84 research outputs found
Analysis of spin density wave conductivity spectra of iron pnictides in the framework of density functional theory
The optical conductivity of LaFeAsO, BaFeAs, SrFeAs, and
EuFeAs in the spin-density wave (SDW) state is investigated within
density functional theory (DFT) in the framework of spin-polarized generalized
gradient approximation (GGA) and GGA+U. We find a strong dependence of the
optical features on the Fe magnetic moments. In order to recover the small Fe
magnetic moments observed experimentally, GGA+ with a suitable
choice of negative on-site interaction was considered. Such
an approach may be justified in terms of an overscreening which induces a
relatively small U compared to the Hund's rule coupling J, as well as a strong
Holstein-like electron-phonon interaction. Moreover, reminiscent of the fact
that GGA+ with a positive is a simple approximation
for reproducing a gap with correct amplitude in correlated insulators, a
negative can also be understood as a way to suppress magnetism
and mimic the effects of quantum fluctuations ignored in DFT calculations. With
these considerations, the resulting optical spectra reproduce the SDW gap and a
number of experimentally observed features related to the antiferromagnetic
order. We find electronic contributions to excitations that so far have been
attributed to purely phononic modes. Also, an orbital resolved analysis of the
optical conductivity reveals significant contributions from all Fe 3d orbitals.
Finally, we observe that there is an important renormalization of kinetic
energy in these SDW metals, implying that the effects of correlations cannot be
neglected.Comment: 8 pages, 4 figures; recalculated spectra for U_eff=-1.9 eV for better
comparison to experimental results, added discussion of the role of U and J
in LDA+
Paramagnetism in the kagome compounds (Zn,Mg,Cd)Cu(OH)Cl
Frustrated magnetism on the kagome lattice has been a fertile ground for rich
and fascinating physics, ranging from experimental evidence of a spin liquid to
theoretical predictions of exotic superconductivity. Among experimentally
realized spin- kagome magnets, herbertsmithite, kapellasite, and
haydeeite [(Zn,Mg)Cu(OH)Cl] are all well described by a
three-parameter Heisenberg model, but they exhibit distinctly different
physics. We address the problem using a pseudofermion functional
renormalization-group approach and analyze the low-energy physics in the
experimentally accessible parameter range. Our analysis places kapellasite and
haydeeite near the boundaries between magnetically ordered and disordered
phases, implying that slight modifications could dramatically affect their
magnetic properties. Inspired by this, we perform \textit{ab initio} density
functional theory calculations of (Zn,Mg,Cd)Cu (OH)Cl at
various pressures. Our results suggest that by varying pressure and composition
one can traverse a paramagnetic regime between different magnetically ordered
phases.Comment: Published version. Main paper (7 pages, 5 figures) and Supplemental
material (7 pages, 4 figures, 3 tables). Change in titl
Das Schachprojekt FUSc#
Die AG Schachprogrammierung der Freien Universität Berlin wurde im Oktober
2002 gegründet und forscht seitdem auf dem Gebiet der Schachprogrammierung und
beschäftigt sich mit der Entwicklung des Schachmotors FUSc#. Im Laufe der Zeit
wurden verschiedene Brettdarstellungen und Algorithmen getestet. Mit einer
nichtrekursiven Alpha-Beta-Variante in Matrixform konnte eine
vielversprechende Leistungssteigerung erreicht werden, dem gegenüber stand
aber ein großer Grad an Codekomplexität. Als wichtigstes Forschungsziel bei
dem seit Februar 2004 neu entwickelten Schachmotor DarkFUSc# gilt die
Verwendung von Lernalgorithmen. Durch die Kombination von TD-Leaf(l) und einer
komplexen Stellungsklassifikation wurde ein neues Lernverfahren entwickelt.
