arXiv subjects

Mauro S. Ferreira

Publications and source records attributed to Mauro S. Ferreira.

At least 19 recordsLinked to original sources

arXiv2026

From rings to resonance: an inverse method links biophotonic structural color to inverse photonic glasses

Structural color arises from the interaction of light with nanoscale structures and is widespread in nature. As structural complexity increases, the mechanisms governing coloration become progressively less understood. The optical response of periodic photonic crystals with a spatially periodic refractive index is well described by Bloch theory, whereas that of photonic glasses composed of randomly assembled uniform spheres is more subtle yet well studied. In contrast, disordered photonic networks found in many beetles are among the most complex natural photonic architectures, and the fundamental relationships between their structure and color remain unclear. Here, we use an inverse method to identify the structural features encoded in the reflectance spectrum. By comparing the spectrum of an unknown system with a database of simulated spectra from computer-generated photonic networks, we infer its structural properties. Applying this approach to both simulated networks and the biophotonic network responsible for the blue coloration of the weevil Pachyrhynchus congestus mirabilis, we identify rings and pores as the key local scattering motifs. Their characteristic sizes govern the spectral position of the reflectance peak, whereas short-range disorder controls its width. The inverse method reveals clear spectral signatures of short-range order, whereas the influence of hyperuniformity and primitive similarity appears comparatively weak. This suggests that blue structural coloration is governed by local scattering mechanisms rather than photonic band-gap effects. We propose an analogy to an inverse photonic glass, in which pores and rings act as correlated local resonators. This perspective provides new design principles for bio-inspired structural-color materials.

physics.optics↗

arXiv2026

Inferring stealthy hyperuniform correlations from quantum transport

Stealthy hyperuniform disordered systems exhibit strongly suppressed long-wavelength fluctuations, producing correlated disorder with unusual consequences for wave propagation. A central quantity characterizing these systems is the stealthiness parameter $χ$, which controls the range of excluded Fourier components in the disorder spectrum. However, in realistic settings, the microscopic disorder configuration may not be directly accessible, making it challenging to determine $χ$ from structural information alone. Here, we propose a conductance-based inverse protocol to recover stealthy hyperuniform correlations from transport data. As a proof of concept, we study spinless fermions in a one-dimensional tight-binding chain connected to clean semi-infinite leads, with on-site disorder generated by imposing a stealthy spectrum $S(k)=Θ(|k|-K)$, where $K=2πχ$. The energy-dependent transmittance is computed using a recursive Green's function method and compared with target spectra through a misfit function defined over an energy window. We show that the position of the sharp drop separating high- and low-transmittance regions is strongly controlled by $χ$, while the disorder strength $W$ mainly affects the absolute magnitude of the transmittance. As a result, the misfit function displays a clear minimum close to the target stealthy parameter. Our results demonstrate that transmittance spectra can serve as fingerprints of stealthy hyperuniform disorder, providing a practical route to infer correlated-disorder parameters from transport measurements.

cond-mat.mes-hall↗

arXiv2026

Anomalous Localization in Magnetically Doped Two-Dimensional Topological Insulators

Two-dimensional topological insulators (2DTIs) harbor spin-polarized edge states that are topologically protected by time-reversal symmetry against non-magnetic structural disorder. However, coupling to magnetic impurities breaks this symmetry, inducing backscattering and destroying perfect quantization. While the impact of isolated dilute magnetic impurities is well understood, the transport properties in the presence of dense, disordered ensembles of magnetic moments remain poorly understood. In this work, we develop an analytical framework, supported by extensive numerical simulations, that captures the behavior of edge transport in two-dimensional topological insulators (2DTIs) with a finite concentration of magnetic impurities. We predict the onset of Anderson localization and uncover an anomalous localization regime characterized by a sub-exponential decay of the conductance, scaling as $\ln {\cal G} \propto -\sqrt{L}$, where $L$ is the system length. Furthermore, we demonstrate that the transport exhibits a universal scaling behavior governed solely by the effective impurity concentration. Applying our model to Mn-doped HgTe quantum wells, we find excellent agreement with experimental data. These findings provide a theoretical foundation for understanding anomalous localization phenomena in magnetically doped topological phases.

cond-mat.mes-hall↗

arXiv2026

Inverse determination of light-matter coupling in disordered systems from transmittance spectra

