744 research outputs found
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
Trigonometric Sutherland systems and their Ruijsenaars duals from symplectic reduction
Besides its usual interpretation as a system of indistinguishable
particles moving on the circle, the trigonometric Sutherland system can be
viewed alternatively as a system of distinguishable particles on the circle or
on the line, and these 3 physically distinct systems are in duality with
corresponding variants of the rational Ruijsenaars-Schneider system. We explain
that the 3 duality relations, first obtained by Ruijsenaars in 1995, arise
naturally from the Kazhdan-Kostant-Sternberg symplectic reductions of the
cotangent bundles of the group U(n) and its covering groups
and , respectively. This geometric interpretation
enhances our understanding of the duality relations and simplifies Ruijsenaars'
original direct arguments that led to their discovery.Comment: 34 pages, minor additions and corrections of typos in v
Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
The spin structure and spin dynamics of excitons in an ensemble of
(In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both
direct and indirect band gap dots, are studied. Time-resolved and spectral
selective techniques are used to distinguish between the direct and indirect
QDs. The exciton fine structure is studied by means of optical alignment and
optical orientation techniques in magnetic fields applied in the Faraday or
Voigt geometries. A drastic difference in emission polarization is found for
the excitons in the direct QDs involving a -valley electron and the
excitons in the indirect QDs contributed by an -valley electron. We show
that in the direct QDs the exciton spin dynamics is controlled by the
anisotropic exchange splitting, while in the indirect QDs it is determined by
the hyperfine interaction with nuclear field fluctuations. The anisotropic
exchange splitting is determined for the direct QD excitons and compared with
model calculations
Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
The spin structure and spin dynamics of excitons in an ensemble of
(In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both
direct and indirect band gap dots, are studied. Time-resolved and spectral
selective techniques are used to distinguish between the direct and indirect
QDs. The exciton fine structure is studied by means of optical alignment and
optical orientation techniques in magnetic fields applied in the Faraday or
Voigt geometries. A drastic difference in emission polarization is found for
the excitons in the direct QDs involving a -valley electron and the
excitons in the indirect QDs contributed by an -valley electron. We show
that in the direct QDs the exciton spin dynamics is controlled by the
anisotropic exchange splitting, while in the indirect QDs it is determined by
the hyperfine interaction with nuclear field fluctuations. The anisotropic
exchange splitting is determined for the direct QD excitons and compared with
model calculations
Electronic structure, magnetic and optical properties of intermetallic compounds R2Fe17 (R=Pr,Gd)
In this paper we report comprehensive experimental and theoretical
investigation of magnetic and electronic properties of the intermetallic
compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these
two systems was probed by optical measurements in the spectral range of 0.22-15
micrometers. On top of that charge carriers parameters (plasma frequency and
relaxation frequency) and optical conductivity s(w) were determined.
Self-consistent spin-resolved bandstructure calculations within the
conventional LSDA+U method were performed. Theoretical interpetation of the
experimental s(w) dispersions indicates transitions between 3d and 4p states of
Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical
optical conductivity coincides well with our experimental data. Calculated by
LSDA+U method magnetic moments per formula unit are found to be in good
agreement with observed experimental values of saturation magnetization.Comment: 16 pages, 5 figures, 1 tabl
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
Relating Gauge Theories via Gauge/Bethe Correspondence
In this note, we use techniques from integrable systems to study relations
between gauge theories. The Gauge/Bethe correspondence, introduced by Nekrasov
and Shatashvili, identifies the supersymmetric ground states of an N=(2,2)
supersymmetric gauge theory in two dimensions with the Bethe states of a
quantum integrable system. We make use of this correspondence to relate three
different quiver gauge theories which correspond to three different
formulations of the Bethe equations of an integrable spin chain called the tJ
model.Comment: 30 pages, published in JHEP. LaTeX problem correcte
On superconducting and magnetic properties of iron-oxypnictides
Pairing symmetry in oxypnictides, a new family of multiband high-Tc
superconductors, is partially imposed by the positions of multiple Fermi
pockets, which itself can give rise to new order parameters, such as s+,-
states or the state of dx^2-y^2 symmetry. Other pairing states may appear on
small pockets for long range interactions, but they are expected to be
sensitive to defects. We identify the competing antiferromagnetic order with
the triplet exciton transition in the semi- metallic background and discuss
whether its coexistence with superconductivity explains the doping dependence
of Tc.Comment: Fig1b replace
A note on instanton counting for N=2 gauge theories with classical gauge groups
We study the prepotential of N=2 gauge theories using the instanton counting
techniques introduced by Nekrasov. For the SO theories without matter we find a
closed expression for the full prepotential and its string theory gravitational
corrections. For the more subtle case of Sp theories without matter we discuss
general features and compute the prepotential up to instanton number three. We
also briefly discuss SU theories with matter in the symmetric and antisymmetric
representations. We check all our results against the predictions of the
corresponding Seiberg-Witten geometries.Comment: 24 pages, LaTeX. v2: refs added. v3: typos correcte
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