Transport through two-level quantum dots weakly coupled to ferromagnetic leads

Spin-dependent transport through a two-level quantum dot in the sequential tunneling regime is analyzed theoretically by means of a real-time diagrammatic technique. It is shown that the current, tunnel magnetoresistance, and shot noise (Fano factor) strongly depend on the transport regime, providing a detailed information on the electronic structure of quantum dots and their coupling to external leads. When the dot is asymmetrically coupled to the leads, a negative differential conductance may occur in certain bias regions, which is associated with a super-Poissonian shot noise. In the case of a quantum dot coupled to one half-metallic and one nonmagnetic lead, one finds characteristic Pauli spin blockade effects. Transport may be also suppressed when the dot levels are coupled to the leads with different coupling strengths. The influence of an external magnetic field on transport properties is also discussed.Comment: 12 pages, 8 figure

Splitting efficiency and interference effects in a Cooper pair splitter based on a triple quantum dot with ferromagnetic contacts

We theoretically study the spin-resolved subgap transport properties of a Cooper pair splitter based on a triple quantum dot attached to superconducting and ferromagnetic leads. Using the Keldysh Green's function formalism, we analyze the dependence of the Andreev conductance, Cooper pair splitting efficiency, and tunnel magnetoresistance on the gate and bias voltages applied to the system. We show that the system's transport properties are strongly affected by spin dependence of tunneling processes and quantum interference between different local and nonlocal Andreev reflections. We also study the effects of finite hopping between the side quantum dots on the Andreev current. This allows for identifying the optimal conditions for enhancing the Cooper pair splitting efficiency of the device. We find that the splitting efficiency exhibits a nonmonotonic dependence on the degree of spin polarization of the leads and the magnitude and type of hopping between the dots. An almost perfect splitting efficiency is predicted in the nonlinear response regime when the energies of the side quantum dots are tuned to the energies of the corresponding Andreev bound states. In addition, we analyzed features of the tunnel magnetoresistance (TMR) for a wide range of the gate and bias voltages, as well as for different model parameters, finding the corresponding sign changes of the TMR in certain transport regimes. The mechanisms leading to these effects are thoroughly discussed

Spin-resolved dynamical conductance of correlated large-spin magnetic molecules

The finite-frequency transport properties of a large-spin molecule attached to ferromagnetic contacts are studied theoretically in the Kondo regime. The focus is on the behavior of the dynamical conductance in the linear response regime, which is determined by using the numerical renormalization group method. It is shown that the dynamical conductance depends greatly on the magnetic configuration of the device and intrinsic parameters of the molecule. In particular, conductance exhibits characteristic features for frequencies corresponding to the dipolar and quadrupolar exchange fields resulting from the presence of spin-dependent tunneling. Moreover, a dynamical spin accumulation in the molecule, associated with the off-diagonal-in-spin component of the conductance, is predicted. This spin accumulation becomes enhanced with increasing the spin polarization of the leads, and it results in a nonmonotonic dependence of the conductance on frequency, with local maxima occurring for characteristic energy scales

Manipulating spins of magnetic molecules: Hysteretic behavior with respect to bias voltage

Formation of a magnetic hysteresis loop with respect to a bias voltage is investigated theoretically in a spin-valve device based on a single magnetic molecule. We consider a device consisting of two ferromagnetic electrodes bridged by a carbon nanotube, acting as a quantum dot, to which a spin-anisotropic molecule is exchange coupled. Such a coupling allows for transfer of angular momentum between the molecule and a spin current flowing through the dot, and thus, for switching orientation of the molecular spin. We demonstrate that this current-induced switching process exhibits a hysteretic behavior with respect to a bias voltage applied to the device. The analysis is carried out with the use of the real-time diagrammatic technique in the lowest-order expansion of the tunnel coupling of the dot to electrodes. The influence of both the intrinsic properties of the spin-valve device (the spin polarization of electrodes and the coupling strength of the molecule to the dot) and those of the molecule itself (magnetic anisotropy and spin relaxation) on the size of the magnetic hysteresis loop is discussed