The Revista Cubana de Física (RCF, Cuban Journal of Physics) is a biannual open access journal (e-ISSN 2224-7939, p-ISSN 0253-9268) published by the Cuban Physical Society (SCF) and the School of Physics of the University of Havana (FF-UH) solely funded by its publishers (without charges to the authors). Three types of articles are considered for publications: Regular Original Articles, Original Short Articles and Review Articles. Likewise, the RCF includes other several sections as: Moments of the Cuban Physics, For Physics and Non-Physics, News and Obituaries. Concerning the language, manuscripts in Spanish as well as in English are welcome in Microsoft Office Word (.doc or .docx) and LaTex (.tex) formats. The peer-review is blind and performed by external reviewers outside the editorial team of the RCF, the SCF or the FF-UH.
Sociedad Cubana de Física & Facultad de Física de la Universidad de La Habana
Computational Optimization of A Stochastic Model for Simulating Apoptotic Signaling
The development of mathematical models to simulate biological processes is essential for understanding the complexity of living systems, allowing for predictions and virtual experiments that would be difficult to carry out under real conditions. In this work, we present a mathematical model to simulate the process of apoptosis or programmed cell death, using the Gillespie algorithm. Additionally, an optimization of this model is proposed, which reduces computation time and enables the model to simulate apoptosis in the cells of a tumor under topical treatment. Results are also presented demonstrating that both models are equivalent. The optimization reduced the execution time of the simulation by two days.
Copyright (c) 2025 Cuban Physical Society & Faculty of Physics of the University of Havana https://creativecommons.org/licenses/by-nc/4.0
S-shaped or sigmoid curves can be defined as the solutions of autonomous first-order differential equations that satisfy four conditions. Without solving the equations, we demonstrate that the solutions of the logistic family and the Smith-Birch model satisfy these conditions. We introduce two generalizations of the Smith-Birch, whose solutions are identified as S-shaped for some range of variation of the parameters. The new models introduced here predict the spread of the disease better than traditional logistic family models for time series of the cumulative number of cases for the first 61 days of the COVID-19 pandemic in some countries.
Mathematical Treatment of the Canonical Finite State Machine for the Ising Model
The complete framework for the minimal deterministic automata construction of the one-dimensional Ising model is presented. The approach follows the known treatment of the Ising model as a Markov random field, where the local characteristic is usually obtained from the stochastic matrix. The problem is the inverse relation or how to get the stochastic matrix from the local characteristics given via the transfer matrix treatment. The obtained expressions allow for performing complexity-entropy analysis of particular instances of the Ising model. Two examples are discussed: the 1/2-spin nearest neighbour and next nearest neighbours Ising model.
The Role of Clay Charge in The Mobility of Compensating Cations
The charge of clays plays an important role in the mobility of compensating cations and in swelling processes. In this work we have developed a method to generate Lithium Fluorhectorite (Li-Fh) clay models with a charge of -1.2e and a non-homogeneous charge distribution. This charge is closer to the experimentally reported value. We used this approach to study their interaction with water using Molecular Dynamics (MD) simulations. The MD simulations showed that the Li+ diffusion coefficient increases by two orders of magnitude with decreasing clay charge. Population analyses and Li+ coordination indicate a greater interaction of the cations with water molecules with decreasing clay charge, leading to a deformation in the stacking of clay layers in the 010 model. These results highlight the important influence of clay charge on cation dynamics and structural behavior, providing insight into delamination and swelling mechanisms.
Metal-Insulator Transition described by NOFT
The metal–insulator transition (MIT) is a fundamental phenomenon in condensed matter physics and a hallmark of strong electronic correlations. Hydrogen-based systems offer a simple yet powerful model for investigating the MIT, as their insulating behavior arises purely from electron–electron interactions. In this work, we study finite hydrogen clusters with cubic geometries using Natural Orbital Functional Theory (NOFT), a method capable of accurately describing correlated systems beyond mean-field approaches. We focus on two key signatures of the MIT: the fundamental energy gap and the harmonic average of the atomic one-particle reduced density matrix. Our results show that NOFT captures the transition from insulating to metallic behavior as the interatomic distance decreases. By extrapolating the energy gap to the thermodynamic limit, we estimate a critical distance rc ~1.2 Å, in excellent agreement with quantum Monte Carlo benchmarks. These findings demonstrate the reliability of NOFT for describing strong correlation effects in large-scale models.
