98 research outputs found
Magnetoelastic coupling in iron
Exchange interactions in {\alpha}- and {\gamma}-Fe are investigated within an
ab-initio spin spiral approach. We have performed total energy calculations for
different magnetic structures as a function of lattice distortions, related
with various cell volumes and the Bain tetragonal deformations. The effective
exchange parameters in {\gamma}-Fe are very sensitive to the lattice
distortions, leading to the ferromagnetic ground state for the tetragonal
deformation or increase of the volume cell. At the same time, the
magnetic-structure-independent part of the total energy changes very slowly
with the tetragonal deformations. The computational results demonstrate a
strong mutual dependence of crystal and magnetic structures in Fe and explain
the observable "anti-Invar" behavior of thermal expansion coefficient in
{\gamma}-Fe.Comment: Submitted to Phys. Rev.
Quadratic operators used in deducing exact ground states for correlated systems: ferromagnetism at half filling provided by a dispersive band
Quadratic operators are used in transforming the model Hamiltonian (H) of one
correlated and dispersive band in an unique positive semidefinite form coopting
both the kinetic and interacting part of H. The expression is used in deducing
exact ground states which are minimum energy eigenstates only of the full
Hamiltonian. It is shown in this frame that at half filling, also dispersive
bands can provide ferromagnetism in exact terms by correlation effects .Comment: 7 page
Theory of quasiparticle spectra for Fe, Co, and Ni: bulk and surface
The correlated electronic structure of iron, cobalt and nickel is
investigated within the dynamical mean-field theory formalism, using the newly
developed full-potential LMTO-based LDA+DMFT code. Detailed analysis of the
calculated electron self-energy, density of states and the spectral density are
presented for these metals. It has been found that all these elements show
strong correlation effects for majority spin electrons, such as strong damping
of quasiparticles and formation of a density of states satellite at about -7 eV
below the Fermi level. The LDA+DMFT data for fcc nickel and cobalt (111)
surfaces and bcc iron (001) surface is also presented. The electron self energy
is found to depend strongly on the number of nearest neighbors, and it
practically reaches the bulk value already in the second layer from the
surface. The dependence of correlation effects on the dimensionality of the
problem is also discussed.Comment: 15 pages, 24 figure
Model of charge and magnetic order formation in itinerant electron systems
We propose a simple model of charge and/or magnetic order formation in
systems containing both localized and itinerant electrons coupled by the
on-site, spin-dependent interaction that represents Coulomb repulsion and
Hund's rule (a generalized Falicov-Kimball model). Ground state properties of
the model are analyzed on the square lattice on a basis of the phase diagrams
that have been constructed rigorously, but in a restricted configurational
space. For intermediate values of the coupling constants there are considerable
ranges of itinerant electron densities where phases with complex charge and
magnetic structures of the localized electrons have lower energy than the
simplest antiferro- and ferromagnetic ones. A strong tendency towards the
antiferromagnetic coupling between spins of localized electrons has been
observed close to half-filling for any density of localized electrons,
including situations where the magnetic ions are diluted. For small band
fillings the ferromagnetic coupling between localized spins is predominant.Comment: 13 pages, 5 figure
Wave packet dynamics in hole Luttinger systems
For hole systems with an effective spin 3/2 we analyzed analytically and
numerically the evolution of wave packets with the different initial
polarizations. The dynamics of such systems is determined by the
Luttinger Hamiltonian. We work in the space of arbitrary superposition of
light- and heavy-hole states of the "one-particle system". For 2D packets we
obtained the analytical solution for the components of wave function and
analyzed the space-time dependence of probability densities as well as angular
momentum densities. Depending on the value of the parameter ( is
the average momentum vector and is the packet width) two scenarios of
evolution are realized. For the initial wave packet splits into two
parts and the coordinates of packet center experience the transient
oscillations or {\it Zitterbewegung} (ZB) as for other two-band systems. In the
case when remains
almost cylindrically symmetric and the ripples arise at the circumference of
wave packet. The ZB in this case is absent. We evaluated and visualized for
different values of parameter the space-time dependence of angular momentum
densities, which have the multipole structure. It was shown that the average
momentum components can precess in the absence of external or effective
magnetic fields due to the interference of the light- and heavy hole states.
For localized initial states this precession has a transient character.Comment: 9 pages, 8 gigur
Correlated band theory of spin and orbital contributions to Dzyaloshinskii-Moriya interactions
A new approach for calculations of Dzyaloshinskii-Moriya interactions in
molecules and crystals is proposed. It is based on the exact perturbation
expansion of total energy of weak ferromagnets in the canting angle with the
only assumption of local Hubbard-type interactions. This scheme leads to a
simple and transparent analytical expression for Dzyaloshinskii-Moriya vector
with a natural separation into spin and orbital contributions. The main problem
was transferred to calculations of effective tight-binding parameters in the
properly chosen basis including spin-orbit coupling. Test calculations for
LaCuO give the value of canting angle in a good agreement with
experimental data.Comment: 4 pages, 1 figur
Comment on "Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory"
A comment about importance of Anderson's orthogonality catastrophe for
projective Quantum Monte Carlo methods.Comment: Replaced by final versio
Nonlinear screening of charge impurities in graphene
It is shown that a ``vacuum polarization'' induced by Coulomb potential in
graphene leads to a strong suppression of electric charges even for undoped
case (no charge carriers). A standard linear response theory is therefore not
applicable to describe the screening of charge impurities in graphene. In
particular, it overestimates essentially the contributions of charge impurities
into the resistivity of graphene.Comment: 3 pages, 1 figure; final version as published in the journa
Current-induced phase transition in ballistic Ni nanocontacts
Local phase transition from ferromagnetic to paramagnetic state in the region
of the ballistic Ni nanocontacts (NCs) has been experimentally observed. We
found that contact size reduction leads to an increase in the bias voltage at
which the local phase transition occurs. Presented theoretical interpretation
of this phenomena takes into the account the specificity of the local heating
of the ballistic NC and describes the electron's energy relaxation dependences
on the applied voltage. The experimental data are in good qualitative and
quantitative agreement with the theory proposed.Comment: 8 pages, 2 figure
Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure
We extend the solid-state nudged elastic band method to handle a
non-conserved order parameter - in particular, magnetization, that couples to
volume and leads to many observed effects in magnetic systems. We apply this
formalism to the well-studied magneto-volume collapse during the
pressure-induced transformation in iron - from ferromagnetic body-centered
cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a
bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state
enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and
coherent stress occurs at the transition state, which has a form of a cusp on
the potential-energy surface (yet all the atomic and cell degrees of freedom
are continuous); the calculated pressure jump of 25 GPa is related to the
observed 25 GPa spread in measured coexistence pressures arising from
martensitic and coherency stresses in samples. Our results agree with
experiments, but necessarily differ from those arising from drag and restricted
parametrization methods having improperly constrained or uncontrolled degrees
of freedom.Comment: 7 pages, 7 figure
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