We investigate quantum inverse problems in one-dimensional (1D) electronic disordered systems strongly coupled to optical cavities. More specifically, we consider the Anderson and the Aubry-Andre-Harper models connected to electronic reservoirs and embedded in a single-mode optical cavity. The light-matter interaction enables photon-assisted hopping processes that significantly modify the transmittance spectrum. Within the nonequilibrium Green's function formalism, we implement an inversion-based approach capable of accurately extracting the electron-photon coupling strength directly from transmittance spectra. While cavity coupling acts as a minor perturbation within the Anderson model, yielding broad yet precise parameter estimates, its influence is markedly different in the Aubry-André-Harper model. The latter exhibits a sharp metal-insulator transition in 1D, thus resulting in more pronounced cavity-induced spectral changes. This renders even more accurate inverse solutions, offering unparalleled precision in the characterization of low-dimensional disordered systems. Altogether, our results demonstrate that the quantum inverse problem provides a robust diagnostic tool for quantum materials, particularly effective for systems exhibiting metal-insulator transitions.

cond-mat.mes-hall↗

arXiv2025

Characterizing Memristive Nanowire Network Models via a Unified Computational Framework

Randomly self-assembled nanowire networks (NWNs) are dynamical systems in which junctions between two nanowires can be modelled as memristive units viewed as adaptive resistors with memory. Various memristive models have been proposed to capture the complex mechanics of these junctions. Here, we showcase a novel computational framework named Memristive Nanowire Network Simulator (MemNNetSim) to simulate and analyze random memristive NWNs in a unified approach. Implemented using the Python programming language, MemNNetSim allows for the analysis of static and dynamic scenarios of NWNs under arbitrary memristive models. This provides a versatile foundation to build upon in further work, such as reservoir dynamics with NWNs, which has seen increased interest due to the interconnected architecture of NWNs. In this work, we introduce the package, demonstrate its utility in simulating NWNs, and then test advanced scenarios in which it can aid in the exploratory analysis of these systems, particularly in learning how to use NWNs as a physical reservoir in reservoir computing applications.

cond-mat.mes-hall↗

arXiv2025

Edge disorder and magnetism in graphene nanoribbons: an inverse modelling approach

It is difficult to completely eliminate disorder during the fabrication of graphene-based nanodevices. From a simulation perspective, it is straightforward to determine the electronic transport properties of disordered devices if complete information about the disorder and the Hamiltonian describing it is available. However, to do the reverse and determine information about the nature of the disorder purely from transport measurements is a far more difficult task. In this work, we apply a recently developed inversion technique to identify important structural information about edge-disordered zigzag graphene nanoribbons. The inversion tool decodes the electronic transmission spectrum to obtain the overall level of edge vacancies in this type of device. We also consider the role of spin-polarised states at the ribbon edges and demonstrate that, in addition to edge roughness, the inversion procedure can also be used to detect the presence of magnetism in such nanoribbons. We finally show that if the transmission for both spin orientations is available, for example by using ferromagnetic contacts in a transport measurement, then additional structural information about the relative concentration of defects on each edge can be derived.

cond-mat.mes-hall↗

arXiv2023

Bayesian Time-Lapse Full Waveform Inversion using Hamiltonian Monte Carlo

Time-lapse images carry out important information about dynamic changes in Earth's interior which can be inferred using different Full Waveform Inversion (FWI) schemes. The estimation process is performed by manipulating more than one seismic dataset, associated with the baseline and monitors surveys. The time-lapse variations can be so minute and localised that quantifying the uncertainties becomes fundamental to assessing the reliability of the results. The Bayesian formulation of the FWI problem naturally provides confidence levels in the solution, but evaluating the uncertainty of time-lapse seismic inversion remains a challenge due to the ill-posedness and high dimensionality of the problem. The Hamiltonian Monte Carlo (HMC) can be used to effectively sample over high dimensional distributions with affordable computational efforts. In this context, we propose a probabilistic Bayesian sequential approach for time-lapse FWI using the HMC method. Our approach relies on the integration of the baseline survey information as prior knowledge in the monitor estimation. We compare the proposed methodology with a parallel scheme in perfect and perturbed acquisition geometry scenarios. We also investigate the correlation effect between baseline and monitor samples in the propagated uncertainties. The results show that our strategy provides accurate times-lapse estimates with errors of similar magnitude to the parallel methodology.