Reduced Density Matrix Functional Theory
Radioactivity Levels and Radiation Hazard in Sands from Cuban Beaches
This study assesses the natural radioactivity levels of 226Ra, 232Th, and 40K in sand samples from six Cuban white-sand beaches to evaluate potential radiological risks. Samples were analyzed using high-resolution gamma spectrometry after a one-month equilibrium period. Activity concentrations were determined via key gamma transitions of daughter nuclides. Results show that all measured values of activity, radium equivalent (Raeq), absorbed dose rate (DR), and gamma index (Iγ) are below internationally recommended safety thresholds. La Coloma beach exhibited the highest 232Th activity, slightly exceeding the global mean, but still within safe radiological limits. The calculated gamma index values (0.04–0.52) and dose rates (3–33 nGy·h⁻¹) confirm minimal radiological risk. This work provides a radiological baseline for Cuban beach sands and supports the need for further studies in other major resort areas across the island
Pyroelectric Response and Figures of Merit of Lead-Free Ferroelectric Ceramics
(Bi0.5Na0.5)1-xBaxTiO3 were synthetized via the conventional solid-state reaction sintering method (considering x = 16 and 18 at%) and polarized under an electric field of 2 kV/mm. The pyroelectric behaviour of the studied samples has been analysed by the static method, from the temperature dependence of the pyroelectric current. The figures of merit FV, FD, Fi and FE were investigated in a wide temperature range. The composition with x = 18 at% showed better results than that of x = 16 at% and other reported ceramic systems, especially for FV and FE parameters. A high thermal stability through the studied temperature range was obtained, which is a relevant behaviour for applications involving high-temperature sensors. The results have suggested that (Bi0.5Na0.5)1-xBaxTiO3 is a suitable candidate for pyroelectric sensors and energy harvesting applications.
Towards the STM: The Story Behind The First Atomic Resolution Image in Cuba
Scanning tunneling microscopy is a widely used tool in nanoscience and nanotechnology research. This technique has the advantage of allowing the study of surface layers without damaging or destroying them and of achieving atomic resolution. Its simplicity compared to other scientific instruments has allowed different laboratories to build their own microscopes. Here we present how the STM at the University of Havana was built and how it achieved atomic resolution for the first time in Cuba.
Photodissociation Dynamics of the Collinear Van Der Waals Complex NeBr2(B, ν=14-34)
We perform trajectory-based simulations of the dissociative dynamics of the collinear van der Waals complex NeBr2(B,ν) following photoexcitation to the B electronic state and for varying vibrational excitations of the dihalogen molecule in the range ν=14-34. The quantum dynamics of unimolecular decay is mapped onto a classical-like dynamics in an extended phase space. This methodology allows us to compute the time-dependent dissociation probability, intramolecular energy transfer mechanisms, and transient vibrational state distributions of the linear triatomic system, which are challenging to access experimentally using modern pump-probe spectroscopy techniques. The computed quantities can be used to refine interaction potential models at collinear geometries, which are less well characterized than the T-shaped isomer.
Study of the Rotation Dynamics of a Rigid Body through the accelerometer of a Mobile Device
The introduction of the mobile devices in the teaching of physics has gained importance in recent times due to the ample range of sensors they contain and their ability to accomplish measurements in real time. The study of the dynamics of a rigid body rotation using the accelerometer of a smart phone was tackled. The problem is a modification of the standard laboratory device consisting in a crossbeam that rotates in a horizontal plane around a vertical axis. Through an ad hoc Android application, it was possible to visualize the behavior and to measure the radial and tangential components to a linear acceleration. Moreover, the moment of inertia and the friction torque of the crossbeam was calculated. Finally, we discuss some particular cases of the main problem that shows the potential of these devices for physics teaching.
