Kondo Peak Research Articles

STM probing of local oscillations of the Fano-Kondo effect: a Doniach-Sunjic approach for the Kondo peak

The use of the scanning tunneling microscope (STM) for the investigation of Kondo adatoms on normal metallic surfaces reveals a Fano-Kondo behavior of the conductance as a function of the tip bias. In this work, the Doniach-Sunjic expression is used to describe the Kondo peak and we analyze the effect of a complex Fano phase, arising from an external magnetic field, on the conductance pattern. It is demonstrated that such phase generates local oscillations of the Fano-Kondo line shape and can lead to the suppression of anti-resonances.

Spin-polarized STM for a Kondo adatom

We investigate the bias dependence of the tunneling conductance between a spin-polarized(SP) scanning tunneling microscope (STM) tip and the surface conduction states of anormal metal with a Kondo adatom. Quantum interference between tip–host metal andtip–adatom–host metal conduction paths is studied in the full range of the Fano parameterq. The spin-polarized STM gives rise to a splitting of the Kondo peak and asymmetry in thezero-bias anomaly, depending on the lateral tip–adatom distance. For increasing lateraldistances, the Kondo peak splitting shows a strong suppression and the spin-polarizedconductance exhibits the standard Fano–Kondo profile.

Kondo Effect in Carbon Nanotube Quantum Dot in a Magnetic Field

The out-of-equilibrium electron transport of carbon nanotube semiconducting quantum dot placed in a magnetic field is studied in the Kondo regime by means of the non-equilibrium Green functions. The equation of motion method is used. For parallel magnetic field the Kondo peak splits in four peaks, following the simultaneous splitting of the orbital and spin states. For perpendicular field orientation the triple peak structure of density of states is observed with the central peak corresponding to orbital Kondo effect and the satellites reflecting the spin and spin-orbital fluctuations.

Phonon-assisted Kondo effect in single-molecule quantum dots coupled to ferromagnetic leads

Based on the infinite- U Anderson model spin-polarized transport through the tunnel magnetoresistance (TMR) system of single-molecule quantum dot is investigated under the interplay of strong electron correlation and electron–phonon (e–ph) coupling. The spectral density and the nonlinear differential conductance are studied using the extended non-equilibrium Green's function method through calculating the dot-level splitting self-consistently. The results exhibit that a serial of peaks emerge on the two sides of the main Kondo peak for the antiparallel magnetic configuration of electrodes, while for the parallel case both the main and phonon-assisted satellite Kondo peaks all split up into two asymmetric peaks even at zero-bias. Correspondingly, the nonlinear differential conductance displays a set of satellite-peaks around the Kondo-peak in the presence of the e–ph interaction. Furthermore, extra maxima and minima appear in the TMR curve. The TMR alternates between the positive and the negative values along with the variation of bias voltage.

Nonequilibrium Kondo effect and Andreev reflection through double quantum dots with spin-flip scattering

The Kondo effect and the Andreev reflection tunneling through a normal (ferromagnet)–double quantum dots–superconductor hybrid system is examined in the low temperature by using the nonequilibrium Green's function technique in combination with the slave-boson mean-field theory. The interplay of the Kondo physics and the Andreev bound state physics can be controlled by varying the interdot hopping strength. The Andreev differential conductance is mainly determined by the competition between Kondo states and Andreev states. The spin-polarization of the ferromagnetic electrode increases the zero-bias Kondo peak. The spin-flip scattering influences the Kondo effect and the Andreev reflection in a nontrivial way. For the ferromagnetic electrode with sufficiently large spin polarization, the negative Andreev differential conductance is found when the spin flip strength in the double quantum dots is sufficiently strong.

Spin-Polarized Transport through Parallel Double Quantum Dots Coupled to Ferromagnetic Leads

We theoretically study the spin-polarized transport phenomena of the parallel double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. The Hamiltonian is solved by means of the equation-of-motion approach. We analyse the transmission probability of this system in both the equilibrium and nonequilibrium cases, and our results reveal that the transport properties show some noticeable characteristics depending upon both the spin-polarized strength p and the value of the magnetic flux φ. Moreover, in the parallel configuration, the position of the Kondo peak shifts while it remains unchanged for the antiparallel configuration. These effects might have some potential applications in spintronics.

