arXiv subjects

Sérgio L. Morelhão

Publications and source records attributed to Sérgio L. Morelhão.

15 recordsLinked to original sources

arXiv2026

Itinerant antiferromagnetism in the antagonistic pair compound Y$_4$Co$_3$Ag

Low dimensional crystallographic motifs have long been associated with desirable physical properties. The confinement of electrons to low dimensions is thought to enhance quantum fluctuations and may promote correlated phenomena. Here, using the antagonistic pair concept, we add Y to the immiscible Co-Ag pair to discover Y$_4$Co$_3$Ag. This compound adopts a monoclinic $I$2/$m$ structure consisting of Y channels that are filled by one-dimensional zigzag and hexagonal Co chains, which extend along the crystallographic $b$-axis with no nearest neighbor contacts between Co and Ag atoms. Transport, magnetic, and specific heat measurements reveal that Y$_4$Co$_3$Ag orders antiferromagnetically at $T_N=14.9$ K with an effective magnetic moment $μ_{\text{eff}}$ = 1.4 $μ_{\text{B}}$/Co. Specific heat measurements show only a small entropy loss on the order of $0.1\,R\ln2$ associated with magnetic order, and magnetization isotherms, in DC fields up to 70 kOe at 1.8 K and in pulsed fields up to 600 kOe at 500 mK, indicate a small ordered moment of less than 0.2 $μ_B$/Co. Taken together, our results imply the presence of small, itinerant moments and strong fluctuations in Y$_4$Co$_3$Ag, suggesting that Y$_4$Co$_3$Ag may be a promising candidate material to investigate itinerant magnetic interactions in a quasi-one dimensional system.

cond-mat.str-el↗

arXiv2025

Structural phase transitions in the van der Waals ferromagnets Fe$_x$Pd$_{y}$Te$_2$

Here, we provide a detailed study of the crystal structure and physical properties of the recently discovered vdW ferromagnet FePd$_2$Te$_2$. We find this compound has a relatively wide width of formation, and grow single crystals with compositions Fe$_x$Pd$_{y}$Te$_2$ where $x$ ranges from 0.9 to 1.1 and $y$ from 1.8 to 2.5, respectively. Temperature-dependent X-ray diffraction and transport measurements reveal that a first-order structural transition occurs in the range of $T$ = 360-420 K. Above the transition, the compounds with Pd fraction $y>2$ adopt a disordered derivative of the tetragonal FeTe structure, with the Fe layer showing mixed Fe/Pd occupancy and the extra Pd atoms partially occupying interstitial sites. Below 370 K, the structure is incommensurately modulated. For $y<2$, the composition Fe$_{1.1}$Pd$_{1.8}$Te$_2$ has monoclinic symmetry at room temperature that is consistent with the reported structure of FePd$_2$Te$_2$. This phase undergoes a structural transition at 420 K for which the high temperature structure is yet to be determined; however, based on the similarities with the $y > 2$ compounds, we speculate that this composition also adopts a tetragonal structure above 420\,K. All compounds investigated in the Fe$_x$Pd$_{y}$Te$_2$ series show metallic behavior, with magnetic characterization indicating that they are easy-plane, hard, ferromagnets with $T_C$ spanning 98--180 K. Both the critical temperature for the structural transition and the Curie temperature are moderately suppressed with increasing Pd fraction $y$ and corresponding decreasing Fe fraction $x$, indicating that synthetic control over $x$ and $y$ paves way for the further exploration of these compounds.

cond-mat.str-el↗

arXiv2024

Role of filler lanthanide ions on lattice dynamics of phosphide skutterudites RFe$_4$P$_{12}$ (R = La, Ce, and Pr) from first principles

Phosphide skutterudites primarily show promise for thermoelectric applications due to their chemical stability at high temperatures and relatively low cost. Ion doping and band gap engineering have been used to enhance their typically poor thermoelectric performance, opening avenues for practical applications. Herein, we report a comparative lattice dynamics study on the impact of filler and temperature on the structural and vibrational properties of RFe$_4$P$_{12}$ (R = La, Ce, and Pr) skutterudites. Calculations are performed within the quasi-harmonic approximation, and the results are critically compared against experimental data and other \textit{ab initio} calculations. We found gaps between the heat-carrying acoustic and optical modes, a-o gaps, of approximately 4, -2, and 0.01\,cm$^{-1}$ for La, Ce, and Pr compounds, respectively. These results suggest a filler-induced reduction in the a-o gap is attributed to the softening of the optical modes instead of the conventionally considered upward shift of acoustic modes proposed in the \textit{rattling} scenario. The distinct softening of the optical modes is rationalized by the stiffening of chemical bonds between the filler and host lattice.