Quasi-classical Dynamics of Hydrogen Molecules Trapped Inside Fullerene Cages
We perform molecular dynamics simulations of hydrogen molecules inside fullerene cages, incorporating quantum effects via the Feynman-Hibbs effective potential method. The distance between hydrogen atoms in the molecule is kept fixed by using the constraint dynamics algorithm. We evaluate the energetic properties and the influence of quantum effects for hydrogen molecules in fullerene cages of varying size and geometry (Cn, n=24, 28, 60, 70), and within a wide range of thermodynamics conditions (i.e., from T=130 K to T=320 K). We compute the temperature dependence of quantities such as the translational and rotational kinetic energies, the total energy and the contribution of quantum effects. It is found that quantum corrections to the total energy are significant even at room temperature. We discuss the possible influence of these properties on the hydrogen storage capacity of these materials.
The Two-state Diffusion Problem with Resetting
The concept of stochastic resetting emerged in 2011, introducing a simple modification to [15] the one-dimensional Random Walk that leads to a stationary distribution and where the first-passage time can be easily optimized. In this context, we will investigate what happens when modeling these phenomena in a framework where a Brownian particle alternates between different states. We will derive the Fokker-Planck equations of the model and analyze the behavior of the probability density of the particle positions for long times and solve the corresponding first-passage time problem. Contrasting with the two-state Brownian motion, we will observe how resetting ensures a stationary behavior in the long run leading to a finite expectation for the first-passage problem. Finally, we will demonstrate how the existence of this expectation guarantees minimum values by varying the resetting or state-change parameters.
Particle Fluence Estimation for Samples Irradiated at a Long-Term Exposure Station (NICA Complex, Dubna)
The prototype of the Target Station for Long-Term Exposure was assembled after all detectors of BM@N Experiment. During collection of the physical data with the 124Xe54 beam of 3.8 GeV kinetic energy at BM@N, different samples were irradiated. Data was analyzed for precise determination of the intensity, the fluence and the absorbed dose for irradiated materials. The beam intensity and profile distributions were determined for each sample. From the obtained intensity, the fluence was calculated for each irradiated sample. Then, the absorbed dose of irradiated materials was be calculated. The study was performed within the ARIADNA Collaboration.
A Finite Element Method Exploration of Technologically Relevant Geometries in PiezoelectricEnergy-Harvesting PZT-4
In a piezoelectric material, any external action that can generate a mechanical deformation could be a potential source of electrical energy. This phenomenon is studied for the development of microscale devices, using as external agents the hydraulic pressure of a fluid, the movement of the human being and others. Nowadays, the main researches are directed to the optimization of the properties of the developed piezoelectric and also to the simulation of different situations that allow the optimization of the energy harvesting devices. The present paper shows some simulations, which have been made by using the finites elements method to simulate the energy harvesting; the results are presented for a commercial piezoelectric, PZT-4.
Dynamic Cavity Method in Fully-Asymmetric Models
The physics of disordered systems is a broad and constantly evolving field. In this work, we focus on the study of discrete variable models with asymmetric interactions, in particular the fully-asymmetric ferromagnet and the fully-asymmetric Sherrington-Kirkpatrick. We use the cavity master equation, a well-known technique for the out-of-equilibrium dynamics, to derive average equations describing the time evolution of the magnetization and the energy in these models. In this way, we recovered previous results for the magnetization known from the literature and obtained new equations for the energy. With this work, we contribute to establish the cavity master equation as one of the most relevant techniques in the study of out-of-equilibrium systems and clarify its relationship with previous methods.
DFT study of BaKN3 and BaRbN3 Perovskites: Revealing their Mechanical, Optoelectronic, and Magnetic Properties
The full-potential linearized augmented plane waves (FP-LAPW) method, which is based entirely on density functional theory (DFT), was used to investigate the structural, electronic, magnetic, optical, and elastic properties of BaXN3(X=K, Rb). This method also employs the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) and a modified Beck Johnson TB-mBJ potential in the exchange correlation term. When the resulting structural properties were examined, the findings showed that our compounds are more stable when they are configured as ferromagnetic materials. The total and partial density of state curves were used to assess the contributions of the various bands. Additionally, we discovered that the total magnetic moment is an integer of 6 µB, confirming the half-metallic nature. The mechanical stability of these compounds has been discovered. The elastic parameters are obtained, including the elastic constants, bulk modulus, anisotropy factor, Poisson's ratio, and Pugh's ratio.
