J. Barnaś

Large enhancement of thermoelectric effects in a double quantum dot system due to interference and Coulomb correlation phenomena

Thermoelectric effects in a double quantum dot system coupled to external magnetic/nonmagnetic leads are investigated theoretically. The basic thermoelectric transport characteristics, like thermopower, electronic contribution to heat conductance, and the corresponding figure of merit, have been calculated in terms of the linear response theory and Green function formalism in the Hartree-Fock approximation for Coulomb interactions. An enhancement of the thermal efficiency (figure of merit ZT) due to Coulomb blockade has been found. The magnitude of ZT is further considerably enhanced by quantum interference effects. Both the Coulomb correlations and interference effects lead to strong violation of the Wiedemann-Franz law. The influence of spin-dependent transport and spin bias on the thermoelectric effects (especially on Seebeck and spin Seebeck effects) is also analyzed.

Kondo effect in the presence of itinerant-electron ferromagnetism studied with the numerical renormalization group method.

The Kondo effect in quantum dots (QDs) - artificial magnetic impurities - attached to ferromagnetic leads is studied with the numerical renormalization group (NRG) method. It is shown that the QD level is spin-split due to presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally-screened state, describable by Fermi liquid theory and a generalized Friedel sum rule, and transport in the unitary limit is not spin dependent.

Thermoelectric effects in silicene nanoribbons

Transport and thermoelectric coefficients (including also spin thermopower) of silicene nanoribbons with zigzag edges are investigated by {\it ab-initio} numerical methods. Local spin density of such nanoribbons reveals edge magnetism. Like in graphene, one finds antiferromagnetic and ferromagnetic ordering, with spin polarization at one edge antiparallel or parallel to that at the other edge, respectively. Thermoelectric properties, especially the Seebeck coefficient, significantly depend on the electronic band structure and are enhanced when the Fermi level is in the energy gap. However, these thermoelectric properties are significantly reduced when the phonon contribution to the heat conductance is included. This phonon contribution has been calculated numerically by two different methods. Transition from antiferromagnetic to ferromagnetic states leads to a large magnetoresistance as well as to a considerable magnetothermopower. Thermoelectric parameters in the antiparallel configuration, when spin polarization in the left part of the nanoribbon is opposite to that in the right part, are also analyzed.

Magnetic switching of a single molecular magnet due to spin-polarized current

Magnetic switching of a single molecular magnet (SMM) due to spin-polarized current flowing between ferromagnetic metallic leads (electrodes) is investigated theoretically. Magnetic moments of the leads are assumed to be collinear and parallel to the magnetic easy axis of the molecule. Electrons tunneling through the barrier between magnetic leads are coupled to the SMM via exchange interaction. The current flowing through the system, as well as the spin relaxation times of the SMM, are calculated from the Fermi golden rule. It is shown that spin of the SMM can be reversed by applying a certain voltage between the two magnetic electrodes. Moreover, the switching may be visible in the corresponding current-voltage characteristics.

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.

Spin polarized transport through a single-molecule magnet : Current-induced magnetic switching

Magnetic switching of a single molecule magnet (SMM) due to spin polarized current is investigated theoretically. The charge transfer between the electrodes takes place via the lowest unoccupied molecular orbital (LUMO) of the SMM. Generally, the double occupancy of the LUMO level, and hence a finite on-site Coulomb repulsion, is allowed. Owing to the exchange interaction between electrons in the LUMO level and the SMMs spin, the latter can be reversed. The perturbation approach (Fermi golden rule) is applied to calculate current-voltage characteristics. The influence of Coulomb interactions on the switching process is also analyzed.

Tunnel magnetoresistance of quantum dots coupled to ferromagnetic leads in the sequential and cotunneling regimes

We study electronic transport through quantum dots weakly coupled to ferromagnetic leads with collinear magnetization directions. Tunneling contributions of first and second order in the tunnel-coupling strength are taken into account. We analyze the tunnel magnetoresistance (TMR) for all combinations of linear and nonlinear response, at or off resonance, with an even or odd dot-electron number. Different mechanisms for transport and spin accumulation of the various regimes give rise to different TMR behavior.

Spin effects in single-electron tunnelling

An important consequence of the discovery of giant magnetoresistance in metallic magnetic multilayers is a broad interest in spin-dependent effects in electronic transport through magnetic nanostructures. An example of such systems are tunnel junctions—single-barrier planar junctions or more complex ones. In this review we present and discuss recent theoretical results on electron and spin transport through ferromagnetic mesoscopic junctions including two or more barriers. Such systems are also called ferromagnetic single-electron transistors. We start from the situation when the central part of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle. Transport characteristics then reveal single-electron charging effects, including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations. Single-electron ferromagnetic transistors based on semiconductor quantum dots and large molecules (especially carbon nanotubes) are also considered. The main emphasis is placed on the spin effects due to spin-dependent tunnelling through the barriers, which gives rise to spin accumulation and tunnel magnetoresistance. Spin effects also occur in the current–voltage characteristics, (differential) conductance, shot noise, and others. Transport characteristics in the two limiting situations of weak and strong coupling are of particular interest. In the former case we distinguish between the sequential tunnelling and cotunnelling regimes. In the strong coupling regime we concentrate on the Kondo phenomenon, which in the case of transport through quantum dots or molecules leads to an enhanced conductance and to a pronounced zero-bias Kondo peak in the differential conductance.

Spin effects in ferromagnetic single-electron transistors

Electron tunneling in ferromagnetic single-electron transistors is considered theoretically in the sequential tunneling regime. A formalism is developed, which operates in a two-dimensional space of states, instead of one-dimensional space used in the spinless case. It is shown that spin fluctuations can be significantly larger than the charge fluctuations. The influence of discrete energy spectrum of a small central electrode on tunneling current, charge and spin accumulation, charge and spin fluctuations, and on tunnel magnetoresistance is analyzed in detail. Two different scales are found in the bias dependence of the basic transport characteristics; the shorter one originates from the discrete energy spectrum and the longer one from discrete charging of the central electrode. The features due to discrete spectrum and discrete charging disappear at high temperatures.

Kondo Effect in Quantum Dots Coupled to Ferromagnetic Leads

Abstract The Kondo effects in transport through a quantum dot (QD) coupled to ferromagnetic leads are shown to be modified by the spin polarization of the electrodes. For parallel alignment of the magnetization of the leads the zero-bias anomaly in the differential conductance is split even in the absence of an external magnetic field. For antiparallel alignment the peaks are split only in the presence of a magnetic filed, but show a characteristic asymmetry.