1,052 research outputs found
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
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
Ferromagnetism within the periodic Anderson model: A new approximation scheme
We introduce a new approach to the periodic Anderson model (PAM) that allows
a detailed investigation of the magnetic properties in the Kondo as well as the
intermediate valence regime. Our method is based on an exact mapping of the PAM
onto an effective medium strong-coupling Hubbard model. For the latter, the
so-called spectral density approach (SDA) is rather well motivated since it is
based on exact results in the strong coupling limit. Besides the T=0 phase
diagram, magnetization curves and Curie temperatures are presented and
discussed with help of temperature-dependent quasiparticle densities of state.
In the intermediate valence regime, the hybridization gap plays a major role in
determining the magnetic behaviour. Furthermore, our results indicate that
ferromagnetism in this parameter regime is not induced by an effective
spin-spin interaction between the localized levels mediated by conduction
electrons as it is the case in the Kondo regime. The magnetic ordering is
rather a single band effect within an effective f-band.Comment: 13 pages, 16 figures, Phys. Stat. Sol. in pres
Low density approach to the Kondo-lattice model
We propose a new approach to the (ferromagnetic) Kondo-lattice model in the
low density region, where the model is thought to give a reasonable frame work
for manganites with perovskite structure exhibiting the "colossal
magnetoresistance" -effect. Results for the temperature- dependent
quasiparticle density of states are presented. Typical features can be
interpreted in terms of elementary spin-exchange processes between itinerant
conduction electrons and localized moments. The approach is exact in the zero
bandwidth limit for all temperatures and at T=0 for arbitrary bandwidths,
fulfills exact high-energy expansions and reproduces correctly second order
perturbation theory in the exchange coupling.Comment: 11 pages, 7 figures, accepted by PR
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
Spin-polarized tunneling through a thin-film
The effect of spin-disorder scattering on perpendicular transport in a
magnetic monolayer is considered within the single-site Coherent Potential
Approximation (CPA). The exchange interaction between a conduction electron and
localized moment of the magnetic ion is treated with the use of the
model. Electron-spin polarization is evaluated in the tunnel current which
comes from the different densities of spin-up, spin-down conduction electrons
at the Fermi level in a ferromagnetic semiconductor (EuS). Calculated results
are compared with some tunneling experiments.Comment: 5 pages, LaTex, 2 EPS figure
Spin-polarized tunneling currents through a ferromagnetic insulator between two metallic or superconducting leads
Using the Keldysh formalism the tunneling current through a hybrid structure
where a confined magnetic insulator (I) is sandwiched between two non-magnetic
leads is calculated. The leads can be either normal metals (M) or
superconductors (S). Each region is modelled as a single band in tight-binding
approximation in order to understand the formation of the tunneling current as
clearly as possible. The tunneling process itself is simulated by a
hybridization between the lead and insulator conduction bands. The insulator is
assumed to have localized moments which can interact with the tunneling
electrons. This is described by the Kondo Lattice Model (KLM) and treated
within an interpolating self-energy approach. For the superconductor the
mean-field BCS theory is used. The spin polarization of the current shows a
strong dependence both on the applied voltage and the properties of the
materials. Even for this idealized three band model there is a qualitative
agreement with experiment.Comment: 15 pages, 23 figures, accepted for publication in PR
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