arXiv subjects

A. R. Rocha

Publications and source records attributed to A. R. Rocha.

At least 19 recordsLinked to original sources

arXiv2025

Determining Exciton Binding Energy and Reduced Effective Mass in Metal Tri-Halide Perovskites from Optical and Impedance Spectroscopy Measurements

Accurate determination of the exciton binding energy and reduced effective mass in halide perovskites is of utmost importance for the selective design of optoelectronic devices. Although these properties are currently determined by several spectroscopic techniques, complementary theoretical models are often required to bridge macroscopic and microscopic properties. Here, we present a novel method to determine these quantities while fully accounting for polarization effects due to carrier interactions with longitudinal optical phonons. Our approach estimates the exciton-polaron binding energy from optical absorption measurements using a recently developed Elliott based Band Fluctuations model. The reduced effective mass is obtained via the Pollmann-Buttner exciton-polaron model, which is based on the Frohlich polaron framework, where the strength of the electron-phonon interaction arises from changes in the dielectric properties. The procedure is applied to the family of perovskites ABX3 (A = MA, FA, Cs; B = Pb; X = I, Br, Cl), showing excellent agreement with high field magnetoabsorption and other optical-resolved techniques. The results suggest that the Pollmann-Buttner model offers a robust and novel approach for determining the reduced effective mass in metal tri-halide perovskites and other polar materials exhibiting free exciton bands.

cond-mat.mtrl-sci↗

arXiv2022

Efficient hot carrier dynamics in near-infrared photocatalytic metals

Photoexcited metals can produce highly-energetic hot carriers whose controlled generation and extraction is a promising avenue for technological applications. While hot carrier dynamics in Au-group metals have been widely investigated, a microscopic description of the dynamics of photoexcited carriers in the mid-infrared and near-infrared Pt-group metals range is still scarce. Since these materials are widely used in catalysis and, more recently, in plasmonic catalysis, their microscopic carrier dynamics characterization is crucial. We employ \emph{ab initio} many-body perturbation theory to investigate the hot carrier generation, relaxation times, and mean free path in bulk Pd and Pt. We show that the direct optical transitions of photoexcited carriers in this metals are mainly generated in the near-infrared range. We also find that the electron-phonon mass enhancement parameter for Pt is 16 $\%$ higher than Pd, a result that help explains several experimental results showing diverse trends. Moreover, we predict that Pd (Pt) hot electrons possess total relaxation times of up to 35 fs (24 fs), taking place at approximately 0.5 eV (1.0 eV) above the Fermi energy. Finally, an efficient hot electron generation and extraction can be achieved in nanofilms of Pd (110) and Pd (100) when subject to excitation energies ranging from 0.4 to 1.6 eV.

cond-mat.mtrl-sci↗

arXiv2022

Half-integer anomalous currents in 2D materials from a QFT viewpoint

Charge carriers in Dirac/Weyl semi-metals exhibit a relativistic-like behavior. In this work we propose a novel type of intrinsic half-integer Quantum Hall effect in 2D materials, thereby also offering a topological protection mechanism for the current. Its existence is rooted in the 2D parity anomaly, without any need for a perpendicular magnetic field. We conjecture that it may occur in disturbed honeycomb lattices where both spin degeneracy and time reversal symmetry are broken. These configurations harbor two distinct gap-opening mechanisms that, when occurring simultaneously, drive slightly different gaps in each valley, causing a net anomalous conductivity when the chemical potential is tuned to be between the distinct gaps. Some examples of promising material setups that fulfill the prerequisites of our proposal are also listed to motivate looking for the effect at the numerical and experimental level.

