arXiv subjects

M. S. Figueira

Publications and source records attributed to M. S. Figueira.

At least 37 records · Page 2Linked to original sources

arXiv2020

Topological isoconductance signatures in Majorana nanowires

We consider transport properties of a hybrid device composed by a quantum dot placed between normal and superconducting reservoirs, and coupled to a Majorana nanowire: a topological superconducting segment hosting Majorana zero-modes at the opposite ends. It is demonstrated that if topologically protected (nonoverlapping) Majorana zero-modes are formed in the system, zero-bias Andreev conductance through the dot exhibits isoconductance profiles with the shape depending on the spin asymmetry of the coupling between a dot and a topological superconductor. Otherwise, for the topologically trivial situation corresponding to the formation of Andreev bound states, the conductance is insensitive to the spin polarization and the isoconductance signatures disappear. This allows to propose an experimental protocol for distinguishing between isolated Majorana zero-modes and Andreev bound states.

cond-mat.supr-con↗

arXiv2019

Spin-dependent zero-bias peak in a hybrid nanowire-quantum dot system: Distinguishing isolated Majorana fermions from Andreev bound states

Hybrid system composed by a semiconducting nanowire with proximity-induced superconductivity and a quantum dot at the end working as spectrometer was recently used to quantify the so-called degree of Majorana nonlocality [Deng et al., Phys.Rev.B, 98, 085125 (2018)]. Here we demonstrate that spin-resolved density of states of the dot responsible for zero-bias conductance peak strongly depends on the separation between the Majorana bound states (MBSs) and their relative couplings with the dot and investigate how the charging energy affects the spectrum of the system in the distinct scenarios of Majorana nonlocality (topological quality). Our findings suggest that spin-resolved spectroscopy of the local density of states of the dot can be used as a powerful tool for discriminating between different scenarios of the emergence of zero-bias conductance peak.

cond-mat.mes-hall↗

arXiv2019

Chiral magnetic chemical bonds in molecular states of impurities in Weyl semimetals

We demonstrate that chirality of the electron scattering in Weyl semimetals leads to the formation of magnetic chemical bonds for molecular states of a pair of impurities. The effect is associated with the presence of time-reversal symmetry breaking terms in the Hamiltonian which drive a crossover from s- to p-wave scattering. The profiles of the corresponding molecular orbitals and their spin polarizations are defined by the relative orientation of the lines connecting two Weyl nodes and two impurities. The magnetic character of the molecular orbitals and their tunability open the way for using doped Weyl semimetals for spintronics and realization of qubits.

cond-mat.mes-hall↗

arXiv2018

Braiding of edge states in narrow zigzag graphene nanoribbons: effect of the third neighbors hopping

We study narrow zigzag graphene nanoribbons (ZGNRs), employing density functional theory (DFT) simulations and the tight-binding (TB) method. The main result of these calculations is the braiding of the conduction and valence bands, generating Dirac cones for non-commensurate wave vectors $\vec{k}$. Employing a TB Hamiltonian, we show that the braiding is generated by the third-neighbor hopping (N3). We calculate the band structure, the density of states and the conductance, new conductance channels are opened, and the conductance at the Fermi energy assumes integer multiples of the quantum conductance unit $G_{o} = 2e^{2}/h$. We also investigate the satisfaction of the Stoner criterion by these ZGNRs. We calculate the magnetic properties of the fundamental state employing LSDA (spin-unrestricted DFT) and we confirm that ZGNRs with $N=(2,3)$ do not satisfy the Stoner criterion and as such the magnetic order could not be developed at their edges. These results are confirmed by both tight-binding and LSDA calculations.

cond-mat.mes-hall↗

arXiv2018

Tuning of heat and charge transport by Majorana fermions

We investigate theoretically thermal and electrical conductances for the system consisting of a quantum dot (QD) connected both to a pair of Majorana fermions residing the edges of a Kitaev wire and two metallic leads. We demonstrate that both quantities reveal pronounced resonances, whose positions can be controlled by tuning of an asymmetry of the couplings of the QD and a pair of MFs. Similar behavior is revealed for the thermopower, Wiedemann-Franz law and dimensionless thermoelectric figure of merit. The considered geometry can thus be used as a tuner of heat and charge transport assisted by MFs.

