791 research outputs found
Electronic Structure of New LiFeAs High-Tc Superconductor
We present results of it ab initio LDA calculations of electronic structure
of "next generation" layered ironpnictide High-Tc superconductor LiFeAs
(Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently
studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely
close to the Fermi level its electronic properties are also determined ma inly
by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs
are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of
the Fe-3d density o f states and Fermi surface.Comment: 4 pages, 5 figures; Electronic structure improved with respect to new
experimental crystal structure dat
Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors
This work is provoked by recent discovery of new class prototype systems
AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors
(Tc=36K). Here we report ab initio LDA results for electronic structure of the
AFFeAs systems. We provide detailed comparison between electronic properties of
both new systems and reference LaOFeAs (La111) compound. In the vicinity of the
Fermi level all three systems have essentially the same band dispersions.
However for iron fluoride systems F(2p) states were found to be separated in
energy from As(4p) ones in contrast to La111, where O(2p) states strongly
overlaps with As(4p). Thus it should be more plausible to include only Fe(3d)
and As(4p) orbitals into a realistic noninteracting model than for La111.
Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads
to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in
smaller value of the density of states at the Fermi level in the case of Ca
compound. So to some extend Ca system reminds RE111 with later Rare Earths.
However Fermi surface of new fluorides is found to be nearly perfect
two-dimensional. Also we do not expect strong dependence of superconducting
properties with respect to different types of A substitutes.Comment: 5 pages, 4 figure
Novel multiple-band superconductor SrPt2As2
We present LDA calculated electronic structure of recently discovered
superconductor SrPt2As2 with Tc=5.2K. Despite its chemical composition and
crystal structure are somehow similar to FeAs-based high-temperature
superconductors, the electronic structure of SrPt2As2 is very much different.
Crystal structure is orthorhombic (or tetragonal if idealized) and has layered
nature with alternating PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr
ions. The Fermi level is crossed by Pt-5d states with rather strong admixture
of As-4p states. Fermi surface of SrPt2As2 is essentially three dimensional,
with complicated sheets corresponding to multiple bands. We compare SrPt2As2
with 1111 and 122 representatives of FeAs-class of superconductors, as well as
with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of
superconductivity in SrPt2As2 is also presented.Comment: 5 pages, 4 figure
Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P
We report LDA calculated band structure, densities of states and Fermi
surfaces for recently discovered Pt-pnictide superconductors APt3P
(A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is
essentially three dimensional, in contrast to Fe pnictides and chalcogenides.
LDA calculated Sommerfeld coefficient agrees rather well with experimental
data, leaving little space for very strong coupling superconductivity,
suggested by experimental data on specific heat of SrPt3P. Elementary estimates
show, that the values of critical temperature can be explained by rather weak
or moderately strong coupling, while the decrease of superconducting transition
temperature Tc from Sr to La compound can be explained by corresponding
decrease of total density of states at the Fermi level N(E_F). The shape of the
density of states near the Fermi level suggests that in SrPt3P electron doping
(such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g.
partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F)
and possibly Tc.Comment: 5 pages, 5 figure
Electronic and Magnetic Structure of Possible Iron Based Superconductor BaFe2Se3
We present results of LDA calculations (band structure, densities of states,
Fermi surfaces) for possible iron based superconductor BaFe2Se3 (Ba123) in
normal (paramagnetic) phase. Results are briefly compared with similar data on
prototype BaFe2As2 and (K,Cs)Fe2Se2 superconductors. Without doping this system
is antiferromagnetic with T_N^{exp}~250K and rather complicated magnetic
structure. Neutron diffraction experiments indicated the possibility of two
possible spin structures (antiferromagnetically ordered "plaquettes" or
"zigzags"), indistinguishable by neutron scattering. Using LSDA calculated
exchange parameters we estimate Neel temperatures for both spin structures
within the molecular field approximation and show \tau_1 ("plaquettes") spin
configuration to be more favorable than \tau_2 ("zigzags").Comment: 5 pages, 4 figure
Realistic modeling of strongly correlated electron systems: An introduction to the LDA+DMFT approach
The LDA+DMFT approach merges conventional band structure theory in the local
density approximation (LDA) with a state-of-the-art many-body technique, the
dynamical mean-field theory (DMFT). This new computational scheme has recently
become a powerful tool for ab initio investigations of real materials with
strong electronic correlations. In this paper an introduction to the basic
ideas and the set-up of the LDA+DMFT approach is given. Results for the
photoemission spectra of the transition metal oxide La_{1-x}Sr_xTiO_3, obtained
by solving the DMFT-equations by quantum Monte-Carlo (QMC) simulations, are
presented and are found to be in very good agreement with experiment. The
numerically exact DMFT(QMC) solution is compared with results obtained by two
approximative solutions, i.e., the iterative perturbation theory and the
non-crossing approximation.Comment: 15 pages, 3 figures, SCES-Y2K Conference Proceeding
Ab initio exchange interactions and magnetic properties of Gd2Fe17 iron sublattice: rhombohedral vs. hexagonal phases
In the framework of the LSDA+U method electronic structure and magnetic
properties of the intermetallic compound Gd2Fe17 for both rhombohedral and
hexagonal phases have been calculated. On top of that, ab initio exchange
interaction parameters within the Fe sublattice for all present nearest and
some next nearest Fe ions have been obtained. It was found that for the first
coordination sphere direct exchange interaction is ferromagnetic. For the
second coordination sphere indirect exchange interaction is observed to be
weaker and of antiferromagnetic type. Employing the theoretical values of
exchange parameters Curie temperatures Tc of both hexagonal and rhombohedral
phases of Gd2Fe17 within Weiss mean-field theory were estimated. Obtained
values of Tc and its increase going from the hexagonal to rhombohedral crystal
structure of Gd2Fe17 agree well with experiment. Also for both structures
LSDA+U computed values of total magnetic moment coincide with experimental
ones.Comment: 20 pages, 2 figures; V2 as published in PR
Consistent LDA'+DMFT approach to electronic structure of transition metal oxides: charge transfer insulators and correlated metals
We discuss the recently proposed LDA'+DMFT approach providing consistent
parameter free treatment of the so called double counting problem arising
within the LDA+DMFT hybrid computational method for realistic strongly
correlated materials. In this approach the local exchange-correlation portion
of electron-electron interaction is excluded from self consistent LDA
calculations for strongly correlated electronic shells, e.g. d-states of
transition metal compounds. Then the corresponding double counting term in
LDA+DMFT Hamiltonian is consistently set in the local Hartree (fully localized
limit - FLL) form of the Hubbard model interaction term. We present the results
of extensive LDA'+DMFT calculations of densities of states, spectral densities
and optical conductivity for most typical representatives of two wide classes
of strongly correlated systems in paramagnetic phase: charge transfer
insulators (MnO, CoO and NiO) and strongly correlated metals (SrVO3 and
Sr2RuO4). It is shown that for NiO and CoO systems LDA'+DMFT qualitatively
improves the conventional LDA+DMFT results with FLL type of double counting,
where CoO and NiO were obtained to be metals. We also include in our
calculations transition metal 4s-states located near the Fermi level missed in
previous LDA+DMFT studies of these monooxides. General agreement with optical
and X-ray experiments is obtained. For strongly correlated metals
LDA+DMFT results agree well with earlier LDA+DMFT calculations and
existing experiments. However, in general LDA'+DMFT results give better
quantitative agreement with experimental data for band gap sizes and oxygen
states positions, as compared to the conventional LDA+DMFT.Comment: 13 pages, 11 figures, 1 table. In v2 there some additional
clarifications are include
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