arXiv subjects

Gilberto Nakamura

Publications and source records attributed to Gilberto Nakamura.

6 recordsLinked to original sources

arXiv2026

Acoustic Resonance Distribution for Core-Shell Scatterers

Acoustic metamaterials can exhibit unusual effective properties through mechanisms such as energy localization and resonant behavior in their constituent building blocks. Here, we investigate the internal acoustic energy of fluid core-shell spheres and map how resonances emerge from the interplay between material contrasts and shell geometry. The resulting phase diagram reveals an organized resonant landscape across distinct impedance-contrast regimes. From this structure, we obtain two compact predictive relations: one for resonance existence and another for spectral recurrence of successive peaks. Together, they enable the rapid identification of material and geometric combinations associated with targeted resonance responses. The framework is further tested against representative systems from the literature, reproducing their observed behavior. These results provide a physics-based route for screening and tailoring resonant core-shell building blocks before full-wave numerical simulations.

physics.app-ph↗

arXiv2025

Restoring detailed balance in non-Hermitian Markov processes

Stochastic processes out-of-equilibrium often involve asymmetric contributions that break detailed balance and lead to non-monotonic entropy production, limiting thermodynamic interpretations and inference techniques. Here we use Dyson maps to restore monotonic entropy growth in those processes, allowing the use of standard tools from statistical physics, providing a general and computationally tractable method applicable to a broad class of Markovian systems.

cond-mat.stat-mech↗

arXiv2024

Trapped acoustic energy and resonances in spherical scatterers

The effectiveness of biochemical antivirals are vulnerable to mutations, motivating physical approaches. Recent experiments with ultrasound reveal viral disruption at MHz frequencies, yet the mechanism remains unclear. We model viruses as fluid-like inclusions and analyze internal acoustic fields. Exact solutions reveal that impedance mismatch induces resonances even for subwavelength scatterers. Beyond the monopole, higher-order modes trap significant energy with significant resonance peak broadening even in absence of explicit dissipation mechanisms. These findings suggest internal acoustic resonances as a mechanism for viral destabilization and acoustic metamaterial applications.

physics.app-ph↗

arXiv2024

On the duration of face-to-face contacts

The analysis of social networks, in particular those describing face-to-face interactions between individuals, is complex due to the intertwining of the topological and temporal aspects. We revisit them both, using public data recorded by the sociopatterns wearable sensors in some very different sociological environments, putting particular emphasis on the contact duration timelines. As well known, the distribution of the contact duration for all the interactions within a group is broad, with tails that resemble each other, but not precisely, in different contexts. By separating each interacting pair, we find that the fluctuations of the contact duration around the mean-interaction time follow however a very similar pattern. This common robust behavior is observed on 7 different datasets. It suggests that, although the set of persons we interact with and the mean-time spent together, depend strongly on the environment, our tendency to allocate more or less time than usual with a given individual is invariant, i.e. governed by some rules that lie outside the social context. Additional data reveal the same fluctuations in a baboon population. This new metric, which we call the relation *contrast*, can be used to build and test agent-based models, or as an input for describing long duration contacts in epidemiological studies.

physics.soc-ph↗

arXiv2023

How long do we stand our colleagues? A universal behavior in face-to-face relations

We compare face-to-face interaction data recorded by wearable sensors in various sociological environments. The interactions among individuals display a clear environment-dependent diversity in agreement with previous analyses. The contact durations follow heavy-tailed distributions although not exactly of power-law type as previously suggested. Guided by the common patterns observed for each relation, we introduce a variable named the duration contrast, which reveals a common behavior among all datasets. This suggests that our tendency to spend more or less time than usual with a given individual in a face-to-face relation is not governed by social rules but by a common human trait. Additional data shows that it is the same for baboons. Furthermore, we propose a new kind of model to describe the contacts in a given relation based on the recently introduced concept of Levy Geometric Graphs. It reproduces the data at an impressive level. The associated Levy index is found to be alpha = 1.1 on all the datasets, suggesting a universal law for primates and opening many exciting perspectives.

physics.soc-ph↗

arXiv2020

Effective epidemic model for COVID-19 using accumulated deaths

The severe acute respiratory syndrome COVID-19 has been in the center of the ongoing global health crisis in 2020. The high prevalence of mild cases facilitates sub-notification outside hospital environments and the number of those who are or have been infected remains largely unknown, leading to poor estimates of the crude mortality rate of the disease. Here we use a simple model to describe the number of accumulated deaths caused by COVID-19. The close connection between the proposed model and an approximate solution of the SIR model provides a system of equations whose solutions are robust estimates of epidemiological parameters. We find that the crude mortality varies between $10^{-4}$ and $10^{-3}$ depending on the severity of the outbreak which is lower than previous estimates obtained from laboratory confirmed patients. We also estimate quantities of practical interest such as the basic reproduction number and the expected number of deaths in the asymptotic limit with and without social distancing measures and lockdowns, which allow us to measure the efficiency of these interventions.

q-bio.PE↗