physics.geo-ph↗

arXiv2023

Acoustic Full Waveform Inversion with Hamiltonian Monte Carlo Method

Full-Waveform Inversion (FWI) is a high-resolution technique used in geophysics to evaluate the physical parameters and construct subsurface models in a noisy and limited data scenario. The ill-posed nature of the FWI turns this a challenging problem since more than one model can match the observations. In a probabilistic way, solving the FWI problem demands efficient sampling techniques to infer information on parameters and to estimate the uncertainties in high-dimensional model spaces. We investigate the feasibility of applying the Hamiltonian Monte Carlo (HMC) method in the acoustic FWI by a reflection setup containing different noise level data. We propose a new strategy for tuning the mass matrix based on the acquisition geometry of the seismic survey. Our methodology significantly improves the ability of the HMC method in reconstructing reasonable seismic models with affordable computational efforts.

physics.geo-ph↗

arXiv2019

Time-resolved impurity-invisibility in graphene nanoribbons

We investigate time-resolved charge transport through graphene nanoribbons supplemented with adsorbed impurity atoms. Depending on the location of the impurities with respect to the hexagonal carbon lattice, the transport properties of the system may become invisible to the impurity due to the symmetry properties of the binding mechanism. This motivates a chemical sensing device since dopants affecting the underlying sublattice symmetry of the pristine graphene nanoribbon introduce scattering. Using the time-dependent Landauer--B{ü}ttiker formalism, we extend the stationary current-voltage picture to the transient regime, where we observe how the impurity invisibility takes place at sub-picosecond time scales further motivating ultrafast sensor technology. We further characterize time-dependent local charge and current profiles within the nanoribbons, and we identify rearrangements of the current pathways through the nanoribbons due to the impurities. We finally study the behavior of the transients with ac driving which provides another way of identifying the lattice-symmetry breaking caused by the impurities.

cond-mat.mes-hall↗

arXiv2018

Emergence of Winner-takes-all Connectivity Paths in Random Nanowire Networks

Nanowire networks are promising memristive architectures for neuromorphic applications due to their connectivity and neurosynaptic-like behaviours. Here, we demonstrate a self-similar scaling of the conductance of networks and the junctions that comprise them. We show this behavior is an emergent property of any junction-dominated network. A particular class of junctions naturally leads to the emergence of conductance plateaus and a "winner-takes-all" conducting path that spans the entire network, and which we show corresponds to the lowest-energy connectivity path. These results point to the possibility of independently addressing memory or conductance states in complex systems and is expected to have important implications for neuromorphic devices based on reservoir computing.

cond-mat.mtrl-sci↗

arXiv2017

Oxide-mediated self-limiting recovery of field effect mobility in plasma-treated MoS$_2$

Precise tunability of electronic properties of 2D nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS$_2$) has long been thought to be detrimental to the electrical performance of the material. Here we show that the mobility and conductivity of MoS$_2$ can be precisely controlled and improved by systematic exposure to oxygen:argon plasma, and characterise the material utilising advanced spectroscopy and microscopy. Through complementary theoretical modelling which confirms conductivity enhancement, we uncover the role of a two-dimensional phase of molybdenum trioxide (2D-MoO$_3$) in improving the electronic behaviour of the material. Deduction of the beneficial role of MoO$_3$ will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

cond-mat.mtrl-sci↗

arXiv2015

Sublattice Segregation of Hydrogen Adsorbates in Carbon Nanotubes

Recent experimental observations have reported that, rather than randomly distributed, nitrogen atoms may prefer to be located on one of the two sub-lattices of graphene. It has been suggested that such a preference may present a possible avenue to tailor the band gap of graphene whilst maintaining its excellent electronic transport properties. Among the proposed mechanisms to explain this effect is the suggestion that long-range inter-impurity interactions mediated by the conduction electrons of graphene may give rise to the asymmetry between sub-lattice occupations. Electron-mediated interactions are known to be prevalent not only between N atoms but also between impurities that are adsorbed to a specific location within the hexagonal structure, namely to the top of the carbon atom. Furthermore, this interaction is known to become more long-ranged as the dimensionality of the system is lowered. For that reason, in this paper we investigate whether a similar sub-lattice asymmetry appears in the case of metallic carbon nanotubes doped with Hydrogen adatoms. Our results indicate that similar sub-lattice asymmetries are observable and even more pronounced in small-diameter CNTs with a dilute concentration of adsorbates, diminishing with increasing diameter or impurity concentrations.

