arXiv subjects

A. Carvalho

Publications and source records attributed to A. Carvalho.

At least 19 recordsLinked to original sources

arXiv2026

Closed-form approximations of fundamental quantities of Lemaitre-Tolman-Bondi cosmologies from Symbolic Regression: I. Results on the Garcia-Bellido-Haugb{\o}lle parameterization

We introduce a novel set of analytic approximations for five fundamental functions in spherically symmetric, inhomogeneous Lemaitre-Tolman-Bondi (LTB) cosmologies, derived via Symbolic Regression (SR). Focusing on the constrained Garcia-Bellido-Haugboelle (GBH) parameterization, we sample the four-dimensional LTB parameter space using the bubble LTB numerical code and apply SR to reconstruct closed-form expressions for the radial and transverse scale factors A_parallel(r,t) and A_perp(r,t), the corresponding Hubble functions H_parallel(r,t) and H_perp(r,t), and the angular diameter distance D_A(z). Our best-fit formulas reproduce the numerical data with high precision: the relative mean error across all quantities remains below 0.3 percent, except for the radial Hubble function, where it reaches 1.4 percent. These compact expressions enable rapid evaluation of LTB predictions, supporting fast parameter scans, likelihood analyses, and model comparisons without time-consuming integrations. We provide explicit coefficients and discuss the domain of validity, demonstrating that SR-driven approximations can serve as robust surrogates for exact LTB solutions in both theoretical investigations and observational analyses.

astro-ph.CO↗

arXiv2025

The Outbursting YSOs Catalogue (OYCAT)

YSOs can display unpredictable and high-amplitude rises in brightness that can last from a few months to possibly over 100 years. These types of outbursts are explained by large changes in the mass accretion rate from the disk onto the central star. The outbursts support to a model of star formation (episodic accretion) where stars would spend most of their lifetimes accreting at low rates, and gain most of their mass through these short-lived accretion outbursts. The universality of episodic accretion, as well as its potential impact on stellar and planetary formation are still under debate. Improvement on the statistics of the members of the eruptive class is needed to better understand the episodic accretion phenomenon and its universality across different mass regimes and environments. In this paper we collect published information on the spectroscopic and photometric characteristics of 174 YSOs confirmed to belong to the eruptive variable class. We classify these objects into five different sub-classes (we find 49 FUor, 20 FUor-like, 16 EX Lupi-type, 81 Peculiar/V1647 Ori-like/MNors and 8 Periodic YSOs). The classification follows what has been done previously in the literature, and it is not an attempt to redefine these classes. In addition, we present a list of 18 embedded, and 6 massive YSOs, as additional categories of eruptive variable YSOs. Due to the complexity and/or faintness of these systems, it is hard to place them into the original classification scheme of this class of variable YSOs. Finally, we present a separate list of 355 candidate eruptive variable YSOs, which either lack spectroscopic information or the available spectroscopic data is not sufficient for an unambiguous classification. The online catalogue of confirmed and candidate eruptive YSOs will be maintained and updated in the future to serve as an important reference for the star formation community.

astro-ph.SR↗

arXiv2025

Holonomic quantum computation on graphene from Atiyah-Singer index theorem

We investigate the emergence of geometric phases in graphene-based nanostructures through the lens of the Atiyah-Singer index theorem. By modeling low-energy quasiparticles in curved graphene geometries as Dirac fermions, we demonstrate that topological defects arising from the insertion of pentagonal or heptagonal carbon rings generate effective gauge fields that induce quantized Berry phases. We derive a compact expression for the geometric phase in terms of the genus and number of open boundaries of the structure, providing a topological classification of zero-energy modes. This framework enables a deeper understanding of quantum holonomies in graphene and their potential application in holonomic quantum computation. Our approach bridges discrete lattice models with continuum index theory, yielding insights that are both physically intuitive and experimentally accessible.

cond-mat.mes-hall↗

arXiv2025

Conformal Geometry and Regularization of Disclinations by a Cosmological Constant in $(2+1)$ Dimensions

