arXiv subjects

M. Pacheco

Publications and source records attributed to M. Pacheco.

At least 19 recordsLinked to original sources

arXiv2023

Fano-Andreev effect in a T-shaped Double Quantum Dot in the Coulomb blockade regime

We studied the effects of superconducting quantum correlations in a system consisting of two quantum dots, two normal leads, and a superconductor. Using the non-equilibrium Green's functions method, we analyzed the transmission, density of states, and differential conductance of electrons between the normal leads. We found that the superconducting correlations resulted in Fano-Andreev interference, which is characterized by two anti-resonance line shapes in all of these quantities. This behavior was observed in both equilibrium and non-equilibrium regimes and persisted even when Coulomb correlations were taken into account using the Hubbard-I approximation. It is worth noting that the robustness of this behavior against these conditions has not been studied previously in the literature.

cond-mat.mes-hall↗

arXiv2022

Topological bands in the PdSe$_2$ pentagonal monolayer

The electronic structure of monolayer pentagonal palladium diselenide (PdSe2) is analyzed from the topological band theory perspective. Employing first-principles calculations, effective models and symmetry indicators, we find that the low-lying conduction bands are topologically nontrivial, protected by time reversal and crystalline symmetries. Numerical evidence supporting the nontrivial character of the bands is presented. Furthermore, we obtain a relevant physical response from the topological viewpoint, such as the spin Hall conductivity.

cond-mat.mes-hall↗

arXiv2020

Two-dimensional Weyl points and nodal lines in pentagonal materials and their optical response

Two-dimensional pentagonal structures based on the Cairo tiling are the basis of a family of layered materials with appealing physical properties. In this work we present a theoretical study of the symmetry-based electronic and optical properties of these pentagonal materials. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, their electronic band structures and the local spin textures in momentum space are analyzed. Our results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, we discuss the linear and nonlinear optical features of some penta-materials, such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.

cond-mat.mtrl-sci↗

arXiv2019

Symmetry-protected metallic and topological phases in penta-materials

We analyze the symmetry and topological features of a family of materials closely related to penta-graphene, derived from it by adsorption or substitution of different atoms. Our description is based on a novel approach, called topological quantum chemistry, that allows to characterize the topology of the electronic bands, based on the mapping between real and reciprocal space. In particular, by adsorption of alkaline (Li or Na) atoms we obtain a nodal line metal at room temperature, with a continuum of Dirac points around the perimeter of the Brillouin zone. This behavior is also observed in some substitutional derivatives of penta-graphene, such as penta-PC$_2$. Breaking of time-reversal symmetry can be achieved by the use of magnetic atoms; we study penta-MnC$_2$, which also presents spin-orbit coupling and reveals a topological insulator phase. We find that for this family of materials, symmetry is the source of protection for metallic and nontrivial topological phases that can be associated to the presence of fractional band filling, spin-orbit coupling and time-reversal symmetry breaking.

cond-mat.mtrl-sci↗

arXiv2017

Tight-binding model for opto-electronic properties of penta-graphene nanostructures

We present a tight-binding parametrization for penta-graphene that correctly describes its electronic band structure and linear optical response. The set of parameters is validated by comparing to ab-initio density functional theory calculations for single-layer penta-graphene, showing a very good global agreement. We apply this parameterization to penta-graphene nanoribbons, achieving an adequate description of quantum-size effects. Additionally, a symmetry-based analysis of the energy band structure and the optical transitions involved in the absorption spectra is introduced, allowing for the interpretation of the optoelectronic features of these systems.

cond-mat.mtrl-sci↗

arXiv2017

Spin and charge caloritronics in bilayer graphene flakes with magnetic contacts

We investigate the coupling of spin and thermal currents as a means to rise the thermoelectric efficiency of nanoscale graphene devices. We consider nanostructures composed of overlapping graphene nanoribbons with ferromagnetic contacts in different magnetic configurations. Our results show that the charge Seebeck effect is greatly enhanced when the magnetic leads are in an antiparallel configuration, due to the enlargement of the transport gap. However, for the optimization of the charge figure of merit ZT it is better to choose a parallel alignment of the magnetization in the leads, because the electron-hole symmetry is broken in this magnetic configuration. We also obtain the spin-dependent Seebeck coefficient and spin figure of merit. In fact, the spin ZT can double its value with respect to the charge ZT for a wide temperature range, above 300 K. These findings suggest the potential value of graphene nanosystems as energy harvesting devices employing spin currents.