Density-functional theory atomic and molecular physics
Correlating Texture and Electromechanical Properties of Activated Carbons
This study presents a simplified experimental method for evaluating the electro-mechanical behavior of activated carbons derived from agro-industrial residues such as sugarcane bagasse, coconut shells, peanut hulls, and rice husks. Using a two-point electrical measurement under controlled uniaxial pressure, we introduce and analyze the parameter ρm/ρ, which shows a strong linear correlation with pressure across all samples. The effective conductivity (σₑ) is modeled using an effective-medium approximation, revealing two competing effects of pressure: improved grain contact at low pressures and grain fracture at high pressures. The derived expression for σₑ(P) incorporates four key parameters that act as a fingerprint for each material. Experimental results confirm the model's predictive capability and highlight the influence of material composition, notably SiO₂ content, on conductivity. This approach provides a reliable, low-cost tool for preliminary assessment of the textural and conductive properties of activated carbons, with potential implications for their optimization in electrical applications.
Historical and Recent Observations of Comets in Cuba
Cuba has a long-standing tradition in comet observation, dating back to the 19th century when figures like José Joaquín de Ferrer recorded comet movements, including the 1807 comet. These efforts continued into the 20th century, with notable events such as the telescopic observation of Halley's Comet in 1910 by Jesuit priest Mariano Gutiérrez-Lanza. His observations contributed to cometary science and alleviated public fears of a collision between the comet and Earth. Today, Cuban amateur astronomers, using digital tools and social media, continue this legacy. Recent observations of comets reflect Cuba's enduring passion for astronomy and highlight its growing interest in this topic.
Instructing Talent Students in Physics: A Challenge
The teaching of Physics in Cuba is in one of its worst moments since the 1960s, at least at the pre-university level. In this article, we highlight the importance of serving students who show talent for this subject at that educational level. In particular, we propose some ideas of very little economic cost that could contribute to the better performance of these students, but also of all young people with a vocational inclination towards Physics and related sciences in Cuba.
The 2024 Physics Nobel Prize: The Foundations of Artificial Intelligence
The 2024 Nobel Prize in Physics was jointly awarded to John Hop eld and Geoffrey Hinton "for foundational discoveries and inventions that enable machine learning with artificial neural networks". Here we try to explain the meaning of this statement and why we consider their contribution to be an important advance for Physics. This prize is probably the effective demonstration that once again in History, Physics is more than the study of inanimate natural objects.
Construction and Automation of a Growth System for Ferroelectric Thin Films
The fabrication of thin films is fundamental for technological development, considering the continuous miniaturization of the devices being developed. Ferroelectric thin films receive special attention due to their superior energy storage properties and electrocaloric effect, among others, compared to bulk ceramics. Various growth methods have been developed for their production: physical, chemical, and physical-chemical. The dip-coating technique is one of the least expensive, easy to implement and use, and allows for control over the microstructure of the deposited layer. In this work, a dip-coating growth system was designed, built, and automated for the production of ferroelectric thin films. A precursor solution for the films was also prepared using the sol-gel method. Subsequently, the growth of the films was carried out, and the obtained sample was analyzed using SEM, providing detailed information about its morphology.
