47 research outputs found
Itinerant in-plane magnetic fluctuations and many-body correlations in NaCoO
Based on the {\it ab-initio} band structure for NaCoO we derive the
single-electron energies and the effective tight-binding description for the
bands using projection procedure. Due to the presence of the
next-nearest-neighbor hoppings a local minimum in the electronic dispersion
close to the point of the first Brillouin zone forms. Correspondingly,
in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates dominant itinerant in-plane
ferromagnetic fluctuations above certain critical concentration , in
agreement with neutron scattering data. Below the magnetic susceptibility
shows a tendency towards the antiferromagnetic fluctuations. We further analyze
the many-body effects on the electronic and magnetic excitations using various
approximations applicable for different ratio.Comment: 10 page
Electronic theory for itinerant in-plane magnetic fluctuations in NaCoO
Starting from {\it ab-initio} band structure for NaCoO, we derive the
single-electron energies and the effective tight-binding description for the
bands using a projection procedure. We find that due to the presence
of the next-nearest-neighbor hoppings a local minimum in the electronic
dispersion close to the point of the first Brillouin zone forms.
Therefore, in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates itinerant in-plane
ferromagnetic state above certain critical concentration , in agreement
with neutron scattering data. Below the magnetic susceptibility shows a
tendency towards the antiferromagnetic fluctuations. We estimate the value of
within the rigid band model and within the Hubbard model
with infinite on-site Coulomb repulsion consistent with the experimental phase
diagram.Comment: 4 pages, 4 figures; LDA calculations were done with Na in the
symmetric 2d position contrary to the 6h position in a previous version of
this pape
Competition of multiband superconducting and magnetic order in ErNi2B2C observed by Andreev reflection
Point contacts (PC) Andreev reflection dV/dI spectra for the
antiferromagnetic (T_N =6K) superconductor (Tc=11K) ErNi2B2C have been measured
for the two main crystallographic directions. Observed retention of the Andreev
reflection minima in dV/dI up to Tc directly points to unusual superconducting
order parameter (OP) vanishing at Tc. Temperature dependence of OP was obtained
from dV/dI using recent theory of Andreev reflection including pair-breaking
effect. For the first time existence of a two superconducting OPs in ErNi2B2C
is shown. A distinct decrease of both OPs as temperature is lowered below T_N
is observed.Comment: 5 pages, 5 figs, to be published in Europhys. Let
The influence of defects on magnetic properties of fcc-Pu
The influence of vacancies and interstitial atoms on magnetism in Pu has been
considered in frames of the Density Functional Theory (DFT). The relaxation of
crystal structure arising due to different types of defects was calculated
using the molecular dynamic method with modified embedded atom model (MEAM).
The LDA+U+SO (Local Density Approximation with explicit inclusion of Coulomb
and spin-orbital interactions) method in matrix invariant form was applied to
describe correlation effects in Pu with these types of defects. The
calculations show that both vacancies and interstitials give rise to local
moments in -shell of Pu in good agreement with experimental data for
annealed Pu. Magnetism appears due to destroying of delicate balance between
spin-orbital and exchange interactions.Comment: 13 pages, 4 figure
Electronic structure and magnetic state of transuranium metals under pressure
Electronic structure of bcc Np, fcc Pu, Am, and Cm pure metals under pressure
has been investigated employing the LDA+U method with spin-orbit coupling
(LDA+U+SO). Magnetic state of the actinide ions was analyzed in both LS and jj
coupling schemes to reveal the applicability of corresponding coupling bases.
It was demonstrated that whereas Pu and Am are well described within the jj
coupling scheme, Np and Cm can be described appropriately neither in {m-sigma},
nor in {jmj} basis, due to intermediate coupling scheme realizing in these
metals that requires some finer treatment. The LDA+U+SO results for the
considered transuranium metals reveal bands broadening and gradual 5f electron
delocalization under pressure.Comment: 5 pages, 5 figure
Pseudogap Value in the Energy Spectrum of LaOFeAs: Fixed Spin Moment Treatment
The experimental data available up to date in literature corresponding to the
paramagnetic - spin density wave transition in nonsuperconducting LaOFeAs are
discussed. In particular, we pay attention that upon spin density wave
transition there is a relative decrease of the density of states on the Fermi
level and a pseudogap formation. The values of these quantities are not
properly described in frames of the density functional theory. The agreement of
them with experimental estimations becomes more accurate with the use of fixed
spin moment procedure when iron spin moment is set to experimental value.
