideal fault-tolerant large-scale quantum computer will not be available soon, many papers focus on noisy intermediate-scale quantum (NISQ) devices, how to characterize them through various benchmarking protocols, and how to make them useful through error mitigation or variational algorithms. Most probably, anyone who ends up opening this thesis is already an expert in certain topics, but they can still find some “out of the box" elements in one chapter or another. The results presented in the thesis might not be directly useful, but hopefully, these elements can provide insights and inspiration. The main body of this thesis is organized into six independent chapters. Each chapter is complete and self-consistent. All these chapters share a similarity: They provide some mathematical ideas of quantum computing in one aspect or another. Since part of the research is conducted in collaboration, the pronoun “we" is used throughout the work. In the first chapter, we focus on actions that transform quantum states into quantum states. We begin with a more theoretical approach. These actions naturally process a group structure. And we ask, under what conditions is this group finite? Given an arbitrary finite group, how can we construct these actions? How can these actions be useful to quantum computing? The set of these actions is named "Quantum representation of finite group," and answers to these questions are provided in the main text. Then, in the second chapter, we switch to a more practical scenario. The inspiration came from a problem that appeared in post-quantum cryptography, the dihedral coset problem, which uses quantum algorithms to recover a secret of the group hidden in quantum states. We simplified this problem and designed an interactive protocol for verifying the computational capacity of a quantum device. Holding a small quantum computer, the verifier prepares quantum state samples that encode a secret and sends them one by one to the prover via a one-way quantum channel. The prover then uses his quantum device to recover the secret with a certain probability. The device’s capacity is then reflected in the accuracy of the solution. This interactive protocol can be used locally on a single device as a benchmarking protocol. In the third chapter, we explore quantum entanglement. We present a new method for visualizing entanglement during a quantum evolution, such as the execution of a quantum algorithm. We name this method "Entanglement trajectory" and show that it can provide a "fingerprint" for each quantum process. We determine the boundaries of these trajectories with Random Matrix Theory and use different examples and entanglement measurements to show that this trajectory has an intrinsic quantum meaning and provides more information about the system. In the fourth chapter, we elaborate on the entanglement presented in a succession of numbers. First, we give a numerical and analytical description of the average entropy of succession, which can provide a standard for studying any successions. Then, we simulate the example of k-almost primes and its unions for different system sizes and explore their particular feature. In the fifth chapter, we will show three different methods of using a quantum annealer to solve Multivariate Quadratic (MQ) problems. In the sixth chapter, we present a data encoder in the fixed Hamming weight subspace, which is parameter-optimal and can be extended to a sparse and binary data encoder. We will give a small summary for each chapter. At the end of this thesis, we will provide an overall conclusion for our readers as “take-home messages".
A multiscale study on multivalence targeting
(Universitat de Barcelona, 2025-12-01) Xie, Zhendong; Battaglia, Giuseppe; Samitier i Martí, Josep; Universitat de Barcelona. Facultat de Física
[eng] Precision drug design utilizes detailed biological information, such as genetic profiles, cellular phenotypes, and molecular expression patterns, to develop therapeutics that are highly specific, effective, and safe. In this work, we demonstrate the potential of nanomedicine in the development of precision drugs. We establish a set of design principles to guide nanoparticle selectivity, grounded in thermodynamics, kinetics, and pharmacokinetics. Nanomedicine can leverage the multivalent effect to enhance binding avidity. To better understand nanoparticle–cell interactions, we developed a thermodynamic model called the phenotypic targeting theory (PAT). Generally, a larger number of low-affinity ligands can effectively increase binding selectivity. We applied PAT to study poly(2-(methacryloyloxy)ethyl phosphorylcholine)-poly(2-(diisopropylamino)ethyl methacrylate) (PMPC-PDPA) nanoparticles with various topologies. The model predicts the binding fraction and association constant for different nanoparticle architectures. Based on PAT, we analyzed the selectivity parameter of spherical nanoparticles as a function of particle radius and the degree of polymerization of a single PMPC chain. Since PMPC targets three distinct receptors, we also explored how changes in receptor density affect the binding fraction. These results provide a quantitative framework for optimizing nanoparticle design for selective targeting. We extended the PAT to include the targeting of large receptors, such as the low-density lipoprotein receptor-related protein 1 (LRP1) and the mannose receptor (MRC1/CD206). When targeting such large receptors, both osmotic steric penalty and compression steric penalty must be taken into account. Additionally, a steric penalty arises from unbound ligands. When an excessive number of functional ligands is used, this steric penalty increases, leading to a reduction in binding strength, ultimately resulting in negligible or no binding. This steric effect is influenced by both the number of functional ligands and an interference parameter. To account for these factors, we redefined the selectivity parameter as a function of the number of ligands rather than the number of receptors. We then analyzed this revised selectivity parameter in relation to binding energy, the interference parameter, the steric penalty per unbound ligand, and the number of available binding sites. After developing the PAT, a thermodynamic model, we extended it to describe the binding kinetics of nanoparticle-cell interactions. We investigated the binding kinetics of PMPC nanoparticles with various topologies, fitting experimental data using a mathematical model. By integrating PAT with a kinetic framework, we clarified key parameters governing multivalent interactions, including the on-rate, dissociation rate, and endocytosis rate. In addition, we explored the intracellular trafficking of PMPC nanoparticles with different topologies to gain deeper insight into their cellular fate. We also applied the model to fit the binding kinetics between mUNO-decorated nanoparticles, functionalized with varying ligand densities, and THP-1-derived macrophages. To expand the applicability of our model to more complex biological scenarios, we developed an advanced framework that accounts for binding kinetics, endocytosis, exocytosis, receptor recycling, and receptor synthesis. Additionally, we introduced a chaotic dynamics model to simulate receptor oscillation behavior observed in certain experimental conditions. We anticipate that this chaos-based model can be further refined and applied to better interpret uptake experiments involving dynamic receptor regulation. To quantify therapeutic efficiency, we developed a physiologically based pharmacokinetic (PBPK) model to describe the absorption, distribution, metabolism, and excretion (ADME) of nanoparticles. We applied this model to two distinct scenarios. The first involved non-specific interactions using PMET (poly(methionine)) nanoparticles, while the second was based on the Phenotypic Targeting Theory (PAT) applied to PMPC nanoparticles. For the PMET nanoparticles, we quantified key pharmacokinetic parameters and analyzed their influence on targeting selectivity. In the case of PMPC nanoparticles, we proposed a strategy to tune nanoparticle design parameters in order to enhance targeting selectivity, ensuring that the nanoparticles preferentially interact with specific cell types. We aim to use in silico methods, including thermodynamic, kinetic, and pharmacokinetic modeling, to assist researchers in designing more selective nanomedicines and gaining deeper insights into nanoparticle-cell interactions.
