arXiv subjects

R. M. Ribeiro

Publications and source records attributed to R. M. Ribeiro.

17 recordsLinked to original sources

arXiv2025

Electrically Tunable Interband Collective Excitations in Biased Bilayer and Trilayer Graphene

Collective excitations of charged particles under the influence of an electromagnetic field give rise to a rich variety of hybrid light-matter quasiparticles with unique properties. In metals, intraband collective response manifested by negative permittivity leads to plasmon-polaritons with extreme field confinement, wavelength squeezing, and potentially low propagation losses. In contrast, photons in semiconductors commonly couple to interband collective response in the form of exciton polaritons, which give rise to completely different polaritonic properties, described by a superposition of the photon and exciton and an anti-crossing of the eigenstates. In this work, we identify the existence of plasmon-like collective excitations originating from the interband excitonic response of biased bilayer and trilayer graphene, in the form of graphene-exciton-polaritons (GEPs). We find that GEPs possess electrically tunable polaritonic properties and discover that such excitations follow a universal dispersion law for all surface polaritons in 2D excitonic systems. Accounting for nonlocal corrections to the excitonic response, we find that the GEPs exhibit confinement factors that can exceed those of graphene plasmons, and with moderate losses. These predictions of plasmon-like interband collective excitations in biased graphene systems open up new research avenues for tunable polaritonic phenomena based on excitonic systems, and the ability to control and manipulate such phenomena at the atomic scale.

cond-mat.mes-hall↗

arXiv2022

Two limit cases of twisted hBN bilayers and their excitonic response

In this paper we discuss the optical response due to the excitonic effect of two types of hBN bilayers: AB and AA'. Understanding the properties of these bilayers is of great utility to the study of twisted bilayers at arbitrary angles, since these two configurations correspond to the limit cases of 0 and 60 degree rotation. To obtain the excitonic response we present a method to solve a four-band Bethe-Salpeter equation, by casting it into a 1D problem, thus greatly reducing the numerical burden of the calculation when compared with strictly 2D methods. We find results in good agreement with ab initio calculations already published in the literature for the AA' bilayer, and predict the excitonic conductivity of the AB bilayer, which remains largely unstudied. The main difference in the conductivity of these two types of bilayers is the appearance of a small, yet well resolved, resonance between two larger ones in the AB configuration. This resonance is due to a mainly interlayer exciton, and is absent in the AA' bilayer. Also, the conductivity of the AB bilayer is due to both intralayer and interlayer excitons and is dominated by p-states, while intralayer states are the relevant ones for the AA' configuration, like in a monolayer. The effect of introducing a bias in the AA' bilayer is also discussed.

cond-mat.mes-hall↗

arXiv2018

Excitons in hexagonal boron nitride single-layer: a new platform for polaritonics in the ultraviolet

The electronic and optical properties of 2D hexagonal boron nitride are studied using first principle calculations. GW and BSE methods are employed in order to predict with better accuracy the excited and excitonic properties of this material. We determine the values of the band gap, optical gap, excitonic binding energies and analyse the excitonic wave functions. We also calculate the exciton energies following an equation of motion formalism and the Elliot formula, and find a very good agreement with the GW+BSE method. The optical properties are studied for both the TM and TE modes, showing that 2D hBN is a good candidate to polaritonics in the UV range. In particular it is shown that a single layer of h-BN can act as an almost perfect mirror for ultraviolet electromagnetic radiation.

cond-mat.mes-hall↗

arXiv2017

Excitonic effects in the optical properties of 2D materials: An equation of motion approach

