R. Świrkowicz

Spin effects in electron tunneling through a quantum dot coupled to noncollinearly polarized ferromagnetic leads

Spin-dependent transport through an interacting single-level quantum dot coupled to ferromagnetic leads with noncollinear magnetizations is analyzed theoretically. The transport properties and average spin of the dot are investigated within the nonequilibrium Green function technique based on the equation of motion in the Hartree-Fock approximation. Numerical results show that Coulomb correlations on the dot and strong spin polarization of the leads significantly enhance precession of the average dot spin around the effective molecular field created by the external electrodes. Moreover, they also show that spin precession may lead to negative differential conductance in the voltage range between the two relevant threshold voltages. Nonmonotonous angular variation of electric current and change in sign of the tunnel magnetoresistance are also found. It is also shown that the diodelike behavior in asymmetrical junctions with one electrode being half-metallic is significantly reduced in noncollinear configurations.

The Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations: effects due to electron–phonon coupling

We study the Kondo effect in a quantum dot coupled to ferromagnetic leads and analyze its properties as a function of the spin polarization of the leads. Based on a scaling approach, we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation of motion technique, we study nonlinear transport through the dot. For parallel alignment, the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.Spin-polarized transport through a quantum dot strongly coupled to ferromagnetic electrodes with noncollinear magnetic moments is analysed theoretically in terms of the nonequilibrium Green function formalism based on the equation of motion method. Electrons in the dot are assumed to be coupled to a phonon bath. The influence of electron–phonon coupling on tunnelling current, linear and nonlinear conductance, and tunnel magnetoresistance is studied in detail. Variation of the main Kondo peaks and phonon satellites with the angle between the magnetic moments of the leads is analysed.

Spin-polarized transport through a single-level quantum dot in the Kondo regime

Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter U. Exchange interaction between the dot and ferromagnetic electrodes is taken into account via an effective molecular field. The following situations are analysed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization of electron states at the Fermi level, and (iii) one of the two electrodes is nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak in the density of states (DOS) becomes spin-split when the total exchange field acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads to splitting and suppression of the corresponding zero-bias anomaly in the differential conductance.

Interference effects and Fano resonance in transport across a two dot system in the Kondo regime

Transport across a double dot system of a special geometry with two channels accessible for tunneling electrons is theoretically studied in a region of low temperatures corresponding to the Kondo regime, and interference effects are analyzed. The spectral function and the linear conductance are calculated using the Green function formalism based on the equation of motion method. It is shown that interference processes strongly influence the spectral function. Moreover, due to interference effects the conductance, which is close to zero in a low energy region, shows at higher gate voltages a well defined Fano resonance. A development of the Fano peak as the system undergoes a gradual transition from the serial configuration to the geometry with two dots partially connected to both electrodes is demonstrated. The position and intensity of the Fano resonance strongly depend on the inter-dot tunneling rate. The influence of temperature on the resonance is also discussed.

Transport through a quantum dot subject to spin and charge bias

Abstract Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron–phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.

Conductance of a double quantum dot system in the Kondo regime in the presence of inter-dot coupling and channel mixing effects

Electron transport through a parallel double quantum dot is theoretically studied in the Kondo regime with the use of the non-equilibrium Green function formalism based on the equation of motion method. An influence of inter-dot tunnel coupling t and channel mixing effects on the orbital (spinless) Kondo phenomenon is analysed in the linear and nonlinear transport regimes. Both effects lead to a considerable suppression of the conductance in the Kondo regime. In a system with dots capacitively coupled (t = 0) the differential conductance shows a zero-bias peak whose intensity diminishes gradually with mixing effects included. When tunnel coupling between dots is taken into account () the orbital Kondo resonance splits and the intensities of both components are strongly influenced by channel mixing. The linear conductance calculated as a function of a dot level position E0 is strongly suppressed in the Kondo regime but at higher values of E0 a relatively well-pronounced side peak appears whose intensity increases with the coupling rate t. We consider the side peak maximum as originated from interference processes in the system.