6,039 research outputs found
Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion
With a variational three--body calculation we study the role of the interplay
between the onsite Coulomb, Hund's rule, and superexchange interactions on the
spinwave excitation spectrum of itinerant ferromagnets. We show that
correlations between a Fermi sea electron--hole pair and a magnon result in a
very pronounced zone boundary softening and strong deviations from the
Heisenberg spinwave dispersion. We show that this spin dynamics depends
sensitively on the Coulomb and exchange interactions and discuss its possible
relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid
communication
Tunneling resonances in quantum dots: Coulomb interaction modifies the width
Single-electron tunneling through a zero-dimensional state in an asymmetric
double-barrier resonant-tunneling structure is studied. The broadening of steps
in the -- characteristics is found to strongly depend on the polarity of
the applied bias voltage. Based on a qualitative picture for the
finite-life-time broadening of the quantum dot states and a quantitative
comparison of the experimental data with a non-equilibrium transport theory, we
identify this polarity dependence as a clear signature of Coulomb interaction.Comment: 4 pages, 4 figure
Resonant Tunneling through Multi-Level and Double Quantum Dots
We study resonant tunneling through quantum-dot systems in the presence of
strong Coulomb repulsion and coupling to the metallic leads. Motivated by
recent experiments we concentrate on (i) a single dot with two energy levels
and (ii) a double dot with one level in each dot. Each level is twofold
spin-degenerate. Depending on the level spacing these systems are physical
realizations of different Kondo-type models. Using a real-time diagrammatic
formulation we evaluate the spectral density and the non-linear conductance.
The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure
Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors
We develop a theory of the magnetization dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. We describe the effects of
the strong carrier spin relaxation on the nonlinear optical response by using
the Lindblad semigroup method. We demonstrate magnetization control during
femtosecond timescales via the interplay between circularly polarized optical
excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin
interactions. Our results show a light-induced magnetization precession and
relaxation for the duration of the optical pulse.Comment: 4 pages, 2 figure
Zero-bias anomaly in cotunneling transport through quantum-dot spin valves
We predict a new zero-bias anomaly in the differential conductance through a
quantum dot coupled to two ferromagnetic leads with antiparallel magnetization.
The anomaly differs in origin and properties from other anomalies in transport
through quantum dots, such as the Kondo effect. It occurs in Coulomb-blockade
valleys with an unpaired dot electron. It is a consequence of the interplay of
single- and double-barrier cotunneling processes and their effect on the spin
accumulation in the dot. The anomaly becomes significantly modified when a
magnetic field is applied.Comment: 4 pages, 3 figure
Cotunneling through a quantum dot coupled to ferromagnetic leads with noncollinear magnetizations
Spin-dependent electronic transport through a quantum dot has been analyzed
theoretically in the cotunneling regime by means of the second-order
perturbation theory. The system is described by the impurity Anderson
Hamiltonian with arbitrary Coulomb correlation parameter . It is assumed
that the dot level is intrinsically spin-split due to an effective molecular
field exerted by a magnetic substrate. The dot is coupled to two ferromagnetic
leads whose magnetic moments are noncollinear. The angular dependence of
electric current, tunnel magnetoresistance, and differential conductance are
presented and discussed. The evolution of a cotunneling gap with the angle
between magnetic moments and with the splitting of the dot level is also
demonstrated.Comment: accepted for publication in Eur. Phys. J.
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