Slave-boson Mean-field Approach Research Articles

Electron–phonon interaction and electronic correlations in transport through electrostatically and tunnel coupled quantum dots

We investigate two equivalent capacitively and tunnel coupled quantum dots, each coupled to its own pair of leads. Local Holstein type electron–phonon coupling at the dots is assumed. To study many-body effects we use the finite-U mean-field slave boson approach. For vanishing interdot interaction, weak e–ph coupling and finite tunneling, molecular orbital spin Kondo effects occur for single electron or single hole occupations. Phonons influence both correlations and tunneling and additionally they shift the energies of the dots. Depending on the dot energies and the strength of electron–phonon coupling, the system is occupied by a different number of electrons that effectively interact with each other repulsively or attractively leading to a number of different ground states of DQD. Among them are Kondo-like states with spin, orbital or charge correlations resulting from polaron cotunneling processes and states with magnetic intersite correlations.

Metal-insulator Transitions in Three-component Falicov-Kimball Model within Coherent Potential Approximation

We apply the coherent potential approximation to study the three-component Falicov - Kimball model, in which single-component and two-component fermionic particles are mixed in an optical lattice. In the model, the heavy single-component fermionic particles are localized while the light two-component fermionic particles can hop in the lattice. At half-filling, two transitions from an insulator via a metallic state to a Mott insulator are found with increasing the particle correlations. By contrast, at third-filling, only one transition from the metallic state to the Mott insulating phase is observed for strong repulsive interactions. Our results are consistent with those obtained by the dynamical mean field theory as well as by the slave boson mean field approach.

Impact of electron-phonon coupling on electron transport through T-shaped arrangements of quantum dots in the Kondo regime.

We calculate the conductance through strongly correlated T-shaped molecular or quantum dot systems under the influence of phonons. The system is modelled by the extended Anderson–Holstein Hamiltonian. The finite-U mean-field slave boson approach is used to study many-body effects. Phonons influence both interference and correlations. Depending on the dot unperturbed energy and the strength of electron–phonon interaction, the system is occupied by a different number of electrons that effectively interact with each other repulsively or attractively. This leads, together with the interference effects, to different spin or charge Fano–Kondo effects.

Transport through strongly correlated triple quantum dot

Strong electron correlations are discussed for the three capacitively coupled quantum dots, each of which is connected to a separate pair of electrodes. The finite-U mean field slave boson approach is used. The analysis is carried out for both repulsive and attractive intra- and inter-dot interactions. Depending on the ratio and the sign of interaction parameters and occupation, either charge ordered states or different spin, spin–charge and charge Kondo resonances arise.

Kondo effects in small-bandgap carbon nanotube quantum dots.

We study the magnetoconductance of small-bandgap carbon nanotube quantum dots in the presence of spin–orbit coupling in the strong-correlations regime. A finite-U slave-boson mean-field approach is used to study many-body effects. Different degeneracies are restored in a magnetic field and Kondo effects of different symmetries arise, including SU(3) effects of different types. Full spin–orbital degeneracy might be recovered at zero field and, correspondingly, the SU(4) Kondo effect sets in. We point out the possibility of the occurrence of electron–hole Kondo effects in slanting magnetic fields, which we predict to occur in magnetic fields with an orientation close to perpendicular. When the field approaches a transverse orientation a crossover from SU(2) or SU(3) symmetry into SU(4) is observed.

Enhanced Negative Nonlocal Conductance in an Interacting Quantum Dot Connected to Two Ferromagnetic Leads and One Superconducting Lead

In this paper, we investigate the electronic transport properties of a quantum dot (QD) connected to two ferromagnetic leads and one superconducting lead in the Kondo regime by means of the finite-U slave boson mean field approach and the nonequilibrium Green function technique. In this three-terminal hybrid nanodevice, we focus our attention on the joint effects of the Kondo correlation, superconducting proximity pairing, and spin polarization of leads. It is found that the superconducting proximity effect will suppress the linear local conductance (LLC) stemming from the weakened Kondo peak, and when its coupling is bigger than the tunnel-coupling of two normal leads, the linear cross conductance (LCC) becomes negative in the Kondo region. Regarding the antiparallel configuration, increasing spin polarization further suppresses LLC but enhances LCC, i.e., causing larger negative values of LCC, since it is beneficial for the emergence of cross Andreev reflection. On the contrary, for the parallel configuration, with increasing spin polarization, the LLC decreases and greatly widens with the appearance of shoulders, and eventually splits into four peaks, while the LCC decreases relatively rapidly to the normal conductance.

Intra- and inter-shell Kondo effects in carbon nanotube quantum dots

The linear response transport properties of carbon nanotube quantum dot in the strongly correlated regime are discussed. The finite-U mean field slave boson approach is used to study many-body effects. Magnetic field can rebuilt Kondo correlations, which are destroyed by the effect of spin-orbit interaction or valley mixing. Apart from the field induced revivals of SU(2) Kondo effects of different types: spin, valley or spin-valley, also more exotic phenomena appear, such as SU(3) Kondo effect. Threefold degeneracy occurs due to the effective intervalley exchange induced by short-range part of Coulomb interaction or due to the intershell mixing. In narrow gap nanotubes the full spin-orbital degeneracy might be recovered in the absence of magnetic field opening the condition for a formation of SU(4) Kondo resonance.

