arXiv subjects

G. B. Martins

Publications and source records attributed to G. B. Martins.

At least 19 recordsLinked to original sources

arXiv2025

Kondo effect in a two-dimensional electron gas in the Persistent Spin Helix regime

The Kondo effect arises from many-body interactions between localized magnetic impurities and conduction electrons, affecting electronic properties at low temperatures. In this study, we investigate the Kondo effect within a two-dimensional electron gas subjected to strong spin-orbit coupling in and out of the persistent spin helix regime, a state characterized by a long spin lifetime due to SU(2) symmetry recovery. Using the numerical renormalization group approach, we systematically analyze the influence of spin-orbit coupling strength and the orientation of an external magnetic field on the spectral properties of the impurity. Our findings reveal an entrancing interplay between spin-orbit coupling and the magnetic field, leading to key phenomena such as splitting of the hybridization function, asymmetry in the spectral function of the impurity, and significant tunability of the Kondo temperature due to spin orbit. These results provide valuable insights into the delicate balance between spin-orbit and external magnetic field effects in quantum impurity systems, contributing to a deeper understanding of spintronics and quantum manipulation in low-dimensional materials.

cond-mat.str-el↗

arXiv2024

Identifying an effective model for the two-stage-Kondo regime: Numerical renormalization group results

A composite impurity in a metal can explore different configurations, where its net magnetic moment may be screened by the host electrons. An example is the two-stage Kondo (TSK) system, where screening occurs at successively smaller energy scales. Alternatively, impurities may prefer a local singlet disconnected from the metal. This competition is influenced by the system's couplings. A double quantum dot T-shape geometry, where a "hanging" dot is connected to current leads only via another dot, allows experimental exploration of these regimes. Differentiating the two regimes has been challenging. This study provides a method to identify the TSK regime in such a geometry. The TSK regime requires a balance between the inter-dot coupling ($t_{01}$) and the coupling of the quantum dot connected to the Fermi sea ($Γ_0$). Above a certain ratio, the system transitions to a molecular regime, forming a local singlet with no Kondo screening. The study identifies a region in the $t_{01}$--$Γ_0$ parameter space where a pure TSK regime occurs. Here, the second Kondo stage properties can be described by a single impurity Anderson model with effective parameters. By examining the magnetic susceptibility of the hanging quantum dot, a single parameter, $Γ_{\rm eff}$, can simulate this susceptibility accurately. This effective model also provides the hanging quantum dot's spectral function accurately within a limited parameter range, defining the true TSK regime. Additionally, spin correlations between the quantum dots show universal behavior in this parameter range. These findings can guide experimental groups in selecting parameter values to place the system in either the TSK regime or the crossover to the molecular regime.

cond-mat.str-el↗

arXiv2024

The cumulant Green's functions method for the single impurity Anderson model

Using the cumulant Green's functions method (CGFM), we study the single impurity Anderson model (SIAM). The CGFM starting point is a diagonalization of the SIAM Hamiltonian expressed in a semi-chain form, containing N sites, viz., a correlated site (simulating an impurity) connected to the remaining N-1 uncorrelated conduction-electron sites. An exact solution can be obtained since the complete system has few sites. That solution is employed to calculate the atomic Green's functions and the approximate cumulants used to obtain the impurity and conduction Green's functions for the SIAM, and no self-consistency loop is required. We calculated the density of states, the Friedel sum rule, and the impurity occupation number, all benchmarked against results from the numerical renormalization group (NRG). One of the main insights obtained is that, at very low temperatures, only four atomic transitions contribute to generating the entire SIAM density of states, regardless of the number of sites in the chain and the model's parameters and different regimes: Empty orbital, mixed-valence, and Kondo. We also pointed out the possibilities of the CGFM as a valid alternative to describe strongly correlated electron systems like the Hubbard and t-J models, the periodic Anderson model, the Kondo and Coqblin-Schrieffer models, and their variants.