cond-mat.mtrl-sci↗

arXiv2023

Proper usage of Scherrer's and Guinier's formulas in X-ray analysis of size distribution in systems of monocrystalline CeO2 nanoparticles

Small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) techniques are widely used as analytical tools in the optimization and control of nanomaterial synthesis processes. In crystalline nanoparticle systems with size distribution, the discrepant size values determined by using SAXS and XRD still lacks a well-established description in quantitative terms. To address fundamental questions, the isolated effect of size distribution is investigated by SAXS and XRD simulation in polydisperse systems of virtual nanoparticles. It quantitatively answered a few questions, among which the most accessible and reliable size values and what they stand for regarding the size distribution parameters. When a finite size distribution is introduced, the two techniques produce differing results even in perfectly crystalline nanoparticles. Once understood, the deviation in resulting size values can, in principle, resolve two parameters size distributions of crystalline nanoparticles. To demonstrate data analysis procedures in light of this understanding, XRD and SAXS experiments were carried out on a series of powder samples of cubic ceria nanoparticles. Besides changes in the size distribution related to the synthesis parameters, proper comparison of XRD and SAXS results revealed particle-particle interaction effects underneath the SAXS intensity curves. It paves the way for accurate and reliable methodologies to assess size, size dispersion, and degree of crystallinity in synthesized nanoparticles.

cond-mat.mtrl-sci↗

arXiv2022

A simple recipe to create three-dimensional reciprocal space maps

Combinations of advanced X-ray sources and zero-noise detector of enormous dynamic range have significantly increased the opportunity of mapping the reciprocal space of crystal lattices. It is particularly important in the design of new devices based on epitaxial thin films. In this work, we present a simple approach to the three-dimensional geometry involved in plotting reciprocal space maps, along with an example of computer codes to allow X-ray users to write their own codes. Experimental data used to illustrate an application of the codes are from antimony telluride epitaxial film on barium difluoride substrate.

cond-mat.mtrl-sci↗

arXiv2020

Morphology Control in van der Waals Epitaxy of Bismuth Telluride Topological Insulators

Bismuth telluride have regained significant attention as a prototype of topological insulator. Thin films of high quality have been investigated as a basic platform for novel spintronic devices. Low mobility of bismuth and high desorption coefficient of telluride compose a scenario where growth parameters have drastic effects on structural and electronic properties of the films. Recently [J. Phys. Chem. C 2019, 123, 24818-24825], a detailed investigation has been performed on the dynamics of defects in epitaxial films of this material, revealing the impact of film/substrate lattice misfit on the films' lateral coherence. Very small lattice misfit (<0.05%) are expected to have no influence on quality of epitaxial system with atomic layers weakly bonded to each other by van der Waals forces, contrarily to what was observed. In this work, we investigate the correlation between lattice misfit and size and morphology of the film crystalline domains. Three-dimensional reciprocal-space maps of film Bragg reflections obtained with synchrotron X-rays are used to visualize the spatial conformation of the crystallographic domains through film thickness, while atomic force microscopy images provide direct information of the domains morphology at the film surface.

cond-mat.mtrl-sci↗

arXiv2020

Phonon Scattering Mechanism in Thermoelectric Materials Revised via Resonant X-ray Dynamical Diffraction

Engineering of thermoelectric materials requires an understanding of thermal conduction by lattice and electronic degrees of freedom. Filled skutterudites denote a large family of materials suitable for thermoelectric applications where reduced lattice thermal conduction attributed to localized low-frequency vibrations (rattling) of filler cations inside large cages of the structure. In this work, a multi-wavelength method of exploiting X-ray dynamical diffraction in single crystals of CeFe$_4$P$_{12}$ is presented and applied to resolve the atomic amplitudes of vibrations. The results suggest that the vibrational dynamics of the whole filler-cage system is the actual active mechanism behind the optimization of thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv2019

Usage of Scherrer's formula in X-ray diffraction analysis of size distribution in systems of monocrystalline nanoparticles

In the supporting information file of the article Controlled Formation and Growth Kinetics of Phase-Pure, Crystalline BiFeO3 Nanoparticles (Crystal Growth & Design 2019), there is a description on how to use Scherrer equation for in situ X-ray diffraction analysis of crystallization processes investigated in the article. That description led to a necessary revaluation on the current understanding of the usage of Scherrer equation for analyzing size distributions, as discussed in this work.

