867 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
Spin-filter effect of the europium chalcogenides: An exactly solved many-body model
A model Hamiltonian is introduced which considers the main features of the
experimental spin filter situation as s-f interaction, planar geometry and the
strong external electric field. The proposed many-body model can be solved
analytically and exactly using Green functions.
The spin polarization of the field-emitted electrons is expressed in terms of
spin-flip probabilities, which on their part are put down to the exactly known
dynamic quantities of the system.
The calculated electron spin polarization shows remarkable dependencies on
the electron velocity perpendicular to the emitting plane and the strength of
s-f coupling. Experimentally observed polarization values of about 90% are well
understood within the framework of the proposed model.Comment: accepted (Physical Review B); 10 pages, 11 figures;
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Magnetic phase diagram of the Kondo lattice model with quantum localized spins
The magnetic phase diagram of the ferromagnetic Kondo lattice model is
determined at T=0 in 1D, 2D, and 3D for various magnitudes of the quantum
mechanical localized spins ranging from S=1/2 to classical spins. We consider
the ferromagnetic phase, the paramagnetic phase, and the
ferromagnetic/antiferromagnetic phase separated regime. There is no significant
influence of the spin quantum number on the phase boundaries except for the
case S=1/2, where the model exhibits an instability of the ferromagnetic phase
with respect to spin disorder. Our results give support, at least as far as the
low temperature magnetic properties are concerned, to the classical treatment
of the S=3/2-spins in the intensively investigated manganites, for which the
ferromagnetic Kondo-lattice model is generally employed to account for
magnetism.Comment: 8 pages, 6 figure
Ferromagnetism in the Periodic Anderson Model: A Comparison of Spectral Density Approximation (SDA), Modified Alloy Analogy (MAA) and Modified Perturbation Theory (MPT)
We compare different approximation schemes for investigating ferromagnetism
in the periodic Anderson model. The use of several approximations allows for a
detailed analysis of the implications of the respective methods, and also of
the mechanisms driving the ferromagnetic transition. For the Kondo limit, our
results confirm a previously proposed mechanism leading to ferromagnetic order,
namely an RKKY exchange mediated via the formation of Kondo screening clouds in
the conduction band. The contrary case is found in the intermediate-valence
regime. Here, the bandshift correction ensuring a correct high-energy expansion
of the self-energy is essential. Inclusion of damping effects reduces stability
of the ferromagnetic phase.Comment: 11 pages, 7 figures, accepted by Eur. Phys. J.
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