cond-mat.str-el↗

arXiv2024

Quantum impurity with 2/3 local moment in 1D quantum wires: an NRG study

We study a Kondo state that is strongly influenced by its proximity to an w^-1/2 singularity in the metallic host density of states. This singularity occurs at the bottom of the band of a 1D chain, for example. We first analyze the non-interacting system: A resonant state e_d, located close to the band singularity, suffers a strong `renormalization', such that a bound state is created below the bottom of the band in addition to a resonance in the continuum. When e_d is positioned right at the singularity, the spectral weight of the bound state is 2/3, irrespective of its coupling to the conduction electrons. The interacting system is modeled using the Single Impurity Anderson Model, which is then solved using the Numerical Renormalization Group method. We observe that the Hubbard interaction causes the bound state to suffer a series of transformations, including level splitting, transfer of spectral weight, appearance of a spectral discontinuity, changes in binding energy (the lowest state moves farther away from the bottom of the band), and development of a finite width. When e_d is away from the singularity and in the intermediate valence regime, the impurity occupancy is lower. As e_d moves closer to the singularity, the system partially recovers Kondo regime properties, i.e., higher occupancy and lower Kondo temperature T_K. The impurity thermodynamic properties show that the local moment fixed point is also strongly affected by the existence of the bound state. When e_d is close to the singularity, the local moment fixed point becomes impervious to charge fluctuations (caused by bringing e_d close to the Fermi energy), in contrast to the local moment suppression that occurs when e_d is away from the singularity. We also discuss an experimental implementation that shows similar results to the quantum wire, if the impurity's metallic host is an armchair graphene nanoribbon.

cond-mat.str-el↗

arXiv2022

Ferromagnetism in armchair graphene nanoribbon heterostructures

We study the properties of flat-bands that appear in a heterostructure composed of strands of different widths of graphene armchair nanoribbons. One of the flat-bands is reminiscent of the one that appears in pristine armchair nanoribbons and has its origin in a quantum mechanical destructive interference effect, dubbed `Wannier orbital states' by Lin et al. in Phys. Rev. B 79, 035405 (2009). The additional flat-bands found in these heterostructures, some reasonably closer to the Fermi level, seem to be generated by a similar interference process. After doing a thorough tight-binding analysis of the band structures of the different kinds of heterostructures, focusing in the properties of the flat-bands, we use Density Functional Theory to study the possibility of magnetic ground states when placing, through doping, the Fermi energy close to the different flat-bands. Our DFT results confirmed the expectation that these heterostructures, after being appropriately hole-doped, develop a ferromagnetic ground state that seems to require, as in the case of pristine armchair nanoribbons, the presence of a dispersive band crossing the flat-band. In addition, we found a remarkable agreement between the tight-binding and DFT results for the charge density distribution of the so-called Wannier orbital states.

cond-mat.mes-hall↗

arXiv2021

Spin-orbit coupling effects over thermoelectric transport properties in quantum dots

We study the effects caused by Rashba and Dresselhaus spin-orbit coupling over the thermoelectric transport properties of a single-electron transistor, viz., a quantum dot connected to one-dimensional leads. Using linear response theory and employing the numerical renormalization group method, we calculate the thermopower, electrical and thermal conductances, dimensionless thermoelectric figure of merit, and study the Wiedemann-Franz law, showing their temperature maps. Our results for all those properties indicate that spin-orbit coupling drives the system into the Kondo regime. We show that the thermoelectric transport properties, in the presence of spin-orbit coupling, obey the expected universality of the Kondo strong coupling fixed point. In addition, our results show a notable increase in the thermoelectric figure of merit, caused by the spin-orbit coupling in the one-dimensional quantum dot leads.

