arXiv subjects

Jan Martinek

Publications and source records attributed to Jan Martinek.

26 records · Page 2Linked to original sources

arXiv2005

Nonequilibrium Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads

We study the Kondo effect in the electron transport through a quantum dot coupled to ferromagnetic leads, using a real-time diagrammatic technique which provides a systematic description of the nonequilibrium dynamics of a system with strong local electron correlations. We evaluate the theory in an extension of the `resonant tunneling approximation', introduced earlier, by introducing the self-energy of the off-diagonal component of the reduced propagator in spin space. In this way we develop a charge and spin conserving approximation that accounts not only for Kondo correlations but also for the spin splitting and spin accumulation out of equilibrium. We show that the Kondo resonances, split by the applied bias voltage, may be spin polarized. A left-right asymmetry in the coupling strength and/or spin polarization of the electrodes significantly affects both the spin accumulation and the weight of the split Kondo resonances out of equilibrium. The effects are observable in the nonlinear differential conductance. We also discuss the influence of decoherence on the Kondo resonance in the frame of the real-time formulation.

cond-mat.mes-hall↗

arXiv2004

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 dot's occupation and accumulated spin and discuss the influence of the dot's spin on the transmission. A negative differential conductance and a nontrivial dependence of the conductance on the angle between the lead magnetizations is predicted.

cond-mat.mes-hall↗

arXiv2004

Spin current through a tunnel junction

We derive an expression for the spin-current through a tunnel barrier in terms of many-body Green's functions. The spin current has two contributions. One can be associated with angular-momentum transfer by spin-polarized charge currents crossing the junction. If there are magnetic moments on both sides of the tunnel junction, due to spin accumulation or ferromagnetic ordering, then there is a second contribution related to the exchange coupling between the moments.

cond-mat.mes-hall↗

arXiv2004

Quantum Dots Attached to Ferromagnetic Leads: Exchange Field, Spin Precession, and Kondo Effect

Spintronics devices rely on spin-dependent transport behavior evoked by the presence of spin-polarized electrons. Transport through nanostructures, on the other hand, is dominated by strong Coulomb interaction. We study a model system in the intersection of both fields, a quantum dot attached to ferromagnetic leads. The combination of spin-polarization in the leads and strong Coulomb interaction in the quantum dot gives rise to an exchange field acting on electron spins in the dot. Depending on the parameter regime, this exchange field is visible in the transport either via a precession of an accumulated dot spin or via an induced level splitting. We review the situation for various transport regimes, and discuss two of them in more detail.

cond-mat.mes-hall↗

arXiv2003

Indirect Exchange Interaction between two Quantum Dots in an Aharonov-Bohm Ring

We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two spins located at two quantum dots embedded in an Aharonov-Bohm (AB) ring. In such a system the RKKY interaction, which oscillates as a function of the distance between two local spins, is affected by the flux. For the case of the ferromagnetic RKKY interaction, we find that the amplitude of AB oscillations is enhanced by the Kondo correlations and an additional maximum appears at half flux, where the interaction is switched off. For the case of the antiferromagnetic RKKY interaction, we find that the phase of AB oscillations is shifted by pi, which is attributed to the formation of a singlet state between two spins for the flux value close to integer value of flux.

cond-mat.mes-hall↗

arXiv2003

Interaction-driven spin precession in quantum-dot spin valves

We analyze spin-dependent transport through spin valves composed of an interacting quantum dot coupled to two ferromagnetic leads. The spin on the quantum dot and the linear conductance as a function of the relative angle $θ$ of the leads' magnetization directions is derived to lowest order in the dot-lead coupling strength. Due to the applied bias voltage spin accumulates on the quantum dot, which for finite charging energy experiences a torque, resulting in spin precession. The latter leads to a non-trivial, interaction-dependent, $θ$-dependence of the conductance. In particular, we find that the spin-valve effect is reduced for all $θ\neq π$.

cond-mat.mes-hall↗

arXiv2002

Nonequilibrium spin fluctuations in single-electron transistors

We show that nonequilibrium spin fluctuations significantly influence the electronic transport in a single-electron transistor, when the spin relaxation on the island is slow compared to other relaxation processes, and when size effects play a role. To describe spin fluctuations we generalize the `orthodox' tunneling theory to take into account the electron spin, and show that the transition between consecutive charge states can occur via a high-spin state. This significantly modifies the shape of Coulomb steps and gives rise to additional resonances at low temperatures. Recently some of our predictions were confirmed by Fujisawa et al. [Phys. Rev. Lett. 88, 236802 (2002)], who demonstrated experimentally the importance of nonequilibrum spin fluctuations in transport through quantum dots.

cond-mat.mes-hall↗

arXiv2002

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

We propose a new 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 to study higher order (co-)tunneling processes in the strong nonequlibrium situation.

cond-mat.mes-hall↗