Transport & Nanostructures Group

Our latest paper of 2024 “Lévy flight for electrons in graphene in the presence of regions with enhanced spin-orbit coupling”  has just appeared in Physical Review B.

In this work, we propose an electronic Lévy glass built from graphene nanoribbons in the presence of regions with enhanced spin-orbit coupling. Although electrons in graphene nanoribbons present a low spin-orbit coupling strength, it can be increased by a proximity effect with an appropriate substrate. We consider graphene nanoribbons with different edge types, which contain circular regions with a tunable Rashba spin-orbit coupling, whose diameter follows a power-law distribution. We find that spin-orbital clusters induce a transition from superdiffusive to diffusive charge transport, similar to what we recently reported for nanoribbons with electrostatic clusters [Phys. Rev. B 107, 155432 (2023)]. We also investigate spin polarization in the spin-orbital Lévy glasses, and show that a finite spin polarization can be found only in the superdiffusive regime. In contrast, the spin polarization vanishes in the diffusive regime, making the electronic Lévy glass a useful device whose electronic transmission and spin polarization can be controlled by its Fermi energy. Finally, we apply a multifractal analysis to charge transmission and spin polarization, and find that the transmission time series in the superdiffusive regime are multifractal, while they tend to be monofractal in the diffusive regime. In contrast, spin polarization time series are multifractal in both regimes, characterizing a marked difference between mesoscopic fluctuations of charge transport and spin polarization in the proposed electronic Lévy glass.

The results stem from the work of new Ph.D student Diego B. Fonseca, which is co-supervised by Anderson L. R. Barbosa at UFRPE.

Our latest publication “Length and torsion dependence of thermal conductivity in twisted graphene nanoribbons” has just been accepted for publication in Physical Review Materials.

In this work we investigate the dependence of the thermal conductivity (TC) of twisted graphene nanoribbons (TGNRs) on the number of applied turns to the GNR by calculating more precise and mathematically well defined geometric parameters related to the TGNR shape, namely, its twist and writhe. We show that the dependence of the TC on twist is not a simple function of the number of turns initially applied to a straight GNR. In fact, we show that the TC of TGNRs requires at least two parameters to be properly described. Our conclusions are supported by atomistic molecular dynamics simulations to obtain the TC of suspended TGNRs prepared under different values of initially applied turns and different sizes of their suspended part. Among possible choices of parameter pairs, we show that TC can be appropriately described by the initial number of turns and the initial twist density of the TGNRs.

The work is a collaboration with Alexandre Fonseca at UNICAMP.

Our first paper of 2024 “Tuning the thermal conductivity of silicon phononic crystals via different defect motifs: Implications for thermoelectric devices and photovoltaics” has just been accepted for publication in ACS Applied Nano Materials, where it will be part of a Forum Focused on South American Authors jointly published with ACS Applied Materials & Interfaces. This joint special issue will be a collection of articles published in a single issue of the journal authored by leaders of applied materials in South America.

In this publication we investigate the thermal conductivity of silicon phononic crystals, materials with a periodic arrangement of modifications which can be tailored to control their thermal conductivity. The introduction of intermediate-sized pores in silicon membranes can reduce their lattice thermal conductivity and increase their thermoelectric efficiency, as long as the pores are not too small to interfere with electron transport, nor too large to cause structural instabilities in the material. We consider thin silicon membranes and structures with holes of different sizes and shapes, forming phononic crystals with different defect motifs, and calculate their thermal conductivity along the [110] and [-110] crystalline directions, with the homogeneous non-equilibrium molecular dynamics method. We find that for the hole sizes considered the thermal conductivity is a logarithmically decreasing function of the defect area, independent of defect shape and transport direction, and verify that the thermal conductivity of silicon phononic crystals is ultimately limited by the neck size, which is the smallest distance between two adjacent defects, in agreement with the literature. However, our results also show that the dependence of the conductivity with neck size follows a power law along the [110] direction, but shows an exponential dependence along the [-110] direction. We attribute this difference to the scattering of phonons by surface dimers which originate in the 2×1 reconstruction of the silicon [001] surface, and are oriented along the [-110], acting as resonators which can scatter phonon more efficiently along that direction. Those findings are relevant for the design of thermoelectric devices and thermal barriers in photovoltaic cells.