We investigate the effect of a cosmological constant $\Lambda$ on the geometry generated by a two-dimensional disclination in a conformal metric framework. For $\Lambda>0$, we obtain an exact analytic solution of the Liouville-type equation, which regularizes the defect core, preserves the topological charge, and yields a compact space with finite volume and positive curvature. For $\Lambda<0$, the solution must be obtained numerically and asymptotically approaches $R \to 3\Lambda < 0$, producing an open hyperbolic geometry with divergent volume. In both regimes, the curvature profile is governed solely by the disclination strength $\alpha$, while the sign of $\Lambda$ dictates the global phase: compact and confined for $\Lambda>0$, hyperbolic and delocalized for $\Lambda<0$. This establishes a clear geometric dichotomy and shows that the cosmological constant provides a natural analytic regularization beyond cutoff-based treatments, with implications for analog gravity and two-dimensional condensed matter systems.

gr-qc↗

arXiv2025

Euclid: Forecasts on $Λ$CDM consistency tests with growth rate data

The large-scale structure (LSS) of the Universe is an important probe for deviations from the canonical cosmological constant $Λ$ and cold dark matter ($Λ$CDM) model. A statistically significant detection of any deviations would signify the presence of new physics or the breakdown of any number of the underlying assumptions of the standard cosmological model or possible systematic errors in the data. In this paper, we quantify the ability of the LSS data products of the spectroscopic survey of the Euclid mission, together with other contemporary surveys, to improve the constraints on deviations from $Λ$CDM in the redshift range $0<z<1.75$. We consider both currently available growth rate data and simulated data with specifications from Euclid and external surveys, based on $Λ$CDM and a modified gravity (MoG) model with an evolving Newton's constant (denoted $μ$CDM), and carry out a binning method and a machine learning reconstruction, based on genetic algorithms (GAs), of several LSS null tests. Using the forecast Euclid growth data from the spectroscopic survey in the range $0.95<z<1.75$, we find that in combination with external data products (covering the range $0<z<0.95$), Euclid will be able to improve on current constraints of null tests of the LSS on average by a factor of eight when using a binning method and a factor of six when using the GAs. Our work highlights the need for synergies between Euclid and other surveys, but also the usefulness of statistical analyses, such as GAs, in order to disentangle any degeneracies in the cosmological parameters. Both are necessary to provide tight constraints over an extended redshift range and to probe for deviations from the $Λ$CDM model.

astro-ph.CO↗

arXiv2021

Computational methods for 2D materials modelling

Materials with thickness ranging from a few nanometers to a single atomic layer present unprecedented opportunities to investigate new phases of matter constrained to the two-dimensional plane.Particle-particle Coulomb interaction is dramatically affected and shaped by the dimensionality reduction, driving well-established solid state theoretical approaches to their limit of applicability. Methodological developments in theoretical modelling and computational algorithms, in close interaction with experiments, led to the discovery of the extraordinary properties of two-dimensional materials, such as high carrier mobility, Dirac cone dispersion and bright exciton luminescence, and inspired new device design paradigms. This review aims to describe the computational techniques used to simulate and predict the optical, electronic and mechanical properties of two-dimensional materials, and to interpret experimental observations. In particular, we discuss in detail the particular challenges arising in the simulation of two-dimensional constrained fermions, and we offer our perspective on the future directions in this field.

cond-mat.mes-hall↗

arXiv2020

Higgs boson potential at colliders: status and perspectives

This document summarises the current theoretical and experimental status of the di-Higgs boson production searches, and of the direct and indirect constraints on the Higgs boson self-coupling, with the wish to serve as a useful guide for the next years. The document discusses the theoretical status, including state-of-the-art predictions for di-Higgs cross sections, developments on the effective field theory approach, and studies on specific new physics scenarios that can show up in the di-Higgs final state. The status of di-Higgs searches and the direct and indirect constraints on the Higgs self-coupling at the LHC are presented, with an overview of the relevant experimental techniques, and covering all the variety of relevant signatures. Finally, the capabilities of future colliders in determining the Higgs self-coupling are addressed, comparing the projected precision that can be obtained in such facilities. The work has started as the proceedings of the Di-Higgs workshop at Colliders, held at Fermilab from the 4th to the 9th of September 2018, but it went beyond the topics discussed at that workshop and included further developments.

