Brazilian Journal of Physics最新文献

We investigate the interplay between photonic and phononic spectral dynamics in a cavity optomechanical system driven by radiation pressure coupling. Employing a semi-classical Hamiltonian framework, we derive the quantum Langevin equations governing the photon and phonon mode fluctuations, enabling explicit calculation of their noise spectra. The system comprises a high-finesse optical microcavity with a movable mirror acting as a mechanical oscillator, irradiated by a red-detuned 1064-nm laser. As the optomechanical coupling strength G increases to its maximum (G_m), the photon noise spectrum (S_a(omega )) exhibits significant amplification and broadening, accompanied by a suppression and frequency shift in the phonon spectrum (S_q(omega )). This reciprocal behavior confirms energy transfer from the mechanical to the optical mode, consistent with laser cooling principles. Our analysis reveals that the effective mechanical frequency (omega _{eff}) and damping rate (gamma _{eff}) are renormalized under enhanced optomechanical coupling, leading to spectral imbalance and cooling rates proportional to (S_q(omega )-S_q(-omega )). Notably, the mechanical mode’s effective temperature is reduced by several orders of magnitude, demonstrating sub-millikelvin cooling capabilities. These results highlight the critical role of coupling strength in optimizing cooling efficiency and photon-mediated control, with implications for quantum metrology, state engineering, and noise suppression in hybrid quantum systems.

Multinucleon transfer (MNT) reactions offer a promising pathway for the production and study of neutron-rich nuclei in the heavy and superheavy element regions. The majority of undiscovered nuclei in these regions are expected to exhibit extremely short half-lives and production cross-sections well below the microbarn range, thus demanding highly efficient separation and detection techniques to enable their identification. In this context, to assess the applicability of the Separator for Heavy ELement Spectroscopy SHELS for MNT studies, several collision systems involving Pb and Bi targets were investigated. The results demonstrate that asymmetric reaction systems can be successfully employed with SHELS to study the above-target MNT products. Approximately 40 directly populated (alpha)-decaying nuclei were identified, involving up to an eight proton transfer from projectile to target, with cross-sections down to the nanobarn level and half-lives as short as a few microseconds. These findings highlight the potential of SHELS, especially when operated with optimized settings and high intensity beams, for future investigations. The ongoing construction of the dedicated wide acceptance kinematic separator STAR, along with the modernization of the U400 cyclotron to deliver higher intensity beams, is expected to significantly enhance MNT based research and facilitate access to previously unreachable nuclei.

Continuous-variable quantum key distribution (CV-QKD) has emerged as a promising approach for secure quantum communication, offering advantages such as high key generation rates, compatibility with standard telecommunication infrastructure, and potential for integration on photonic chips. This review provides an accessible introduction to the theory of CV-QKD, aimed at researchers entering this rapidly developing field. We focus on fundamental concepts, key protocols, and security analysis essential for understanding CV-QKD systems, with a special emphasis on prepare-and-measure protocols using coherent states under asymptotic security conditions. We explain their equivalence to entanglement-based protocols and detail the security proof framework against collective attacks, encompassing both Gaussian and discrete modulation schemes. We also briefly address more advanced topics, including measurement-device-independent CV-QKD and finite-size security analysis. This work is motivated by Brazil’s growing investment in quantum communication technologies. By presenting a clear learning path from basic concepts to advanced topics, this work aims to equip newcomers with the essential tools to engage with current research in CV-QKD, thereby supporting the training of a new generation of researchers in this strategic field.

