arXiv subjects

Sergio L. Morelhao

Publications and source records attributed to Sergio L. Morelhao.

12 recordsLinked to original sources

arXiv2026

Benchtop Momentum-Resolved Phonon Spectroscopy: Unlocking Lattice Dynamics via X-ray Diffuse Scattering

Momentum-resolved phonon spectroscopy is essential for understanding the thermal, electronic, and structural properties of materials, yet it traditionally necessitates access to large-scale synchrotron or neutron facilities. We report a paradigm shift by demonstrating the accurate measurement and quantitative analysis of Thermal Diffuse Scattering (TDS) patterns using a commercially available benchtop Small/Wide-Angle X-ray Scattering (SAXS/WAXS) instrument. By utilizing a low-energy (8 keV) X-ray source and a custom-integrated vacuum geometry, we acquired characteristic TDS patterns in silicon (111) and (100) slabs. We demonstrate that these benchtop images contain sufficient information to extract interatomic force constants (IFCs) through global optimization algorithms, achieving a fitting accuracy comparable to those obtained from simulated data with statistical noise. Crucially, this capability provides access to phonon information across the entire Brillouin Zone, a critical component of lattice dynamics unavailable via standard laboratory techniques like first-order Raman spectroscopy. This low-barrier approach makes synchrotron-level lattice dynamics characterization accessible to virtually any research laboratory, enabling new avenues for the study of intrinsic strain and anisotropic lattice dynamics in emerging material systems.

cond-mat.mtrl-sci↗

arXiv2026

Statistical Disorder in MBE-Grown AlGaAs/GaAs Superlattices for Quantum Bragg Mirrors using Synchrotron X-ray Diffraction

AlGaAs/GaAs superlattices grown by Molecular Beam Epitaxy (MBE) are foundational for advanced optoelectronic devices, including Quantum Bragg Mirror (QBM) infrared detectors. The performance of these devices critically depends on achieving near-perfect periodicity and abrupt interfaces, however, intrinsic statistical fluctuations during MBE growth introduce nanoscale structural disorder that can degrade device efficiency. In this study, we present a comprehensive methodology for quantifying this disorder in a 203-layer QBM device. High-resolution structural characterization was performed using high-energy (25 keV) synchrotron X-ray diffraction. By coupling a recursive dynamical diffraction formalism with an ensemble simulated annealing refinement, we extracted statistically robust, layer-by-layer thickness profiles. Our analysis reveals highly systematic, material-specific deviations from the nominal design: all AlGaAs barrier layers were consistently thinner than nominal by 0.3-0.6 nm. Furthermore, the sequential thickness profile successfully identified a significant 35 nm deficit in the final macroscopic top contact layer and a 40 nm deficit in the first GaAs layer. Achieving a statistical precision of about 0.2 to 0.6 nm (approximately 1-2 atomic monolayers), this non-destructive diagnostic approach provides directly actionable feedback for MBE flux calibration protocols.

cond-mat.mes-hall↗

arXiv2021

Simulation of X-ray diffraction in Mn$_x$Bi$_2$Te$_{3+x}$ epitaxic films

Disordered heterostructures stand as a general description for compounds that are part of homologous series such as bismuth chalcogenides. In device engineering, van der Waals epitaxy of these compounds is very promising for applications in spintronic and quantum computing. Structural analysis methods are essential to control and improve their synthesis in the form of thin films. Recently, X-rays tools have been proposed for structural modeling of disordered heterostructures [arXiv:2107.12280]. Here, we further evaluate the use of these tools to study the compound Mn$_x$Bi$_2$Te$_{3+x}$ in the grazing incidence region of the reflectivity curves, as well as the effect of thickness fluctuation in the wide angle region.

cond-mat.mtrl-sci↗

arXiv2021

X-ray diffraction tools for structural modeling of epitaxic films of an intrinsic antiferromagnetic topological insulator

Synthesis of new materials demands structural analysis tools suited to the particularities of each system. Van der Waals (vdW) materials are fundamental in emerging technologies of spintronics and quantum information processing, in particular topological insulators and, more recently, materials that allow the phenomenological exploration of the combination of non-trivial electronic band topology and magnetism. Weak vdW forces between atomic layers give rise to composition fluctuations and structural disorder that are difficult to control even in a typical binary topological insulators such as Bi2Te3. The addition of a third element as in MnBi2Te4 makes the epitaxy of these materials even more chaotic. In this work, statistical model structures of thin films on single crystal substrates are described. It allows the simulation of X-ray diffraction in disordered heterostructures, a necessary step towards controlling the epitaxial growth of these materials. On top of this, the diffraction simulation method described here can be readily applied as a general tool in the field of design new materials based on stacking of vdW bonded layers of distint elements.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic Spatial Resolution Limit in Analyzer-Based X-Ray Phase Contrast Imaging Technique