cond-mat.mes-hall↗

arXiv2016

Photon-assisted transport in bilayer graphene flakes

The electronic conductance of graphene-based bilayer flake systems reveal different quantum interference effects, such as Fabry-Pérot resonances and sharp Fano antiresonances on account of competing electronic paths through the device. These properties may be exploited to obtain spin-polarized currents when the same nanostructure is deposited above a ferromagnetic insulator. Here we study how the spin-dependent conductance is affected when a time-dependent gate potential is applied to the bilayer flake. Following a Tien-Gordon formalism we explore how to modulate the transport properties of such systems via appropriate choices of the $ac$-field gate parameters. The presence of the oscillating field opens the possibility of tuning the original antiresonances for a large set of field parameters. We show that interference patterns can be partially or fully removed by the time-dependent gate voltage. The results are reflected in the corresponding weighted spin polarization which can reach maximum values for a given spin component. We found that differential conductance maps as functions of bias and gate potentials show interference patterns for different $ac$-field parameter configurations. The proposed bilayer graphene flake systems may be used as a frequency detector in the THz range.

cond-mat.mes-hall↗

arXiv2016

Fano-Andreev effect in T-shape double-quantum-dot in the Kondo regime

In the present work, we investigate the electronic transport through a T-shape double quantum dot system coupled to two normal leads and to one superconducting lead. We explore the interplay between Kondo and Andreev states due to proximity effects. We find that Kondo resonance is modified by the Andreev bound states, which manifest through Fano antiresonances in the local density of states of the embedded quantum dot and normal transmission. This means that there is a correlation between Andreev bound states and Fano resonances that is robust under the influence of high electronic correlation. We have also found that the dominant couplings at the quantum dots are characterized by a crossover region that defines the range where the Fano-Kondo and the Andreev-Kondo effect prevail in each quantum dot. Likewise, we find that the interaction between Kondo and Andreev bound states has a notable influence on the Andreev transport.

cond-mat.mes-hall↗

arXiv2015

Electron Confinement Induced by Diluted Hydrogen-like Ad-atoms in Graphene Ribbons

We report the electronic properties of two-dimensional systems made of graphene nanoribbons which are patterned with ad-atoms in two separated regions. Due to the extra electronic confinement induced by the presence of the impurities, we find resonant levels, quasi-bound and impurity-induced localized states, which determine the transport properties of the system. Regardless of the ad-atom distribution in the system, we apply band-folding procedures to simple models and predict the energies and the spatial distribution of those impurity-induced states. We take into account two different scenarios: gapped graphene and the presence of randomly distributed ad-atoms in a low dilution regime. In both cases the defect-induced resonances are still detected. Our findings would encourage experimentalist to synthesize these systems and characterize their quasi-localized states employing, for instance, scanning tunneling spectroscopy (STS). Additionally, the resonant transport features could be used in electronic applications and molecular sensor devices.

cond-mat.mes-hall↗

arXiv2014

Bound states in the continuum: localization of Dirac-like fermions

We report the formation of bound states in the continuum for Dirac-like fermions in structures composed by a trilayer graphene flake connected to nanoribbon leads. The existence of this kind of localized states can be proved by combining local density of states and electronic conductance calculations. By applying a gate voltage, the bound states couple to the continuum, yielding a maximum in the electronic transmission. This feature can be exploited to identify bound states in the continuum in graphene-based structures.

cond-mat.mes-hall↗

arXiv2013

Molecular sensor based on graphene nanoribbons

In this work we study thermoelectric properties of graphene nanoribbons with side-attached organic molecules. By adopting a single-band tight binding Hamiltonian and the Green's function formalism, we calculated the transmission and Seebeck coefficients for different hybrid systems. The corresponding thermopower profiles exhibit a series of sharp peaks at the eigenenergies of the isolated molecule. We study the effects of the temperature on the thermoelectric response, and we consider random configurations of molecule distributions, in different disorder regimes. The main characteristics of the thermopower are not destroyed under temperature and disorder, indicating the robustness of the system as a proposed molecular thermo-sensor device.

cond-mat.mes-hall↗

arXiv2013

Electronic and optical properties of carbon nanodisks and nanocones

A theoretical study of the electronic properties of nanodisks and nanocones is presented within the framework of a tight-binding scheme. The electronic densities of states and absorption coefficients are calculated for such structures with different sizes and topologies. A discrete position approximation is used to describe the electronic states taking into account the effect of the overlap integral to first order. For small finite systems, both total and local densities of states depend sensitively on the number of atoms and characteristic geometry of the structures. Results for the local densities of charge reveal a finite charge distribution around some atoms at the apices and borders of the cone structures. For structures with more than 5000 atoms, the contribution to the total density of states near the Fermi level essentially comes from states localized at the edges. For other energies the average density of states exhibits similar features to the case of a graphene lattice. Results for the absorption spectra of nanocones show a peculiar dependence on the photon polarization in the infrared range for all investigated structures.