Transport through capacitively coupled embedded and T-shape quantum dots in the Kondo range

Strong electron correlations resulting from spin and charge pseudospin fluctuations together with interference effects are discussed in asymmetric set of capacitively coupled dots: embedded and side attached (EDTD). Finite - U mean field slave boson approach is used. The many body resonance exhibits SU(4) Kondo character at the embedded dot and SU(4) Kondo-Fano in the T-shaped arm. For the deep dot levels characteristic temperatures of these resonances coincide with the temperatures of corresponding resonances in symmetric double dot systems, embedded or side attached respectively. Linear transport properties through embedded or side attached dots are identical with linear transport characteristics of corresponding symmetric dot systems. Breaking of spin symmetry in one of the arms or in both allows a crossover to resonances of lower symmetries: SU(3) or SU(2). We also show the possibility of gate control of spin filtering in EDTD with polarized electrodes.

Spin-orbital and spin Kondo effects in parallel coupled quantum dots

Strong electron correlations and interference effects are discussed in parallel-coupled single-level and orbitally doubly degenerate quantum dots. The finite-U mean-field slave boson approach is used to study many-body effects. The analysis is carried out in a wide range of parameter space including both atomic-like and molecular-like Kondo regimes and taking into account various perturbations, like interdot tunneling, interdot interaction, mixing of the electrode channels and exchange interaction. We also discuss the influence of singularities of electronic structure and the impact of polarization of electrodes. Special attention is paid to potential spintronic applications of these systems showing how current polarization can be controlled by adjusting interference conditions and correlations by gate voltage. Simple proposals of double dot spin valve and bipolar electrically tunable spin filter are presented.

Mott transitions in a three-component Falicov-Kimball model: A slave boson mean-field study

Metal-insulator transitions in a three-component Falicov-Kimball model are investigated within the Kotliar-Ruckenstein slave boson mean-field approach. The model describes a mixture of two interacting fermion atom species loaded into an optical lattice at ultralow temperature. One species is two-component atoms, which can hop in the optical lattice, and the other is single-component atoms, which are localized. Different correlation-driven metal-insulator transitions are observed depending on the atom filling conditions and local interactions. These metal-insulator transitions are classified by the band renormalization factors and the double occupancies of the atom species. The filling conditions and the critical value of the local interactions for these metal-insulator transitions are also analytically established. The obtained results not only are in good agreement with the dynamical mean-field theory for the three-component Falicov-Kimball model but also clarify the nature and properties of the metal-insulator transitions in a simple physics picture.

Mott transition in the dynamic Hubbard model within slave boson mean-field approach

At zero temperature, the Kotliar–Ruckenstein slave boson mean-field approach is applied to the dynamic Hubbard model. In this paper, the influences of the dynamics of the auxiliary boson field on the Mott transition are investigated. At finite boson frequency, the Mott-type features of the Hubbard model is found to be enhanced by increasing the pseudospin coupling parameter g. For sufficiently large pseudospin coupling g, the Mott transition occurs even for modest values of the bare Hubbard interaction U. The lack of electron–hole symmetry is highlighted through the quasiparticle weight. Our results are in good agreement with the ones obtained by two-site dynamical mean-field theory and determinant quantum Monte Carlo simulation.

Kondo effect of a cobalt adatom on a zigzag graphene nanoribbon

Based on ab-initio calculations we discuss Kondo effect due to Co adatom on graphene zigzag nanoribbon. Co atom located at hollow site behaves as spin S = 1/2 impurity with dxz and dyz orbitals contributing to magnetic moment. Dynamical correlations are analyzed with the use of complementary approximations: mean field slave boson approach, noncrossing approximation and equation of motion method. The impact of interplay between spin and orbital degrees of freedom together with the effect of peculiarities of electronic and magnetic structure of nanoribbon on many-body resonances is examined.

Band filling and correlation controlling electronic properties and magnetism in KxFe2−ySe2: a slave boson study

We investigate the electronic and magnetic properties of KxFe2−ySe2 materials at different band fillings utilizing the multi-orbital Kotliar–Ruckenstein slave boson mean-field approach. We find that the ground state of KFe2Se2 is a paramagnetic (PM) bad metal with intermediate correlation, in contrast with the previous antiferromagnetic (AFM) results obtained by the local density approximation. Our PM metallic ground state suggests that KFe2Se2 is the parent phase of superconducting KxFe2−ySe2, supporting a recent scanning tunneling spectroscopy experiment. For pure Fe2+-based systems, the ground state is a striped AFM (SAFM) metal with a spin density wave gap partially opened near the Fermi level. In comparison, for Fe3+-based compounds, besides SAFM, a Néel AFM metal without orbital ordering is observed, and an orbital selective Mott phase (OSMP) accompanied by an intermediate-spin to high-spin transition is also found, giving a possible scenario of an OSMP in KxFe2−ySe2. These results demonstrate that the band filling and correlation control the Fermi surface topology, electronic state and magnetism in KxFe2−ySe2.