Fabrication, Characterization, and Performance of Graphene Oxide-Based Filtration Devices
Graphene oxide (GO) has been the subject of numerous studies in recent years due to the anomalous water transport through its membranes, which consist of layered, laminated structures with micrometre dimensions. The discovery of this phenomenon opened up the possibility of using GO in a variety of applications related to processes involving nanofluids and nanofiltration, as well as its potential integration in separation methods in industries through the use of GO in various filtration elements or membrane filtration systems. In the most accepted current model, a few layers of water fill two-dimensional (2D) capillaries in an interconnected structure formed by GO sheets. Recent studies have shown that high pressures can induce slight chemical modifications in GO, mainly involving adsorbed water molecules. For economic reasons, it is desirable to work with high flow rates and membranes that are as thin as possible. To primarily achieve these conditions, GO-based filtration elements modified under high pressure were fabricated in this work. Various samples were characterised by FTIR, XRD, SEM, Raman, AFM, and UV-Vis spectroscopy. A nanostructured device was obtained, capable of filtering for over 29 hours with stability and efficacy in different aqueous solutions of organic dyes and clay particle suspensions up to the detection limit of UV-Vis spectroscopy. Additionally, it exhibited a rejection rate of 90% to 100% for trivalent and divalent cations in saline solutions. A comparative study on graphene oxide treated directly with a metal and a polymeric material under high pressures is presented. The presence of the metal at high pressures led to a partial reduction of the GO, which increased its electrical conductivity and caused significant surface modification. Conversely, the presence of the polymer helped preserve functional groups both on the surface and in the inner layers of the devices, which, along with the effect of high pressure, resulted in a structure that allowed for a constant flow of saline solutions containing organic and inorganic contaminants.
Graphene Oxide; Water Filtration; Nanofluids
We consider the features of the translatory motion of a rocket. It is found that its trajectory can have an inflection point or even contain an S-shaped section in the case of the relatively low initial speed. Both these features are explained by the fact that initially, the vertical component of the constant thrust is less than the gravity force. However, due to fuel consumption, it becomes greater than gravity. The times corresponding to the trajectory local extrema are determined using the Lambert W-function.
Image Quality Study in Breast Tomography with Synchrotron Radiation
The SYRMA-3D project is dedicated to carrying out basic research to lay the groundwork for implementing breast computed tomography (bCT) using synchrotron radiation, in routine clinical settings in the near future (bCT-SR). This article focuses on the optimization of studies (image quality vs. patient dose). Three ex vivo samples were scanned at the SYRMEP station of the ELETTRA Synchrotron (Trieste, Italy) with the free space propagation technique with monochromatic beam and phase contrast, for 64 different acquisition conditions. So, for each of the energies used (25, 28, 32 and 35 keV); acquisitions were made at four distances from the sample to the detector, (0.2 m, 1.65 m, 3.7 m and 9 m), and for each of these conditions, four conditions of average glandular dose (MGD) imparted were used 20, 5, 2 and 1 mGy. Images were acquired with a photon-counting PIXIRAD detector. The Contrast-Noise ratio and Image Contrast, as indicators of image quality, were evaluated for the 5 central slices of the 64 tomographic acquisitions, in the variant that includes phase recovery (PhR). The results indicated that several protocols provided relevant three-dimensional
Copyright (c) 2024 Cuban Physical Society & Faculty of Physics of the University of Havana https://creativecommons.org/licenses/by-nc/4.0
Development of an Automated System for Successive Ionic Layer Adsorption and Reaction (SILAR) Technique at Room Temperature
In this work, we present the design, fabrication, and automation of a customized and affordable system capable of performing the thin film deposition techniques of Dip-Coating and SILAR at room temperature. To automate the system, parts from disused equipment were reused, new pieces were fabricated through additive manufacturing, and open-source software was also utilized. The device was controlled using an Arduino UNO R3. Additionally, graphical interface was developed in Python with the PyQt library to adjust the motion settings and transmit parameters to the Arduino via serial communication. We validated the system by fabricating layers of Cu2O and CuO, subsequently comparing their structural and morphological properties with findings previously reported in the literature.
