992 research outputs found
The ground state phase diagram of the diluted ferromagnetic Kondo-lattice model
We investigate the existence of several (anti-)ferromagnetic phases in the
diluted ferromagnetic Kondo-lattice model, i.e. ferromagnetic coupling of local
moment and electron spin. To do this we use a coherent potential approximation
(CPA) with a dynamical alloy analogy. For the CPA we need effective potentials,
which we get first from a mean-field approximation. To improve this treatment
we use in the next step a more appropriate moment conserving decoupling
approach and compare both methods. The different magnetic phases are modelled
by defining two magnetic sublattices. As a result we present zero-temperature
phase diagrams according to the important model parameters and different
dilutions.Comment: accepted for publication in Journal of Physics: Condensed Matte
Ferromagnetism and non-local correlations in the Hubbard model
We study the possibility and stability of band-ferromagnetism in the
single-band Hubbard model for the simple cubic (SC) lattice. A non-local
self-energy is derived within a modified perturbation theory. Results for the
spectral density and quasiparticle density of states are shown with special
attention to the effects of k-dependence. The importance of non-local
correlations for the fulfillment of the Mermin-Wagner theorem is our main
result. A phase digram showing regions of ferromagnetic order is calculated for
the three dimensional lattice. Besides, we show results for the optical
conductivity and prove that already the renormalized one-loop contribution to
the conductivity cancels the Drude peak exactly in case of a local self-energy
which is not anymore true for a non-local self-energy.Comment: 11 pages, 10 figures, accepted for publication in PR
On the origin of temperature dependence of interlayer exchange coupling in metallic trilayers
We study the influence of collective magnetic excitations on the interlayer
exchange coupling (IEC) in metallic multilayers. The results are compared to
other models that explain the temperature dependence of the IEC by mechanisms
within the spacer or at the interfaces of the multilayers. As a main result we
find that the reduction of the IEC with temperature shows practically the same
functional dendence in all models. On the other hand the influence of the
spacer thickness, the magnetic material, and an external field are quite
different. Based on these considerations we propose experiments, that are able
to determine the dominating mechanism that reduces the IEC at finite
temperatures.Comment: 8 pages, 7 figures, accepted for PR
Electronic properties of EuB6 in the ferromagnetic regime: Half-metal versus semiconductor
To understand the halfmetallic ferromagnet EuB6 we use the Kondo lattice
model for valence and conduction band. By means of a recently developed
many-body theory we calculate the electronic properties in the ferromagnetic
regime up to the Curie temperature. The decreasing magnetic order induces a
transition from halfmetallic to semiconducting behavior along with a band
broadening. We show the temperature dependence of the quasiparticle density of
states and the quasiparticle dispersion as well as the effective mass, the
number of carriers and the plasma frequency which are in good agreement with
the experimental data
Exact results on the Kondo-lattice magnetic polaron
In this work we revise the theory of one electron in a ferromagnetically
saturated local moment system interacting via a Kondo-like exchange
interaction. The complete eigenstates for the finite lattice are derived. It is
then shown, that parts of these states lose their norm in the limit of an
infinite lattice. The correct (scattering) eigenstates are calculated in this
limit. The time-dependent Schr\"odinger equation is solved for arbitrary
initial conditions and the connection to the down-electron Green's function and
the scattering states is worked out. A detailed analysis of the down-electron
decay dynamics is given.Comment: 13 pages, 9 figures, accepted for publication in PR
Quantum effects in the quasiparticle structure of the ferromagnetic Kondo lattice model
A new ``Dynamical Mean-field theory'' based approach for the Kondo lattice
model with quantum spins is introduced. The inspection of exactly solvable
limiting cases and several known approximation methods, namely the second-order
perturbation theory, the self-consistent CPA and finally a moment-conserving
decoupling of the equations of motion help in evaluating the new approach. This
comprehensive investigation gives some certainty to our results: Whereas our
method is somewhat limited in the investigation of the J<0-model, the results
for J>0 reveal important aspects of the physics of the model: The energetically
lowest states are not completely spin-polarized.A band splitting, which occurs
already for relatively low interaction strengths, can be related to distinct
elementary excitations, namely magnon emission (absorption) and the formation
of magnetic polarons. We demonstrate the properties of the ferromagnetic Kondo
lattice model in terms of spectral densities and quasiparticle densities of
states.Comment: 19 pages, 4 figure
Ferromagnetic Kondo-Lattice Model
We present a many-body approach to the electronic and magnetic properties of
the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The
coupling between itinerant conduction electrons and localized magnetic moments
leads, on the one hand, to a distinct temperature-dependence of the electronic
quasiparticle spectrum and, on the other hand, to magnetic properties, as
e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly
influenced by the band electron selfenergy and therewith in particular by the
carrier density. We present results for the single-band Kondo-lattice model in
terms of quasiparticle densities of states and quasiparticle band structures
and demonstrate the density-dependence of the self-consistently derived Curie
temperature. The transition from weak-coupling (RKKY) to strong-coupling
(double exchange) behaviour is worked out.
The multiband model is combined with a tight-binding-LMTO bandstructure
calculation to describe real magnetic materials. As an example we present
results for the archetypal ferromagnetic local-moment systems EuO and EuS. The
proposed method avoids the double counting of relevant interactions and takes
into account the correct symmetry of atomic orbitals.Comment: 15 pages, 10 figure
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