cond-mat.mes-hall↗

arXiv2020

Disorder information from conductance: a quantum inverse problem

It is straightforward to calculate the conductance of a quantum device once all its scattering centers are fully specified. However, to do this in reverse, i.e., to find information about the composition of scatterers in a device from its conductance, is an elusive task. This is particularly more challenging in the presence of disorder. Here we propose a procedure in which valuable compositional information can be extracted from the seemingly noisy spectral conductance of a two-terminal disordered quantum device. In particular, we put forward an inversion methodology that can identify the nature and respective concentration of randomly-distributed impurities by analyzing energy-dependent conductance fingerprints. Results are shown for graphene nanoribbons as a case in point using both tight-binding and density functional theory simulations, indicating that this inversion technique is general, robust and can be employed to extract structural and compositional information of disordered mesoscopic devices from standard conductance measurements.

cond-mat.mes-hall↗

arXiv2017

Anomalous temperature dependence of the band-gap in Black Phosphorus

Black Phosphorus (BP) has gained renewed attention due to its singular anisotropic electronic and optical properties that might be exploited for a wide range of technological applications. In this respect, the thermal properties are particularly important both to predict its room temperature operation and to determine its thermoelectric potential. From this point of view, one of the most spectacular and poorly understood phenomena is, indeed, the BP temperature-induced band-gap opening: when temperature is increased the fundamental band-gap increases instead of decreasing. This anomalous thermal dependence has also been observed, recently, in its monolayer counterpart. In this work, based on \textit{ab-initio} calculations, we present an explanation for this long known, and yet not fully explained, effect. We show that it arises from a combination of harmonic and lattice thermal expansion contributions, which are, in fact, highly interwined. We clearly narrow down the mechanisms that cause this gap opening by identifying the peculiar atomic vibrations that drive the anomaly. The final picture we give explains both the BP anomalous band-gap opening and the frequency increase with increasing volume (tension effect).

cond-mat.mtrl-sci↗

arXiv2017

Electron-phonon scattering effects on electronic and optical properties of orthorhombic GeS

Group-VI monochalcogenides are attracting a great deal of attention due to their peculiar anisotropic properties. Very recently, it has been suggested that GeS could act as a promissory absorbing material with high input-output ratios, relevant features for designing prospective optoelectronic devices. In this work, we use the \emph{ab-initio} many body perturbation theory to study the role of the electron-phonon coupling on orthorhombic GeS. We identify the vibrational modes that efficiently couple with the electronic states responsible for giving rise to the first and second excitonic state. We also study the finite-temperature optical absorption and show that even at $T\to0K$, the role of the electron-phonon interaction is crucial to properly describe the main experimental excitation peaks position and width. Our results suggest that the electron-phonon coupling is essential to properly describe the optical properties of the monochalcogenides family.

cond-mat.mtrl-sci↗

arXiv2016

Edge phonons in black phosphorus

Exfoliated black phosphorus has recently emerged as a new two-dimensional crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have potentially important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and the behavior of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.

cond-mat.mtrl-sci↗

arXiv2016

Charge storage in oxygen deficient phases of TiO$_2$: defect Physics without defects

Defects in semiconductors can exhibit multiple charge states, which can be used for charge storage applications. Here we consider such charge storage in a series of oxygen deficient phases of TiO$_2$, known as Magnéli phases. These Ti$_n$O$_{2n-1}$ Magnéli phases present well-defined crystalline structures, i. e., their deviation from stoichiometry is accommodated by changes in space group as opposed to point defects. We show that these phases exhibit intermediate bands with the same electronic quadruple donor transitions akin to interstitial Ti defect levels in TiO$_2$-rutile. Thus, the Magnéli phases behave as if they contained a very large pseudo-defect density: $\frac{1}{2}$ per formula unit Ti$_n$O$_{2n-1}$. Depending on the Fermi Energy the whole material will become charged. These crystals are natural charge storage materials with a storage capacity that rivals the best known supercapacitors.

