arXiv subjects

J. P. Ramos-Andrade

Publications and source records attributed to J. P. Ramos-Andrade.

13 recordsLinked to original sources

arXiv2026

Effects of random vacancies on the spin-dependent thermoelectric properties of silicene nanoribbon

The spin-dependent thermoelectric properties of silicene nanoribbon heterostructures are investigated, in which the central conductor contains a random distribution of vacancies and is connected to two pristine leads of the same material, placed in proximity to ferromagnetic insulators. The magnetic moments of the leads are analyzed in both parallel and antiparallel configurations. A tight-binding Hamiltonian and the Green's function formalism are employed to calculate the spin-resolved thermoelectric properties of the system as functions of geometrical confinement and vacancy concentration. The results demonstrate an enhancement in charge and spin-dependent thermopower, resulting in an improved thermoelectric efficiency at room temperature, which overcomes the limitations imposed by the classical Wiedemann-Franz law. These findings indicate that defective silicene nanoribbons are promising platforms for the development of efficient thermoelectric and spin-caloritronic devices.

cond-mat.mes-hall↗

arXiv2024

Bound States in the Continuum in a Double Whispering Gallery Resonator

In this work, we investigate the single-photon transport through two whispering gallery resonators (WGRs) coupled to a one-dimensional waveguide. Using Green's function formalism, we compute the transmission spectra and the photonic density of states (DOS) for the stationary states. We also obtain the formation of two types of bound states in the continuum (BICs). The first kind is localized into the WGR and are symmetry-protected BICs. In contrast, the second depends on the distance between resonators through the waveguide and is of the Fabry-Perot kind. These BICs are represented as Dirac delta functions in the local density of states. Additionally, we show that quasi-BICs manifest as sharp resonances in photonic transmission due to small symmetry-breaking perturbations. Furthermore, we investigate the dynamics of a single-photon wave packet interacting with the WGRs and analyze the mechanism for storing the wave packet in the structure formed by the WGRs and the finite waveguide between them.

quant-ph↗

arXiv2024

Fano-Rashba effect in the presence of Majorana bound states

In this paper, we investigate the influence of Majorana bound states on the Fano-Rashba effect in a two-channel Fano-Anderson model. Employing Green's function formalism and the equation of motion method, we compute the transmission through the quantum dot and the density of states. Our analysis reveals that the Majorana bound states, localized at the ends of the topological superconductor nanowire, penetrate into the quantum dot, thereby altering the interference pattern in the electronic transmission profile through it, resulting from their interaction with the bound states in the continuum. Furthermore, we explore the robustness of the bound state in the continuum concerning its connection to Majorana bound states and the energy induced by the magnetic field. We posit that our findings contribute to a deeper comprehension of the Fano-Rashba effect in a two-channel quantum dot coupled to a topological superconducting nanowire.

cond-mat.mes-hall↗

arXiv2023

Bound states in the continuum and Majorana zero modes in a double quantum dot interferometer: Ghost-Fano Majorana effect

We investigate the transport properties through a nanostructure composed of parallel double quantum dots coupled to two normal contacts. Additionally, each quantum dot is connected to a topological superconducting nanowire, hosting Majorana zero modes at its ends. A magnetic flux threading across the area enclosed by the interferometer is considered. First, we investigate the physical quantities of the system employing Green's function formalism. We find that the emergence of bound states appears in symmetric configurations of topological superconducting nanowires, i.e., depending on their lengths and coupling energies to the quantum dots. Also, we find a transport suppression anomaly as a function of the magnetic flux in the same symmetric configurations mentioned above. Besides, we find that the magnetic flux controls both the projection of Majorana zero modes and of the bound states in the continuum into the density of states and the linear conductance, suggesting that only by switching this parameter can we manipulate both bound states.

cond-mat.mes-hall↗

arXiv2022

Kondo effect in a quantum dot embedded between topological superconductors

In this article, we study the quantum transport through a single-level quantum-dot in Kondo regime, coupled to current leads and embedded between two one-dimensional topological superconductors, each hosting Majorana zero modes at their ends. The Kondo effect in the quantum dot is modeled by mean-field finite-$U$ auxiliary bosons approximation and solved by using the non-equilibrium Green's function approach. First, we calculate the density of states of the quantum dot, and then both the current and the differential conductance through the quantum dot in order to characterize the interplay between the Kondo resonance and Majorana zero modes. The results reveal that the presence of Majorana zero modes modifies the Kondo resonance exhibiting an anti-resonance structure in the density of states, leading to obtain spin-resolved behavior of the measurable current and differential conductance. We believe our findings could be helpful to understand the behavior of the Kondo effect in connection with Majorana zero modes.

cond-mat.mes-hall↗

arXiv2021

Josephson and persistent currents in a quantum ring between topological superconductors

In this work, we investigate the spectra in an Aharonov-Bohm quantum-ring interferometer forming a Josephson junction between two topological superconductors (TSC) nanowires. The TSCs host Majorana bound states at their edges, and both the magnetic flux and the superconducting phase difference between the TSCs are used as control parameters. We use a tight-binding approach to model the quantum ring coupled to both TSCs, described by the Kitaev effective Hamiltonian. We solve the problem by means of exact numerical diagonalization of the Bogoliubov-de Gennes (BdG) Hamiltonian and obtain the spectra for two sizes of the quantum ring as a function of the magnetic flux and the phase difference between the TSCs. Depending on the size of the quantum ring and the coupling, the spectra display several patterns. Those are denoted as line, point and undulated nodes, together with flat bands, which are topologically protected. The first three patterns can be possibly detected by means of persistent and Josephson currents. Hence, our results could be useful to understand the spectra and their relation with the behavior of the current signals.

