arXiv subjects

Fernando L. Freire Jr.

Publications and source records attributed to Fernando L. Freire Jr..

3 recordsLinked to original sources

arXiv2025

Absorbance marker: Detection of quantum geometry and spread of Wannier function in disordered 2D semiconductors

The optical absorbance of 2D semiconductors is generalized to individual lattice sites through the topological marker formalism, yielding an absorbance marker. This marker allows to investigate the atomic scale variation of absorbance caused by impurities, thereby quantifies the influence of disorder on the quantum geometry and the spread of Wannier functions of valence band states. Applying this marker to transition metal dichalcogenides reveals a very localized suppression of absorbance caused by potential impurities, rendering a reduction of absorbance in the macroscopic scale proportional to the impurity density, in good agreement with the experimental results of plasma-treated WS$_{2}$.

cond-mat.str-el↗

arXiv2024

Detecting the spread of valence band Wannier functions by optical sum rules

The spread of valence band Wannier functions in semiconductors and insulators is a characteristic property that gives a rough estimation of how insulating is the material. We elaborate that the gauge-invariant part of the spread can be extracted experimentally from optical conductivity and absorbance, owing to their equivalence to the quantum metric of the valence band states integrated over momentum. Because the quantum metric enters the matrix element of optical conductivity, the spread of valence band Wannier functions in the gapped 3D materials can be obtained from the frequency-integration of the imaginary part of the dielectric function. We demonstrate this practically for typical semiconductors like Si and Ge, and for topological insulators like Bi$_{2}$Te$_{3}$. In 2D materials, the spread of Wannier functions in the valence bands can be obtained from the absorbance divided by frequency and then integrated over frequency. Applying this method to graphene reveals a finite spread caused by intrinsic spin-orbit coupling, which may be detected by absorbance in the microwave range. The absorbance of twisted bilayer graphene in the millimeter wave range can be used to detect the formation of the flat bands and quantify their quantum metric. Finally, we apply our method to hexagonal transition metal dichalcogenides MX$_{2}$ (M = Mo, W; X = S, Se, Te) and demonstrate how other effects like substrate, excitons, and higher energy bands can affect the spread of Wannier function.

cond-mat.mtrl-sci↗

arXiv2019

New method for characterization of magnetic nanoparticles by scanning magnetic microscopy

In this paper, we present a new method for the magnetic characterization of bulk materials, microstructures, and nanostructures. We investigated the magnetic and morphological properties of two colloidal dispersions of iron oxide (Fe3O4) magnetic nanoparticles (MNPs), synthesized by chemical precipitation (co-precipitation) and pulsed laser ablation (PLA) in liquid, by scanning magnetic microscopy (SMM) applied to a small sample with mass on the order of tens of μg. We evaluated the performance of this technique by comparing magnetization curves and measurements obtained with commercial magnetometers, considered standard. The errors obtained for the saturation and remanent magnetization were approximately 0.18 Am2/kg and 0.6 Am2/kg, respectively. The average size distribution of the NPs estimated from the magnetization curve measurements is consistent with the results obtained by traditional transmission electron microscopy (TEM). The technique can be extended to measure and analyze magnetization curves (hysteresis loops), thus enabling an even more accurate estimation of overall NP sizes.

physics.app-ph↗