244 research outputs found
Spin-polarized scanning tunneling microscopy of half-metallic ferromagnets: Non-quasiparticle contributions
The role of the many-body (spin-polaronic) effects in the scanning tunneling
spectroscopy of half-metallic ferromagnets (HMF) is considered. It is shown
that the non-quasiparticle (NQP) states exist in the majority or minority spin
gap in the presence of arbitrary external potential and, in particular, at the
surfaces and interfaces. Energy dependence of the NQP density of states is
obtained in various models of HMF, an important role of the hybridization
nature of the energy gap being demonstrated. The corresponding temperature
dependence of spin polarization is calculated. It is shown that the NQP states
result in a sharp bias dependence of the tunneling conductance near zero bias.
Asymmetry of the NQP states with respect to the Fermi energy provides an
opportunity to separate phonon and magnon peaks in the inelastic spectroscopy
by STM.Comment: 13 pages, 6 figure
Electron states and magnetic phase diagrams of strongly correlated systems
Various auxiliary-particle approaches to treat electron correlations in
many-electron models are analyzed. Applications to copper-oxide layered systems
are discussed. The ground-state magnetic phase diagrams are considered within
the Hubbard and - exchange (Kondo) models for square and simple cubic
lattices vs. band filling and interaction parameter. A generalized Hartree-Fock
approximation is employed to treat commensurate ferro-, antiferromagnetic, and
incommensurate (spiral) magnetic phases, and also magnetic phase separation.
The correlations are taken into account within the Hubbard model by using the
slave-boson approach. The main advantage of this approach is correct estimating
the contribution of doubly occupied states number and therefore the
paramagnetic phase energy.Comment: Physics of Metals and Metallography, special issue, 4 page
Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics
A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally.Comment: 16 pages, RevTeX, 4 Postscript figures. Extended versio
Exciton effects in a scaling theory of intermediate valence and Kondo systems
An interplay of the Kondo scattering and exciton effects (d-f Coulomb
interaction) in the intermediate valence systems and Kondo lattices is
demonstrated to lead to an essential change of the scaling behavior in
comparison with the standard Anderson model. In particular, a marginal regime
can occur where characteristic fluctuation rate is proportional to flow cutoff
parameter. In this regime the "Kondo temperature" itself is strongly
temperature dependent which may give a key to the interpretation of
controversial experimental data for heavy fermion and related systems.Comment: 4 pages, 4 figure
Scaling picture of magnetism formation in the anomalous f-systems: interplay of the Kondo effect and spin dynamics
Formation of magnetically ordered state in the Kondo lattices is treated
within the degenerate exchange and Coqblin-Schrieffer models. The Kondo
renormalizations of the effective coupling parameter, magnetic moment and spin
excitation frequencies are calculated within perturbation theory. The results
of one-loop scaling consideration of the magnetic state in Kondo lattices are
analyzed. The dependence of the critical values of the bare model parameters on
the type of the magnetic phase and space dimensionality is investigated.
Renormalization of the effective Kondo temperature by the interatomic exchange
interactions is calculated. An important role of the character of spin dynamics
(existence of well-defined magnon excitations, presence of magnetic anisotropy
etc.) is demonstrated. The regime of strongly suppressed magnetic moments,
which corresponds to magnetic heavy-fermion system, may occur in a rather
narrow parameter region only. At the same time, in the magnetically ordered
phases the renormalized Kondo temperature depends weakly on the bare coupling
parameter in some interval. The critical behavior, corresponding to the
magnetic transition with changing the bare coupling parameter, is
investigated. In the vicinity of the strong coupling regime, the spectrum of
the Bose excitations becomes softened. Thus on the borderline of magnetic
instability the Fermi-liquid picture is violated in some temerature interval
due to scattering of electrons by these bosons. This may explain the fact that
a non-Fermi-liquid behavior often takes place in the heavy-fermion systems near
the onset of magnetic ordering.Comment: 20 pages, RevTeX, 13 figure
On the self-consistent spin-wave theory of layered Heisenberg magnets
The versions of the self-consistent spin-wave theories (SSWT) of
two-dimensional (2D) Heisenberg ferro- and antiferromagnets with a weak
interlayer coupling and/or magnetic anisotropy, that are based on the
non-linear Dyson-Maleev, Schwinger, and combined boson-pseudofermion
representations, are analyzed. Analytical results for the temperature
dependences of (sublattice) magnetization and short-range order parameter, and
the critical points are obtained. The influence of external magnetic field is
considered. Fluctuation corrections to SSWT are calculated within a
random-phase approximation which takes into account correctly leading and
next-leading logarithmic singularities. These corrections are demonstrated to
improve radically the agreement with experimental data on layered perovskites
and other systems. Thus an account of these fluctuations provides a
quantitative theory of layered magnets.Comment: 46 pages, RevTeX, 7 figure
On the derivation of the t-J model: electron spectrum and exchange interactions in narrow energy bands
A derivation of the t-J model of a highly-correlated solid is given starting
from the general many-electron Hamiltonian with account of the
non-orthogonality of atomic wave functions. Asymmetry of the Hubbard subbands
(i.e. of ``electron'' and ``hole''cases) for a nearly half-filled bare band is
demonstrated. The non-orthogonality corrections are shown to lead to occurrence
of indirect antiferromagnetic exchange interaction even in the limit of the
infinite on-site Coulomb repulsion. Consequences of this treatment for the
magnetism formation in narrow energy bands are discussed. Peculiarities of the
case of ``frustrated'' lattices, which contain triangles of nearest neighbors,
are considered.Comment: 4 pages, RevTe
Tunable spin transport in CrAs: role of correlation effects
Correlation effects on the electronic structure of half-metallic CrAs in
zinc-blende structure are studied for different substrate lattice constants.
Depending on the substrate the spectral weight of the non-quasiparticle states
might be tuned from a well developed value in the case of InAs substrate to an
almost negligible contribution for the GaAs one. A piezoelectric material that
would allow the change in the substrate lattice parameters opens the
possibility for practical investigations of the switchable (tunable)
non-quasiparticle states. Since the latter are important for the tunneling
magnetoresistance and related phenomena it creates new opportunities in
spintronics.Comment: 12 pages, 3 figures, 2 tables. accepted PRB 71, 1 (2005
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