cond-mat.mes-hall↗

arXiv2020

Anisotropic Kondo screening induced by spin-orbit coupling in quantum wires

Using the numerical renormalization group (NRG) method we study a magnetic impurity coupled to a quantum wire with Rashba and Dresselhaus spin-orbit coupling (SOC) in an external magnetic field. We consider the low-filling regime with the Fermi energy close to the bottom of the band and report the results for local static and dynamic properties in the Kondo regime. In the absence of the field, local impurity properties remain isotropic in spin space despite the SOC-induced magnetic anisotropy of the conduction band. In the presence of the field, clear fingerprints of anisotropy are revealed through the strong field-direction dependence of the impurity spin polarization and spectra, in particular of the Kondo peak height. The detailed behavior depends on the relative magnitudes of the impurity and band $g$-factors. For the case of impurity $g$-factor somewhat lower than the band $g$-factor, the maximal Kondo peak suppression is found for field oriented along the effective SOC field axis, while for a field perpendicular to this direction we observe a compensation effect (``revival of the Kondo peak''): the SOC counteracts the Kondo peak splitting effects of the local Zeeman field. We demonstrate that the SOC-induced anisotropy, measurable by tunneling spectroscopy techniques, can help to determine the ratio of Rashba and Dresselhaus SOC strengths in the wire.

cond-mat.str-el↗

arXiv2020

Reentrant Kondo effect in a quantum impurity coupled to a metal-semiconductor hybrid contact

Using NRG, we show that a system containing a quantum impurity (QI), strongly coupled to a semiconductor (gap $2 Δ$) and weakly coupled to a metal, displays a 'reentrant' Kondo stage at low temperatures. The NRG analysis of the corresponding Single Impurity Anderson Model (SIAM) shows that the reentrant stage is characterized by a second sequence of SIAM fixed points: free orbital (FO) > local moment (LM) > strong coupling (SC). In the first stage, the SC fixed point (Kondo temperature $T_{K1}$) is unstable, while the second stage exhibits a much lower Kondo temperature $T_{K2}$, associated to a stable SC fixed point. The results indicate that the reentrant Kondo screening is associated to an effective SIAM, with an effective repulsion $U_{eff}$. This low temperature effective SIAM, which we dub as 'reentrant' SIAM, behaves as a 'replica' of the high temperature (bare) SIAM. The intuitive picture that emerges is that the first Kondo state develops through impurity screening by semiconducting electrons, while the second Kondo state involves screening by metallic electrons, once the semiconducting electrons are out of reach to thermal excitations ($T < Δ$) and only the metallic spectral weight inside the gap is available for impurity screening. In addition, we analyze a hybrid system formed by a QI `sandwiched' between an armchair graphene nanoribbon (AGNR) and a scanning tunneling microscope (STM) tip, with respective couplings set to reproduce the generic model described above. The energy gap in the AGNR can be externally tuned by an electric-field-induced Rashba spin-orbit interaction. We analyzed this system for realistic parameter values, using NRG, and concluded that the reentrant SIAM, with its associated second stage Kondo, is worthy of experimental investigation.

cond-mat.str-el↗

arXiv2017

Multiband superconductivity in ${\rm BiS_2}$-based layered compounds

A mean-field treatment is presented of a square lattice two-orbital-model for ${\rm BiS_2}$ taking into account intra- and inter-orbital superconductivity. A rich phase diagram involving both types of superconductivity is presented as a function of the ratio between the couplings of electrons in the same and different orbitals (${\rm η= V_{XX}/V_{XY}}$) and electron doping $x$. With the help of a quantity we call orbital-mixing ratio, denoted as $R(ϕ)$, the phase diagram is analyzed using a simple and intuitive picture based on how $R(ϕ)$ varies as electron doping increases. The predictive power of $R(ϕ)$ suggests that it could be a useful tool in qualitatively (or even semi-quantitatively) analyzing multiband superconductivity in BCS-like superconductors.

cond-mat.supr-con↗

arXiv2017

The $SU(4)-SU(2)$ crossover and spin filter properties of a double quantum dot nanosystem