Development of 3D printing technologies for manufacturing microfluidic devices for bioengineering applications
(Universitat de Barcelona, 2025-06-20) Subirada, Francesc; Samitier i Martí, Josep; Rodriguez Trujillo, Romen; Universitat de Barcelona. Facultat de Física
[eng] Advancements in 3D printing have revolutionized microfluidic device fabrication, offering unprecedented flexibility and precision in the design and production of complex geometries. This thesis presents a comprehensive study utilizing a custom-made DLP-SLA 3D printer and commercially available resins to develop and evaluate microfluidic devices for applications in particle separation and fluid mixing under laminar flow conditions. A detailed analysis of printing errors was conducted, highlighting the challenges posed by over-curing effects, which significantly impact the ability to fabricate closed microchannels with high dimensional accuracy. The research involved a dual approach of computational simulations and experimental validations to optimize device performance. Spiral microfluidic devices were designed and tested for particle separation, achieving efficient separation of particles larger than 75 µm under laminar flow conditions, with Reynolds numbers carefully analyzed to ensure optimal performance. Additionally, staggered herringbone mixers were developed to enhance mixing efficiency through chaotic advection. The results demonstrated effective mixing of fluids, confirming the versatility of these devices in achieving homogenous mixing even at low Reynolds numbers. This work underscores the potential of DLP-SLA 3D printing in fabricating both open molds for PDMS replication and fully enclosed microchannels, offering a scalable and customizable alternative to traditional lithographic methods. The findings contribute to advancing the field of microfluidics, providing insights into the interplay between fabrication techniques, material properties, and device functionality. Future applications of these devices extend to biomedical diagnostics, chemical synthesis, and lab-on-a-chip systems, highlighting their relevance and impact in both research and industry.
Leveraging metabolism as a cellular state reporter for liver health and disease via hyperpolarized magnetic resonance spectroscopy
(Universitat de Barcelona, 2025-04-29) Herrero Gómez, Alba; Marco Rius, Irene; Ramón Azcón, Javier; Universitat de Barcelona. Facultat de Física
[eng] Tissue-engineered cellular models represent an innovative, ethical, and controlled research tool, offering an ideal platform to study disease mechanisms and test emerging therapies in vitro. However, their application is limited by the difficulty associated with reproducing the metabolic environment of native organs. Hyperpolarized Magnetic Resonance Spectroscopic Imaging (HP-MRSI) has emerged as a technique capable of evaluating cellular metabolism non-invasively. Despite its potential, the integration of HP-MRSI with tissue engineering has not yet been achieved, mainly due to technical challenges related to their compatibility and the reproducibility of metabolic environments in vitro. This thesis explores these incompatibilities, identifying the main challenges to combine tissue engineering with HP-MRSI. A milestone of this work is the creation and characterization of 3D cellular models and culture platforms compatible with HP-MRSI. Furthermore, this study has successfully identified metabolic signatures of Metabolic-Associated Fatty Liver Disease (MAFLD) in vivo, opening new possibilities to reproduce them in vitro, using HP-MRSI to model disease progression in the laboratory. The conclusions of this thesis establish a foundation for the future use of HP-MRSI in 3D cell models, with the potential to reduce dependence on animal models in preclinical research. Through the application of HP-MRSI to in vitro studies, this work paves the way for more efficient and ethical research into metabolic diseases to develop new treatments. The integration of these technologies represents a significant advancement in the field of metabolic imaging and biomedical research.
Solution-processed Engineering Strategies for Chalcogenide Thermoelectric Nanomaterials
(2025-02-20) Nan, Bingfei; Cabot i Codina, Andreu; Universitat de Barcelona. Facultat de Física
[eng] The chapters of this PhD thesis cover the work carried out in the period 2020-2024 by the PhD candidate Bingfei Nan at the Catalonia Institute for Energy Research (IREC) in Sant Adrià de Besòs, Barcelona, funded by China Scholarship Council (No. 202004910311). The thesis is mainly devoted to the development the high-performance and precisely controllable chalcogenide thermoelectric (TE) nanomaterials via a solution-processed bottom-up engineering strategy. This thesis is divided into five chapters. Basic research background, universal TE concepts, synthetic methods and various objectives are comprehensively introduced in the Chapter 1. The core experimental research work from Chapter 2 to Chapter 5 is presented one by one, involving a series of solution synthesis and characterization of chalcogenide TE building blocks. The work in this dissertation is based on the overall purpose of bottom-up solution processing and developing high-performance TE materials, and specifically aims to eliminate some of the major challenges in this field through the following strategies: 1) lattice thermal conductivity is reduced by various defects produced by secondary phases causing phonon scattering; 2) improvement of the Seebeck coefficient is achieved by increasing band convergence; 3) electrical conductivity is improved by increasing carrier concentration in the matrix by introducing metal nanoparticles (NPs) in a modulation doping scheme. I detail a thiol-free SnTe precursor that can be thermally decomposed to produce SnTe-Cu2SnTe3 by introducing Cu1.5Te in Chapter 2. The presence of Cu in SnTe and the segregation of the semimetallic Cu2SnTe3 phase effectively optimize the electrical conductivity while reducing the lattice thermal conductivity without affecting the Seebeck coefficient. In Chapter 3, NaSbSe2 nanocrystals (NCs) alloyed with SnTe NPs can adjust carrier concentration and band convergence. The SnTe-NaSbSe2 alloys induce multiscale defects that are beneficial to the reduction of lattice thermal conductivity. A novel colloidal quaternary Ag2SbBiSe4 is presented in Chapter 4. A modulation doping strategy based on the blending of semiconductor Ag2SbBiSe4 NCs and metallic Sn NCs is demonstrated to control the charge carrier concentration and carrier mobility. In chapter 5, I present a room temperature, aqueous-phase synthesis approach to generate Ag2Se and Bi2S3 particles, and to incorporate Bi2S3 into the Ag2Se matrix to improve the Seebeck coefficient and power factor.