cond-mat.mes-hall↗

arXiv2017

The role of the short range magnetic correlations on the gap opening of the topological Kondo insulators

In this work we investigate the effects of the short range magnetic correlations on the gap opening of the topological Kondo insulators. We consider an additional narrow band to the otherwise completely localized f-electrons, by adding a term to the periodic Anderson model which allows a small hopping of the localized electrons between neighboring sites of the lattice. This new model is adequate to study a novel class of intermetallic 4f and 5f orbitals materials: the Kondo topological insulators, whose paradigmatic material is the compound SmB6. For simplicity, we consider a version of the periodic Anderson model on a two dimensional square lattice. The starting point of the model is the 4f-Ce ions orbitals, with J=5/2 multiplet in the presence of spin-orbit coupling. We present results of the correlation functions and we show that the short range antiferromagnetic correlations favors the formation of magnetic moments on the atoms, and at the same time the existence of these moments opens the possibility of the spin-flip scattering by the conduction electrons, generating the Kondo effect, which contribute to the opening of a gap in the density of states of the system. We also calculate the phase diagram which shows that, as we vary the Ef level position from the empty regime to the Kondo regime, the system develops several phases: metallic, band insulator and Kondo topological insulator. The band structure calculated shows that the model develops a strong topological insulator.

cond-mat.str-el↗

arXiv2016

Unveiling Majorana Quasiparticles by a Quantum Phase Transition: Proposal of a Current Switch

We propose a theoretical approach based on an interferometer composed by two quantum dots asymmetrically coupled to isolated Majorana quasiparticles (MQPs), lying on the edges of two topological Kitaev chains, respectively via couplings $(t+Δ)$ and $(Δ-t)$. This setup enables us to probe MQPs in a quite distinct way from the zero-bias peak feature. Most importantly, the system behaves as a current switch made by two distinct paths: (i) for the upper dot connected to both chains, the device perceives both MQPs as an ordinary fermion and the current crosses solely the lower dot, since current in the upper dot is prevented due to the presence of the superconducting gap; and (ii) by suppressing slightly the hybridization of the upper dot with one chain, the current is abruptly switched to flow through this dot, once a trapped electron as a bound state in the continuum (BIC) (Phys. Rev. B 93, 165116 (2016)) appears in the lower dot. Such a current switch between upper and lower dots characterizes the Quantum Phase Transition (QPT) proposed here, being the ratio $t/Δ$ the control parameter of the transition. This QPT is associated with a change from an ordinary fermionic excitation regime to a MQP in the interferometer, which enables not only the fundamental revealing of MQPs, but also yields a current switch assisted by them.

cond-mat.mes-hall↗

arXiv2016

Fano fingerprints of Majoranas in Kitaev dimers of superconducting adatoms

We investigate theoretically a Fano interferometer composed by STM and AFM tips close to a Kitaev dimer of superconducting adatoms, in which the adatom placed under the AFM tip, encloses a pair of Majorana fermions (MFs). For the binding energy $Δ$ of the Cooper pair delocalized into the adatoms under the tips coincident with the tunneling amplitude $t$ between them, namely $Δ$ = $t$, we find that only one MF beneath the AFM tip hybridizes with the adatom coupled to the STM tips. As a result, a gate invariance feature emerges: the Fano profile of the transmittance rises as an invariant quantity depending upon the STM tips Fermi energy, due to the symmetric swap in the gate potential of the AFM tip.

cond-mat.mes-hall↗

arXiv2015

Quantum phase transition triggering magnetic BICs in graphene

Graphene hosting a pair of collinear adatoms in the phantom atom configuration has pseudogap with cubic scaling on energy, $Δ\propto|\varepsilon|^{3}$ which leads to the appearance of spin-degenerate bound states in the continuum (BICs) [Phys. Rev. B 92, 045409 (2015)]. In the case when adatoms are locally coupled to a single carbon atom the pseudogap scales linearly with energy, which prevents the formation of BICs. In this Letter, we explore the effects of non-local coupling characterized by the Fano factor of interference $q_{0},$ tunable by changing the slope of the Dirac cones in the graphene band-structure. We demonstrate that three distinct regimes can be identified: i) for $q_{0} q_{c2}$ the cubic scaling of the pseudogap with energy $Δ\propto|\varepsilon|^{3}$ characteristic to the phantom atom configuration is restored and the phase with non-magnetic BICs is recovered. The phase with magnetic BICs can be described in terms of an effective intrinsic exchange field of ferromagnetic nature between the adatoms mediated by graphene monolayer. We thus propose a new type of quantum phase transition resulting from the competition between the states characterized by spin-degenerate and magnetic BICs.