In the intersection of electrical and biomedical engineering, high-performance insulation materials are crucial for ensuring the safe and reliable operation of medical devices. Due to its excellent dielectric properties, chemical stability, and biocompatibility, fluorinated ethylene propylene has demonstrated significant potential in high-frequency and high-voltage medical applications. However, under intense electric fields, the microstructure of fluorinated ethylene propylene can undergo evolution, leading to degradation of its insulation performance and posing risks to device safety. Conventional experimental methods struggle to capture the real-time nanoscale response of materials to electric fields. To address this limitation, a first-principles study based on density functional theory was conducted to systematically investigate the microscopic mechanisms by which external electric fields influence the molecular structure, electronic properties, polarization behavior, and vibrational spectra of fluorinated ethylene propylene. The results indicate that the applied electric field induces conformational changes, enhances bond polarization, narrows the energy gap, and alters the frontier molecular orbitals of the polymer, thereby affecting charge carrier dynamics and trap state distribution. Additionally, shifts in characteristic infrared absorption peaks reveal the electric field’s modulation of bond vibrational modes. Collectively, these microscopic insights elucidate the fundamental physical mechanisms underlying the insulation performance degradation of fluorinated ethylene propylene under strong electric fields, providing a theoretical foundation for assessing insulation reliability and guiding material optimization in medical electronic devices.

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This paper presents a statistical approach based on Principal Component Analysis (PCA) applied to Fourier transform infrared (FT-IR) spectra to investigate the relative strength of intermolecular interactions in liquid crystal (LC) complexes. Liquid crystal complexes: 4-(4-Pyridyl)benzylidene-4′-n-tetradecylaniline (PyB14A): n-fluorobenzoic acid (nFBAs, where n n= 2,3, and 4) are considered for this investigation. Here, PCA is employed on the FT-IR spectral data of the pure and complex systems to analyze the vibrational frequencies and their corresponding transmittances of the functional groups. This analysis yields the principal components that reveal variations in the relative strength of intermolecular interactions within the PyB14A: nFBA complex systems. The method is applied to both the fundamental and fingerprint regions of the FT-IR spectra. The results demonstrate that PCA in combination with FT-IR spectroscopy works as an effective tool for identifying and comparing the relative strength of intermolecular interactions in hydrogen-bonded LC complexes.

The preformation probability of the alpha-particle is a crucial factor in accurately describing decay half-lives, yet it is often treated with model-dependent assumptions and limited parameter constraints. This study aims to comprehensively investigate the alpha-particle preformation probabilities across a wide range of nuclei using a diverse set of microscopic interaction potentials, and to assess the model dependence and uncertainty associated with potential parameters. Using the Bohr–Sommerfeld quantization condition and the WKB approximation, we analyze 263 alpha-emitting nuclei from (^{106})Te to (^{261})Bh, employing 16 interaction potentials derived from the M3Y, DDM3Y, CDM3Y, and BDM3Y models. These double folding potentials incorporate both direct and exchange components, constructed using a Gaussian density for the alpha-particle and daughter nucleus densities obtained from the Hartree–Fock–Bogoliubov (HFB) calculations with the BSk2 Skyrme force. Preformation factors are extracted from experimental half-lives, and average values are determined for even–even, even–odd, odd–even, and odd–odd nuclei. The arithmetic average preformation probabilities across all interaction potentials are found to be (overline{P}_{ee} = 0.083), (overline{P}_{oo} = 0.044), (overline{P}_{eo} = 0.055), and (overline{P}_{oe} = 0.050). The theoretical alpha half-lives calculated using these average values are compared with experimental alpha half-lives, yielding the smallest rms deviations for the M3Y-Reid-ZR and M3Y-Paris-ZR models. The sensitivity of the inner turning point (r_2) to the potential model and decay energy is also analyzed. Uncertainties due to the global quantum number G and Coulomb radius (r_C) are taken into account. This work provides a statistically grounded evaluation of alpha preformation probabilities using a broad set of microscopic interaction potentials and highlights the crucial role of the inner turning point in determining alpha decay half-lives.

