arXiv subjects

Alexandre R. Rocha

Publications and source records attributed to Alexandre R. Rocha.

13 recordsLinked to original sources

arXiv2025

Tunable Interlayer Excitons in Bilayer Graphene Nanoribbons

Vertically stacked van der Waals structures are promising platforms that enable layer engineering, opening new avenues for the quantum control of elementary excitations, including optically generated bound electron-hole pairs. Here we employ excited-state density functional calculations to demonstrate strong interlayer excitonic coupling in one-dimensional van der Waals nanostructures derived from armchair graphene nanoribbons. The excitonic response exhibits prominent peaks in the near-infrared range, mainly attributed to intralayer excitons, while interlayer excitations with absorption peak strengths of up to 13\% of the maximum absorption are also observed. Both type-I and type-II band alignments are found, which promote the formation of intralayer and interlayer excitons. Notably, interlayer excitons in these systems exhibit long-lived radiative lifetimes at room temperature, ranging from 1 nanosecond to 9.4 microseconds. Our calculations suggest the potential to tune the excitonic response and lifetimes of bilayer graphene nanoribbons via careful engineering of the stacking order.

cond-mat.mes-hall↗

arXiv2025

First-Principles Nanocapacitor Simulations of the Optical Dielectric Constant in Water Ice

We introduce a combined density functional theory (DFT) and non-equilibrium Green's function (NEGF) framework to compute the capacitance of nanocapacitors and directly extract the dielectric response of a sub-nanometer dielectric under bias. We identify that at the nanoscale conventional capacitance evaluations based on stored charge per unit voltage suffer from an ill-posed partitioning of electrode and dielectric charge. This partitioning directly impacts the geometric definition of capacitance through the capacitor width, which in turn makes the evaluation of dielectric response uncertain. This ambiguous separation further induces spurious interfacial polarizability when analyzed via maximally localized Wannier functions. Focusing on crystalline ice, we develop a robust charge-separation protocol that yields unique capacitance-derived polarizability and dielectric constants, unequivocally demonstrating that confinement neither alters ice's intrinsic electronic response nor its insensitivity to proton order. Our results lay the groundwork for rigorous interpretation of capacitor measurements in low-dimensional dielectric materials.

cond-mat.mes-hall↗

arXiv2025

Hexagonal ice density dependence on inter atomic distance changes due to nuclear quantum effects

Hexagonal ice ($\rm{I_h}$), the most common structure of ice, displays a variety of fascinating properties. Despite major efforts, a theoretical description of all its properties is still lacking. In particular, correctly accounting for its density and interatomic interactions is of utmost importance as a stepping stone for a deeper understanding of other properties. Deep potentials are a recent alternative to investigate the properties of {\iceIH}, which aims to match the accuracy of \textit{ab initio} simulations with the simplicity and scalability of classical molecular dynamics. This becomes particularly significant if one wishes to address nuclear quantum effects. In this work, we use machine learning potentials obtained for different exchange and correlation functionals to simulate the structural and vibrational properties of {\iceIH}. We show that most functionals overestimate the density of ice compared to experimental results. Furthermore, a quantum treatment of the nuclei leads to even further distancing from experiments. We understand this by highlighting how different inter-atomic interactions play a role in obtaining the equilibrium density. In particular, different from water clusters and bulk water, nuclear quantum effects lead to stronger H-bonds in {\iceIH}.

cond-mat.mtrl-sci↗

arXiv2025

Optical properties of TiS$_3$ as a novel thin film for single-junction and tandem solar cells

Sub-micrometer thin films are promising platforms for emerging flexible photovoltaic devices. Although the current market already produces efficient solar cells, the average wafer thickness of these devices remains far from the sub-micrometer scale, making them susceptible to cracking under bending stress and thus precluding their use in flexible device applications. Due to its earth abundance, non-toxicity, and low elastic modulus, titanium trisulfide (TiS$_3$) has emerged as a promising alternative for flexible device applications. Here, using excited-state density functional calculations combined with the transfer matrix approach, we perform an optical analysis and assess the efficiency of a prototype photovoltaic device based on sub-micrometer TiS$_3$ thin films. Using optical constants obtained from our first-principles calculations, we evaluate the photovoltaic response of a single-junction device in the radiative limit, finding that a 140-nm-thick active layer achieves a maximum power conversion efficiency of approximately 22%. Additionally, we investigate tandem solar cells that incorporate TiS$_3$ into perovskite thin films, and find that the lower and upper power conversion efficiencies range from approximately 18% to 33%. Overall, our results suggest great potential for using TiS$_3$ thin films as an active layer in the design of highly efficient flexible solar cells.

