2,978 research outputs found
Nonlocal correlations in the vicinity of the - phase transition in iron within a DMFT plus spin-fermion model approach
We consider nonlocal correlations in iron in the vicinity of the
- phase transition within the spin-rotationally-invariant
dynamical mean-field theory (DMFT) approach, combined with the recently
proposed spin-fermion model of iron. The obtained nonlocal corrections to DMFT
yield a decrease of the Curie temperature of the phase, leading to an
agreement with its experimental value. We show that the corresponding nonlocal
corrections to the energy of the phase are crucially important to
obtain the proximity of energies of and phases in the
vicinity of the iron - transformation.Comment: 5 pages, 2 figure
Effect of density of states peculiarities on Hund's metal behavior
We investigate a possibility of Hund's metal behavior in the Hubbard model
with asymmetric density of states having peak(s). Specifically, we consider the
degenerate two-band model and compare its results to the five-band model with
realistic density of states of iron and nickel, showing that the obtained
results are more general, provided that the hybridization between states of
different symmetry is sufficiently small. We find that quasiparticle damping
and the formation of local magnetic moments due to Hund's exchange interaction
are enhanced by both, the density of states asymmetry, which yields stronger
correlated electron or hole excitations, and the larger density of states at
the Fermi level, increasing the number of virtual electron-hole excitations.
For realistic densities of states these two factors are often interrelated
because the Fermi level is attracted towards peaks of the density of states. We
discuss the implication of the obtained results to various substances and
compounds, such as transition metals, iron pnictides, and cuprates.Comment: 7 pages, 7 figure
Momentum-dependent susceptibilities and magnetic exchange in bcc iron from supercell DMFT calculations
We analyze the momentum- and temperature dependences of the magnetic
susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by
a combination of density functional theory and dynamical mean-field theory
(DFT+DMFT). By considering a general derivation of the orbital-resolved
effective model for spin degrees of freedom for Hund's metals, we relate
momentum-dependent susceptibilities in the paramagnetic phase to the magnetic
exchange. We then calculate non-uniform orbital-resolved susceptibilities at
high-symmetry wave vectors by constructing appropriate supercells in the DMFT
approach. Extracting the irreducible parts of susceptibilities with respect to
Hund's exchange interaction, we determine the corresponding orbital-resolved
exchange interactions, which are then interpolated to the whole Brillouin zone.
Using the spherical model we estimate the temperature dependence of the
resulting exchange between local moments.Comment: 18 pages, 6 figure
Baryon Asymmetry in a Heavy Moduli Scenario
In some models of supersymmetry breaking, modulus fields are heavy enough to
decay before BBN. But the large entropy produced via moduli decay significantly
dilutes the preexisting baryon asymmetry of the universe. We study whether
Affleck-Dine mechanism can provide enough baryon asymmetry which survives the
dilution, and find several situations in which desirable amount of baryon
number remains after the dilution. The possibility of non-thermal dark matter
is also discussed. This provides the realistic cosmological scenario with heavy
moduli.Comment: 36 pages, 5 figures; added a reference; v3: minor correction
Dirac phase leptogenesis
I present here a concise summary of the preprint arXiv:0707.3024, written in
collaboration with A. Anisimov and P. Di Bari. There we discuss leptogenesis
when {\em CP} violation stems exlusively from the Dirac phase in the PMNS
mixing matrix. Under this assumption it turns out that the situation is very
constrained when a hierarchical heavy right-handed (RH) neutrino spectrum is
considered: the allowed regions are small and the final asymmetry depends on
the initial conditions. On the other hand, for a quasi-degenerate spectrum of
RH neutrinos, the {\em CP} asymmetry can be enhanced and the situation becomes
much more favorable, with no dependence on the initial conditions.
Interestingly, in the extreme case of resonant leptogenesis, in order to match
the observed baryon asymmetry of the Universe, we obtain a lower bound on \sin
\q_{13} which depends on the lightest active neutrino mass m_1.Comment: 3 pages, 2 figures, contribution to the proceedings of the TAUP 07
conference, Sep. 11-15, Sendai, Japa
Non-leptonic two-body decays of the Bc meson in light-front quark model and QCD factorization approach
We study exclusive non-leptonic two-body
decays with (pseudoscalar or vector meson) being factored out in QCD
factorization approach. The non-leptonic decay amplitudes are related to the
product of meson decay constants and the form factors for semileptonic
decays. As inputs in obtaining the branching ratios for a large set of
non-leptonic decays, we use the weak form factors for the semileptonic
decays in the whole kinematical region and the
unmeasured meson decay constants obtained from our previous light-front quark
model. We compare our results of the branching ratios with those of other
theoretical studies.Comment: 11 pages, 3 figures, minor corrections, version to appear in PR
Pressure-driven metal-insulator transition in BiFeO from Dynamical Mean-Field Theory
A metal-insulator transition (MIT) in BiFeO under pressure was
investigated by a method combining Generalized Gradient Corrected Local Density
Approximation with Dynamical Mean-Field Theory (GGA+DMFT). Our paramagnetic
calculations are found to be in agreement with experimental phase diagram:
Magnetic and spectral properties of BiFeO3 at ambient and high pressures were
calculated for three experimental crystal structures , and
. At ambient pressure in the phase, an insulating gap of 1.2
eV was obtained in good agreement with its experimental value. Both and
phases have a metal-insulator transition that occurs simultaneously with
a high-spin (HS) to low-spin (LS) transition. The critical pressure for the
phase is 25-33 GPa that agrees well with the experimental observations.
The high pressure and temperature phase exhibits a metallic
behavior observed experimentally as well as in our calculations in the whole
range of considered pressures and undergoes to the LS state at 33 GPa where a
to transition is experimentally observed. The
antiferromagnetic GGA+DMFT calculations carried out for the structure
result in simultaneous MIT and HS-LS transitions at a critical pressure of 43
GPa in agreement with the experimental data
Theory of optically forbidden d-d transitions in strongly correlated crystals
A general multiband formulation of linear and non-linear optical response
functions for realistic models of correlated crystals is presented. Dipole
forbidden d-d optical transitions originate from the vertex functions, which we
consider assuming locality of irreducible four-leg vertex. The unified
formulation for second- and third-order response functions in terms of the
three-leg vertex is suitable for practical calculations in solids. We
illustrate the general approach by consideration of intraatomic spin-flip
contributions, with the energy of 2J, where J is a Hund exchange, in the
simplest two-orbital model.Comment: 9 pages, 4 figures, to appear in J. Phys. Cond. Matte
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