J. Martinek

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.

Theory of Transport through Quantum-Dot Spin Valves in the Weak-Coupling Regime

We develop a theory of electron transport through quantum dots that are weakly coupled to ferromagnetic leads. The theory covers both the linear and nonlinear transport regime, takes non-collinear magnetization of the leads into account, and allows for an externally-applied magnetic field. We derive generalized rate equations for the dots occupation and accumulated spin and discuss the influence of the dots spin on the transmission. A negative differential conductance and a nontrivial dependence of the conductance on the angle between the lead magnetizations is predicted.

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 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.

Regular atomic narrowing of Ni, Fe, and v nanowires resolved by two-dimensional correlation analysis

We present a novel statistical method for the study of stable atomic configurations in breaking nanowires based on the 2D cross-correlation analysis of conductance versus electrode separation traces. Applying this method, we can clearly resolve the typical evolutions of the conductance staircase in some transition metal nanojunctions (Ni, Fe, V) up to high conductance values. In these metals our analysis demonstrates a very well ordered atomic narrowing of the nanowire, indicating a very regular, stepwise decrease of the number of atoms in the minimal cross section of the junction, in contrast to the majority of the metals. All these features are hidden in traditional conductance histograms.

SHOT NOISE IN FERROMAGNETIC SINGLE-ELECTRON TUNNELING DEVICES

Frequency-dependent current noise in ferromagnetic double junctions with a Coulomb blockade is studied theoretically in the limit of sequential tunneling. Two different relaxation processes are found in the correlations between spin-polarized tunneling currents: low-frequency spin fluctuations and high-frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lead to a negative differential resistance. We also show that large spin noise activated in the range of negative differential resistance gives rise to a significant enhancement of the current noise.

Spin accumulation in ferromagnetic single-electron transistors in the cotunneling regime

We propose a method of direct detection of spin accumulation, which overcomes problems of previous measurement schemes. A spin-dependent current in a single-electron transistor with ferromagnetic electrodes leads to spin accumulation on the metallic island. The resulting spin splitting of the electrochemical potentials of the island, because of an additional shift by the charging energy, can be detected from the spacing between two resonances in the current-voltage characteristics. The results were obtained in the framework of a real-time diagrammatic approach which allows us to study higher-order (co)tunneling processes in the strong nonequlibrium situation.

Gate-controlled spin splitting in quantum dots with ferromagnetic leads in the Kondo regime

The effect of a gate voltage

Nonequilibrium Kondo effect in a quantum dot coupled to ferromagnetic leads

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