cond-mat.str-el↗

arXiv2014

Spin-dependent beating patterns in thermoelectric properties: Filtering the carriers of the heat flux in a Kondo adatom system

We theoretically investigate the thermoelectric properties of a spin-polarized two-dimensional electron gas hosting a Kondo adatom hybridized with an STM tip. Such a setup is treated within the single-impurity Anderson model in combination with the atomic approach for the Green's functions. Due to the spin dependence of the Fermi wavenumbers the electrical and thermal conductances, together with thermopower and Lorenz number reveal beating patterns as function of the STM tip position in the Kondo regime. In particular, by tuning the lateral displacement of the tip with respect to the adatom vicinity, the temperature and the position of the adatom level, one can change the sign of the Seebeck coefficient through charge and spin. This opens a possibility of the microscopic control of the heat flux analogously to that established for the electrical current.

cond-mat.str-el↗

arXiv2014

Interference effects induced by Andreev bound states in a hybrid nanostructure composed by a quantum dot coupled to ferromagnetic and superconductor leads

In this work, it is considered a nanostructure composed by a quantum dot coupled to two ferromagnets and a superconductor. The transport properties of this system are studied within a generalized mean-field approximation taking into account proximity effects and spin-flip correlations within the quantum dot. It is shown that the zero-bias transmittance for the co-tunneling between the ferromagnetic leads presents a dip whose height depends on the relative orientation of the magnetizations. When the superconductor is coupled to the system, electron-hole correlations between different spin states leads to a resonance in the place of the dip appearing in the transmittance. Such an effect is accompanied by two anti-resonances explained by a leakage of conduction channels from the co-tunneling to the Andreev transport. In the non-equilibrium regime, correlations within the quantum dot introduce a dependence of the resonance condition on the finite bias applied to the ferromagnetic leads. However, it is still possible to observe signatures of the same interference effect in the electrical current.

cond-mat.mes-hall↗

arXiv2013

Dimensionality effects in the LDOS of ferromagnetic hosts probed via STM: spin-polarized quantum beats and spin filtering

We theoretically investigate the local density of states (LDOS) probed by a STM tip of ferromagnetic metals hosting a single adatom and a subsurface impurity. We model the system via the two-impurity Anderson Hamiltonian. By using the equation of motion with the relevant Green functions, we derive analytical expressions for the LDOS of two host types: a surface and a quantum wire. The LDOS reveals Friedel-like oscillations and Fano interference as a function of the STM tip position. These oscillations strongly depend on the host dimension. Interestingly, we find that the spin-dependent Fermi wave numbers of the hosts give rise to spin-polarized quantum beats in the LDOS. While the LDOS for the metallic surface shows a damped beating pattern, it exhibits an opposite behavior in the quantum wire. Due to this absence of damping, the wire operates as a spatially resolved spin filter with a high efficiency.

cond-mat.str-el↗

arXiv2012

Non-Zeeman splitting for a spin-resolved STM with a Kondo adatom in a spin-polarized two-dimensional electron gas

We theoretically investigate the spin-resolved local density of states (SR-LDOS) of a spin-polarized two-dimensional electron gas in the presence of a Kondo adatom and a STM probe. Using Green function formalism and the atomic approach in the limit of infinite Coulomb correlation, it is found an analytical SR-LDOS expression in the low temperature regime of the system. This formal result is given in terms of phase shifts originated by the adatom scattering and Fano interference. The SR-LDOS is investigated as a function of the probe position and different Fano factors. Our findings provide an alternative way to spin-split the Kondo resonance without the use of huge magnetic fields, typically necessary in adatom systems characterized by large Kondo temperatures. We observe a non-Zeeman spin-splitting of the Kondo resonance in the total LDOS, with one spin-component pinned around the host Fermi level. Interestingly, this result is in accordance to recent experimental data reported in Phys. Rev. B 82, 020406(R) (2010).

