arXiv subjects

Miguel Carvalho Sequeira

Publications and source records attributed to Miguel Carvalho Sequeira.

4 recordsLinked to original sources

arXiv2026

Understanding the anisotropic response of $β$-Ga$_2$O$_3$ to ion implantation

While $β$-Ga$_2$O$_3$ is considered a promising wide bandgap semiconductor, the impact of ion-induced defect formation and anisotropic elasticity remains poorly understood. Here, we combine a simulation and experiment X-ray diffraction (XRD) study of the strain-stress dynamics induced by ion implantation into $β$-Ga$_2$O$_3$ single-crystals with different surface orientations. The strain accumulation in the out-of-plane direction is observed by XRD to occur in an anisotropic manner, with compressive strain along the [010] direction and tensile strain along the directions perpendicular to (100) and (001). An anisotropic stress/strain accumulation model is proposed and probed via Molecular Dynamics (MD), showing an excellent agreement with the experiments. For higher damage levels, pole figures obtained both experimentally and by MD via a novel reciprocal-space projection method reveal an orientation-independent $β$-to-$γ$ phase transition, with a fixed crystallographic relationship between the polymorphs. By exploring the strain-stress dynamics in anisotropic systems, this work establishes a method to directly compare macroscale diffraction experiments and atomistic simulations and opens a new path to engineer the properties of such systems utilizing their anisotropic response to ion implantation/irradiation.

cond-mat.mtrl-sci↗

arXiv2025

Microtubes and nanomembranes by ion-beam-induced exfoliation of $β$-Ga$_{2}$O$_{3}$

This paper reports an innovative process to fabricate $β$-Ga$_{2}$O$_{3}$ microtubes and nanomembranes based on ion implantation in (100)-oriented single-crystals. We show that, under specific flux and fluence conditions, the irradiation-induced strain profile promotes the detachment and rolling-up of a thin surface layer, forming a microtube. The strain-disorder interplay was investigated in detail for Cr-implanted $β$-Ga$_{2}$O$_{3}$ with a range of complementary methods, showing an excellent agreement between experimental and simulation data, and suggesting an exfoliation mechanism that is correlated with the anisotropic nature of the $β$-Ga$_{2}$O$_{3}$ monoclinic system and its easy-cleavage planes. Moreover, these microtubes can be unrolled upon a subsequent annealing step, resulting in nanomembranes with bulk-like crystalline quality that can be transferred to other substrates. The recovery of the implantation-induced damage under thermal annealing has also been studied, showing a remarkable recovery at moderate temperatures (~500 °C). This observation underscores the potential of this method for the scalable production of nanomembranes with improved reproducibility compared to conventional mechanical exfoliation techniques. Importantly, such exfoliation can be done employing different ions, providing simultaneous $β$-Ga$_{2}$O$_{3}$ doping, chosen to control the structural, optical, magnetic and electrical properties of the nanomembranes, thus tailoring them to fit the desired applications.

cond-mat.mtrl-sci↗

arXiv2009

Modified theories of gravity with non-minimal curvature-matter coupling

In this contribution one examines the generalization of the $f(R)$ theories of gravity where one introduces a non-minimal coupling between curvature and matter. This model has new and interesting features. %, specially concerning the energy exchange between the matter fields and the curvature. However, as any modified gravity theories, these may give origin to anomalies which might turn the theory physically meaningless. In this respect, one undertakes a study on the energy conditions and the Dolgov-Kawasaki criterion specific of this new model.

gr-qc↗

arXiv2009

Energy Conditions and Stability in $f(R)$ theories of gravity with non-minimal coupling to matter

Recently, in the context of $f(R)$ modified theories of gravity, a new type of model has been proposed where one directly couples the scalar curvature to matter. As any model in $f(R)$ theory, there are certain conditions which have to be satisfy in order to ensure that the model is viable and physically meaningful. In this paper, one considers this new class of models with curvature-matter coupling and study them from the point of view of the energy conditions and of their stability under the Dolgov-Kawasaki criterion.

gr-qc↗