Dipòsit Digital de la Universitat de Barcelona

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El Dipòsit Digital de la Universitat de Barcelona és el repositori institucional que conté en format digital els materials derivats de l'activitat docent, investigadora i institucional de la comunitat universitària.

Enviaments recents

Vortex dynamics in Bose-Einstein condensates
(2026-06) Lin, Yixue; Juliá-Díaz, Bruno; Briongos-Merino, Héctor
The theoretical framework used to understand the Bose-Einstein condensate (BEC) phenomenon is presented, with emphasis on the case with strong interactions, the so-called Thomas-Fermi regime. Superfluidity, a consequence of BEC important for the understanding of liquid helium, is characterized by the presence of quantized vortices. A heuristic ansatz for quantized vortices is presented, aiming to study them in a simulation, where the numerical implementation is based on the split-step Fourier method. Imaginary-time evolution is performed for the free vortex and one displaced vortex case. With the objective of studying the dynamics of the latter, real time evolution is performed. Finally, the results are briefly discussed
Spatial characterization of thickness in thin films deposited by thermal evaporation
(2026-06) Lara Rodríguez, Raül; Galceran Vercher, Regina
This work analyzes the spatial thickness distribution of aluminum thin films deposited onto a glass substrate via thermal evaporation. A methodology combining a gridded lift-off mask with optical mapping enabled precise large-area topographic characterization. The experimental data were fitted to the Knudsen cosine law (R2 = 0.92), confirming a radial thickness decay with a maximum central thickness of d0 = (93.0 ± 0.5) nm. The empirical beaming parameter (n = 0.9 ± 0.3) is highly consistent with an ideal planar surface source (n = 1). This demonstrates that the crucible used distributes the atomic flux predictably, acting as a flat emitting area without inducing severe collimation. Finally, the preservation of this distinct directional profile is attributed to the high-vacuum regime, which prevents the intermolecular flux dispersion typical of higher working pressures.
Weak Interactions as a mechanism against Gravitational Collapse in Primordial Baryonic Stars
(2026-06) Lampurlanés Rosell, Joan; Espriu, D. (Domènec)
This work investigates whether weak interactions mediated by Z-boson exchange can provide a stabilising pressure against gravitational collapse (GC) in primordial baryonic stars. Building on previous works, we completed the equation of state by incorporating the degeneracy pressure of an ultra-relativistic fermionic gas. A crossover density 𝑛* ≈ 9 · 1010 fm−3 is identified, marking the transition between weak-interaction-dominated and degeneracy-dominated regimes. The Tolman-Oppenheimer-Volkoff equation is solved numerically with a boundary condition derived from the thermal radiation pressure of the primordial plasma, yielding a universal stellar radius of approximately 15 meters. Stability analysis via the turning point theorem reveals that all stable configurations have central densities well below the crossover density, demonstrating that weak interactions do not play a relevant structural role in these compact objects.
The role of pH in the light-harvesting properties of PC645
(2026-06) Lafuente Puig, Jia Laura; Franzese, Giancarlo; Curutchet Barat, Carles E.
The PC645 is a pigment-protein complex that plays a key role in light harvesting algae by absorbing photons and facilitating energy transfer within the photosynthetic apparatus. In this work, we investigate the relation between pH and the light harvesting mechanisms of PC645. We perform classical molecular dynamics (MD) simulations combined with quantum/molecular mechanical polarizable (QM/MMPol) models to derive the exciton Hamiltonian of the system. Using this Hamiltonian, we compute the absorption spectrum of the complex and simulate the energy transfer dynamics between pigments. Our results reveal two effects of acidic pH: a decrease in the site energy of one of the pigments of the complex and a slower Electronic Energy Transfer (EET) rate, a parameter which quantifies how quickly energy is transferred between pigments. This preliminary work establishes the basis for future research, suggesting avenues for improvement and more advanced simulations to be performed.
Phase transition and evolutionary mechanisms in a grand canonical minority game
(2026-06) Jordà i Coll, Enrique; Montero Torralbo, Miquel
We consider the grand canonical minority game as a model for financial markets. We first characterise the phase transition of the standard minority game and the grand canonical variant between a symmetric, unpredictable phase and an asymmetric, predictable phase and identify the critical point. Then we look at two evolutionary mechanisms: competitive selection and cooperation through imitation. Through overcrowding, competition destroys the asymmetric phase. Limited imitation recovers the asymmetric phase and reduces volatility relative to competitive selection. These results suggest that, in this model, coordination is not maximised by selecting the most successful agents but by preserving sufficient strategic heterogeneity