Desenvolupament i implementació de tècniques de microscòpia òptica per la mesura 3D de superfícies de manufactura additiva
(Universitat de Barcelona, 2025-07-18) Vilar Solé, Narcís; Duocastella Solà, Martí; Carles, Guillem; Universitat de Barcelona. Facultat de Física
[cat] La manufactura additiva ha revolucionat els paradigmes tradicionals de fabricació, oferint una llibertat de disseny i unes capacitats de personalització sense precedents. No obstant això, les complexitats inherents a aquests processos de manufactura requereixen tècniques de metrologia òptica precises i fiables per garantir la integritat dels components fabricats. Aquesta tesi està enfocada en la investigació de la metrologia òptica tradicional i d’avantguarda, introduint tècniques i sistemes òptics innovadors dissenyats per abordar els desafiaments de la fabricació additiva. L’estudi comença amb una anàlisi dels processos de fabricació additiva existents, avaluant-ne els punts forts i febles. Posteriorment, es valoren les tècniques de metrologia més avançades, identificant-ne les limitacions per capturar geometries intrincades i propietats dels materials de fabricació additiva. Per abordar les limitacions crítiques de la metrologia tradicional en la manufactura additiva, aquesta tesi introdueix un nou sistema òptic que millora l’eficiència i la velocitat de mesura. La metrologia tradicional afronta reptes com l’elevada rugositat superficial i les variacions en la reflectivitat, que dificulten la precisió i l’escalabilitat en els processos de manufactura additiva. El sistema proposat integra un disseny òptic personalitzat, incloent-hi una lent de tub dissenyada a mida per mitigar aquests desafiaments. Amb aquesta millora en les capacitats de microscòpia, aquesta tesi tanca la bretxa entre les tècniques de metrologia existents i els requisits creixents de la manufactura additiva. A més, es presenten diversos dissenys òptics per a un objectiu de microscopi, especialment personalitzats per abordar els reptes únics en la inspecció de peces fabricades additivament. A la segona part es presenta una tècnica innovadora de metrologia òptica que permet velocitats de captura topogràfica sense precedents, superant significativament el rendiment dels mètodes convencionals. La integració d’aquest mètode en els fluxos de treball de la manufactura additiva es facilita mitjançant tècniques del control del focus axial, com l’enfocament remot i les lents varifocals. A més, la recerca explora els principis fonamentals del mètode, aprofitant propietats òptiques com la interferència i la polarització, alhora que es demostra la seva versatilitat mitjançant la implementació en un microscopi confocal. Aquestes aportacions milloren col·lectivament l’estat de l’art en la metrologia òptica. En conclusió, aquesta tesi doctoral fa contribucions significatives als camps de la fabricació additiva i la metrologia òptica a través d’avanços innovadors en sistemes òptics, el desenvolupament de dissenys d’objectius de microscopi personalitzats i la introducció d’una nova tècnica de metrologia òptica per a la captura topogràfica.
Few Interacting Particles in Low-Dimensional Quantum Systems: From Harmonic Oscillators to Fractal Lattices
(Universitat de Barcelona, 2025-02-21) Rojo Francàs, Abel; Juliá-Díaz, Bruno; Universitat de Barcelona. Facultat de Física
[eng] Quantum mechanics is a fascinating field to explore, where its effects are usually non- trivial and counterintuitive. In this Thesis, we focus on systems with a small number of particles in low dimensions, where the quantum effects are enhanced. We consider experimentally realistic systems, considering either an external harmonic confinement or a lattice potential. These systems can be created nowadays in ultracold atom laboratories worldwide, where they can control the dimensionality, the external potential, as well as the number of particles and the interactions. Our goal is to understand the static and dynamic properties of different systems involving few particles and a wide range of interactions. In particular, we focus on the one-dimensional harmonic trap, fractal lattices, and one-dimensional three-site lattices. In each system, we consider a contact interaction potential defined by a delta function in the continuum space or an on-site interaction for the lattices. In addition, we compute different properties, such as energy spectra, densities, pair correlations, mean square displacement, and population of each site, in both static and time-dependent cases. We combine analytical and numerical techniques to study the different systems. In particular, the numerical part is mostly done using exact diagonalization techniques. With this approach, we can only examine systems with few particles due to the intrinsic limitations of the method, although it provides the capacity to obtain any property needed. The case of the harmonic oscillator trap includes an additional limitation due to the exact diagonalization method, as the basis must be truncated, resulting in upper-bound energies. We show how to correct this error and obtain a better estimate of the energy and the density by using the analytical solution of the two-particle system. We demonstrate that this correction works for a larger number of particles by computing results for up to eight particles. For the systems with harmonic oscillator confinement, we compute the energy spectrum as a function of the interaction strength, the density for different interaction values, and pair correlations. We present the results for the symmetric SU (N ) case and then study systems with broken interaction symmetry. We present different interaction configurations, and we explore the ground state structure, where the correlations play an important role. We also consider the impurity case, where one particle interacts with all the bath particles but the bath particles do not interact with each other. We show that the impurity system can be mapped to an effective one particle in a double-well model, showing two phases: the miscible and the immiscible. Afterwards, we study fractal lattice systems, where the sites and the tunneling connections are configured in a non-standard scheme creating an effective finite representation of a fractal structure. Under this situation, we explore the effect on the transport of a single-particle obtaining the diffusion exponent and relating it to spectral properties. We demonstrate that the fractal slows down the motion of the particle and that this effect is robust against a random potential. Using the slower dynamics, we show how the fractal system can preserve information about the initial phase of the wavefunction for much longer times than a regular lattice. We also explore the entanglement of a two-particle system and how the interactions affect entanglement creation. Finally, we study a three-site lattice where each site has a different energy and the couplings are time-dependent. We implement the spatial adiabatic passage protocol, that transfers a single particle from the first to the third site, and generalize it for a few particle systems with interactions. Due to the interactions, the adiabaticity of the protocol is lost, but we demonstrate that it is possible to populate the third state for certain interaction strength values. As a result, since the final state populates the most energetic site of the system, we propose this setup as a quantum battery model.