Heat Treatments, Effects on Microstructure
Advances in the Conceptual Design of a Small Modular Reactor of the Integrated Pressure Water Type
The development and deployment of small nuclear reactors (SMRs) as part of the new generations of nuclear reactors has gained significant importance in recent years. The advantages of SMRs is due not only to their ability to generate low-emission energy, but essentially to their modularity characteristics and their small dimensions. The TRI-structural-ISOtropic (TRISO) fuel, with proven advantages in graphite-gas type nuclear reactors, has been researched for use in PWRs. To obtain high proliferation resistance, extended fuel cycles for SMRs have been proposed. To obtain conceptual designs of SMR cores with a large cycle length, using low fuel enrichment, without shuffle, and with a relatively small core size is a challenge. In this work, a study was carried out to consider improvements in the behavior of the fuel cycles of a conceptual design of a SMR type iPWR, using TRISO fuel. These improvements are related to increasing reactor core power while maintaining extended fuel cycles and, on the other hand, considering different core zones with different fuel compositions, varying the packaging fraction, the last with the aim of to enlarge the fuel cycles and maintaining a flattened radial power distribution.
Theory, and simulation of nuclear reactors
Thermo-Hydraulic Analysis of A Small Modular Reactor of Type IPWR
Small Modular Reactors (SMRs) are an innovative option for power generation. This type of reactor offers numerous benefits and uses the implementation of fully ceramic microencapsulated fuel (FCM) in an integral pressurized water reactor (iPWR) with SMR characteristics has been studied previously. FCM has excellent thermal and irradiation resistance properties and is composed of a SiC matrix with dispersed TRISO fuel particles. To perform the thermo-hydraulic analysis of the core of an SMR using FCM as fuel, a computational model was built using CFD codes. The model describes a typical fuel assembly of the reactor and the main thermo-hydraulic parameters are calculated for the increased power values of 65, 80, and 100 MWt. To improve the thermo-hydraulic behavior of the core, it was considered to vary the values of mass flow, core inlet temperature, and system pressure.
Cooling and Heat Recovery of Nuclear Reactor
Use of Convolution Method at Linear Intervals in X-ray Diffraction Profiles
A mathematical formalism for calculating the intrinsic profile of a diffraction maximum through a convolution process using a set of linear functions by intervals is presented. This convolution process is implemented in the formalism for the direct solution of a diffraction pattern of a layered crystalline structure affected by planar defects. It allows the correlation length (ΔC) to be calculated, which describes the degree of disorder in this type of crystalline structure. The obtained result is validated through the Warren–Averbach method for microstructural analysis
Copyright (c) 2024 The Cuban Journal of Physics https://creativecommons.org/licenses/by-nc/4.0/
Evaluation of the Absorbed Dose in Brachitherapy with 192Ir Sources in Heterogeneous Medium, Using Optically Stimulated Luminescence Dosimeter and Monte Carlo Simulation
The study aims to evaluate the absorbed dose (Dabs) in high-dose-rate brachytherapy treatments with a source of 192Ir, considering the heterogeneities of the tissues, types of applicators and patient dimensions. To this end, isodose calculations by Monte Carlo (MC) simulation and a test of accuracy of the dose delivered to a simulator object were performed, comparing the Dabs planned in the treatment planning system (TPS) with the dose recorded by the optically stimulated luminescence dosimeter. It was found that the TPS adequately calculates the distribution of Dabs when the medium is water; however, it is necessary to include libraries that consider heterogeneities in order to perform a more precise calculation of the distribution of Dabs in patients. It is concluded that performing a CM simulation is a good option to test whether the TPS calculates the Dabs in the presence of heterogeneities.
Monte Carlo methods in biological physics
Copyright (c) 2024 Revista Cubana de Física https://creativecommons.org/licenses/by-nc/4.0
Molecular Dynamics Simulations of the Diffusion of Li Cations in Lithium Fluorhectorite Clay Models
We study, using the Molecular Dynamics method, the mobility of Lithium compensation cations in two clay models that are in contact with a water reservoir. The preferential site of Li+ is the center of the hexagonal ring of the structure. The simulations show that Li+ cations can leave their initial positions and move in four different ways. These movements occur over short time intervals, suggesting a jump diffusion mechanism. Furthermore, our simulations have highlighted that Li+ can coordinate with the framework oxygens and with water molecules, and the solvation will depend on the position of the cation. Likewise, the cations close to the surface are the ones that diffuse the most.
Copyright (c) 2024 Sociedad Cubana de Física & Facultad de Física de la Universidad de La Habana https://creativecommons.org/licenses/by-nc/4.0/