cond-mat.str-el↗

arXiv2010

Green's Functions for the Anderson model: the Atomic Approximation

We consider the cumulant expansion of the PAM employing the hybridization as perturbation (Phys. Rev. B 50, 17933 (1994)), and we obtain formally exact one-electron Green's functions (GF). These GF contain effective cumulants that are as difficult to calculate as the original GF, and the Atomic Approximation consists in substituting the effective cumulants by the ones that correspond to the atomic case, namely by taking a conduction band of zeroth width and local hybridization. This approximation has already been used for the case of infinite electronic repulsion U (Phys. Rev. B 62, 7882 (2000)), and here we extend the treatment to the case of finite U. The method can also be applied to the single impurity Anderson model (SIAM), and we give explicit expressions of the approximate GF both for the PAM and the SIAM.

cond-mat.str-el↗

arXiv2009

The atomic approach of the Anderson model for the U finite case: application to a quantum dot

In the present work we apply the atomic approach to the single impurity Anderson model (SIAM). A general formulation of this approach, that can be applied both to the impurity and to the lattice Anderson Hamiltonian, was developed in a previous work (arXiv:0903.0139v1 [cond-mat.str-el]). The method starts from the cumulant expansion of the periodic Anderson model (PAM), employing the hybridization as perturbation. The atomic Anderson limit is analytically solved and its sixteen eigenenergies and eigenstates are obtained. This atomic Anderson solution, which we call the (AAS), has all the fundamental excitations that generate the Kondo effect, and in the atomic approach is employed as a seed to generate the approximate solutions for finite U. The width of the conduction band is reduced to zero in the AAS, and we choose its position so that the Friedel sum rule (FSR) be satisfied, close to the chemical potential. We perform a complete study of the density of states of the SIAM in all the relevant range of parameters: the empty dot, the intermediate valence (IV-regime),the Kondo and the magnetic regime. In the Kondo regime we obtain a density of states that characterizes well the structure of the Kondo peak. To shown the usefulness of the method we have calculated the conductance of a quantum dot, side coupled to a conduction band.

cond-mat.str-el↗

arXiv2003

Heavy fermion d-wave superconductivity: a X-boson approach

From an extension of the periodic Anderson model (PAM) in the $ U=\infty $ limit taking into account the effect of a nearest neighbor attractive interaction between $ f $-electrons, we compare the obtained superconducting phase diagram of a two dimensional d-wave superconductor with the results obtained for an isotropic s-wave superconductor employing the X-boson method.

cond-mat.supr-con↗

arXiv2003

Heavy fermion superconductivity: X-boson treatment

We use the X-boson method to study the heavy fermion superconductivity employing an extension of the periodic Anderson model in the $ U = \infty $ limit with a nearest neighbor attractive interaction between the localized $ f $-electrons and we show that higher values of the hybridization parameter implies in lower values associated to the maximum of the superconducting critical temperature $ T_{c} $, indicating that the real charge transfer between bands tends to destabilizes the Cooper pairing. Moreover, we show that superconductivity are constrained to the vicinity of a range of occupations where the $ f $-band density of states at the Fermi level $ ρ_{f} (μ) $ have sufficiently high values, though superconductivity was found both for configurations where the system present intermediate valence and heavy fermion behavior, as experimentally observed. Finally, as the total occupation is raised and for increased hybridization parameter $ V $ there is a transition insulator-superconductor, which is related to the existence of a hybridization gap that cannot be obtained by the slave-boson method.

cond-mat.supr-con↗

arXiv2003

Finite band effects over the linear conductance through a quantum wire coupled with a quantum dot: X-boson treatment

We study the electronic transport through a quantum wire (QW) with a strong side coupled quantum dot (QD). We obtain a linear conductance with lateral peaks when the gate voltage, is located near the edge of the conduction band. The calculated density of states shows, that these peaks are associated with renormalized localized levels out side of the conduction band. These results are compatibles with recent experimental results.

cond-mat.mes-hall↗