Advanced Transition Metal Oxide Nanomaterials for Energy Technologies
(Universitat de Barcelona, 2025-02-10) Yang, Linlin; Cabot i Codina, Andreu; Martinez Alanis, Paulina R.; Universitat de Barcelona. Facultat de Física
[eng] Transition metal oxides (TMOs) are compounds formed from transition metals and oxygen, known for their complex and versatile structures, including diverse crystal forms and multiple oxidation states. This versatility gives TMOs unique electronic, magnetic, optical, and thermal properties, making them valuable in advanced technological applications. The ability of TMOs to engage in redox reactions, coupled with their stability abundant availability, and cost-effectiveness, makes them ideal for applications such as electrochromic devices, catalysis, and batteries. Electrochromic smart windows (ESWs) offer an attractive option for regulating indoor lighting conditions. Electrochromic materials based on ion insertion/desertion mechanisms also present the possibility for energy storage, thereby increasing overall energy efficiency and adding value to the system. However, current electrochromic electrodes suffer from performance degradation, long response time, and low coloration efficiency (CE). In Chapter 2, I detail the synthesis mechanism of defect-engineered brookite titanium dioxide (TiO2) nanorods (NRs) with different lengths and investigate their electrochromic performance as potential energy storage materials. The controllable synthesis of TiO2 NRs with inherent defects, along with smaller impedance and higher carrier concentrations, significantly enhanced their electrochromic performance, including improved resistance to degradation, shorter response times, and enhanced CE. The electrochromic performance of TiO2 NRs, particularly longer ones, is characterized by fast switching speeds (20 s for coloration and 12 s for bleaching), high CE (84.96 cm2 C−1 at a 600 nm wavelength), and good stability, highlighting their potential for advanced electrochromic smart window applications based on Li+ ion intercalation. This work was published in Small in 2023. Direct urea fuel cells (DUFCs), generating electric power from the electrooxidation of urea have great potential as a cost-effective technology to simultaneously treat urea-containing wastewater and produce electricity. DUFCs release only gaseous products, not generating new waste, and they are characterized by a relatively high theoretically open circuit voltage (OCV) of 1.147 V, similar to that of hydrogen fuel cells. However, the relatively low OCVs and peak power densities realized so far have hindered their commercialization. Therefore, improving the OCVs and peak power densities of DUFCs using low-cost and abundant transition oxide nanoparticles as catalysts is required to ensure practical significance. In Chapter 3, I detail the production of self-supported electrodes consisting of NiO nanosheets vertically grown on CuO nanowires and use them to realize the urea oxidation reaction (UOR). Such electrodes show excellent UOR performance requiring 1.39 V vs. reversible hydrogen electrode (RHE) to achieve 100 mA cm-2. Besides, DUFCs provide OCV and power densities up to 0.88 V and 11.35 mW cm-2. Electrochemical characterization and Raman spectroscopy prove the formation of NiOOH to enable the UOR. Mott-Schottky analysis and ultraviolet photoelectron spectroscopy show the NiO/CuO p-p heterostructure to facilitate the charge transfer from CuO nanowires to NiO nanosheets. Besides, at a local level, density functional theory calculations show that the presence of CuO modulates the electronic states of Ni at the very NiOOH/CuO interface, which results in stretched Ni-O bonds and a uniquely elongated N-H bond of urea that favor its oxidation. This work was published in Nano Energy in 2023. Rechargeable aqueous zinc-air batteries (ZABs) have emerged as a promising candidate technology for energy storage applications owing to their high energy density, safety, and environmental friendliness. However, ZABs are limited by the sluggish kinetics of the multiple electron-proton coupling processes involved in the oxygen evolution reaction (OER) that takes place at the air cathode during ZAB charging. Therefore, the development of transition oxide nanoparticles as highly efficient, low- cost, and durable OER catalysts is crucial for the realization of high-performance ZABs, among other electrochemical technologies. Beyond optimizing electronic energy levels, the modulation of the electronic spin configuration is an effective strategy, often overlooked, to boost activity and selectivity in a range of catalytic reactions, including the OER. This electronic spin modulation is frequently accomplished using external magnetic fields, which makes it impractical for real applications. In Chapter 4, I detail the synthesis of Ni/MnFe2O4 heterojunctions and apply them in OER. I investigate the spin modulations of the reconstrued NiOOH/MnFeOOH during the OER by the heterojunctions without an external magnetic field. NiOOH/MnFeOOH shows a high spin state of Ni, which regulates the OH− and O2 adsorption energy and enables spin alignment of oxygen intermediates. As a result, NiOOH/MnFeOOH electrocatalysts provide excellent OER performance with an overpotential of 261 mV at 10 mA cm−2. Besides, rechargeable ZABs based on Ni/MnFe2O4 show a high OCV of 1.56 V and excellent stability for more than 1000 cycles. This outstanding performance is rationalized using density functional theory calculations, which show that the optimal spin state of both Ni active sites and oxygen intermediates facilitates spin-selected charge transport, optimizes the reaction kinetics, and decreases the energy barrier to the evolution of oxygen. This work was published in Adv. Mater. in 2024. The main conclusions of this thesis and some perspectives for future work are presented in the last