cond-mat.mtrl-sci↗

arXiv2024

Probing Water-Electrified Electrode interfaces: Insights from Au and Pd

The water/electrode interface under an applied bias potential is a challenging out-of-equilibrium phenomenon, which is difficult to accurately model at the atomic scale. In this study, we employ a combined approach of Density Functional Theory (DFT) and non-equilibrium Green's function (NEGF) methods to analyze the influence of an external bias on the properties of water adsorbed on Au(111) and Pd(111) metallic electrodes. Our results demonstrate that while both Au and Pd-electrodes induce qualitatively similar structural responses in adsorbed water molecules, the quantitative differences are substantial, driven by the distinct nature of water-metal bonding. Our findings underscore the necessity of quantum-mechanical modeling for accurately describing electrochemical interfaces.

cond-mat.mtrl-sci↗

arXiv2024

Suppression of the Mott insulating phase in the particle-hole asymmetric Hubbard model

We explore the phase diagram of the Mott metal-insulator transition (MIT), focusing on the effects of particle-hole asymmetry (PHA) in the single-band Hubbard model. Our dynamical mean-field theory (DMFT) study reveals that the introduction of PHA in the model significantly influences the critical temperature ($T_c$) and interaction strength ($U_c$), as well as the size of the co-existence region of metallic and insulating phases at low temperatures. Specifically, as the system is moved away from particle-hole symmetry, $T_c$ decreases and $U_c$ increases, indicating a suppression of the insulating phase and the strengthening of the metallic behavior. Additionally, the first-order transition line between metallic and insulating phases is better defined in the model with PHA, leading to a reduced co-existence region at $T<T_c$. Moreover, we propose that the MIT can be characterized by the charge density, which serves as a viable alternative to zero-frequency spectral density typically used in DMFT calculations. Our findings provide new insights into the role of particle-hole asymmetry in the qualitative and quantitative characterization of the MIT even in a very simple system.

cond-mat.str-el↗

arXiv2024

Screened hydrogen model of excitons in semiconducting nanoribbons

The optical response of quasi-one-dimensional systems is often dominated by tightly bound excitons, that significantly influence their basic electronic properties. Despite their importance for device performance, accurately predicting their excitonic effects typically requires computationally demanding many-body approaches. Here, we present a simplified model to describe the static macroscopic dielectric function, which depends only on the width of the quasi-one-dimensional system and its polarizability per unit length. We show that at certain interaction distances, the screened Coulomb potential is greater than its bare counterpart, which results from the enhanced repulsive electron-electron interactions. As a test case, we study fourteen different nanoribbons, twelve of them armchair graphene nanoribbons of different families. Initially, we devised a simplified equation to estimate the exciton binding energy and extension that provides results comparable to those from the full Bethe-Salpeter equation, albeit for a specific nanoribbon family. Then, we used our proposed screening potential to solve the 1D Wannier-Mott equation, which turn out to be broad approach, that is able to predict binding energies that match quite well the ones obtained with the Bethe-Salpeter equation, irrespective of the nanoribbon family.

cond-mat.mes-hall↗

arXiv2023

Photovoltaic efficiency of transition metal dichalcogenides thin films by ab initio excited-state methods

Transition metal dichalcogenides (TMDCs) have garnered significant interest in optoelectronics, owing to their scalability and thickness-dependent electrical and optical properties. In particular, thin films of TMDCs could be used in photovoltaic devices. In this work, we employ $ab$ $initio$ many-body perturbation theory within $G_0W_0$-BSE approach to accurately compute the optoelectronic properties of thin films of 2H-TMDCs composed of Mo, W, S, and Se. Subsequently, we evaluate their photovoltaic performance including exciton recombination effects, and show this is a key ingredient. We obtain efficiencies of up to 29\% for a 200-nm thick film of \ce{WSe2}, thus providing an upper limit. We also include other \textcolor{black}{phenomenological} recombination mechanisms that could be present in current samples. This slightly reduces efficiencies, indicating that even with current synthesis technologies, there is still potential for further enhancement of TMDCs' performance in photovoltaic applications.

cond-mat.mtrl-sci↗

arXiv2023

Many-body effects on the quasiparticle band structure and optical response of single-layer penta-NiN$_2$