cond-mat.mtrl-sci↗

arXiv2015

DFT+U simulation of the Ti${}_4$O${}_7$-TiO${}_2$ interface

The formation of conducting channels of Ti${}_4$O${}_7$ inside TiO${}_2$-based memristors is believed to be the origin for the change in electric resistivity of these devices. While the properties of the bulk materials are reasonably known, the interface between them has not been studied up to now mostly due to their different crystalline structures. In this work we present a way to match the interfaces between TiO${}_2$ and Ti${}_4$O${}_7$ and subsequently the band offset between these materials is obtained from density functional theory based calculations. The results show that while the valence band is located at the Ti${}_4$O${}_7$, the conduction band is found at the TiO${}_2$ structure, resulting into a type II interface. In this case, the Ti${}_4$O${}_7$ would act as a donor to the TiO${}_2$ matrix.

cond-mat.mtrl-sci↗

arXiv2014

A study of Ti${}_n$O${}_{2n-1}$ Magnéli phases using Density Functional Theory

Defects in the rutile TiO${}_2$ structures have been extensively studied, but the intrinsic defects of the oxygen deficient Ti${}_n$O${}_{2n-1}$ phases have not been given the same amount of consideration. Those structures, known as Magnéli phases, are characterized by the presence of ordered planes of oxygen vacancies, also known as shear-planes, and it has been shown that they form conducting channels inside TiO-based memristor devices. Memristors are excellent candidates for a new generation of memory devices in the electronics industry. In this paper we present DFT-based electronic structure calculations for Ti${}_n$O${}_{2n-1}$ Magnéli structures using PBESol+U ($0 \leq U \leq 5$ eV) and HSE functionals, showing that intrinsic defects present in these structures are responsible for the appearance of states inside the bandgap, which can act as intrinsic dopants for the enhanced conductivity of TiO${}_2$ memristive devices.

cond-mat.mtrl-sci↗

arXiv2014

A microscopic model for hydrated biological tissues

The present work presents a density-functional microscopic model of soft biological tissue. The model was based on a prototype molecular structure from experimentally resolved collagen peptide residues and water clusters and has the objective to capture some well-known experimental features of soft tissues. It was obtained the optimized geometry, binding and coupling energies and dipole moments. The results concerning the stability of the confined water clusters, the water-water and water-collagen interactions within the CLBM framework were successfully correlated to some important trends observed experimentally in inflammatory tissues.

cond-mat.soft↗

arXiv2011

Spin-filtering and Disorder Induced Giant Magnetoresistance in Carbon Nanotubes: Ab Initio Calculations

Nitrogen-doped carbon nanotubes can provide reactive sites on the porphyrin-like defects. It's well known that many porphyrins have transition metal atoms, and we have explored transition metal atoms bonded to those porphyrin-like defects in N-doped carbon nanotubes. The electronic structure and transport are analyzed by means of a combination of density functional theory and recursive Green's functions methods. The results determined the Heme B-like defect (an iron atom bonded to four nitrogens) as the most stable and with a higher polarization current for a single defect. With randomly positioned Heme B-defects in a few hundred nanometers long nanotubes the polarization reaches near 100% meaning an effective spin filter. A disorder induced magnetoresistance effect is also observed in those long nanotubes, values as high as 20000% are calculated with non-magnectic eletrodes.

cond-mat.mtrl-sci↗

arXiv2010

Mimicking Nanoribbon Behavior Using a Graphene Layer on SiC

We propose a natural way to create quantum-confined regions in graphene in a system that allows large-scale device integration. We show, using first-principles calculations, that a single graphene layer on a trenched region of $[000\bar{1}]$ $SiC$ mimics i)the energy bands around the Fermi level and ii) the magnetic properties of free-standing graphene nanoribbons. Depending on the trench direction, either zigzag or armchair nanoribbons are mimicked. This behavior occurs because a single graphene layer over a $SiC$ surface loses the graphene-like properties, which are restored solely over the trenches, providing in this way a confined strip region.