Enhanced Optical Response in Arrays of Multifunctional Plasmonic Nanostructures
(Universitat de Barcelona, 2025-04-11) Rodríguez Álvarez, Javier; Fraile Rodríguez, Arantxa; Labarta, Amílcar; Universitat de Barcelona. Facultat de Física
[eng] This thesis is devoted to the study of two plasmonic nanostructures that present a 3-fold symmetry, namely, inverted honeycomb lattices of bars and twisted stacks of triskelion nanostructures. These systems are particularly interesting due to the inherent geometric frustration originated by the mis- match between dipolar excitations, and the odd parity associated with the 3-fold symmetry. The combination of FDTD simulations with the fabrica- tion and subsequent far-field and near-field optical characterization of such structures allows for a multifaceted description of the system. In this thesis, a remarkable agreement between experiments and simulation is achieved, demonstrating the effectiveness of this combined approach in elucidating the response of plasmonic systems. This thesis begins with a fundamental overview of the field, laying the groundwork for the essential concepts necessary for understanding the main findings presented in subsequent sections. These results are discussed in detail through the publications derived from this research. Furthermore, simulation methodologies, nanofabrication techniques, and characterization methods are introduced, as they form the core of the research presented herein. Publications I and II are devoted to the study of inverted honeycomb plas- monic lattices. Here we prove the potential of such structures as refractive index sensors, taking advantage of the sharp SLR and the well-defined spec- tral dependence with the refractive index. The general sensing capabilities of this SLR can easily be expanded due to the out-of-plane electric field of hot spots spanning hundreds of nanometers away from the structure surface, providing a huge potential sensing volume. From a fundamental point of view, we have successfully characterized the plasmonic response of this system through state-of-the-art EELS experiments and FDTD simulations. By using EELS, both bright and dark modes can be detected. In particular, we present the first observation of resonances with an anti-ferroelectric arrangement of the dipolar excitations of the slits in the honeycomb lattice that occur with such spatial periodicity so that their unit cell has twice the area of the honeycomb lattice. The samples presented in this part have been fabricated by EBL and specially dedicated FIB milling using Au ions to avoid contamination of the sample. Publications III and IV focus on the study of two stacked triskelia nanostruc- tures, and their response as a function of the geometry of the structure, in particular, the twist angle between them. The triskelion motif is character- ized by its 3-fold symmetry and inherent two-dimensional chirality in 2D. This system holds two coupled plasmonic resonances tunable by control- ling the angle between both triskelia. We have demonstrated that a simple bonding-antibonding model is insufficient to fully elucidate the behavior of these two resonances. Instead, we have observed a continuous evolution of the excited modes as a function of the angle between the elements. Further insight into the combination of such resonances with SLR is proposed. The fabrication of this structure by successive EBL over large areas and high degree of alignment are also detailed.
Development of a Neurovascular Microfluidic Model with an Endothelial Barrier-Integrity Sensor for Pharmaceutical Innovation in Neurodegenerative Diseases
(Universitat de Barcelona, 2025-03-14) Palma Florez, Sujey; Mir Llorente, Mònica; Lagunas Targarona, Anna; Universitat de Barcelona. Facultat de Física
[eng] Neurodegenerative diseases represent a significant health challenge, as no cure is currently available. The unique anatomy of the blood-brain barrier, which limits drug delivery to the brain, as well as the lack of predictive pre-clinical models, contribute to delays in the drug discovery process. This research presents the development of two distinct 3D models aimed at advancing pharmaceutical innovation for neurodegenerative diseases. We designed a blood-brain barrier-on-a-chip device that incorporates trans-endothelial electrical resistance electrodes for real-time assessment of barrier integrity. The 3D microfluidic design involves the co-culture of brain endothelial cells, pericytes, and astrocytes, before being fully characterized to verify the proper development of the blood-brain barrier. The developed platform was then utilized to evaluate the permeability and toxicity of novel nanotherapeutic agents for neurodegenerative diseases. Furthermore, a more advanced in vitro model for drug discovery of neurodegenerative diseases was assembled. This model incorporated human induced-neurons along with other cell types to create a neurovascular-unit in vitro. Specifically, we co-cultured human induced neurons with oligodendrocytes, astrocytes, pericytes, and endothelial cells within a microfluidic device. Finally, we monitored the release of neurofilament light, a potential biomarker of neuronal degeneration, in response to exposure to two different neurotoxic agents. Quantifying neurofilament light release enables assessment of neurodegenerative progression and evaluation of potential therapeutic interventions to mitigate disease advancement. The blood-brain barrier-on-a-chip and neurovascular-unit-on-a-chip models represent a versatile and scalable platform that offers a cost-effective, human-relevant alternative to traditional animal models. These platforms facilitate drug screening and accelerate the discovery of treatments for neurodegenerative diseases.