cond-mat.mes-hall↗

arXiv2014

Green Functions of Graphene: An Analytic Approach

In this article we derive the lattice Green Functions (GFs) of graphene using a Tight Binding Hamiltonian incorporating both first and second nearest neighbour hoppings and allowing for a non-orthogonal electron wavefunction overlap. It is shown how the resulting GFs can be simplified from a double to a single integral form to aid computation, and that when considering off-diagonal GFs in the high symmetry directions of the lattice this single integral can be approximated very accurately by an algebraic expression. By comparing our results to the conventional first nearest neighbour model commonly found in literature, it is apparent that the extended model leads to a sizeable change in the electronic structure away from the linear regime. As such, this article serves as a blueprint for researchers who wish to examine quantities where these considerations are importa

cond-mat.mes-hall↗

arXiv2014

Sublattice asymmetry of impurity doping in graphene: A review

In this review we highlight recent theoretical and experimental work on sublattice asymmetric doping of impurities in graphene, with a focus on substitutional Nitrogen dopants. It is well known that one current limitation of graphene in regards to its use in electronics is that in its ordinary state it exhibits no band gap. By doping one of its two sublattices preferentially it is possible to not only open such a gap, which can furthermore be tuned through control of the dopant concentration, but in theory produce quasi-ballistic transport of electrons in the undoped sublattice, both important qualities for any graphene device to be used competetively in future technology. We outline current experimental techniques for synthesis of such graphene monolayers and detail theoretical efforts to explain the mechanisms responsible for the effect, before suggesting future research directions in this nascent field.

cond-mat.mes-hall↗

arXiv2014

RKKY interaction between extended magnetic defect lines in graphene

Of fundamental interest in the field of spintronics is the mechanism of indirect exchange coupling between magnetic impurities embedded in metallic hosts. A range of physical features, such as magnetotransport and overall magnetic moment formation, are predicated upon this magnetic coupling, often referred to as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Recent theoretical studies on the RKKY in graphene have been motivated by possible spintronic applications of magnetically doped graphene systems. In this work a combination of analytic and numerical techniques are used to examine the effects of defect dimensionality on such an interaction. We show, in a mathematically transparent manner, that moving from single magnetic impurities to extended lines of impurities effectively reduces the dimensionality of the system and increases the range of the interaction. This has important consequences for the spintronic application of magnetically-doped and we illustrate this with a simple magnetoresistance device.

cond-mat.mes-hall↗

arXiv2014

Sublattice imbalance of substitutionally doped nitrogen in graphene

Motivated by the recently observed sublattice asymmetry of substitutional nitrogen impurities in CVD grown graphene, we show, in a mathematically transparent manner, that oscillations in the local density of states driven by the presence of substitutional impurities are responsible for breaking the sublattice symmetry. While these oscillations are normally averaged out in the case of randomly dispersed impurities, in graphene they have either the same, or very nearly the same, periodicity as the lattice. As a result, the total interaction energy of randomly distributed impurities embedded in the conduction-electron-filled medium does not vanish and is lowered when their configuration is sublattice-asymmetric. We also identify the presence of a critical concentration of nitrogen above which one should expect the sublattice asymmetry to disappear. This feature is not particular to nitrogen dopants, but should be present in other impurities.

cond-mat.mes-hall↗

arXiv2013

Friedel Oscillations in Graphene: Sublattice Asymmetry in Doping

Symmetry breaking perturbations in an electronically conducting medium are known to produce Friedel oscillations (FOs) in various physical quantities of an otherwise pristine material. Here we show in a mathematically transparent fashion that FOs in graphene have a strong sublattice asymmetry. As a result, the presence of impurities and/or defects may impact the distinct graphene sublattices very differently. Furthermore, such an asymmetry an be used to explain the recent observations that Nitrogen atoms and dimers are not randomly distributed in graphene but prefer to occupy one of its two distinct sublattices. We argue that this feature is not exclusive of Nitrogen and that it can be seen with other substitutional dopants.

cond-mat.mes-hall↗

arXiv2013

Indirect Exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) Interactions in Magnetically-Doped Graphene

Magnetically-doped graphene systems are potential candidates for application in future spintronic devices. A key step is to understand the pairwise interactions between magnetic impurities embedded in graphene that are mediated by the graphene conduction electrons. A large number of studies have been undertaken to investigate the indirect exchange, or RKKY, interactions in graphene. Many of these studies report a decay rate faster than expected for a 2-dimensional material and the absence of the usual distance dependent oscillations. In this review we summarize the techniques used to calculate the interaction and present the key results obtained to date. The effects of more detailed parameterisations of the magnetic impurities and graphene host are considered, as are results obtained from ab initio calculations. Since the fast decay of the interaction presents an obstacle to spintronic applications, we focus in particular on the possibility of augmenting the interaction range by a number of methods including doping, spin precession and the application of strain.

cond-mat.mes-hall↗