684 research outputs found
Hubbard Hamiltonian in the dimer representation. Large U limit
We formulate the Hubbard model for the simple cubic lattice in the
representation of interacting dimers applying the exact solution of the dimer
problem. By eliminating from the considerations unoccupied dimer energy levels
in the large U limit (it is the only assumption) we analytically derive the
Hubbard Hamiltonian for the dimer (analogous to the well-known t-J model), as
well as, the Hubbard Hamiltonian for the crystal as a whole by means of the
projection technique. Using this approach we can better visualize the
complexity of the model, so deeply hidden in its original form. The resulting
Hamiltonian is a mixture of many multiple ferromagnetic, antiferromagnetic and
more exotic interactions competing one with another. The interplay between
different competitive interactions has a decisive influence on the resulting
thermodynamic properties of the model, depending on temperature, model
parameters and assumed average number of electrons per lattice site. A
simplified form of the derived Hamiltonian can be obtained using additionally
Taylor expansion with respect to (t-hopping integral between
nearest neighbours, U-Coulomb repulsion). As an example, we present the
expansion including all terms proportional to t and to and we
reproduce the exact form of the Hubbard Hamiltonian in the limit . The nonperturbative approach, presented in this paper, can, in principle, be
applied to clusters of any size, as well as, to another types of model
Hamiltonians.Comment: 26 pages, 1 figure, LaTeX; added reference
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
The temperature dependent bandstructure of a ferromagnetic semiconductor film
The electronic quasiparticle spectrum of a ferromagnetic film is investigated
within the framework of the s-f model. Starting from the exact solvable case of
a single electron in an otherwise empty conduction band being exchange coupled
to a ferromagnetically saturated localized spin system we extend the theory to
finite temperatures. Our approach is a moment-conserving decoupling procedure
for suitable defined Green functions. The theory for finite temperatures
evolves continuously from the exact limiting case. The restriction to zero
conduction band occupation may be regarded as a proper model description for
ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a
simple cubic film cut parallel to the (100) crystal plane, we find some marked
correlation effects which depend on the spin of the test electron, on the
exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure
Temperature-dependent electronic structure and ferromagnetism in the d=oo Hubbard model studied by a modfied perturbation theory
The infinite-dimensional Hubbard model is studied by means of a modified
perturbation theory. The approach reduces to the iterative perturbation theory
for weak coupling. It is exact in the atomic limit and correctly reproduces the
dispersions and the weights of the Hubbard bands in the strong-coupling regime
for arbitrary fillings. Results are presented for the hyper-cubic and an
fcc-type lattice. For the latter we find ferromagnetic solutions. The
filling-dependent Curie temperature is compared with the results of a recent
Quantum Monte Carlo study.Comment: RevTeX, 5 pages, 6 eps figures included, Phys. Rev. B (in press),
Ref. 16 correcte
Metal-insulator transition in EuO
It is shown that the spectacular metal-insulator transition in Eu-rich EuO
can be simulated within an extended Kondo lattice model. The different orders
of magnitude of the jump in resistivity in dependence on the concentration of
oxygen vacancies as well as the low-temperature resistance minimum in
high-resistivity samples are reproduced quantitatively. The huge colossal
magnetoresistance (CMR) is calculated and discussed
Kondo-lattice model: Application to the temperature-dependent electronic structure of EuO(100) films
We present calculations for the temperature-dependent electronic structure
and magnetic properties of thin ferromagnetic EuO films. The treatment is based
on a combination of a multiband-Kondo lattice model with first-principles
TB-LMTO band structure calculations. The method avoids the problem of
double-counting of relevant interactions and takes into account the correct
symmetry of the atomic orbitals. We discuss the temperature-dependent
electronic structures of EuO(100) films in terms of quasiparticle densities of
states and quasiparticle band structures. The Curie temperature T_C of the EuO
films turns out to be strongly thickness-dependent, starting from a very low
value = 15K for the monolayer and reaching the bulk value at about 25 layers
Influence of Spin Wave Excitations on the Ferromagnetic Phase Diagram in the Hubbard-Model
The subject of the present paper is the theoretical description of collective
electronic excitations, i.e. spin waves, in the Hubbard-model. Starting with
the widely used Random-Phase-Approximation, which combines Hartree-Fock theory
with the summation of the two-particle ladder, we extend the theory to a more
sophisticated single particle approximation, namely the
Spectral-Density-Ansatz. Doing so we have to introduce a `screened`
Coulomb-interaction rather than the bare Hubbard-interaction in order to obtain
physically reasonable spinwave dispersions. The discussion following the
technical procedure shows that comparison of standard RPA with our new
approximation reduces the occurrence of a ferromagnetic phase further with
respect to the phase-diagrams delivered by the single particle theories.Comment: 8 pages, 9 figures, RevTex4, accepted for publication in Phys. Rev.
Element-resolved x-ray ferrimagnetic and ferromagnetic resonance spectroscopy
We report on the measurement of element-specific magnetic resonance spectra
at gigahertz frequencies using x-ray magnetic circular dichroism (XMCD). We
investigate the ferrimagnetic precession of Gd and Fe ions in Gd-substituted
Yttrium Iron Garnet, showing that the resonant field and linewidth of Gd
precisely coincide with Fe up to the nonlinear regime of parametric
excitations. The opposite sign of the Gd x-ray magnetic resonance signal with
respect to Fe is consistent with dynamic antiferromagnetic alignment of the two
ionic species. Further, we investigate a bilayer metal film,
NiFe(5 nm)/Ni(50 nm), where the coupled resonance modes of Ni and
NiFe are separately resolved, revealing shifts in the resonance
fields of individual layers but no mutual driving effects. Energy-dependent
dynamic XMCD measurements are introduced, combining x-ray absorption and
magnetic resonance spectroscopies.Comment: 16 pages, 8 figure
Ferromagnetism and Temperature-Driven Reorientation Transition in Thin Itinerant-Electron Films
The temperature-driven reorientation transition which, up to now, has been
studied by use of Heisenberg-type models only, is investigated within an
itinerant-electron model. We consider the Hubbard model for a thin fcc(100)
film together with the dipole interaction and a layer-dependent anisotropy
field. The isotropic part of the model is treated by use of a generalization of
the spectral-density approach to the film geometry. The magnetic properties of
the film are investigated as a function of temperature and film thickness and
are analyzed in detail with help of the spin- and layer-dependent quasiparticle
density of states. By calculating the temperature dependence of the
second-order anisotropy constants we find that both types of reorientation
transitions, from out-of-plane to in-plane (``Fe-type'') and from in-plane to
out-of-plane (``Ni-type'') magnetization are possible within our model. In the
latter case the inclusion of a positive volume anisotropy is vital. The
reorientation transition is mediated by a strong reduction of the surface
magnetization with respect to the inner layers as a function of temperature and
is found to depend significantly on the total band occupation.Comment: 10 pages, 8 figures included (eps), Phys Rev B in pres
The effects of a magnetic barrier and a nonmagnetic spacer in tunnel structures
The spin-polarized transport is investigated in a new type of magnetic tunnel
junction which consists of two ferromagnetic electrodes separated by a magnetic
barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method
and the nearly-free-electron-approximation the dependence of the tunnel
magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer
thickness and the applied bias voltage are studied theoretically. The TMR and
spin polarization show an oscillatory behavior as a function of the spacer
thickness and the bias voltage. The oscillations originate from the quantum
well states in the spacer, while the existence of the magnetic barrier gives
rise to a strong spin polarization and high values of the TMR. Our results may
be useful for the development of spin electronic devices based on coherent
transport.Comment: 15 pages, 5 figure
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