The results stem from Higo de Araujo Oliveira mater’s thesis, and it is the first graduate thesis from our group since we moved to UFPE in 2020. It is also another great result from our ongoing collaboration with Ari Harju and Zheyong Fan, both currently at Varian Medical Systems in Finland.

This week I am giving a presentation at the XI Brazilian Meeting on Simulational Physics. This eleventh edition will take place at Universidade Federal de Minas Gerais, where the meeting started a few decades ago, but it also includes a few online presentations. Traditionally, the BMSP gathers scientists specialized in simulation in the most diverse areas of physics, chemistry, biology and materials science, who present the latest advances in methodology and techniques applied to the study of problems through computer simulations. The meeting also promotes interaction among scientists working in this area, with the goal of advancing the methods and techniques available today.

In my online presentation, entitled “Wave transmission and its fluctuations in superlattices with Lévy-like disorder” I will present some of our recent results on the transmission of Schrödinger, Klein-Gordon and Dirac waves in the presence of disorder following a Lévy distribution, as well as its application in a graphene Lévy glass.

In September I started a sabbatical leave during which I will be at Dipartimento di Fisica of Sapienza Università di Roma. A sabbatical period is an extraordinary opportunity to focus on novel challenging issues while visiting a new institute, and experiencing a new academic atmosphere, free of heavy teaching and administrative duties. For the next 12 months I will be working in the quantum theory of materials group, hosted by Francesco Mauri, focusing on thermal transport in novel materials.

Our first paper of 2023 “Lévy flight for electrons in graphene: Superdiffusive-to-diffusive transport transition”  has just appeared in Physical Review B.

In this work we propose an electronic Lévy glass, analogous to a recent optical realization. To that end, we investigate the transmission of electrons in graphene nanoribbons in the presence of circular electrostatic clusters, whose diameter follow a power-law distribution. We analyze the effect of the electrostatic clusters on the electronic transport regime of the nanoribbons, in terms of its diffusion behavior. Our numerical calculations show that the presence of circular electrostatic clusters induces a transition from Lévy (superdiffusive) to diffusive transport as the energy increases. Furthermore, we argue that in our electronic Lévy glass, superdiffusive transport is an exclusive feature of the low-energy quantum regime, while diffusive transport is a feature of the semiclassical regime. We thus attribute the observed transition to the chiral symmetry breaking, once the energy moves away from the Dirac point of graphene.

The results stem from Diego B. Fonseca mater’s thesis, which is supervised by Anderson L. R. Barbosa at UFRPE.

Our latest publication of 2022 “Wave transmission and its universal fluctuations in one-dimensional systems with Lévy-like disorder: Schrödinger, Klein-Gordon, and Dirac equations”  has just appeared in the Physical Review E.

In this publication we investigate the propagation of electronic waves in one-dimensional systems with Lévy-type disorder. We perform a complete analysis of nonrelativistic and relativistic wave transmission submitted to potential barriers whose width, separation, or both follow Lévy distributions characterized by an exponent 0<α<1. For the first two cases, where one of the parameters is fixed, nonrelativistic and relativistic waves present anomalous localization. However, for the latter case, in which both parameters follow a Lévy distribution, nonrelativistic and relativistic waves present a transition between anomalous and standard localization as the incidence energy increases relative to the barrier height. Moreover, we obtain the localization diagram delimiting anomalous and standard localization regimes, in terms of incidence angle and energy. Finally, we verify that transmission fluctuations, characterized by its standard deviation, are universal, independent of barrier architecture, wave equation type, incidence energy, and angle, further extending earlier studies on electronic localization. We believe that our predictions can be verified in graphene nanoribbons, where Dirac electrons are the main charge carriers.

This publication was done in collaboration with Jonas R.F. Lima and Anderson L. R. Barbosa, both at UFRPE, and we began it before the covid-19 pandemic hit the world! We are all glad to finally see it out there.