hep-ph↗

arXiv2018

Les Houches 2017: Physics at TeV Colliders New Physics Working Group Report

We present the activities of the `New Physics' working group for the `Physics at TeV Colliders' workshop (Les Houches, France, 5--23 June, 2017). Our report includes new physics studies connected with the Higgs boson and its properties, direct search strategies, reinterpretation of the LHC results in the building of viable models and new computational tool developments.

hep-ph↗

arXiv2017

Handbook of LHC Higgs Cross Sections: 4. Deciphering the Nature of the Higgs Sector

This Report summarizes the results of the activities of the LHC Higgs Cross Section Working Group in the period 2014-2016. The main goal of the working group was to present the state-of-the-art of Higgs physics at the LHC, integrating all new results that have appeared in the last few years. The first part compiles the most up-to-date predictions of Higgs boson production cross sections and decay branching ratios, parton distribution functions, and off-shell Higgs boson production and interference effects. The second part discusses the recent progress in Higgs effective field theory predictions, followed by the third part on pseudo-observables, simplified template cross section and fiducial cross section measurements, which give the baseline framework for Higgs boson property measurements. The fourth part deals with the beyond the Standard Model predictions of various benchmark scenarios of Minimal Supersymmetric Standard Model, extended scalar sector, Next-to-Minimal Supersymmetric Standard Model and exotic Higgs boson decays. This report follows three previous working-group reports: Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables (CERN-2011-002), Handbook of LHC Higgs Cross Sections: 2. Differential Distributions (CERN-2012-002), and Handbook of LHC Higgs Cross Sections: 3. Higgs properties (CERN-2013-004). The current report serves as the baseline reference for Higgs physics in LHC Run 2 and beyond.

hep-ph↗

arXiv2016

2D materials and van der Waals heterostructures

The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With new 2D materials, truly 2D physics has started to appear (e.g. absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc). Novel heterostructure devices are also starting to appear - tunneling transistors, resonant tunneling diodes, light emitting diodes, etc. Composed from individual 2D crystals, such devices utilize the properties of those crystals to create functionalities that are not accessible to us in other heterostructures. We review the properties of novel 2D crystals and how their properties are used in new heterostructure devices.

cond-mat.mtrl-sci↗

arXiv2016

Strongly bound Mott-Wannier Excitons in GeS and GeSe monolayers

The excitonic spectra of single layer GeS and GeSe are predicted by ab initio GW-Bethe Salpeter equation calculations. G 0 W 0 calculations for the band structures find a fundamental band gap of 2.85 eV for GeS and 1.70 eV for GeSe monolayer. However, excitons are tightly bound, specially in GeS at the Γ point, where the quasi-particle interactions are so strong that they shift the Γ exciton peak energy into the visible range and below the off-Γ exciton peak. The lowest energy excitons in both materials are excited by light along the zigzag direction and have exciton binding energies of 1.05 eV and 0.4 eV, respectively, but despite the strong binding, the calculated binding energies are in agreement with a Mott-Wannier model.

cond-mat.mes-hall↗

arXiv2016

Les Houches 2015: Physics at TeV colliders - new physics working group report

We present the activities of the 'New Physics' working group for the 'Physics at TeV Colliders' workshop (Les Houches, France, 1-19 June, 2015). Our report includes new physics studies connected with the Higgs boson and its properties, direct search strategies, reinterpretation of the LHC results in the building of viable models and new computational tool developments. Important signatures for searches for natural new physics at the LHC and new assessments of the interplay between direct dark matter searches and the LHC are also considered.