We present a unified description of the excitonic properties of four monolayer transition-metal dichalcogenides (TMDC's) using an equation of motion method for deriving the Bethe-Salpeter equation in momentum space. Our method is able to cope with both continuous and tight-binding Hamiltonians, and is less computational demanding than the traditional first-principles approach. We show that the role of the exchange energy is essential to obtain a good description of the binding energy of the excitons. The exchange energy at the $Γ-$point is also essential to obtain the correct position of the C-exciton peak. Using our model we obtain a good agreement between the Rydberg series measured for WS$_2$. We discuss how the absorption and the Rydberg series depend on the doping. Choosing $r_0$ and the doping we obtain a good qualitative agreement between the experimental absorption and our calculations for WS$_2$. We also derive a semi-analytical version of Ellitot's formula for TMDC's.

cond-mat.mes-hall↗

arXiv2016

Multiple negative differential conductance regions and inelastic phonon assisted tunneling in graphene-hBN-graphene structures

In this paper we study in detail the effect of the rotational alignment between a hexagonal boron nitride (hBN) slab and the graphene layers in the vertical current of a a graphene-hBN-graphene device. We show how for small rotational angles, the transference of momentum by the hBN crystal lattice leads to multiple peaks in the I-V curve of the device, giving origin to multiple regions displaying negative differential conductance. We also study the effect of scattering by phonons in the vertical current an see how the opening up of inelastic tunneling events allowed by spontaneous emission of optical phonons leads to sharp peaks in the second derivative of the current.

cond-mat.mes-hall↗

arXiv2013

Origin of indirect optical transitions in few-layer MoS2, WS2 and WSe2

It has been well established that single layer MX2 (M=Mo,W and X=S,Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX2, there are two valleys along the Γ-K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in few-layer MX2 by examining the temperature-dependent shift of indirect exciton PL. Hihgly anisotropic thermal expansion of the lattice and corresponding evolution of the band structure result in distinct peak shift for indirect transitions involving the K and Λ (midpoint along Γ-K) valleys. We identify the origin of the indirect emission and concurrently determine the relative energy of these valleys. Our results show that the two valleys compete in energy in few-layer WSe2.

cond-mat.mtrl-sci↗

arXiv2013

Band nesting and the optical response of two-dimensional semiconducting transition metal dichalcogenides

We have studied the optical conductivity of two-dimensional (2D) semiconducting transition metal dichalcogenides (STMDC) using ab-initio density functional theory (DFT). We find that this class of materials presents large optical response due to the phenomenon of band nesting. The tendency towards band nesting is enhanced by the presence of van Hove singularities in the band structure of these materials. Given that 2D crystals are atomically thin and naturally transparent, our results show that it is possible to have strong photon-electron interactions even in 2D

cond-mat.mtrl-sci↗

arXiv2012

Enhanced Optical Dichroism of Graphene Nanoribbons

The optical conductivity of graphene nanoribbons is analytical and exactly derived. It is shown that the absence of translation invariance along the transverse direction allows considerable intra-band absorption in a narrow frequency window that varies with the ribbon width, and lies in the THz range domain for ribbons 10-100nm wide. In this spectral region the absorption anisotropy can be as high as two orders of magnitude, which renders the medium strongly dichroic, and allows for a very high degree of polarization (up to ~85) with just a single layer of graphene. The effect is resilient to level broadening of the ribbon spectrum potentially induced by disorder. Using a cavity for impedance enhancement, or a stack of few layer nanoribbons, these values can reach almost 100%. This opens a potential prospect of employing graphene ribbon structures as efficient polarizers in the far IR and THz frequencies.

cond-mat.mes-hall↗

arXiv2012

Efficient graphene-based photodetector with two cavities

We present an efficient graphene-based photodetector with two Fabri-Pérot cavities. It is shown that the absorption can reach almost 100% around a given frequency, which is determined by the two-cavity lengths. It is also shown that hysteresis in the absorbance is possible, with the transmittance amplitude of the mirrors working as an external driving field. The role of non-linear contributions to the optical susceptibility of graphene is discussed.

cond-mat.mes-hall↗

arXiv2011

Zigzag graphene nanoribbon edge reconstruction with Stone-Wales defects

In this article, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab-initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with non-zero amplitudes in both sub-lattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of pristine zigzag ribbon.

