arXiv subjects

Mauro Fernandes Pereira

Publications and source records attributed to Mauro Fernandes Pereira.

5 recordsLinked to original sources

arXiv2026

Geometric closure of classical nucleation theory for magnetic-field-controlled nanoparticle size across magnetic classes

Controlling nanoparticle size during synthesis remains a central challenge in nanoscience, particularly in systems where external magnetic fields are used as continuous control parameters. Existing descriptions of magnetic-field-assisted nucleation are typically material-specific or rely on computationally intensive atomistic methods. Here, we reformulate classical nucleation theory as a geometrically closed thermodynamic framework by introducing a sphere-packing representation of atomic assembly. This construction establishes a direct link between discrete atomic structure and continuum free-energy contributions under applied magnetic fields, yielding a field-driven evolution equation for the critical nucleus size. The resulting theory provides a unified description of nanoparticle nucleation across superparamagnetic, paramagnetic, and diamagnetic systems within a single formalism. It quantitatively reproduces previously unresolved experimental observations for magnetite and nickel nanoparticles, namely the systematic reduction of mean particle size and narrowing of size distributions with increasing magnetic field. In the diamagnetic limit, the framework recovers our earlier analytical susceptibility-based description of silver nanoparticles, in which the field-dependent critical radius is governed by the induced-magnetization contribution to the nucleation free energy. Beyond modeling the reduction of mean particle size with increasing magnetic field, the framework reveals that the narrowing of size distributions emerges naturally from the curvature of the field-modified free-energy landscape. These results establish magnetic-field-assisted nucleation as a geometrically constrained thermodynamic process, providing a computationally efficient route for controlling nanoparticle size across distinct magnetic material classes.

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arXiv2025

Controlling the Size of Nanoparticles Using a Magnetic Field: A Sphere Packing Approach

We present an analytical framework that predicts and controls nanoparticle size through external magnetic fields, uniting first-principles thermodynamics with a sphere packing approach. Calibrated to diamagnetic silver nanoparticles (20 nm at zero field and 5 nm at 250 mT), the model yields a closed-form relation between radius and field that reproduces the observed shift in most-probable size. Within the limits of classical capillarity and spherical demagnetization, the field lowers the nucleation barrier and drives the distribution toward smaller particles. Our results are robust for radii above 3 nm (5740 atoms). Below this scale non-extensive effects likely dominate, as discussed in detail in Supplementary Information. The approach generalizes to both diamagnetic and paramagnetic systems and the limitations expected for very small or ferromagnetically ordered nanoparticles are discussed.

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arXiv2025

Complementary and Asymmetric Tapered Bent Mid-Infrared Waveguide Arrays for Subwavelength-Pitch Integration and Crosstalk Minimization

This paper delivers the first report of a mid-infrared (MIR) waveguide array design that employs complementary and asymmetric tapered Euler-shaped bends. These provide greater fabrication flexibility to achieve subwavelength-pitch integration while reducing crosstalk to below 30 dB across the 3.1 to 3.6 micron wavelength range. Unlike previous designs, which maintained constant waveguide widths, the Euler waveguide bends are characterized by asymmetric and complementary tapered waveguide widths. This approach significantly reduces crosstalk to below 30 dB for both the first and second neighboring waveguides across a 500 nm wavelength range, enhancing the efficiency of optical phased arrays (OPA) with a large field of view, optimizing light propagation and minimizing crosstalk. The waveguide array is fabricated on a silicon-on-insulator platform, with a 2-micron buried oxide layer and a 500 nm-thick silicon layer. The design is highly tolerant to fabrication variations, maintaining consistent performance even with width variations. The spectral responses, simulated using the 3D finite-difference time-domain method, demonstrate negligible coupling and low insertion loss across the wavelength range. This work offers a robust and CMOS-compatible solution for MIR integrated photonic circuits.

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arXiv2023

Photo-acoustic spectroscopy using a quantum cascade laser (QCL) for analysis of ammonia in water solutions

Ammonia (NH$_3$) toxicity, stemming from nitrification, can adversely affect aquatic life and influence the taste and odor of drinking water. This underscores the necessity for highly responsive and accurate sensors to continuously monitor NH$_3$ levels in water, especially in complex environments where reliable sensors have been lacking until this point. Herein, we detail the development of a sensor comprising a compact and selective analyzer with low gas consumption and a timely response, based on photoacoustic spectroscopy. This, combined with an automated liquid sampling system, enables the precise detection of ammonia traces in water. The sensor system incorporates a state-of-the art quantum cascade laser as the excitation source emitting at 9 \textmu m in resonance with the absorption line of NH$_3$ located at 1103.46 cm$^{-1}$. Our instrument demonstrated detection sensitivity at low ppm level for total ammonia nitrogen with response times less than 60 seconds. For the sampling system, an ammonia stripping solution was designed resulting in a prompt full measurement cycle (6.35 mins). A further evaluation of the sensor within a pilot study showed good reliability and agreement with the reference method for real water samples, confirming the potential of our NH$_3$ analyzer for water-quality monitoring applications.

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arXiv2023

Progress in Analytical Solutions for High Order Harmonic Generation in Semiconductor Superlattice Multipliers

In this study, we address the limitations of previous solutions for modeling high-order harmonics in semiconductor superlattices (SSLs). Earlier research proposed a step function ansatz that effectively modeled high-order even and odd harmonics but introduced numerical noise. An upgrade using a logistic function addressed the noise problem but eliminated high-order odd harmonics. To overcome both limitations, we examined the impact of the y-intercept value in the discontinuity of the step function and proposed a modified logistic function as a new ansatz. The modified logistic function delivers accurate results, preserving high-order odd harmonics up to the 50th order similar to the previous ansatz while also eliminating numerical noise. This research contributes to a more efficient and robust analytical approach for modeling SSLs, notably by avoiding time-consuming numerical solutions and enhancing our understanding of nonlinear phenomena in GHz-THz devices.

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