cond-mat.mes-hall↗

arXiv2019

Fingerprints of Majorana bound states in quantum-rings

In this work, we investigate a quantum-ring coupled to a topological superconductor, hosting Majorana bound states (MBSs). We study the MBSs effects over the spectrum and persistent current along the quantum-ring. To obtain physical quantities, we employ the Green's function formalism. We found that the presence of MBSs leads to dramatic changes in the quantum-ring spectrum by inducing particle-hole symmetry. On the other hand, we have obtained a substantial reduction of the persistent current in the strong coupling limit between ring and MBSs depending on the ring parity. These behaviors provide a measurable signature of MBSs. Our findings could be used as additional information on MBSs presence in these quantum systems.

cond-mat.mes-hall↗

arXiv2019

Majorana bound state in the continuum: Coupling between Majorana bound state and quantum dot mediated by continuum

In this work, we consider a single-level quantum dot (QD) and a Majorana bound state (MBS) placed at the end of a topological superconducting nanowire (TSW). Both are coupled to the continuum and do not have a direct connection between them. We addressed the behavior of MBS leaking phenomena and its consequences into the QD physics in non-interacting and Coulomb blockade regime. By employing Green's function formalism via the equation of motion procedure, we calculate the physical quantities of interest. Our results show that the leakage of the MBS into the continuum state is achieved and can alter the physics of Coulomb blockade in the system through continuum-mediated coupling between MBS and QD. As a main consequence, we found a robust and non-trivial mechanism to accomplish a bound state in the continuum in the system.

cond-mat.mes-hall↗

arXiv2019

Fano-Majorana effect and bound states in the continuum on a crossbar-shaped quantum dot hybrid structure

We investigate transport properties through a crossbar-shaped structure formed by a quantum dot (QD) coupled to two normal leads and embedded between two one-dimensional topological superconductors (TSCs). Each TSC hosts Majorana bound states (MBSs) at its ends, which can interact between them with an effective coupling strength. We find a signature of bound states in continuum (BIC) in the MBSs spectral function. By allowing finite inter MBSs coupling, BICs splitting is observed and shows projection in transmission for asymmetric coupling case as cuasi-BICs. As a consequence, we also show that the Fano effect, arising from interference phenomena between MBSs hybridization trough QD, is observed with a half-integer amplitude modulation. We believe our findings can help to better understand the properties of MBSs and their interplay with QDs.

cond-mat.mes-hall↗

arXiv2018

Bound states in the continuum poisoned by Majorana fermions

In this work, we study the bound states in the continuum (BICs) in a system formed by a triple quantum dot array embedded between two one-dimensional topological superconductors, both hosting Majorana bound states (MBSs) at its ends. The results show the formation of BICs with topological characteristics due to the presence of MBSs. The latter is a consequence of the interplay between the BIC arising from quantum dots states by means of energy level symmetry breaking through gate voltages, and MBSs leaked into the quantum dots. The BIC is not observed when both TSCs are in long wire limit, i. e. for vanishing inter MBSs coupling, while it projects into the electronic transmission whenever the inter MBSs couplings are away from zero, regardless if they are different and/or the phase difference between both TSCs. We study the behavior of BICs poisoned by MBSs as a function of the parameters that are controlling the system. We believe our findings could be useful to implement a protection tool for BICs using MBSs based on tunable gate voltages.

cond-mat.mes-hall↗

arXiv2017

Spin-Seebeck effect and spin polarization in a multiple quantum dot molecule

In this work, we study the conductance and the thermoelectric properties of a quantum dot embedded between two metallic leads with a side-coupled triple quantum dot molecule under a magnetic field. We focus on the spin polarization and thermoelectric quantities. Our results show the possibility of design an efficient spin filter device besides a noticeable enhancement of the Seebeck coefficient driven by the asymmetry in the quantum dots energy levels and a tunable pure spin Seebeck effect is obtained. This behavior also holds in the interacting case, where a pure spin Seebeck effect can be obtained for fixed values of the embedded quantum dot energy level. Our findings could lead to the implementation of a new pure spin energy conversion and capable spin filter devices working with weak magnetic fields.

cond-mat.mes-hall↗

arXiv2017

Detecting coupling of Majorana bound states with an Aharonov-Bohm interferometer

We study the transport properties of an interferometer composed by a quantum dot (QD) coupled with two normal leads and two one-dimensional topological superconductor nanowires (TNWs) hosting Majorana bound states (MBS) at their ends. The geometry considered is such that one TNW has both ends connected with the QD, forming an Aharonov-Bohm (AB) interferometer threaded by an external magnetic flux, while the other TNW is placed near the interferometer TNW. This geometry can alternatively be seen as a long wire contacted across a local defect, with possible coupling between independent-MBS. We use the Green's function formalism to calculate the conductance across normal current leads on the QD. We find that the conductance exhibits a half-quantum value regardless of the AB phase and location of the dot energy level, whenever the interferometer configuration interacts with the neighboring TNW. These findings suggest that such a geometry could be used for a sensitive detection of MBS interactions across TNWs, exploiting the high sensitivity of conductance to the AB phase in the interferometer.

cond-mat.mes-hall↗

arXiv2016

Thermoelectric transport through Majorana bound states and violation of Wiedemann-Franz law

We study features of the thermoelectric transport through a Kitaev chain hosting Majorana bound states (MBS) at its ends. We describe the behavior of the Seebeck coefficient and the ZT figure of merit for two different configurations between MBS and normal current leads. We find an important violation of the Wiedemann-Franz law in one of these geometries, leading to sizeable values of the thermoelectric efficiency over a narrow window in chemical potential away from neutrality. These findings could lead to interesting thermoelectric-based MBSs detection devices, via measurements of the Seebeck coefficient and figure of merit.

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