Dynamical diffraction effects always play a role when working with perfect single crystals. The penetration of X-rays respect to the surface normal during diffraction (extinction depth, $1/σ_e$) in perfect single crystals does not have a constant value. The value changes for different angular positions on the crystal diffraction condition. For higher X-ray energies this value can change from few micrometers to tens of millimeters for each different crystal angular position in the small angular range of the diffraction condition. This effect may spread a single point in the object (sample) as a line in the image detector, especially if the crystal is set (or if the sample angularly deviates the beam) at lower diffraction angle positions, where the surface component of X-ray penetration can achieve huge values. Then, for imaging experiments where the dynamical diffraction occurs, such intrinsic property can affect the image resolution. We have modeled and experimentally checked such a dynamical diffraction property using, as example, an Analyzer-based X-ray phase contrast imaging setup (ABI) at two different X-ray energies: 10.7 keV and 18 keV. The results show that our theoretical model is consistent with the measured results. For higher energies the blur effect is enhanced and intrinsically limits the image spatial resolution.

physics.ins-det↗

arXiv2020

Hybrid reflections from multiple x-ray scattering in epitaxial bismuth telluride topological insulator films

Epitaxial films of bismuth telluride topological insulators have received increasing attention due to potential applications in spintronic and quantum computation. One of the most important properties of epitaxial films is the presence of interface defects due to lateral lattice mismatch since electrically active defects can drastically compromise device performance. By describing hybrid reflections in hexagonal bismuth telluride films on cubic substrates, in-plane lattice mismatches were characterized with accuracy at least 20 times better than using other X-ray diffraction methods, providing clear evidence of 0.007\% lateral lattice mismatch, consistent with stress relaxation associated with van der Waals gaps in the film structure.

cond-mat.mtrl-sci↗

arXiv2019

Lateral lattice coherence lengths in thin films of bismuth telluride topological insulators, with overview on polarization factors for X-ray dynamical diffraction in monochromator crystals

In the supporting information file for article Dynamics of Defects in van der Waals Epitaxy of Bismuth Telluride Topological Insulators(J. Phys. Chem. C 2019, 123, 24818-24825, doi: 10.1021/acs.jpcc.9b05377), several topics on X-ray diffraction analysis of thin films were developed or revisited. A simple equation to determine lateral lattice coherence lengths in thin films stands as the main development (section S4 - Lateral lattice coherence length in thin films), while X-ray dynamical diffraction simulation in monochromator crystals stands as an interesting overview on how the ratio between $π$ and $σ$ polarization components is affected by whether diffraction takes place under kinematical or dynamical regime (section S3 - Polarization factor).

cond-mat.mtrl-sci↗

arXiv2018

X-ray tools for van der Waals epitaxy of bismuth telluride topological insulator films

Potential applications in spintronics and quantum computing have motivated much recent research in epitaxial films of bismuth telluride. This system is also an example of van der Waals (vdW) epitaxy where the interface coherence between film and substrate is based on vdW bonds instead of strong ionic or covalent bonds. Effects of lattice mismatch on electrical properties and film structure are more difficult to control due to the weakness of the vdW forces. Here we present a general x-ray diffraction method to investigate in-plane atomic displacements and lateral lattice coherence length in vdW epitaxy. The method is demonstrated in a series of films grown at different temperatures and pressures of additional tellurium sources, revealing strong intercorrelations between the lateral features as well as with the n/p-types of free charge carries.

physics.app-ph↗

arXiv2007

Hybrid reciprocal space for X-ray diffraction in epitaxic layers

Even after several decades of systematic usage of X-ray diffraction as one of the major analytical tool for epitaxic layers, the vision of the reciprocal space of these materials is still a simple superposition of two reciprocal lattices, one from the substrate and another from the layer. In this work, the general theory accounting for hybrid reflections in the reciprocal space of layer/substrate systems is presented. It allows insight into the non-trivial geometry of such reciprocal space as well as into many of its interesting properties. Such properties can be further exploited even on conventional-source X-ray diffractometers, leading to alternative, very detailed, and comprehensive analysis of such materials.

cond-mat.mtrl-sci↗

arXiv2006

Phase signal definition for electromagnetic waves in X-ray crystallography

Diffracted X-ray waves in crystals have relative phases regarding the mathematical format used to describe them. A forward propagating wave can be defined with either negative or positive time evolution, i.e. kr - wt or $wt - kr. Physically measurable quantities are invariant with respect to the choice of definition. This fact has not been clearly emphasized neither extensively explored when deriving well-established equations currently being used in many X-ray diffraction related techniques. Here, the most important equations are generalized and consequences of conflicting undertaken definitions discussed.

cond-mat.mtrl-sci↗

arXiv2005

An X-ray vision of cataract

This work reports the exploitation of diffraction enhanced X-ray imaging (DEI) for studying cataract in addition to the finding of heavy scattering centers of light, probably Ca-rich precipitates in cataractous lenses. DEI selectively probes diffuse-scattering, refraction, and absorption properties of features in the lenses. Fiber cell compaction areas and dilute distribution of precipitates are identifiable, as well as highly absorbing aggregations providing contrast even in the conventional radiography setup. This finding opens new opportunities for clinical diagnosis, for understaning the causes of cataract, and in developing medicines for this disease.

physics.med-ph↗

arXiv2005

On diffraction and absorption of X-rays in perfect crystals

This work reports the incompatibility of the Stokes relation for time reversibility of wave's propagation and the usage of complex atomic form-factor in diffraction theories. The implications behind a diffraction theory that obeys the Stokes relation is discussed.

cond-mat.mtrl-sci↗