cond-mat.mes-hall↗

arXiv2011

Stability of boron nitride bilayers: Ground state energies, interlayer distances, and tight-binding description

We have studied boron nitride monolayer and bilayer band structures. For bilayers, the ground state energies of the different five stackings are computed using DFT in order to determine the most stable configuration. Also, the interlayer distance for the five different types of stacking in which boron-nitride bilayers can be found is determined. Using a minimal tight binding model for the band structures of boron nitride bilayers, the hopping parameters and the onsite energies have been extracted by fitting a tight binding empirical model to the DFT results.

cond-mat.mes-hall↗

arXiv2010

Optical Properties of Strained Graphene

The optical conductivity of graphene strained uniaxially is studied within the Kubo-Greenwood formalism. Focusing on inter-band absorption, we analyze and quantify the breakdown of universal transparency in the visible region of the spectrum, and analytically characterize the transparency as a function of strain and polarization. Measuring transmittance as a function of incident polarization directly reflects the magnitude and direction of strain. Moreover, direction-dependent selection rules permit identification of the lattice orientation by monitoring the van-Hove transitions. These photoelastic effects in graphene can be explored towards atomically thin, broadband optical elements.

cond-mat.mes-hall↗

arXiv2010

Excitonic effects in the optical conductivity of gated graphene

We study the effect of electron-electron interactions in the optical conductivity of graphene under applied bias and derive a generalization of Elliot's formula, commonly used for semiconductors, for the optical intensity. We show that {\it excitonic resonances} are responsible for several features of the experimentally measured mid-infrared response of graphene such as the increase of the conductivity beyond the "universal" value above the Fermi blocked regime, the broadening of the absorption at the threshold, and the decrease of the optical conductivity at higher frequencies. Our results are also in agreement with {\it ab initio} calculations in the neutral regime.

cond-mat.mes-hall↗

arXiv2009

Strained graphene: tight-binding and density functional calculations

We determine the band structure of graphene under strain using density functional calculations. The ab-initio band strucure is then used to extract the best fit to the tight-binding hopping parameters used in a recent microscopic model of strained graphene. It is found that the hopping parameters may increase or decrease upon increasing strain, depending on the orientation of the applied stress. The fitted values are compared with an available parametrization for the dependence of the orbital overlap on the distance separating the two carbon atoms. It is also found that strain does not induce a gap in graphene, at least for deformations up to 10%.

cond-mat.mes-hall↗

arXiv2009

Scanning Tunneling Microscopy currents on locally disordered graphene

We study the local density of states at and around a substituting impurity and use these results to compute current versus bias characteristic curves of Scanning Tunneling Microscopy (STM) experiments done on the surface of graphene. This allow us to detect the presence of substituting impurities on graphene. The case of vacancies is also analyzed. We find that the shape and magnitude of the STM characteristic curves depend on the position of the tip and on the nature of the defect, with the strength of the binging between the impurity and the carbon atoms playing an important role. Also the nature of the last atom of the tip has an influence on the shape of the characteristic curve.

cond-mat.mes-hall↗

arXiv2008

Distortion of the perfect lattice structure in bilayer graphene

We consider the instability of bilayer graphene with respect to a distorted configuration in the same spirit as the model introduced by Su, Schrieffer and Heeger. By computing the total energy of a distorted bilayer, we conclude that the ground state of the system favors a finite distortion. We explore how the equilibrium configuration changes with carrier density and an applied potential difference between the two layers.

cond-mat.mes-hall↗

arXiv2008

Inducing energy gaps in graphene monolayer and bilayer

In this paper we propose a mechanism for the induction of energy gaps in the spectrum of graphene and its bilayer, when both these materials are covered with water and ammonia molecules. The energy gaps obtained are within the range 20-30 meV, values compatible to those found in experimental studies of graphene bilayer. We further show that the binding energies are large enough for the adsorption of the molecules to be maintained even at room temperature.

cond-mat.mtrl-sci↗