Achievement of sensing single spin with the aid of Kondo resonance in quantum dot connected to ferromagnetic electrodes

We theoretically propose the detection of spin states of magnetic atom or molecule in proximity to a quantum dot by the Kondo effect with ferromagnetic electrodes, which can be switched in parallel or antiparallel alignments. The relative orientation of spin to the magnetization of electrode can be evidentially tracked from the spin splitting in Kondo peak of differential conductance. The experimental realization is discussed.

Spin-charge filtering through a spin-orbit coupled quantum dot controlled via an Aharonov-Bohm interferometer

We show that a strongly correlated quantum dot embedded in an Aharonov-Bohm interferometer can be used to filter both charge and spin at zero voltage bias. The magnitude with which the Aharonov-Bohm arm is coupled to the system controls the many-body effects on the quantum dot. When the quantum dot is in the Kondo regime, the flow of charge through the system can be tuned by the phase of the Aharonov-Bohm arm, ${\ensuremath{\varphi}}_{\mathit{AB}}$. Furthermore, when a spin-orbit interaction is present on a Kondo quantum dot, we can control the flow of spin by the spin-orbit phase, ${\ensuremath{\varphi}}_{\mathit{SO}}$. The existence of the Kondo peak at the Fermi energy makes it possible to control the flow of both charge and spin in the zero voltage bias limit.

Vibration-mediated Kondo effect in a single-molecule quantum dot coupled toferromagnetic electrodes

The nonequilibrium Green’s function approach and canonical transformation technique are appliedto spin-polarized electron transport through a single-molecule quantum dot connected to twoferromagnetic leads in the Kondo regime. A joint effect of electron–electron interactions with finiteU and electron–phonon interaction (EPI) leads to a zero-bias anomaly of tunnelingmagnetoresistance (TMR), the phonon-enhanced negative zero-bias TMR being closer toits experimental value. An effective exchange field on the dot in parallel alignment ofmagnetizations results in an EPI-dependent splitting of the Kondo peak andEPI-dependent shifts of Kondo satellites. The strong Franck–Condon blockade due tomolecule vibrations is also discussed.

Asymmetric Kondo resonance splitting transition in T-shaped coupled-quantum-dot devices

Using the slave-boson mean-field approximation, we theoretically study the transport properties of the ground state of the T-shaped coupled quantum dots with two metallic leads by means of one-impurity Anderson Hamiltonian. It is found that when this system is in the equilibrium state, with the increase of the inter-dot coupling, the Kondo resonance is suppressed, and the single Kondo peak of the density of the Kondo quantum dot is split into two asymmetric Kondo peaks. When it is in the non-equilibrium state, when increasing the bias, this asymmetric Kondo splitting is enhanced.

Tunnel Magnetoresistance in Carbon Nanotube Quantum Dot

The out-of-equilibrium transport properties of carbon nanotube quantum dot in the Kondo regime are studied by means of the non-equilibrium Green function. The equation of motion method is used. The in∞uence of the polarization of electrodes and orbital level splitting, as well as leftright asymmetry, on the spin polarizations of difierential conductance are discussed. For zero bias voltage and orbitally degenerate states the SU(4) symmetry of Kondo state is preserved for antiparallel conflguration of polarizations of electrodes, whereas it is broken for parallel. In the former case a suppression of linear conductance with increasing polarization is observed. In the latter the behaviour is nonmonotonic due to splitting of the Kondo peak and bringing closer one of the peaks to the Fermi level with increasing polarization. This gives rise to giant tunnel linear magnetoresistance for large polarization.

