New publication in ACS Applied Nano Materials – Tuning the thermal conductivity of silicon phononic crystals

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.

New Ph.D. student – Diego B. Fonseca

This month we officially welcome a new Ph.D. student in the Transport & Nanostructures group. Diego B. Fonseca joins us after completing his undergraduate and M.Sc. degrees at nearby UFRPE, where he was supervised by our collaborator Anderson L. R. Barbosa, who also will serve as Diego’s co-supervisor at the Physics Graduate Program at UFPE. In fact, the three of us already share one publication in Physical Review B related to his master’s dissertation “Lévy flight for electrons in graphene: Superdiffusive-to-diffusive transport transition“, and another one is on the way. In our group, Diego will continue to work on electron and spin transport in graphene-based nanostructures, and we are sure he will keep up with his excellent work, leading to a successful doctoral degree in a few years.

XI Brazilian Meeting on Simulational Physics

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.

M.Sc. defense – Higo Oliveira

Graduate student Higo de Araujo Oliveira, defended his master’s thesis on Thursday (21.09.2023) in a virtual auditorium via google meet. The thesis, entitled “Thermal conductivity calculation in Si membranes: a Homogeneous Non Equilibrium Molecular Dynamics approach”, employed advanced computer simulation techniques to calculate the thermal conductivity of silicon membranes, including the presence of defects forming the so-called phononic crystals.

Higo was the first member of TNG at UFPE, and has been with us since 2021. During the last 2+ years he mastered fundamental aspects of molecular dynamics simulations and thermal transport calculations. He also has enough results for a publication. We are very proud of his development as a scientist and look forward to his continued success as a doctoral student in our group.

Sabbatical at Sapienza University of Rome

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.

New publication in Physical Review B: Lévy flight for electrons in graphene

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.

Conference and Advanced School on Low-Dimensional Quantum Systems

From March 13 to 24 we are taking part on the Conference and Advanced School on Low-Dimensional Quantum Systems, an event promoted by the International Centre for Theoretical Physics (ICTP Trieste), which takes place in the Facultad de Ciencias Fisicas y Matematicas of the Universidad de Chile in Santiago.

The event will combine a set of lectures and research talks during the two weeks and will be a meeting point for researchers worldwide, nurturing collaborations and exchange of knowledge among researchers, students and postdocs of different continents and subfields. The activities will highlight recent progress in a range of topics including electronic hydrodynamics, dissipative systems and light-matter interaction, low-dimensional devices and disorder, topological materials, and emergent phases in novel materials.

In my talk, entitled “Wave transmission and its universal fluctuations in one-dimensional systems with Lévy-like disorder: Schrödinger, Klein-Gordon, and Dirac equations“, I will discuss recent results from our homonymous paper in Physical Review E last year.

It is my first in-person conference since February 2020, and I am very excited to participate in this amazing event. Many thanks to the support offered by ICTP and to the organization efforts led by Luis E. F. Foà Torres and collaborators.

New article in Physical Review E: Wave transmission in Lévy-disordered systems

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.

Doctoral defense – José Roberto da Silva

Graduate student José Roberto da Silva, defended his Doctoral dissertation on Wednesday (29.06.2022) at 10:00 am, in a virtual auditorium via google meet. The thesis, entitled “Electronic transport in quasiperiodic nanostructures”, employed advances computational techniques such as efficient implementations of Green’s function calculations to calculate the electronic conductance of periodic and quasiperiodic graphene nanoribbons in the presence of defects and other perturbations.

José Roberto has been a member of TNG since 2017. During the last 4+ years he authored one publication, and has enough results for another one. We are very proud of his development as a scientist and look forward to his continued success.

New publication in Micro and Nanostructures

Our latest publication of 2022 “Localization effects in graphene nanoribbons with quasiperiodic hopping modulation”  has just appeared in Micro and Nanostructures (formerly know as Superlattices and Microstructures).

Graphene nanoribbons present remarkable electronic transport properties which can be tailored to specific applications. We consider a quasiperiodic modulation in the electronic hopping of metallic armchair and zigzag graphene nanoribbons, originating a Fibonacci superlattice with two possible hopping parameters. The electronic conductance for various Fibonacci generations is calculated via the recursive Green’s function method. We observe that a quasiperiodic hopping modulation opens a conductance gap at the Fermi level of armchair nanoribbons, leading to a strong electronic localization regime. The localization length initially increases with Fibonacci generation and saturates for high order generations. Considering an energy slightly above the Fermi level, we obtain similar results for the armchair nanoribbon, while the zigzag nanoribbon eventually enters the localization regime for high order Fibonacci generations. Our results demonstrate that a quasiperiodic hopping modulation is a viable way to control the electronic transport in graphene nanoribbons, widening its range of possible application in nanoelectronic devices.

This publication stems from José Roberto’s PhD dissertation at UFRN, which has been co-supervised by Anderson L. R. Barbosa at UFRPE.