hep-ph↗

arXiv2016

Vacancies and oxidation of 2D group-IV monochalcogenides

Point defects in the binary group-IV monochalcogenide monolayers of SnS, SnSe, GeS, GeSe are investigated using density-functional-theory calculations. Several stable configurations are found for oxygen defects, however we give evidence that these materials are less prone to oxidation than phosphorene, with which monochalcogenides are isoelectronic and share the same orthorhombic structure. Concurrent oxygen defects are expected to be vacancies and substitutional oxygen. We show that it is energetically favorable oxygen be incorporated into the layers substituting for a chalcogen (O S/Se defects), and different from most of the other defects investigated, this defect preserves the electronic structure of the material. Thus, we suggest that annealing treatments can be useful for the treatment of functional materials where loss mechanisms due to the presence of defects are undesirable.

cond-mat.mes-hall↗

arXiv2016

Multiferroic Two-Dimensional Materials

The relation between unusual Mexican-hat band dispersion, ferromagnetism and ferroelasticity is investigated using a combination of analytical, first-principles and phenomenological methods. The class of material with Mexican-hat band edge is studied using the $α$-SnO monolayer as a prototype. Such band edge causes a van Hove singularity diverging with $\frac{1}{\sqrt{E}}$, and in p-type material leads to spatial and/or time-reversal spontaneous symmetry breaking. We show that an unexpected multiferroic phase is obtained in a range of hole density for which the material presents ferromagnetism and ferroelasticity simultaneously.

cond-mat.mtrl-sci↗

arXiv2015

Enhanced piezoelectricity and modified dielectric screening of 2-D group-IV monochalcogenides

We use first principles calculations to investigate the lattice properties of group-IV monochalcogenides. These include static dielectric permittivity, elastic and piezoelectric tensors. For the monolayer, it is found that the static permittivity, besides acquiring a dependence on the interlayer distance, is comparatively higher than in the 3D system. In contrast, it is found that elastic properties are little changed by the lower dimensionality. Poisson ratio relating in-plane deformations are close to zero, and the existence of a negative Poisson ratio is also predicted for the GeS compound. Finally, the monolayers shows piezoelectricity, with piezoelectric constants higher than that recently predicted to occur in other 2D-systems, as hexagonal BN and transition metal dichalcogenide monolayers.

cond-mat.mes-hall↗

arXiv2015

Valleytronics in Tin (II) Sulfide

Tin (II) sulfide (SnS) is a layered mineral found in nature. In this paper, we study the two-dimensional form of this material using a combination of \emph{ab initio} calculation and $\mathbf{k}\cdot\mathbf{p}$ theory. In particular, we address the valley properties and the optical selection rules of 2D SnS. Our study reveals SnS as an extraordinary material for valleytronics, where pairs of equivalent valleys are placed along two perpendicular axes, can be selected exclusively with linear polarized light, and can be separated using non-local electrical measurements.

cond-mat.mes-hall↗

arXiv2015

Phosphorene analogues: isoelectronic two-dimensional group-IV monochalcogenides with orthorhombic structure

The group-IV monochalcogenides SnS, GeS, SnSe and GeSe form a family within the wider group of semiconductor `phosphorene analogues'. Here, we used first principles calculations to investigate systematically their structural, electronic and optical properties, analysing the changes associated with the reduction of dimensionality, from bulk to monolayer or bilayer form. We show that all those binary phosphorene analogues are semiconducting, with bandgap energies covering part of the infra-red and visible range, and in most cases higher than phosphorene. Further, we found that they have multiple valleys in the valence and conduction band, with spin-orbit splitting of the order of 19-86 meV.

cond-mat.mes-hall↗

arXiv2015

Atomically thin dilute magnetism in Co-doped phosphorene

Two-dimensional dilute magnetic semiconductors can provide fundamental insights in the very nature of magnetic orders and their manipulation through electron and hole doping. Despite the fundamental physics, due to the large charge density control capability in these materials, they can be extremely important in spintronics applications such as spin valve and spin-based transistors. In this article, we studied a two-dimensional dilute magnetic semiconductors consisting of phosphorene monolayer doped with cobalt atoms in substitutional and interstitial defects. We show that these defects can be stabilized and are electrically active. Furthermore, by including holes or electrons by a potential gate, the exchange interaction and magnetic order can be engineered, and may even induce a ferromagnetic-to-antiferromagnetic phase transition in p-doped phosphorene.

cond-mat.mtrl-sci↗