cond-mat.mes-hall↗

arXiv2013

Fano effect and Andreev bound states in T-shape double quantum dots

In this paper, we investigate the transport through a T-shaped double quantum dot coupled to two normal metal leads left and right and a superconducting lead. Analytical expressions of Andreev transmission and local Density of States of the system at zero temperature have been obtained. We study the role of the superconducting lead in the quantum interferometric features of the double quantum dot. We report for first time the Fano effect produced by Andreev bound states in a side quantum dot. Our results show that as a consequence of quantum interference and proximity effect, the transmission from normal to normal lead exhibits Fano resonances due to Andreev bound states. We find that this interference effect allows us to study the Andreev bound states in the changes in the conductance between two normal leads.

cond-mat.mes-hall↗

arXiv2013

Spin-Polarized Electrons in Bilayer Graphene Flakes

We show that a bilayer graphene flake deposited above a ferromagnetic insulator can behave as a spin-filtering device. The ferromagnetic material induces exchange splitting in the graphene flake, and due to the Fano antiresonances occurring in the transmission of the graphene flake as a function of flake length and energy, it is possible to obtain a net spin current. This happens when an antiresonance for one spin channel coincides with a maximum transmission for the opposite spin. We propose these structures as a means to obtain spin-polarized currents and spin filters in graphene-based systems.

cond-mat.mes-hall↗

arXiv2013

Electronic Properties of Twisted Trilayer Graphene

We study the electronic properties of a twisted trilayer graphene, where two of the layers have Bernal stacking and the third one has a relative rotation with respect to the AB-stacked layers. Near the Dirac point, the AB-twisted trilayer graphene spectrum shows two parabolic Bernal-like bands and a twisted-like Dirac cone. For small twist angles, the parabolic bands present a gap that increases for decreasing rotation angle. There is also a shift in the twisted-like Dirac cone with a similar angle dependence. We correlate the gap in the trilayer with the shift of the Dirac cone in an isolated twisted bilayer, which is due to the loss of electron-hole symmetry caused by sublattice mixing in the rotated geometry. Using a tight-binding and a continuum model, we derive an effective Hamiltonian which accounts for the relevant low-energy properties of this system.

cond-mat.mes-hall↗

arXiv2012

Compact Groups of Galaxies selected by stellar mass: The 2MASS Compact Group Catalogue

We present a photometric catalogue of compact groups of galaxies (p2MCGs) automatically extracted from the 2MASS extended source catalogue. A total of 262 p2MCGs are identified, following the criteria of Hickson (1982), of which 230 survive visual inspection. Only 1/4 of these groups were previously known compact groups (CGs). Among the 144 p2MCGs that have all their galaxies with known redshifts, 85 have 4 or more accordant galaxies. This v2MCG sample of velocity-filtered p2MCGs constitutes the largest sample of CGs catalogued to date, with both well-defined selection criteria and velocity filtering, and is the first CG sample selected by stellar mass. We compared the properties of the 78 v2MCGs with > 3000 km/s with the properties of other CG samples, as well as those (mvCGs) extracted from the semi-analytical model of Guo et al. (2011) run on the high-resolution Millennium-II simulation. In this mvCG sample, 2/3 of the groups are physically dense. The space density of v2MCGs within 6000 km/s is 4 times that of the Hickson sample up to the same distance and with the same criteria used here, but still 40% less than that of mvCGs. The v2MCG constitutes the first group catalogue showing statistically significant signs of wide magnitude gaps (according to Tremaine-Richstone statistics) and centrally located 1st-ranked galaxies, both consistent with the predictions obtained from mvCGs. By virtue of its automatic selection with the popular Hickson criteria, its size, its selection on stellar mass, and its statistical signs of mergers and centrally located brightest galaxies, the v2MCG catalogue appears to be the laboratory of choice to study compact groups of 4 or more galaxies of comparable luminosity. [Abridged]

astro-ph.CO↗

arXiv2010

Flat Bands in Slightly Twisted Bilayer Graphene

The bands of graphite are extremely sensitive to topological defects which modify the electronic structure. In this paper we found non-dispersive flat bands no farther than 10 meV of the Fermi energy in slightly twisted bilayer graphene as a signature of a transition from a parabolic dispersion of bilayer graphene to the characteristic linear dispersion of graphene. This transition occurs for relative rotation angles of layers around $1.5^o$ and is related to a process of layer decoupling. We have performed ab-initio calculations to develop a tight binding model with an interaction Hamiltonian between layers that includes the $π$ orbitals of all atoms and takes into account interactions up to third nearest-neighbors within a layer.

cond-mat.mes-hall↗

arXiv2010

Bound states in the continuum in graphene quantum dot structures

The existence of bound states in the continuum was predicted at the dawn of quantum mechanics by von Neumann and Wigner. In this work we discuss the mechanism of formation of these exotic states and the feasibility to observe them experimentally in symmetrical heterostructures composed by segments of graphene ribbons with different widths forming a graphene quantum dot. We identify the existence of bound states in the continuum in these graphene quantum dot systems by means of local density of states and electronic conductance calculations.

cond-mat.mes-hall↗