The $SU(4)-SU(2)$ crossover, driven by an external magnetic field $h$, is analyzed in a capacitively-coupled double-quantum-dot device connected to independent leads. As one continuously charges the dots from empty to quarter-filled, by varying the gate potential $V_g$, the crossover starts when the magnitude of the spin polarization of the double quantum dot, as measured by $\langle n_{\uparrow}\rangle -\langle n_{\downarrow}\rangle$, becomes finite. Although the external magnetic field breaks the $SU(4)$ symmetry of the Hamiltonian, the ground state preserves it in a region of $V_g$, where $\langle n_{\uparrow}\rangle -\langle n_{\downarrow}\rangle =0$. Once the spin polarization becomes finite, it initially increases slowly until a sudden change occurs, in which $\langle n_{\downarrow}\rangle$ (polarization direction opposite to the magnetic field) reaches a maximum and then decreases to negligible values abruptly, at which point an orbital $SU(2)$ ground state is fully established. This crossover from one Kondo state, with emergent $SU(4)$ symmetry, where spin and orbital degrees of freedom all play a role, to another, with $SU(2)$ symmetry, where only orbital degrees of freedom participate, is triggered by a competition between $gμ_Bh$, the energy gain by the Zeeman-split polarized state and the Kondo temperature $T_K^{SU(4)}$, the gain provided by the $SU(4)$ unpolarized Kondo-singlet state.

cond-mat.str-el↗

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

Applying experimental constraints to a one-dimensional model for BiS2 superconductivity

Recent ARPES measurements [Phys. Rev. B 92, 041113 (2015)] have confirmed the one-dimensional character of the electronic structure of CeO0.5F0.5BiS2, a representative of BiS2-based superconductors. In addition, several members of this family present sizable increase in the superconducting transition temperature Tc under application of hydrostatic pressure. Motivated by these two results, we propose a one-dimensional three-orbital model, whose kinetic energy part, obtained through ab initio calculations, is supplemented by pair-scattering terms, which are treated at the mean-field level. We solve the gap equations self-consistently and then systematically probe which combination of pair-scattering terms gives results consistent with experiment, namely, a superconducting dome with a maximum Tc at the right chemical potential and a sizable increase in Tc when the magnitude of the hoppings is increased. For these constraints to be satisfied multi-gap superconductivity is required, in agreement with experiments, and one of the hoppings has a dominant influence over the increase of Tc with pressure.

cond-mat.supr-con↗

arXiv2014

Magnetic field modulated Kondo effect in a single-magnetic-ion molecule

We study numerically the low-temperature electronic transport properties of a single-ion magnet with uniaxial and transverse spin anisotropies. We find clear signatures of a Kondo effect caused by the presence of a transverse (zero-field) anisotropy in the molecule. Upon applying a transverse magnetic field to the single-ion magnet, we observe oscillations of the Kondo effect due to the presence of diabolical (degeneracy) points of the energy spectrum of the molecule caused by a geometrical phase interference effects similar to those observed in the quantum tunneling of multi-ion molecular nanomagnets. The field-induced lifting of the ground state degeneracy competes with the interference modulation, resulting in some cases in a suppression of the Kondo peak.

cond-mat.mes-hall↗

arXiv2014

Kondo versus indirect exchange: the role of the lattice and the actual range of RKKY interactions in real materials

Magnetic impurities embedded in a metal interact via an effective Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling mediated by the conduction electrons, which is commonly assumed to be long ranged, with an algebraic decay in the inter-impurity distance. However, they can also form a Kondo screened state that is oblivious to the presence of other impurities. The competition between these effects leads to a critical distance above which Kondo effect dominates, translating into a finite range for the RKKY interaction. We study this mechanism on the square and cubic lattices by introducing an exact mapping onto an effective one-dimensional problem that we can solve with the density matrix renormalization group method (DMRG). We show a clear departure from the conventional RKKY theory, that can be attributed to the dimensionality and different densities of states. In particular, for dimension d>1, Kondo physics dominates even at short distances, while the ferromagnetic RKKY state is energetically unfavorable.

cond-mat.str-el↗

arXiv2013

Polarized currents and spatial separation of Kondo state: NRG study of spin-orbital effect in a double QD