Experimental studies have shown that NdFe₄P₁₂ is a metal with a ferromagnetic ordering. The main objective of this study is to investigate its vibrational behavior and its effect on the lattice thermal conductivity and a detailed study on the magnetic and electronic properties as well as to determine its lattice thermal conductivity and thermoelectric figure of merit. Phonon dispersion curve has shown, for the first time, the absence of soft modes, confirming that NdFe₄P₁₂ is dynamically stable. The rattling role of Nd-atoms in the acoustic branches enhances phonon scattering and reduces the lattice thermal conductivity, however, the absence of overlap between the acoustic and optical branches limits this enhancement. This is supported by the calculated lattice thermal conductivity, which is about 6.94 W.m^− 1.K^− 1 at 300 K. Such a value is moderate for a metal but relatively high for thermoelectric applications. Nevertheless, this moderate LTC remains encouraging for future optimization strategies. The found results also showed that NdFe₄P₁₂ is mechanically stable with a low elastic anisotropy. Unlike GGA and GGA + U, which showed that NdFe₄P₁₂ is ferrimagnetic, GGA + U + SOC has confirmed the ferromagnetic behavior. Using the exchange coupling parameters J_ij, the Curie temperature estimated by Mean-Field Approximation (MFA) is 8.04 K.

Brazil’s abundant natural resources, particularly titanium dioxide (TiO(_{2})) and niobium pentoxide (Nb(_{2})O(_{5})), position the country for strategic advances in optoelectronic and photovoltaic technologies, including third-generation solar cells such as dye-sensitized solar cells (DSSCs). In this study, TiO(_{2}) and Nb(_{2})O(_{5}) nanoparticles were co-dispersed at different mass concentrations to fabricate Nb(_{2})O(_{5})–TiO(_{2}) single-layer photoanodes for DSSCs. The films were deposited by spin-coating and doctor-blade techniques onto microscope slides and conductive glass substrates, and characterized by mechanical profilometry, X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and two-point probe electrical measurements. Prototype DSSCs incorporating the different photoanodes were assembled and evaluated by current–voltage measurements under simulated solar illumination and dark conditions. The films are thick, rough, and porous, with phase segregation between TiO(_{2}) and Nb(_{2})O(_{5}), as well as a reduction in average crystallite size upon the addition of Nb(_{2})O(_{5}) nanoparticles. Niobium incorporation promoted an increase in electrical conductivity and yielded DSSC prototypes with improved short-circuit current density, open-circuit voltage, and overall power conversion efficiency relative to devices based on conventional TiO(_{2}) photoanodes. These results indicate that niobium-containing TiO(_{2}) nanostructures enhance the electrical properties of the photoanode and contribute to improved device performance.

We present a historical review of the development and impact of spontaneous parametric down-conversion (SPDC) in Brazil, marking over three decades since the first twin-photon experiments were performed in the country. This article traces the pioneering efforts that initiated the field, highlighting key experiments, institutions, and researchers who contributed to its growth. We discuss seminal works that established SPDC as a fundamental tool in the Brazilian Quantum Optics community, including studies on spatial correlations, entanglement, and decoherence. By presenting a curated sequence of experiments, we offer an overview of how Brazilian research in twin-photon systems has explored profound concepts through fundamental demonstrations, leading to significant international impact. This review also highlights the formation of a strong scientific community and its ongoing efforts to turn fundamental knowledge into quantum applications.

Nanoparticles of nickel ferrite have been synthesized via the sol-gel auto-combustion method, offering a versatile approach to control their structural, optical, and dielectric properties. Structural analysis using X-ray diffraction (XRD) confirmed the formation of single-phase spinel structure. FTIR analysis indicates the presence of Ni-O and Fe-O bond. Scanning electron microscopy (SEM) reveals the existence of high dense agglomerated grains, indicating pore free crystallites on the surface part. The optical properties evaluated through UV-Vis spectroscopy revels a direct energy band gap of 1.53 eV, demonstrating characteristic absorption bands in the visible and near-infrared regions, crucial for applications in photocatalysis and optical devices. Dielectric measurements across varying frequencies and temperatures elucidates the nanoparticles’ capacitive behavior. The minimum values of dielectric constant and dielectric loss are 2.72 and 0.98, respectively at room temperature. From Nyquist plot, it can clearly identify the negative temperature coefficient of resistance (NTCR) behavior because the size of the semicircles decreases as temperature rises, indicating a decrease in resistance. Overall, this comprehensive analysis provides insights into optimizing the synthesis parameters and understanding the multifaceted properties of nickel ferrite nanoparticles synthesized via sol-gel auto-combustion, paving the way for their tailored applications in advanced materials and device engineering.