Nach den gespielten Partien optimiert DarkFUSc# die mehr als 56.000
Koeffizienten seiner komplexen Bewertungsfunktion eigenständig und verbessert
so sein Stellungsspiel von Partie zu Partie. Als Trainings- und Testplattform
für die verschiedenen FUSc#-Varianten (unter anderem existiert inzwischen auch
eine Linux-Version) wurde der Fusch-OnlineSchachServer bereitgestellt, auf dem
Maschinen und Menschen gegeneinander spielen und voneinander lernen können
Oxidative stress stimulates alpha-tocopherol transfer protein in human trophoblast tumor cells BeWo
alpha-Tocopherol transfer protein (alpha-TTP) has been identified as the major intracellular transport protein for the antioxidant vitamin E (alpha-Tocopherol). Expression of alpha-TTP on the reproductive system has been described both in mouse uterus and lately in the human placenta. The aim of this study was to clarify if placental expression of alpha-TTP can be modified by substances causing oxidative reactions. The human choriocarcinoma cell line BeWo was, therefore, treated with two known pro-oxidants. alpha-TTP expression was determined with immunocytochemistry and evaluated by applying a semiquantitative score. The presence of pro-oxidants in BeWo cells induced alpha-TTP expression. We thus hypothesize that stimulation of alpha-TTP expression by oxidative stress, as this was induced by pro-oxidants, could be part of an antioxidant process occurring in the placenta in the aim of enhancing the supply of alpha-Tocopherol. This process could occur both in normal pregnancies, as well as in pregnancy disorders presented with intensified oxidative stress. In that view, this model is proposed for further oxidative stress studies on trophoblast and placenta, on the grounds of clarifying the role of alpha-Tocopherol in pregnancy physiology and pathophysiology
Stability of the spiral spin liquid in MnSc2S4
We investigate the stability of the spiral spin liquid phase in MnSc2S4 against thermal and quantum fluctuations as well as against perturbing effects of longer-range interactions. Employing ab initio density functional theory (DFT) calculations we propose a realistic Hamiltonian for MnSc2S4, featuring second (J2) and third (J3) neighbor Heisenberg interactions on the diamond lattice that are considerably larger than previously assumed. We argue that the combination of strong J2 and J3 couplings reproduces the correct magnetic Bragg peak position measured experimentally. Calculating the spin-structure factor within the pseudofermion functional-renormalization group technique, we find that close to the magnetic phase transition the sizable J3 couplings induce a strong spiral selection effect, in agreement with experiments. With increasing temperature the spiral selection becomes weaker such that in a window around three to five times the ordering temperature an approximate spiral spin liquid is realized in MnSc2S
Signatures of a gearwheel quantum spin liquid in a spin- pyrochlore molybdate Heisenberg antiferromagnet
We theoretically investigate the low-temperature phase of the recently
synthesized LuMoON material, an extraordinarily rare
realization of a three-dimensional pyrochlore Heisenberg
antiferromagnet in which Mo are the magnetic species. Despite a
Curie-Weiss temperature () of K, experiments have
found no signature of magnetic ordering spin freezing down to
K. Using density functional theory, we find that the compound
is well described by a Heisenberg model with exchange parameters up to third
nearest neighbors. The analysis of this model via the pseudofermion functional
renormalization group method reveals paramagnetic behavior down to a
temperature of at least , in agreement with the
experimental findings hinting at a possible three-dimensional quantum spin
liquid. The spin susceptibility profile in reciprocal space shows
momentum-dependent features forming a "gearwheel" pattern, characterizing what
may be viewed as a molten version of a chiral noncoplanar incommensurate spiral
order under the action of quantum fluctuations. Our calculated reciprocal space
susceptibility maps provide benchmarks for future neutron scattering
experiments on single crystals of LuMoON.Comment: Published version. Main paper (6 pages, 3 figures) + Supplemental
Material (4 pages, 3 figures, 1 table
Effects of correlation in LiFeAs
We discuss the role of electronic correlations in the iron-based
superconductor LiFeAs by studying the effects on band structure, mass
enhancements, and Fermi surface in the framework of density functional theory
combined with dynamical mean field theory calculations. We conclude that LiFeAs
shows characteristics of a moderately correlated metal and that the strength of
correlations is mainly controlled by the value of the Hund's rule coupling J.
The hole pockets of the Fermi surface show a distinctive change in form and
size with implications for the nesting properties. Our calculations are in good
agreement with recent angle-resolved photoemission spectroscopy and de Haas-van
Alphen experiments.Comment: This version includes minor additions and shows experimental dHvA
data for easier comparison. Accepted for publication in Physical Review B (in
press). 7 pages, 9 figure
Dynamical cluster approximation within an augmented plane-wave framework: Spectral properties of SrVO
We present a combination of local density approximation (LDA) with the
dynamical cluster approximation (LDA+DCA) in the framework of the
full-potential linear augmented plane-wave method, and compare our LDA+DCA
results for SrVO to LDA with the dynamical mean field theory (LDA+DMFT)
calculations as well as experimental observations on SrVO. We find a
qualitative agreement of the momentum resolved spectral function with
angle-resolved photoemission spectra (ARPES) and former LDA+DMFT results. As a
correction to LDA+DMFT, we observe more pronounced coherent peaks below the
Fermi level, as indicated by ARPES experiments. In addition, we resolve the
spectral functions in the and
sectors of DCA, where band insulating and metallic phases coexist. Our approach
can be applied to correlated compounds where not only local quantum
fluctuations but also spatial fluctuations are important.Comment: 6 pages, 3 figures, accepted in Phys. Rev.
Spontaneous Dissociation of Co2(CO)8 and Autocatalytic growth of Co on SiO2 : A Combined Experimental and Theoretical Investigation
We present experimental results and theoretical simulations of the adsorption
behavior of the metal-organic precursor Co2(CO)8 on SiO2 surfaces after
application of two different pre-treatment steps, namely by air plasma cleaning
or a focused electron beam pre-irradiation. We observe a spontaneous
dissociation of the precursor molecules as well as auto-deposition of cobalt on
the pre-treated SiO2 surfaces. We also find that the differences in metal
content and relative stability of these deposits depend on the pre-treatment
conditions of the substrate. Transport measurements of these deposits are also
presented. We are led to assume that the degree of passivation of the SiO2
surface by hydroxyl groups is an important controlling factor in the
dissociation process. Our calculations of various slab settings using
dispersion corrected density functional theory support this assumption. We
observe physisorption of the precursor molecule on a fully hydroxylated SiO2
surface (untreated surface) and chemisorption on a partially hydroxylated SiO2
surface (pre-treated surface) with a spontaneous dissociation of the precursor
molecule. In view of these calculations, we discuss the origin of this
dissociation and the subsequent autocatalysis.Comment: 22 pages, 8 Figures, In Press Article, Beilstein Journal of
Nanotechnology, 201
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