AC Conductance Through a Vibrating Molecular Dot in KondoRegime

In the present paper, by applying the Lang–Firsov canonicaltransformation and the so-called non-crossing approximation technique,we investigate the joint effects of the electron-phonon interaction andan external alternating gate voltage on the transport of a quantum dotsystem in the Kondo regime. We find that, while the satellite Kondoresonant peaks appear in both the averaged local density of states andthe differential conductance, the main Kondo peak at the Fermi energyis greatly suppressed. These results confirm the previous ones derivedby other methods, such as the equation of motion solution. Furthermore,based on the picture of virtual transition between quasi-eigenstatesin the system, we also give a slightly different explanation on thesephenomena.

Magnetic field dependence of the spin-12and spin-1 Kondo effects in a quantum dot

We study the magnetic field evolution of the spin-$\frac{1}{2}$ and spin-1 Kondo effects in a quantum dot formed from a single-walled carbon nanotube. In the spin-$\frac{1}{2}$ case, the energy of spin-resolved Kondo conductance peaks is proportional to magnetic field at high fields, contrary to recent reports. At lower fields, the energy falls below this linear dependence, in qualitative agreement with theoretical expectations. For even electron occupancy, we observe a zero-bias Kondo peak due to the degeneracy of the spin-1 triplet ground states. Tuning gate voltage within the same Coulomb diamond drives a transition to a singlet ground state. We also independently tune the energy difference between singlet and triplet states with a magnetic field. The Zeeman splitting thus measured confirms the value of the $g$ factor deduced from the high-field behavior of spin-$\frac{1}{2}$ Kondo.

Kondo-Peak Splitting and Coherent Transport in a Single-Electron Transistor

The peculiar behavior of Kondo-peak splitting under a magnetic field and bias can be explained by calculating the nonequilibrium retarded Green's function via the nonperturbative dynamical theory (NDT). In the NDT, the application of a lead-dot-lead system reveals that new resonant tunneling levels are activated near the Fermi level and the conventional Kondo peak at the Fermi level diminishes when a bias is applied. Magnetic field causes asymmetry in the spectral density and transforms the new resonant peak into a major peak whose behavior explains all the features of the nonequilibrium Kondo phenomenon. We also show the mechanism of coherent transport through the new resonant tunneling level.

Kondo effect of a quantum dot coupling indirectly to the conduction electrons

When a quantum dot in the Kondo regime couples to two leads (the conduction electron reservoirs) indirectly through intermediate electron levels, two features are noteworthy concerning the Kondo effect. First, the Kondo peak in the spectrum of local density of states becomes narrower as the coupling to the leads is much larger than the interdot coupling, which is just opposite to the case of direct dot-lead coupling. Secondly, the increment of the coupling to the leads and the deviation of the intermediate levels from the Fermi level can effectively facilitate the formation of the negative differential conductance.

Kondo effect in transport through molecules adsorbed on metal surfaces: From Fano dips to Kondo peaks

The Kondo effect observed in recent scanning tunneling microscopy (STM) experiments on transport through CoPc and TBrPP-Co molecules adsorbed on Au(111) and Cu(111) surfaces, respectively, is discussed within the framework of a simple model [G. Chiappe and E. Louis, Phys. Rev. Lett. 97, 076806 (2006)]. Both molecules show a four lobed structure with Co placed in its center, the active $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}d$ orbital lying at lower energy in the TBrPP-Co molecule. In the Kondo regime, and by varying the ratio $r$ of STM tip/Co to STM tip/lobes hoppings, it is possible to produce a crossover from a conductance Kondo peak (CoPc, high $r$) to a conductance Fano dip (TBrPP-Co, $r$ around 1). The essential role of the internal structure of the molecule in controlling even the entrance into the Kondo regime is highlighted. In particular, it is shown that the crossover from Fano dip to Kondo peak can also be produced, changing the sign of the lobe/lobe hopping while keeping that ratio constant. In the case of $\mathrm{TBrPP}\text{\ensuremath{-}}\mathrm{Co}∕\mathrm{Cu}(111)$, we show that varying the density of states at the metal surface, or distorting the molecule, changes the shape of the Fano dip and the Kondo temperature, and shifts the dip minimum. These features are in line with experimental observations, indicating that our simple model contains the essential physics underlying the transport properties of such complex molecules.