Directed cell migration: forces, shapes, and fluctuations in tissues. From active hydrodynamics to experiments
(Universitat de Barcelona, 2025-03-21) Pi Jaumà, Irina; Casademunt i Viader, Jaume; Universitat de Barcelona. Facultat de Física
[eng] This thesis investigates collective cell migration in epithelial tissues, under the framework of active soft matter physics. We model epithelial tissues as active polar fluids, since their components, the cells, have an internal source of energy and align, polarizing, in order to generate movement. Despite the myriad of extremely complex chemical interactions and signaling cascades within cells, ultimately motion must be governed by the most basic laws of physics. The focus of this thesis, therefore, is to use a simple and phenomenological model, encoding all these complex interactions in terms of the physical forces and mechanical parameters of the tissue. This allows us, in a very simplified but effective way, to have a generic model to model several relevant scenarios in collective cell migration. One of the main focuses of the thesis is the collective durotaxi, a phenomenon by which the migration of cellular aggregates is directed by the rigidity of the extracellular environment. Comparing the model with in vitro experiments, we identify the existence of an optimal stiffness of the substrate that maximizes the migration speed, and we observe that the wetting properties of the aggregates are closely linked to it. In addition, in the same way that the tensile forces made by cells are influenced by the stiffness of the substrate or the environment, we also study the inverse feedback mechanism, that is, how these forces can modify the rigidity of the environment. We find that their stiffening can be a mechanism to induce a jump to a much higher state of traction and trigger the spreading of the tissue. Durotaxi could have implications in fundamental biological processes where there is directed movement of cells, such as in embryonic development or cancerous metastasis. Apart from durotaxis, other factors that contribute to targeted migration are examined, such as the influence of tissue shape and fluctuations. We experimentally demonstrate that an asymmetry in shape can induce spontaneous tissue movement, without requiring global polarization, and we classify different modes of migration according to the parameters of the model. By carrying out epithelial tissue migration experiments, where we control their initial shape, we corroborate some of these predictions, especially the distinction between an anisotropic and an isotropic expansion. Finally, to incorporate the inherent variability of biological systems, we introduce stochastic noise into the model and study the consequences of different noise sources. The noise can be internal in the dynamics of polarity, or external in a coupling parameter between the cells and the substrate, related to the adhesion kinetics of the ligands. Although the model captures well some aspects of the fluctuations of the tensile forces in the experiments, such as their magnitude with the distance from the center of the fabric or the temporal correlations, other aspects remain to be explored in more depth, such as the spatial correlations and the diffusion coefficients of the center of mass of the fabric. In conclusion, this work provides a framework to decipher the physical mechanisms underlying collective cell migration, thus contributing to the knowledge of various biological processes, such as tissue regeneration, wound closure or tumor progression. The development of simple but effective theoretical models can also inspire new experimental designs, allowing a fruitful integration between theory and experiments in our quest to explain nature.
High Entropy Materials as Air Cathodes for Robust Zinc-Air Batteries
(Universitat de Barcelona, 2025-02-10) He, Ren; Cabot i Codina, Andreu; Universitat de Barcelona. Facultat de Física
[eng] This thesis focuses on the development of high-entropy materials (HEMs) as advanced bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The study systematically investigates the synthesis methods, structural properties, and catalytic performance of these materials, with particular emphasis on their application in zinc-air batteries (ZABs). Through the incorporation of transition metals and experiencing surface reconstruction processes, these materials exhibit remarkable catalytic efficiencies and stability. Density functional theory (DFT) calculations provide further insights into the active sites and mechanisms driving the enhanced catalytic activity. This research highlights the potential of high entropy alloys (HEAs) and high-entropy phosphides (HEPs) as next-generation catalysts, paving the way for future advancements in energy storage and conversion technologies. In Chapter 2, I detail the development a low-temperature colloidal method to synthesize CrMnFeCoNi and CuMnFeCoNi HEAs, along with quaternary and ternary alloys. CrMnFeCoNi displays superior bifunctional catalytic performance for both OER and ORR, outperforming CuMnFeCoNi, quaternary alloys, and commercial catalysts like RuO2 and Pt/C. DFT calculations reveal that the incorporation of Cr into the MnFeCoNi matrix lowered the energy barriers for OER and optimized ORR intermediate steps. This material exhibits high power density, specific capacity, and excellent long-term cycling stability when applied as a bifunctional catalyst in ZABs, underscoring its potential for energy storage applications. This work was published in Energy Storage Materials in 2023. Chapter 3 presents the synthesis of FeCoNiMoW HEA by incorporating 4d Mo and 5d W into a 3d FeCoNi matrix using a low-temperature solution-based method. The resulting alloy demonstrates a highly distorted lattice and strong electronic coupling effects. The FeCoNiMoW HEA exhibits excellent catalytic performance for OER with low overpotentials and great bifunctional properties, surpassing commercial catalysts like Pt/C and RuO2. DFT calculations identify Ni as the active site for OER, with Mo and W enhancing oxygen intermediate interactions. The FeCoNiMoW-based ZABs show high power density, specific capacity, and exceptional long-term stability, even 1 in flexible applications, making them a promising candidate for wearable energy devices. This work was published in Advanced Materials in 2023. In Chapter 4, I detail the synthesis of FeCoNiPdWP HEPs via a mild colloidal method, resulting in a homogeneous nanostructure. These HEPs demonstrate exceptional bifunctional catalytic performance for both OER and ORR, with a low overpotential of 227 mV for OER and a half-wave potential of 0.81 V for ORR. The outstanding OER performance is attributed to the reconstructed FeCoNiPdWOOH surface, enriched with high-oxidation-state Fe, Co, and Ni. Pd facilitates OH⁻ adsorption, while W modulates the electronic structure for better oxygen intermediate adsorption. For ORR, surface reconstruction into FeCoNiPdWPOH further enhances performance, with Pd and W maintaining their phosphide environments and Pd as the main active site for ORR. The small energy gap between OER and ORR enables FeCoNiPdWP HEPs to achieve over 700 h of stable operation in ZABs, showcasing their potential for long-term and highly efficient bifunctional catalysis. This work was published in Energy & Environmental Science in 2024. The main conclusions of this thesis and some perspectives for future work are presented in the last.
