524 research outputs found
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
Interplay of the Kondo Effect and Spin-Polarized Transport in Magnetic Molecules, Adatoms and Quantum Dots
We study the interplay of the Kondo effect and spin-polarized tunneling in a
class of systems exhibiting uniaxial magnetic anisotropy, such as magnetic
molecules, magnetic adatoms, or quantum dots coupled to a single localized
magnetic moment. Using the numerical renormalization group method we calculate
the spectral functions and linear conductance in the Kondo regime. We show that
the exchange coupling between conducting electrons and localized magnetic core
generally leads to suppression of the Kondo effect. We also predict a
nontrivial dependence of the tunnel magnetoresistance on the strength of
exchange coupling and on the anisotropy constant.Comment: 4 pages with 4 EPS figures (version as accepted for publication in
Physical Review Letters
Phase diagram and excitations of a Shiba molecule
We analyze the phase diagram associated with a pair of magnetic impurities
trapped in a superconducting host. The natural interplay between Kondo
screening, superconductivity and exchange interactions leads to a rich array of
competing phases, whose transitions are characterized by discontinuous changes
of the total spin. Our analysis is based on a combination of numerical
renormalization group techniques as well as semi-classical analytics. In
addition to the expected screened and unscreened phases, we observe a new
molecular doublet phase where the impurity spins are only partially screened by
a single extended quasiparticle. Direct signatures of the various Shiba
molecule states can be observed via RF spectroscopy.Comment: 13 pages, 7 figure
Effects of different geometries on the conductance, shot noise and tunnel magnetoresistance of double quantum dots
The spin-polarized transport through a coherent strongly coupled double
quantum dot (DQD) system is analyzed theoretically in the sequential and
cotunneling regimes. Using the real-time diagrammatic technique, we analyze the
current, differential conductance, shot noise and tunnel magnetoresistance
(TMR) as a function of both the bias and gate voltages for double quantum dots
coupled in series, in parallel as well as for T-shaped systems. For DQDs
coupled in series, we find a strong dependence of the TMR on the number of
electrons occupying the double dot, and super-Poissonian shot noise in the
Coulomb blockade regime. In addition, for asymmetric DQDs, we analyze transport
in the Pauli spin blockade regime and explain the existence of the leakage
current in terms of cotunneling and spin-flip cotunneling-assisted sequential
tunneling. For DQDs coupled in parallel, we show that the transport
characteristics in the weak coupling regime are qualitatively similar to those
of DQDs coupled in series. On the other hand, in the case of T-shaped quantum
dots we predict a large super-Poissonian shot noise and TMR enhanced above the
Julliere value due to increased occupation of the decoupled quantum dot. We
also discuss the possibility of determining the geometry of the double dot from
transport characteristics. Furthermore, where possible, we compare our results
with existing experimental data on nonmagnetic systems and find qualitative
agreement.Comment: 15 pages, 12 figures, accepted in Phys. Rev.
Underscreened Kondo effect in S=1 magnetic quantum dots: Exchange, anisotropy and temperature effects
We present a theoretical analysis of the effects of uniaxial magnetic
anisotropy and contact-induced exchange field on the underscreened Kondo effect
in S=1 magnetic quantum dots coupled to ferromagnetic leads. First, by using
the second-order perturbation theory we show that the coupling to
spin-polarized electrode results in an effective exchange field
and an effective magnetic anisotropy . Second, we confirm these
findings by using the numerical renormalization group method, which is employed
to study the dependence of the quantum dot spectral functions, as well as
quantum dot spin, on various parameters of the system. We show that the
underscreened Kondo effect is generally suppressed due to the presence of
effective exchange field and can be restored by tuning the anisotropy constant,
when . The Kondo effect can also be restored by
sweeping an external magnetic field, and the restoration occurs twice in a
single sweep. From the distance between the restored Kondo resonances one can
extract the information about both the exchange field and the effective
anisotropy. Finally, we calculate the temperature dependence of linear
conductance for the parameters where the Kondo effect is restored and show that
the restored Kondo resonances display a universal scaling of Kondo
effect.Comment: 13 pages, 9 figures (version as accepted for publication in Physical
Review B
Spin-polarized transport through weakly coupled double quantum dots in the Coulomb-blockade regime
We analyze cotunneling transport through two quantum dots in series weakly
coupled to external ferromagnetic leads. In the Coulomb blockade regime the
electric current flows due to third-order tunneling, while the second-order
single-barrier processes have indirect impact on the current by changing the
occupation probabilities of the double dot system. We predict a zero-bias
maximum in the differential conductance, whose magnitude is conditioned by the
value of the inter-dot Coulomb interaction. This maximum is present in both
magnetic configurations of the system and results from asymmetry in cotunneling
through different virtual states. Furthermore, we show that tunnel
magnetoresistance exhibits a distinctively different behavior depending on
temperature, being rather independent of the value of inter-dot correlation.
Moreover, we find negative TMR in some range of the bias voltage.Comment: 9 pages, 7 figures, accepted in Phys. Rev.
The Influence of Magnetic Anisotropy on the Kondo Effect and Spin-Polarized Transport through Magnetic Molecules, Adatoms and Quantum Dots
Transport properties in the Kondo regime of a nanosystem displaying uniaxial
magnetic anisotropy (such as a magnetic molecule, magnetic adatom or quantum
dot coupled to a localized magnetic moment) are analyzed theoretically. In
particular, the influence of spin-polarized transport through a local orbital
of the system and exchange coupling of conduction electrons to the system's
magnetic core on the Kondo effect is discussed. The numerical renormalization
group method is applied to calculate the spectral functions and linear
conductance in the case of the parallel and antiparallel configurations of the
electrodes' magnetic moments. It is shown that both the magnetic anisotropy as
well as the exchange coupling between electrons tunneling through the
conducting orbital and magnetic core play an important role in formation of the
Kondo resonance, leading generally to its suppression. Specific transport
properties of such a system appear also as a nontrivial behavior of tunnel
magnetoresistance. It is also shown that the Kondo effect can be restored by an
external magnetic field in both the parallel and antiparallel magnetic
configurations.Comment: 14 pages with 10 EPS figures (version as accepted for publication in
Physical Review B
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