Strong electron correlations which are not included into the present
calculation scheme may lead both to the decrease of spin moment and
renormalization of energy spectrum in the vicinity of the Fermi level for
correct description of discussed characteristics
Coulomb Parameter U and Correlation Strength in LaFeAsO
First principles constrained density functional theory scheme in Wannier
functions formalism has been used to calculate Coulomb repulsion U and Hund's
exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend
on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and
corresponding bands are included, computation results in U=3-4 eV, however,
with the basis set restricted to Fe-3d orbitals and bands only, computation
gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local
Density Approximation combined with the Dynamical Mean-Field Theory)
calculation with this parameters results in weakly correlated electronic
structure that is in agreement with X-ray experimental spectra
Coulomb repulsion and correlation strength in LaFeAsO from Density Functional and Dynamical Mean-Field Theories
LDA+DMFT (Local Density Approximation combined with Dynamical Mean-Field
Theory) computation scheme has been used to calculate spectral properties of
LaFeAsO -- the parent compound for new high-T iron oxypnictides. Coulomb
repulsion and Hund's exchange parameters for iron 3d electrons were
calculated using \textit {first principles} constrained density functional
theory scheme in Wannier functions formalism. Resulting values strongly depend
on the number of states taken into account in calculations: when full set of
O-, As-, and Fe-3d orbitals with corresponding bands are included,
computation results in 4 eV and J=0.8 eV. In contrast to that when the
basis set is restricted to Fe-3d orbitals and bands only, computation gives
much smaller parameter values =0.8 eV, =0.5 eV. However, DMFT
calculations with both parameter sets and corresponding to them choice of basis
functions result in weakly correlated electronic structure that is in agreement
with experimental X-ray and photoemission spectra.Comment: 13 pages, 9 figure
Magnetic state of plutonium ion in metallic Pu and its compounds
By LDA+U method with spin-orbit coupling (LDA+U+SO) the magnetic state and
electronic structure have been investigated for plutonium in \delta and \alpha
phases and for Pu compounds: PuN, PuCoGa5, PuRh2, PuSi2, PuTe, and PuSb. For
metallic plutonium in both phases in agreement with experiment a nonmagnetic
ground state was found with Pu ions in f^6 configuration with zero values of
spin, orbital, and total moments. This result is determined by a strong
spin-orbit coupling in 5f shell that gives in LDA calculation a pronounced
splitting of 5f states on f^{5/2} and f^{7/2} subbands. A Fermi level is in a
pseudogap between them, so that f^{5/2} subshell is already nearly completely
filled with six electrons before Coulomb correlation effects were taken into
account. The competition between spin-orbit coupling and exchange (Hund)
interaction (favoring magnetic ground state) in 5f shell is so delicately
balanced, that a small increase (less than 15%) of exchange interaction
parameter value from J_H=0.48eV obtained in constrain LDA calculation would
result in a magnetic ground state with nonzero spin and orbital moment values.
For Pu compounds investigated in the present work, predominantly f^6
configuration with nonzero magnetic moments was found in PuCoGa5, PuSi2, and
PuTe, while PuN, PuRh2, and PuSb have f^5 configuration with sizeable magnetic
moment values. Whereas pure jj coupling scheme was found to be valid for
metallic plutonium, intermediate coupling scheme is needed to describe 5f shell
in Pu compounds. The results of our calculations show that both spin-orbit
coupling and exchange interaction terms in the Hamiltonian must be treated in a
general matrix form for Pu and its compounds.Comment: 20 pages, LaTeX; changed discussion on reference pape
High - Temperature Superconductivity in Iron Based Layered Compounds
We present a review of basic experimental facts on the new class of high -
temperature superconductors - iron based layered compounds like REOFeAs
(RE=La,Ce,Nd,Pr,Sm...), AFe_2As_2 (A=Ba,Sr...), AFeAs (A=Li,...) and FeSe(Te).
We discuss electronic structure, including the role of correlations, spectrum
and role of collective excitations (phonons, spin waves), as well as the main
models, describing possible types of magnetic ordering and Cooper pairing in
these compounds.Comment: 43 pages, 30 figures, review talk on 90th anniversary of Physics
Uspekh