A double quantum dot device, connected to two channels that only see each other through interdot Coulomb repulsion, is analyzed using the numerical renormalization group technique. By using a two-impurity Anderson model, and parameter values obtained from experiment [S. Amasha {\it et al.}, Phys. Rev. Lett. {\bf 110}, 046604 (2013)], it is shown that, by applying a moderate magnetic field, and adjusting the gate potential of each quantum dot, opposing spin polarizations are created in each channel. Furthermore, through a well defined change in the gate potentials, the polarizations can be reversed. This polarization effect is clearly associated to a spin-orbital Kondo state having a Kondo peak that originates from spatially separated parts of the device. This fact opens the exciting possibility of experimentally probing the internal structure of an SU(2) Kondo state.

cond-mat.str-el↗

arXiv2013

Lanczos transformation for quantum impurity problems in d-dimensional lattices: application to graphene nanoribbons

We present a completely unbiased and controlled numerical method to solve quantum impurity problems in d-dimensional lattices. This approach is based on a canonical transformation, of the Lanczos form, where the complete lattice Hamiltonian is exactly mapped onto an equivalent one dimensional system, in the same spirit as Wilson's numerical renormalization. The method is particularly suited to study systems that are inhomogeneous, and/or have a boundary. As a proof of concept, we use the density matrix renormalization group to solve the equivalent one-dimensional problem. The resulting dimensional reduction translates into a reduction of the scaling of the entanglement entropy by a factor $L^{d-1}$, where L is the linear dimension of the original d-dimensional lattice. This allows one to calculate the ground state of a magnetic impurity attached to an LxL square lattice and an LxLxL cubic lattice with L up to 140 sites. We also study the localized edge states in graphene nanoribbons by attaching a magnetic impurity to the edge or the center of the system. For armchair metallic nanoribbons we find a slow decay of the spin correlations as a consequence of the delocalized metallic states. In the case of zigzag ribbons, the decay of the spin correlations depends on the position of the impurity. If the impurity is situated in the bulk of the ribbon, the decay is slow as in the metallic case. On the other hand, if the adatom is attached to the edge, the decay is fast, within few sites of the impurity, as a consequence of the localized edge states, and the short correlation length. The mapping can be combined with ab-initio band structure calculations to model the system, and to understand correlation effects in quantum impurity problems from first principles.

cond-mat.mes-hall↗

arXiv2013

Transport properties of a two impurity system: a theoretical approach

A system of two interacting cobalt atoms, at varying distances, was studied in a recent scanning tunneling microscope experiment by Bork et. al.[Nature Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all experimentally analyzed interatomic distances, the physics observed in these experiments. Our proposal is based on the two-impurity Anderson model, with the inclusion of a two-path geometry for charge transport. This many-body system is treated in the finite-U slave boson mean-field approximation and the logarithmic-discretization embedded-cluster approximation. We physically characterize the different charge transport regimes of this system at various interatomic distances and show that, as in the experiments, the features observed in the transport properties depend on the presence of two impurities but also on the existence of two conducting channels for electron transport. We interpret the splitting observed in the conductance as the result of the hybridization of the two Kondo resonances associated with each impurity.

cond-mat.mes-hall↗

arXiv2012

Kondo temperature and screening extension in a double dot system

In this work we use the Slave Boson Mean Field Approximation at finite U to study the effects of spin-spin correlations in the transport properties of two quantum dots coupled in series to metallic leads. Different quantum regimes of this system are studied in a wide range of parameter space. The main aspects related to the interplay between the half-filling Kondo effect and the antiferromagnetic correlation between the quantum dots are reviewed. Slave boson results for conductance, local density of states in the quantum dots, and the renormalized energy parameters, are presented. As a different approach to the Kondo physics in a double dot system, the Kondo cloud extension inside the metallic leads is calculated and its dependence with the inter-dot coupling is analyzed. In addition, the cloud extension permits the calculation of the Kondo temperature of the double quantum dot. This result is very similar to the corresponding critical temperature $T_c$, as a function of the parameters of the system, as obtained by using the finite temperature extension of the Slave Boson Mean Field Approximation.

cond-mat.mes-hall↗