A multidisciplinary approach to rheological blood characterisation and angiogenesis modelling using microfluidics
(Universitat de Barcelona, 2025-01-24) Ferré Torres, Josep; Hernández Machado, Aurora; Universitat de Barcelona. Facultat de Física
[eng] Blood viscosity plays a critical role in cardiovascular health, influencing hemodynamic processes and disease states. Accurate and rapid measurement of blood rheological properties is essential for diagnostic and therapeutic interventions. Concurrently, understanding the mechanisms of endothelial cell migration is fundamental to elucidating angiogenesis and developing treatments for vascular diseases. This thesis presents a comprehensive study encompassing the design and industrialisation of a novel medical device for blood viscosity measurements alongside the mathematical modelling of collective chemotactic endothelial cell migration. A microfluidic system employing fluid front rheology was developed, featuring a microchannel equipped with electrodes to facilitate efficient and rapid analysis of blood samples within five minutes. This innovative methodology requires minimal sample volumes and provides reliable rheological data, demonstrating significant potential for integration into routine clinical workflows. The device’s capability for high-throughput screening addresses the need for timely diagnostic and therapeutic decision-making in clinical settings. The analysis of plasma viscosity revealed greater variability than anticipated, yet plasma samples predominantly exhibited Newtonian behaviour, aligning with established theoretical models. Factors such as protein concentration, red blood cell lysis, and overall plasma composition contribute to this variability. Blood viscosity measurements indicated substantial variability across different samples, underscoring the complexity arising from the non-Newtonian, shear-thinning nature of blood. Individual differences in haematocrit levels, red blood cell deformability, and aggregation tendencies further complicate the utilisation of blood viscosity as a standalone diagnostic marker. These findings emphasise the necessity for comprehensive patient profiling, including age, sex, medical history, lifestyle habits, and concurrent medications, to enhance the diagnostic accuracy of rheological assessments. The considerable overlap in viscosity profiles between healthy and non-healthy individuals suggests that blood viscosity measurements should be integrated with other clinical data and biomarkers to improve understanding of a patient’s haemodynamic status. Personalised diagnostic approaches are recommended to optimise patient outcomes through targeted interventions. In parallel, the thesis addresses the computational challenges of modelling chemotactic endothelial cell migration on moving and deformable domains. A phase-field method was employed to solve equations governing the dynamics of growing capillaries and the extracellular matrix using a fixed mesh framework. An energy functional accounting for local chemoattraction at the cell membrane was proposed, effectively reproducing actin behaviour observed in experimental studies. A finite difference numerical method was developed, proving efficient, accurate, and robust, facilitating the investigation of cell migration in two-dimensional environments. Simulations demonstrated that migration on flat substrates leads to stationary states of motion, consistent with experimental observations. Introducing obstacles enabled the reproduction of complex migratory behaviours, highlighting the cells’ ability to exploit microenvironment geometry for effective migration. The inclusion of extracellular matrix degradation mechanisms allowed tip cells to navigate towards maximum chemotactic gradients by creating their pathways, simulating the action of matrix metalloproteases. This modelling provides valuable insights into the autonomy of tip cells in directing migration and the interplay between cellular activities and extracellular matrix modifications. This simulation approach offers a new framework for understanding cell migration dynamics within complex environments, emphasising the intricate relationships between cellular behaviour, environmental structure, and matrix remodelling. The model simulates migrating cells and their chemotactic interaction with their surroundings by accurately capturing stationary motion, obstacle navigation, and matrix degradation mechanisms. These insights extend beyond theoretical validation, providing a predictive tool to explore how cells navigate diverse extracellular landscapes. These could inform therapeutic strategies targeting cell migration in tissue regeneration, cancer metastasis, and angiogenesis. Furthermore, this type of modelisation allows state-of-the-art deep learning models to build upon and create systems that would inform the biomaterial engineers of the necessary properties of the surroundings to generate an optimal vasculature network.
Effective field theory methods at high temperature and chemical potential
(Universitat de Barcelona, 2025-03-31) Comadran, Marc; Manuel Hidalgo, Cristina; Universitat de Barcelona. Facultat de Física
[eng] This thesis applies and develops effective field theory methods for the study of plasmas at high temperature and/or density. In the 90s, the theoretical frameworks necessary to study quantum electrodynamics (QED) and quantum chromodynamics (QCD) in these extreme conditions were developed. The tools developed assumed that the mass of the plasma constituents could be neglected. In a first stage of the thesis, we investigate the effects of incorporating small masses associated with the fermionic constituents of the plasma in perturbative calculations, relevant when they are not extremely small compared to the temperature and/or chemical potential that characterizes the plasma. Our study provides a first step to address this impact, calculating small massive corrections to both the photon and gluon polarization tensor, under the hard thermal loop approach. To evaluate these mass corrections, we show the usefulness of effective field theories, in particular, the on-shell effective field theory (OSEFT) for fermions. Next, we analyze the impact of mass corrections in the context of energy loss due to collision of a highly massive and energetic fermion, which passes through a plasma at high temperature and/or density. Let us consider the following two cases: when the constituents of the plasma are electrons, positrons and photons, and also when these are quarks, antiquarks and gluons. Using dimensional regularization, we effectively manage the new divergences arising from the expansion by small masses and demonstrate a consistent cancellation of divergences between hard and soft contributions, obtaining a finite result. Mass corrections for energy loss are determined in the first order with logarithmic precision, extending the foundational work of Braaten and Thoma for massless fermions. In a second stage of the thesis, we develop a new effective field theory, the OSEFT for abelian gauge fields. The final Lagrangian can be formulated in terms of a gauge-invariant vector field without the need to introduce a gauge fixation term. We prove the invariance under reparameterization (RI) of the theory, which means that the Lorentz symmetry is respected. By exploiting RI symmetry, we provide a derivation from early principles of the side-jump effect for photons. We also present applications of photon OSEFT in the context of electron and positron plasmas, for example, in quantum kinetic theory and in perturbative quantum field theory calculations. In addition, we show that when considering small purely quantum effects, the classical definition of polarization ratios, given in terms of the Stokes parameters, loses its Lorentz invariance. We therefore propose a new definition of polarization ratios, which is Lorentz invariant when these small quantum effects are present, relevant in reference systems that are not at rest with respect to the medium and with possible applications in astrophysics and cosmology, where these conditions are common. Finally, we construct a quantum kinetic theory for photons in the presence of a background of axions, at the so-called collision-free limit, from the complete theory of quantum electrodynamics. We show that, in the classical regime, kinetic equations exhibit well-known features of electrodynamics with axions. The formalism we present allows us to systematically calculate how the classical limit is corrected due to small quantum effects. In addition, we address the impact of the axion background on the collective modes of photons that occur in electron and positron plasmas in thermal equilibrium. Notably, the axion background breaks the degeneration of the transverse collective modes, while the longitudinal collective mode, called plasmon, is not affected.
