New paper in Physica A – Majority-vote model with limited visibility

Our latest publication “Majority-vote model with limited visibility: An investigation into filter bubbles” is a little outside our main line of work. Nonetheless, any physics problem is interesting, and opinion formation models are a lot of fun to investigate. Indeed, the dynamics of opinion formation in a society is a complex phenomenon where many variables play essential roles. Recently, the influence of algorithms to filter which content is fed to social networks users has come under scrutiny. Supposedly, the algorithms promote marketing strategies, but can also facilitate the formation of filters bubbles in which a user is most likely exposed to opinions that conform to their own.

In the two-state majority-vote model, an individual adopts an opinion contrary to the majority of its neighbors with probability , defined as the noise parameter. Here, we introduce a visibility parameter  in the dynamics of the majority-vote model, which equals the probability of an individual ignoring the opinion of each one of its neighbors. For  each individual will, on average, ignore the opinion of half of its neighboring nodes. We employ Monte Carlo simulations to calculate the critical noise parameter as a function of the visibility  and obtain the phase diagram of the model. We find that the critical noise is an increasing function of the visibility parameter, such that a lower value of  favors dissensus. Via finite-size scaling analysis we obtain the critical exponents of the model, which are visibility-independent, and show that the model belongs to the Ising universality class. We compare our results to the case of a network submitted to a static site dilution and find that the limited visibility model is a more subtle way of inducing opinion polarization in a social network.

This work is a collaboration with my colleagues André Vilela at Universidade de Pernambuco, Laercio Dias and Luciano Rodrigues at Universidade Federal do Rio Grande do Norte, and H. E. Stanley at Boston University.

BRICS Young Scientist Forum

This week we are taking part in the 5th BRICS Young Scientist Forum, hosted by the South Ural State University, in Chelyabinsk, Russia. We have been selected by the Brazilian Ministry for Science, Technology and Innovation to compose the Brazilian Delegation in the area of Materials Science.

In our presentation, entitled “Computational experiments of heat transport in 2D superlattices” we review some of the recent results obtained within our research group.

New paper in PCCP – Boron-doped nitrogenated holey graphene

Our fourth publication of 2020 “Electronic, optical and thermoelectric properties of boron-doped nitrogenated holey graphene”  has just been published in Physical Chemistry Chemical Physics.

Following-up form our previous publications on the physical properties of nitrogenated holey graphene (NHG), we now employed first principles calculations to investigate the electronic, optical, and thermoelectric properties of ten boron-doped NHG monolayers. We find that most of the proposed structures remain stable during ab initio molecular dynamics simulations, in spite of their increased formation energies. Density functional theory calculations employing a hybrid functional predict band gaps ranging from 0.73 eV to 2.30 eV. In general, we find that boron doping shifts optical absorption towards the visible spectrum, and also reduces light reflection in this region. On the other hand, the magnitude of optical absorption coefficients are reduced. Regarding the thermoelectric properties, we predict that boron doping can enhance the figure of merit ZT of NHG by up to 55%. Our results indicate that boron-doped NHG monolayers may find application in solar cells and thermoelectric devices.

This work results from collaboration with colleagues at Universidade Federal do Rio Grande do Norte, Universidade Federal da Paraíba, and Leibniz Universität Hannover. Most calculations were carried out within our research group at UFRN, and we are grateful for the computational support provided by the local supercomputing center NPAD.

Doctoral defense – Isaac de Macedo Felix

Graduate student Isaac de Macêdo Felix, defended his Doctoral dissertation on Friday (04.08.2020) at 10:00 am, in a virtual auditorium via google meet. The thesis, entitled “Heat conduction in quasiperiodic graphene-hBN superlattice nanoribbons” employed non-equilibrium molecular dynamics simulations to calculate the thermal conductivity of periodic and quasiperiodic graphene-hBN nanoribbons.

Isaac has been a member of TNG since its inception in early 2014, where he also completed his M.Sc. work in 2016. During the last 6 years he authored 4 publications, and has enough results for at least a couple more. We are very proud of his development as a scientist and look forward to his continued success.

The 33rd Annual Center for Simulational Physics Workshop

This week we are in Athens, Georgia for the 33rd edition of the Center for Simulational Physics Workshop, which has the theme “Recent Developments in Computer Simulational Studies in Condensed Matter Physics“. 

This annual workshop series highlights advances in applications, algorithms, and parallel implementations of computer simulation methods for the study of condensed matter systems. Topics of interest include Monte Carlo, molecular dynamics, and other numerical studies of material growth, structural and magnetic phase transitions, polymers, surfaces and interfaces, strongly correlated electron systems and exotic quantum phases, granular flow, diffusion, membranes and protein folding. 

We are honored with the opportunity to deliver an invited talk entitled “Phonon thermal conductivity of 2D materials with MD simulations”, where we will present some of the most recent developments in our research. It is a great pleasure to be back in Athens and the Center for Simulational Physics, where I was a graduate student 15 years ago.

New paper in Carbon – Suppression of coherent thermal transport in quasiperiodic graphene-hBN superlattices

Our third publication of 2020 “Suppression of coherent thermal transport in quasiperiodic graphene-hBN superlattice ribbons”  has just been published in Carbon.

Nanostructured superlattices are promising materials for novel electronic devices due to their adjustable physical properties. Periodic superlattices facilitate coherent phonon thermal transport due to constructive wave interference at the boundaries between the materials. However, it is possible to induce a crossover from coherent to incoherent transport regimes by adjusting the superlattice period. In 2018 we observed such crossover in periodic graphene-boron nitride nanoribbons as the length of individual domains was increased. In general, transport properties are dominated by translational symmetry and the presence of unconventional symmetries leads to unusual transport characteristics. In this work we perform non-equilibrium molecular dynamics simulations to investigate phonon heat transport in graphene-hBN superlattices following the Fibonacci quasiperiodic sequence, which lie between periodic and disordered structures. We show that the quasiperiodicity can suppress coherent phonon thermal transport in these superlattices. This behavior is related to the increasing number of interfaces per unit cell as the Fibonacci generation increases, hindering phonon coherence along the superlattice. The suppression of coherent thermal transport in graphene-hBN superlattices enables a higher degree of control on heat conduction at the nanoscale, and shows potential for application in the design of novel thermal management devices.

We are rather proud of this work. It is an extension of Isaac’s Master’s thesis and also part of his PhD dissertation. It was completely carried out within our research group, and we are grateful for the computational support provided by the supercomputing center at UFRN (NPAD).