Spectral density of an interacting dot coupled indirectly to conducting leads

We study the spectral density of electrons ${\ensuremath{\rho}}_{d\ensuremath{\sigma}}(\ensuremath{\omega})$ in an interacting quantum dot (QD) with a hybridization $\ensuremath{\lambda}$ to a noninteracting QD, which, in turn, is coupled to a noninteracting conduction band. The system corresponds to an impurity Anderson model in which the conduction band has a Lorentzian density of states of width ${\ensuremath{\Delta}}_{2}$. We solved the model using perturbation theory in the Coulomb repulsion $U$ (PTU) up to second order and a slave-boson mean-field approximation (SBMFA). The PTU works surprisingly well near the exactly solvable limit ${\ensuremath{\Delta}}_{2}\ensuremath{\rightarrow}0$. For fixed $U$ and large enough $\ensuremath{\lambda}$ or small enough ${\ensuremath{\Delta}}_{2}$, the Kondo peak in ${\ensuremath{\rho}}_{d\ensuremath{\sigma}}(\ensuremath{\omega})$ splits into two peaks. This splitting can be understood in terms of weakly interacting quasiparticles. Before the splitting takes place, the universal properties of the model in the Kondo regime are lost. Using the SBMFA, simple analytical expressions for the occurrence of split peaks are obtained. For small or moderate ${\ensuremath{\Delta}}_{2}$, the side bands of ${\ensuremath{\rho}}_{d\ensuremath{\sigma}}(\ensuremath{\omega})$ have the form of narrow resonances that were missed in previous studies using the numerical renormalization group. This technique also has shortcomings for properly describing the split Kondo peaks. As the temperature is increased, the intensity of the split Kondo peaks decreases, but it is not completely suppressed at high temperatures.

Selective spin injection controlled by electrical way in ferromagnet/quantum dot/semiconductor system

Selective and large polarization of current injected into semiconductor (SC) is predicted in ferromagnet (FM)/quantum dot (QD)/SC system by varying the gate voltage above the Kondo temperature. In addition, spin-dependent Kondo effect is also revealed below Kondo temperature. It is found that Kondo resonances for up spin state are suppressed with increasing of the polarization P of the FM lead. While the down one is enhanced. The Kondo peak for up spin is disappear at P = 1 .

Electron Transport through Double Quantum Dot: from SU(4) Kondo to SU(2) Symmetry

Electron transport across two capacitively coupled quantum dots in a parallel geometry is theoretically studied in the non-linear response regime with spin and orbital degrees of freedom taken into account and the Kondo effect induced by on-site and inter-dot Coulomb correlations is analyzed. For a system with each dot symmetrically coupled to a separate set of electrodes a well-defined spin and orbital contributions to zero-bias Kondo resonance are obtained. The Kondo peak splits if spin and/or orbital degeneracies are removed. A suppression of the orbital Kondo anomaly due to orbital asymmetry and channel mixing effects is discussed.

Kondo effect and spin-polarized transport through a quantum dot with Rashba spin-orbit interaction

By using the finite-$U$ slave-boson approach, we study theoretically the spin accumulation, spin-dependent transport, and the shot noise in a magnetic semiconductor quantum dot induced by the Rashba spin-orbit interactions (SOI) in the Kondo regime. It is shown that the Rashba SOI causes the splitting of the Kondo peak and more conductance dips appear in the Fano-type conductance. The spin-polarized conductance indicates two peaks locating both sides of ${ϵ}_{d}=\ensuremath{-}U∕2$ (${ϵ}_{d}$ and $U$ represent the dot level and the Coulomb repulsion, respectively). In the large bias limit and ignoring the effect of Rashba SOI, the Fano factor can be tuned by the direct coupling between two leads while not related to the Coulomb repulsion. It is found that the Rashba SOI can increase or decrease the Fano factor, which depends on the dot level and the chemical potentials in both leads. When the dot level is below the chemical potentials, the Fano factor increases in the presence of Rashba SOI; otherwise, the Rashba SOI suppresses the Fano factor.