Heavy Antiferromagnetic Phases in Kondo Lattices

We propose a microscopic physical mechanism that stabilizes the coexistence of the Kondo effect and antiferromagnetism in heavy-fermion systems. We consider a two-dimensional quantum Kondo-Heisenberg lattice model and show that long-range electron hopping leads to a robust antiferromagnetic Kondo state. By using a modified slave-boson mean-field approach we analyze the stability of the heavy antiferromagnetic phase across a range of parameters, and discuss transitions between different phases. Our results may be used to guide future experiments on heavy fermion compounds.

Doping the Kane-Mele-Hubbard model: A slave-boson approach

We study the Kane-Mele-Hubbard model both at half-filling and away from half-filling using a slave-boson mean-field approach at zero temperature. We obtain a phase diagram at half-filling and discuss its connection to recent results from quantum Monte Carlo, cellular dynamical mean field, slave-rotor, and ${Z}_{2}$ mean-field studies. In particular, we find a small window in parameter space where a spin-liquid phase with gapped spin and charge excitations reside. Upon doping, we show the spin-liquid state becomes a superconducting state by explicitly calculating the singlet pairing order parameters. Interestingly, we find an ``optimal'' doping for such superconductivity. Our work reveals some of the phenomenology associated with doping an interacting system with strong spin-orbit coupling and intermediate strength electron-electron interactions.

Theoretical Investigation of the Four-Layered Self-Doped High-TcSuperconductors: Evidence of the Pair Tunneling Effect

Based on a four-layered self-doped t-J type model and the slave-boson mean-field approach, we study theoretically the superconductivity in the electron-doped and hole-doped layers. The neighbor layers are coupled through both the single electron interlayer hopping and pair tunneling effect. The superconducting gap magnitude for the electron-doped band is nearly twice that of the hole-doped one, which contrasts with our previous understanding of the electron-hole asymmetry in high-T(c) superconductors but is consistent with recent angle-resolved photoemission spectroscopy experiments in four-layered materials Ba2Ca3Cu4O8F2. Our results propose that the pair tunneling effect is important to examine the multilayered superconducting materials.

Kondo-Dicke Resonances in Electronic Transport Through Double Quantum Dots

Electronic transport through a system of two quantum dots coupled to external leads is considered theoretically. The dots are assumed to be in the Kondo regime, and the infinite-U mean-field slave-boson approach is used to obtain basic transport characteristics. Density of states and electron transmission probability in the Kondo regime are considered and calculated numerically. Both quantities are shown to exhibit features that are typical of the Dicke resonance.

Charge-order in the quasi-cubic manganites

The CE phase of La1−xCaxMnO3 displays long-range magnetic, charge and orbital order. The magnetic order of the Mn spins arises from the competition of the superexchange and double-exchange interactions. The checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the eg orbitals to the lattice. Using a mean-field slave-boson approach for the eg electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling, we obtain the phase diagram including the A, B, C, CE and G phases.

Slave-boson mean-field study of the dimensional crossover in Sr2RuO4

The anomalous temperature dependence of the c-axis resistivity of Sr2RuO4 is analyzed under the viewpoint of a dimensional crossover which occurs from a lattice of perpendicular one-dimensional chains, belonging to different Ru-4d-orbitals, to a two-dimensional system induced by weak inter-orbital hybridization. This system is described by a two-orbital Hubbard model which is treated within a slave-boson mean-field approach to take correlation effects, such as the spin-charge separation, into account. We obtain the emergence of quasiparticle states at low temperature, a result which can be used to discuss the evolution of the spectral density and the c-axis transport within a tunneling approach. For the latter, a crossover between the low- and high-temperature regime is found in qualitative agreement with experimental observations.

Spin and charge order in the CE phase ofLa1−xCaxMnO3

The CE phase of ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ is stable for $x\ensuremath{\gtrsim}0.5$ and displays long-range magnetic, charge, and orbital orders. The magnetic order of the Mn spins arises from the competition of the superexchange and double-exchange interactions and the checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the ${e}_{g}$ orbitals to the lattice. Using a mean-field slave-boson approach for the ${e}_{g}$ electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling between the ${e}_{g}$ and ${t}_{2g}$ states, we obtain the tight-binding band structure of the CE phase. The unit cell of the CE phase consists of 16 sites. The 32 ${e}_{g}$ bands in the Brillouin zone are grouped into two sets of 16 bands separated by a charge order gap. The phase diagram including the A, B, C, CE, and G phases reproduces the phase sequence observed in ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$.