Transport & Nanostructures Group

Our second publication of 2021 “First-principles investigation of electronic, optical, mechanical and heat transport properties of pentadiamond: A comparison with diamond”  has just appeared in Carbon Trends

Pentadiamond is a carbon allotrope consisting of hybrid sp2 and sp3 atoms, which has recently been predicted to be stable and synthesizable. We employed first-principles calculations to explore the electronic structure, optical characteristics, mechanical response and lattice thermal conductivity of pentadiamond, performing a direct comparison with the corresponding properties in diamond. Density functional theory calculations with the HSE06 functional predicts indirect electronic band gaps of 3.58 eV and 5.27 eV for pentadiamond and diamond, respectively. Pentadiamond shows large absorption in the middle UV region, where diamond does not absorb light, consistent with its smaller band gap. The elastic modulus and tensile strength of pentadiamond are 496 GPa and 60 GPa, respectively, considerably lower than the corresponding values for diamond. Finally, the lattice thermal conductivity was examined by solving the Boltzmann transport equation with anharmonic force constants evaluated via state-of-the-art machine-learning interatomic potentials. We predict a thermal conductivity of 427 W/m-K for pentadiamond, less than one fifth of the corresponding quantity for diamond. Our results provide a useful vision of the intrinsic properties of pentadiamond, but also highlight some of its disadvantages in mechanical strength and heat conduction when compared to diamond.

This work results from collaboration with colleagues at Leibniz Universität Hannover and the Persian Gulf University.

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.

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.

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).