cond-mat.mtrl-sci↗

arXiv2017

X-ray dynamical diffraction in amino acid crystals: a step towards improving structural resolution of biological molecules via physical phase measurements

In this work, experimental and data analysis procedures were developed and applied for studying amino acid crystals by means of X-ray phase measurements. It clearly demonstrated the sensitivity of invariant triplet phases to electronic charge distribution in D-alanine crystal, providing useful information on intermolecular forces for molecular dynamics calculations. Feasibility of phase measurements to investigate radiation damage mechanisms is also discussed on experimental and theoretical grounds.

cond-mat.mtrl-sci↗

arXiv2016

Nanoscale characterization of bismuth telluride epitaxic layers by advanced X-ray analysis

Topological insulator surface properties are strongly correlated to structural properties, requiring high-resolution techniques capable of probing both surface and bulk structures at once. In this work, high flux of synchrotron source, recursive equations for fast X-ray dynamical diffraction simulation, and genetic algorithm for data fitting are combined to reveal the detailed structure of bismuth telluride epitaxic films with thickness ranging from 8 to 168 nm. It includes stacking sequences, thickness and composition of layers in model structures, interface coherence, surface termination and morphology. These results are in agreement with the surface morphology determined by atomic force microscopy. Moreover, by using X-ray data from zero noise area detector to construct three-dimensional reciprocal space maps, insights into the nanostructure of domains and stacking faults in Bi2Te3 films are given.

cond-mat.mtrl-sci↗

arXiv2016

Hybrid reciprocal lattice: application to layer stress appointment in GaAlN/GaN(0001) systems with patterned substrates

Epitaxy of semiconductors is a process of tremendous importance in applied science and optoelectronic industry. Controlling of defects introduced during epitaxial growth is a key point in manufacturing devices of high efficiency and durability. In this work, we demonstrate how useful hybrid reflections are on the study of epitaxial structures with anisotropic strain gradients due to patterned substrates. High accuracy to detect and distinguish elastic and plastic relaxations are one of the greatest advantages of measuring this type of reflection, as well as the fact that it can be exploited in symmetrical reflection geometry on a commercial high-resolution diffractometer.

cond-mat.mtrl-sci↗

arXiv2007

General recursive solution for one dimensional quantum potentials: a simple tool for applied physics

A revision of the recursive method proposed by S.A. Shakir [Am. J.Phys. \textbf{52}, 845 (1984)] to solve bound eigenvalues of the Schrödinger equation is presented. Equations are further simplified and generalized for computing wave functions of any given one-dimensional potential, providing accurate solutions not only for bound states but also for scattering and resonant states, as demonstrated here for a few examples.

quant-ph↗

arXiv2004

Classical and quantum concepts of probability applied to diffraction physics in perfect crystals

By following the trajectories of quantum particles inside a periodic lattice and preserving their classical probabilities for reflection, transmission and absorption at each lattice plane, classical scattering outcomes are obtained. Diffraction phenomena in crystals are reproducible after assigning probability amplitudes to every classical outcome. When applied to X-ray diffraction in reflection geometry, this procedure has provided simple recursive equations to calculated the X-ray reflectivity of crystals with thickness varying since a few atomic layers to infinity. The results are in agreement with the dynamical theory of X-ray diffraction, even when absorption is considered.

cond-mat.mtrl-sci↗

arXiv2004

Automatic physical phasing X-ray crystallography

Phase invariants are important pieces of information about the atomic structures of crystals. There are several mathematical methods in X-ray crystallography to estimate phase invariants. The multi-wave diffraction phenomenon offers a unique opportunity of physically measuring phase invariants. In this work, the underneath principals for developing an automatic procedure to extract accurate phase-invariant values are described. A general systematic procedure is demonstrated, in practice, by analyzing intensity data from a KDP crystal.

cond-mat.mtrl-sci↗

arXiv2004

Energy conservation in approximated solutions of multiple beam scattering problems

Solutions in the form of series expansion, as the Born approximation, are very useful for describing time-independent scattering of quantum particles. In this work, it is mathematically demonstred that such solutions, when applied to multiple beam scattering phenomena, can lead to energy violation where the number of incident and scattered particles is not preserved. General basic conditions for developing consistent solutions of time-dependent multiple beam scattering problems are outlined.

cond-mat.mtrl-sci↗