cond-mat.mtrl-sci↗

arXiv2010

Ab initio study of electron transport in dry poly(G)-poly(C) A-DNA strands

The bias-dependent transport properties of short poly(G)-poly(C) A-DNA strands attached to Au electrodes are investigated with first principles electronic transport methods. By using the non- equilibrium Green's function approach combined with self-interaction corrected density functional theory, we calculate the fully self-consistent coherent I-V curve of various double-strand polymeric DNA fragments. We show that electronic wave-function localization, induced either by the native electrical dipole and/or by the electrostatic disorder originating from the first few water solvation layers, drastically suppresses the magnitude of the elastic conductance of A-DNA oligonucleotides. We then argue that electron transport through DNA is the result of sequence-specific short-range tunneling across a few bases combined with general diffusive/inelastic processes.

cond-mat.mes-hall↗

arXiv2009

Disorder-based graphene spintronics

The use of the spin of the electron as the ultimate logic bit - in what has been dubbed spintronics - can lead to a novel way of thinking about information flow. At the same time single layer graphene has been the subject of intense research due to both its fundamental properties as well as its potential application in nanoscale electronics. While defects can significantly alter the electronic properties of nanoscopic systems, the lack of control can lead to seemingly deleterious effects arising from the random arrangement of such impurities. Here we demonstrate, using {\it ab initio} density functional theory and non-equilibrium Green's functions calculations, that it is possible to obtain perfect spin selectivity in doped graphene nanoribbons to produce a perfect spin filter. We show that initially unpolarized electrons entering the system give rise to 100% polarization of the current due to random disorder. This effect is explained in terms of different localization lengths for each spin channel which together with the well know exponential dependence of the conductance on the length of the device leads to a new mechanism for the spin filtering effect that is enhanced by disorder.

cond-mat.mes-hall↗

arXiv2007

Is magnetoresistance in excess of 1,000 % possible in Ni point contacts?

Electronic transport in nickel magnetic point contacts is investigated with a combination of density functional theory and the non-equilibrium Green functions method. In particular we address the possibility of huge ballistic magnetoresistance in impurity-free point contacts and the effects of oxygen impurities. On-site corrections over the local spin density approximation (LSDA) for the exchange and correlation potential, namely the LDA+U method, are applied in order to account for low-coordination and strong correlations. We show that impurity-free point contacts present magnetoresistance never in excess of 50%. This value can raise up to about 450 % in the case of oxygen contamination. These results suggest that magnetoresistance in excess of 1,000 % can not have solely electronic origin.

cond-mat.mtrl-sci↗

arXiv2006

Conductance oscillations in zigzag platinum chains

Using first principles simulations we perform a detailed study of the structural, electronic and transport properties of monoatomic platinum chains, sandwiched between platinum electrodes. First we demonstrate that the most stable atomic configuration corresponds to a zigzag arrangement that gradually straightens as the chains are stretched. Secondly, we find that the conductance at equilibrium atomic spacing does not oscillate with the number of atoms $n$ in the chain, but instead decreases almost monotonically with $n$. In contrast, the conductances of chains of fixed $n$ oscillate as the end atoms are pulled apart, due to the gradual closing and opening of conductance channels as the chain straightens.

cond-mat.mtrl-sci↗

arXiv2005

The Smeagol method for spin- and molecular-electronics

{\it Ab initio} computational methods for electronic transport in nanoscaled systems are an invaluable tool for the design of quantum devices. We have developed a flexible and efficient algorithm for evaluating $I$-$V$ characteristics of atomic junctions, which integrates the non-equilibrium Green's function method with density functional theory. This is currently implemented in the package {\it Smeagol}. The heart of {\it Smeagol} is our novel scheme for constructing the surface Green's functions describing the current/voltage probes. It consists of a direct summation of both open and closed scattering channels together with a regularization procedure of the Hamiltonian, and provides great improvements over standard recursive methods. In particular it allows us to tackle material systems with complicated electronic structures, such as magnetic transition metals. Here we present a detailed description of {\it Smeagol} together with an extensive range of applications relevant for the two burgeoning fields of spin and molecular-electronics.

cond-mat.mes-hall↗