We present a comprehensive first-principles study on the optoelectronic properties of the single-layer nickel diazenide (penta-NiN$_2$), a recently synthesized Cairo pentagonal 2D semiconductor. We carry out $ab$ $initio$ calculations based on the density-functional theory (DFT) and many-body perturbation theory, within the framework of Green's functions, to describe the quasiparticle properties and analyze the excitonic effects on the optical properties of monolayer penta-NiN$_2$. Our results reveal a quasiparticle band gap of approximately 1 eV within the eigenvalue self-consistent $GW$ approach, corroborating the monolayer penta-NiN$_2$'s potential in optoelectronics. Remarkably, the acoustic phonon-limited carrier mobility for the monolayer penta-NiN$_2$ exhibits an ultra-high hole mobility of $84{\times}10^4$ cm$^2$/V$\cdot$s. Furthermore, our findings indicate that the material's band gap exhibits an anomalous negative dependence on temperature. Despite being a two-dimensional material, monolayer penta-NiN$_2$ presents resonant excitons in its most prominent absorption peak. Therefore, penta-NiN$_2$ boasts compelling and promising properties that merit exploration in optoelectronics and high-speed devices.

cond-mat.mtrl-sci↗

arXiv2019

Ab initio modelling of spin relaxation lengths in disordered graphene nanoribbons†

The spin-dependent transport properties of armchair graphene nanoribbons in the presence of extrinsic spin-orbit coupling induced by a random distribution of Nickel adatoms is studied. By combining a recursive Green's function formalism with density functional theory, we explore the influence of ribbon length and metal adatom concentration on the conductance. At a given length, we observed a significant enhancement of the spin-flip channel around resonances and at energies right above the Fermi level. We also estimate the spin-relaxation length, finding values on the order of tens of micrometers at low Ni adatom concentrations. This study is conducted at singular ribbon lengths entirely from fully ab-initio methods, providing indirectly evidence that the Dyakonov-Perel spin relaxation mechanism might be the dominant at low concentrations as well as the observation of oscillations in the spin-polarization.

cond-mat.mes-hall↗

arXiv2017

Topological Line Defects around Graphene Nanopores for DNA Sequencing

Topological line defects in graphene represent an ideal way to produce highly controlled structures with reduced dimensionality that can be used in electronic devices. In this work we propose using extended line defects in graphene to improve nucleobase selectivity in nanopore-based DNA sequencing devices. We use a combination of QM/MM and non-equilibrium Green's functions methods to investigate the conductance modulation, fully accounting for solvent effects. By sampling over a large number of different orientations generated from molecular dynamics simulations, we theoretically demonstrate that distinguishing between the four nucleobases using line defects in a graphene-based electronic device appears possible. The changes in conductance are associated with transport across specific molecular states near the Fermi level and their coupling to the pore. Through the application of a specifically tuned gate voltage, such a device would be able to discriminate the four types of nucleobases more reliably than that of graphene sensors without topological line defects.

cond-mat.mes-hall↗

arXiv2017

Quantum Hall Effect in Graphene with Interface-Induced Spin-Orbit Coupling

We consider an effective model for graphene with interface-induced spin-orbit coupling and calculate the quantum Hall effect in the low-energy limit. We perform a systematic analysis of the contribution of the different terms of the effective Hamiltonian to the quantum Hall effect (QHE). By analysing the spin-splitting of the quantum Hall states as a function of magnetic field and gate-voltage, we obtain different scaling laws that can be used to characterise the spin-orbit coupling in experiments. Furthermore, we employ a real-space quantum transport approach to calculate the quantum Hall conductivity and investigate the robustness of the QHE to disorder introduced by hydrogen impurities. For that purpose, we combine first-principles calculations and a genetic algorithm strategy to obtain a graphene-only Hamiltonian that models the impurity.

cond-mat.mes-hall↗

arXiv2015

Resistive Switching in Nanodevices

Passing current at given threshold voltages through a metal/insulator/metal sandwich structure device may change its resistive state. Such resistive switching is unique to nanoscale devices, but its underlying physical mechanism remains unknown. We show that the different resistive states are due to different spontaneously charged states, characterized by different `band bending' solutions of Poisson's equation. For an insulator with mainly donor type defects, the low-resistivity state is characterized by a negatively charged insulator due to convex band bending, and the high-resistivity state by a positively charged insulator due to concave band bending; vice versa for insulators with mainly acceptor type defects. These multiple solutions coexist only for nanoscale devices and for bias voltages limited by the switching threshold values, where the system charge spontaneously changes and the system switches to another resistive state. We outline the general principles how this functionality depends on material properties and defect abundance of the insulator `storage medium', and propose a new magnetic memristor device with increased storage capacity.

cond-mat.mtrl-sci↗