Matter Growth in Imperfect Fluid Cosmology

Citation:

Zimdahl W, Velten HES, Algoner WC. Matter Growth in Imperfect Fluid Cosmology. Universe [Internet]. 2019;5 (3).

Abstract:

Extensions of Einstein’s General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The imperfect fluid structure allows us to include, on a phenomenological basis, anisotropic stresses and energy fluxes which are considered as potential signatures for deviations from the cosmological standard Λ -cold-dark-matter ( Λ CDM) model. As an example, we consider the dynamics of a scalar-tensor extension of the standard model, the e Φ Λ CDM model. We constrain the magnitudes of anisotropic pressure and energy flux with the help of redshift-space distortion (RSD) data for the matter growth function f σ 8 .

Website

Publicações Recentes do Departamento de Física

G Vieira C, P H de Oliveira MA, Fernandes WP, Matos MJS, Paniago R, de Carvalho VE, Chacham H, A Soares E. Unveiling Pseudo-Dirac Resonances in the (7×7)R19.1° Phase of Sulfur Segregated on Cu(111) Surface. The Journal of Physical Chemistry C [Internet]. Submitted :null. Publisher's Version

Gontijo RN, Moutinho MVO, Righi A, Chiu P-W, Venezuela P, Pimenta MA. Resonant enhancement of the 2G Raman band in twisted bilayer graphene. Materials Chemistry and Physics [Internet]. 2024 :129279. Publisher's Version Abstract

Raman spectroscopy is an extremely useful tool to characterize graphene systems. The strongest Raman features are the first-order G band and the second-order 2D and 2D′ bands, which are the overtones of the double resonance D and D’ bands. However, the 2G band, which is the overtone of the G band, is not usually observed in the spectra of monolayer graphene and of crystalline graphite. In this work, we present an experimental and theoretical investigation of the resonance Raman spectra in twisted bilayer graphene (TBG) with different twisting angles and using several laser excitation energies in the NIR and visible ranges. We observed that the 2G band is enhanced when the incident photons are in resonance with the transition between the van Hove singularities in the density of states of the TBG. We show that the 2G band has three contributions (2G1, 2G2 and 2G3), that are not dispersive by changing the laser excitation energy. We also present theoretical calculations showing that the 2G1 and 2G2 bands are related to combinations of the in-phase (IP) and out-of-phase (OP) vibrations of the atoms in the different layers. The Raman excitation profiles (REPs) of the 2G peaks are upshifted in comparison with the REP of the G band. This behavior was confirmed theoretically using a graphene tight binding model. We conclude that the different resonance behavior comes from the fact that the G band is a first-order process whereas the 2G band is second-order processes giving rise to overall different resonance conditions.

Freitas WD, Mazzoni MSC, Matos MJS, Galvão BRL. Predicting molecular crystals of polynitrogen (N6) structures with cage-like geometries using ab initio evolutionary algorithm. Chemical Physics Letters [Internet]. 2024;844 :141262. Publisher's Version Abstract

In this work, we employed Density Functional Theory calculations combined with search techniques based on evolutionary algorithms to predict and characterize crystalline structures composed of nitrogen (N6) cage-like molecules. We found stable molecular crystals and a rich phenomenology associated with their behavior under pressure, including atomic rebonding and semiconductor-metal transitions. This investigation resides in the context of high-energy-density materials, since molecular species containing only nitrogen atoms tend to dissociate into N2 molecules, releasing large amounts of energy.

Omwoyo E, Belich H, Fabris JC, Velten H. Black hole lensing in Kerr-de Sitter spacetimes. The European Physical Journal Plus. 2023;138 (11) :1043.

Valentim MR, Matos MJS, Mazzoni MSC. B2N2O2: A wide-bandgap two-dimensional semiconductor featuring a zigzag nitrogen array of bonds. Applied Physics Letters [Internet]. 2023;123 (9) :093103. Publisher's Version Abstract

In this work, we apply a combination of theoretical techniques to characterize a two-dimensional material with formula B2N2O2, featuring a zigzag array of nitrogen atoms. We predict its energetic, thermal, and dynamic stability and determine its electronic properties, including band structure and mobility evaluation for a phonon-mediated mechanism. We show that the compound is a wideband-gap semiconductor, with parabolic band edges and with large electron and hole mobilities within the deformation potential approach. We ascribe this result to the existence of electronic channels defined by the zigzag array of nitrogen bonds, which define the edges of both conduction and valence bands. We also propose a mechanism to synthesize the compound based on oxygen functionalization and application of pressure. Finally, we show that the results can be generalized to represent a family of 2D compounds.

Flecha AC, Bandeira RA, Campos VBG, Silva AVC, Leiras A. Social Network Analysis in disaster management. Production. 2023;33 :e20220046.