New experimental techniques for axion searches in the RADES and BabyIAXO experiments
(Universitat de Barcelona, 2025-02-20) Cogollos Triviño, Cristian; Picatoste Olloqui, Eduardo; Doebrich, Babette; Universitat de Barcelona. Facultat de Física
[eng] Initially proposed as part of the solution to the strong CP problem of the Standard Model of particle physics, axions and axion-like particles appear on several Beyond Standard Model extensions. These pseudoscalar pseudo-Nambu-Goldstone bosons could also be the answer to one of the most puzzling questions on cosmology, the Dark Matter problem. For decades, searches have been performed for finding these elusive particles. The most promising of which deals with the conversion of axions into photons and their observation. Depending on the origin of these axions we can distinguish between two main kind of experiments, haloscopes (for dark matter axions) and helioscopes (for solar axions). In this document we compile three different contributions to axion searches within the same framework that have been inside the scope of the thesis work during the last years. The first comprises the work developed in a proposal that paves the path for haloscope searches at BabyIAXO, in particular a set of 5 meter long cavities are proposed for covering the structures already used at CAST for axion searches. In the second part we expose the characterization and optimization of the radioactive background of the acquisition electronics for the BabyIAXO helioscope. In the last part a general framework for treating multicavity resonators is presented as an extension of the theory developed for the first RADES prototype, based on this idea two new prototypes were designed, manufactured and characterized.
Salt dependent DNA translocation dynamics across nanopores
(Universitat de Barcelona, 2025-04-25) Colchero Truniger, Alejandro; Ritort Farran, Fèlix; Pastor del Campo, Isabel; Universitat de Barcelona. Facultat de Física
[eng] This thesis uses two complementary single-molecule techniques, nanopipette microscopy and optical tweezers, to investigate the impact of various monovalent salts at high concentrations in DNA. The thesis begins by characterizing the conductance and noise properties of nanopipettes in a wide range of concentrations. Once the nanopipettes have been characterized, they are used to conduct DNA translocation experiments to examine how different cations influence their translocation parameters. These experiments also allow us to explore the effects of applied voltage and salt concentration on DNA folding configurations during translocation. In addition, we explore why compact DNA folding configurations have lower dwell times compared to longer folding configurations. Finally, optical tweezers are used to carry out stretching experiments on a DNA fork, providing information on DNA stability under conditions of high ionic strength related to DNA translocation experiments. The thesis is divided into six parts. Part I provides an introduction to the experimental techniques used throughout this work, along with the theoretical frameworks and concepts that will be needed for Parts II, III and IV. This part is divided into three chapters. Chapter 1 is an introduction to the nanofear field. The chapter describes the setup and basic concepts required to perform electrical measurements with nanopipettes and discusses the limits of resolution of the technique. The main sources of noise when experiments with nanopores are carried out are also described. Also, some relevant nanofluidic phenomena are introduced when working at the nanoscale, along with some theoretical foundations on the regulation of surface load. Finally, some previous results of DNA translocation through nanopores are shown. Chapter 2 introduces the optical trap and mini-tweezers configuration that was used to perform the experiments in this thesis. Chapter 3 introduces the basic components and structure of nucleic acids, focusing on DNA, which will be the biomolecule studied throughout the thesis. The chapter concludes by presenting the theoretical foundations of two elastic models used to describe the elastic properties of polynucleotides. Part II contains results of experiments with nanopipettes. It begins with chapter 4, where the conductance of nanopipettes for different salt concentrations is studied, comparing the contributions of conductance in volume and surface area and the effect of pH on surface loading. In addition, two conductivity models are compared to model the conductance of nanopipettes with concentration. In addition, the blinking noise of the nanopipettes is analyzed, exploring how the parameters that describe the noise change with concentration and tension. Chapter 5 presents λ-DNA translocation experiments in different monovalent salts. This chapter investigates the effect of concentration and cation type on translocation parameters such as residence time, electric current blocking, and charge blocking of λ-DNA translocations. The chapter focuses on how the cation-DNA interaction changes with the size of the cation. Chapter 6 studies the different folding configurations that occur during DNA translocation and how they depend on salt tension and concentration. To do this, an analysis of the different levels that occur during λ-DNA translocation and the residence time of the different levels is carried out. The analysis also allows us to extract general data on DNA translocation through nanopipettes. Finally, the causes of the lower dispersion of the dwell time of the more compact folding configurations compared to the longer folding configurations are explored. Part III includes experiments with optical tweezers. In Chapter 7, optical tweezers are used to conduct stretching experiments of long DNA forks at high ionic concentrations of various monovalent salts. The average opening force and the effect of concentration on fork stability are investigated. The chapter concludes with a joint discussion of the results of translocation experiments and optical tweezers at high salt concentrations. Part IV contains the results of a one-month international stay. Chapter 8 includes a brief introduction to the SPRNT (Single-molecule picometer resolution nanopore tweezers) technique, which was used to perform experiments with the Hel308 helicase. In addition, some preliminary experiments of the with the gp41 helicase using SPRNT are presented. Part V contains the final conclusions of the thesis and the future perspectives of the work. Part VI consists of all the Appendices. Appendices E and C complement the results of some of the chapters, while Appendices A, B, F, and D describe in detail the most important experimental protocols and MATLAB codes used in the development of the thesis.

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