140 research outputs found
Magnetic structure of EuFe2As2 determined by single crystal neutron diffraction
Among various parent compounds of iron pnictide superconductors, EuFe2As2
stands out due to the presence of both spin density wave of Fe and
antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal
neutron diffraction studies have been carried out to determine the magnetic
structure of this compound and to investigate the coupling of two magnetic
sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190
K and 19 K, respectively. The ordering of Fe2+ moments is associated with the
wave vector k = (1,0,1) and it takes place at the same temperature as the
tetragonal to orthorhombic structural phase transition, which indicates the
strong coupling between structural and magnetic components. The ordering of Eu
moment is associated with the wave vector k = (0,0,1). While both Fe and Eu
spins are aligned along the long a axis as experimentally determined, our
studies suggest a weak coupling between the Fe and Eu magnetism.Comment: 7 pages, 7 figure
Field induced spin reorientation and giant spin-lattice coupling in EuFe2As2
We have studied a EuFe2As2 single crystal by neutron diffraction under
magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin
reorientation is observed in the presence of a magnetic field along both the a
and c axes, respectively. Above critical field, the ground state
antiferromagnetic configuration of Eu moments transforms into a
ferromagnetic structure with moments along the applied field direction. The
magnetic phase diagram for Eu magnetic sublattice in EuFe2As2 is presented. A
considerable strain (0.9%) is induced by the magnetic field, caused by
the realignment of the twinning structure. Furthermore, the realignment of the
twinning structure is found to be reversible with the rebound of magnetic
field, which suggested the existence of magnetic shape-memory effect. The Eu
moment ordering exhibits close relationship with the twinning structure. We
argue that the Zeeman energy in combined with magnetic anisotropy energy is
responsible for the observed spin-lattice coupling.Comment: 5 pages, 4 figure
Helicity skewed quark distributions of the nucleon and chiral symmetry
We compute the helicity skewed quark distributions and
in the chiral quark-soliton model of the nucleon. This model
emphasizes correctly the role of spontaneously broken chiral symmetry in
structure of nucleon. It is based on the large-N_c picture of the nucleon as a
soliton of the effective chiral lagrangian and allows to calculate the leading
twist quark- and antiquark distributions at a low normalization point. We
discuss the role of chiral symmetry in the helicity skewed quark distributions
and . We show that generalization of soft pion
theorems, based on chiral Ward identities, leads in the region of -\xi < x <
\xi to the pion pole contribution to which dominates at small
momentum transfer.Comment: 22 pages, 5 figure
Pressure dependence of phonon modes across the tetragonal to collapsed tetragonal phase transition in CaFe2As2
The pressure dependence of a large number of phonon modes in CaFe2As2 with
energies covering the full range of the phonon spectrum has been studied using
inelastic x-ray and neutron scattering. The observed phonon frequency changes
are in general rather small despite the sizable changes of the lattice
parameters at the phase transition. This indicates that the bonding properties
are not profoundly altered by the phase transition. The transverse acoustic
phonons propagating along the c-direction are an exception because they stiffen
very significantly in response to the large contraction of the c-axis. The
lattice parameters are found to change significantly as a function of pressure
before, during and after the first-order phase transition. However, the
frequencies change nearly uniformly with the change in the lattice parameters
due to pressure, with no regard specifically to the first-order phase
transition. Density functional theory describes the frequencies in both the
zero pressure and in the collapsed phase in a satisfactory way if based on the
respective crystal structures
Stimulus-preceding negativity in ADHD
Children with ADHD often show disrupted response preparation as indicated by attenuated stimulus-preceding negativity (SPN). This study examined response preparation in a relatively short cue-stimulus interval. No differences in SPN occurred between children with ADHD and their normal peers. A strong positive relationship was found between SPN and mean reaction time in both groups. Children with ADHD are able to mentally prepare themselves for upcoming events in short cue-stimulus intervals. © 2013 Springer-Verlag Wien
Baryons as non-topological chiral solitons
The present review gives a survey of recent developments and applications of
the Nambu--Jona-Lasinio model with and quark flavors for the
structure of baryons. The model is an effective chiral quark theory which
incorporates the SU(N)SU(N)U(1) approximate
symmetry of Quantum chromodynamics. The approach describes the spontaneous
chiral symmetry breaking and dynamical quark mass generation. Mesons appear as
quark-antiquark excitations and baryons arise as non-topological solitons with
three valence quarks and a polarized Dirac sea. For the evaluation of the
baryon properties the present review concentrates on the non-linear
Nambu--Jona-Lasinio model with quark and Goldstone degrees of freedom which is
identical to the Chiral quark soliton model obtained from the instanton liquid
model of the QCD vacuum. In this non-linear model, a wide variety of
observables of baryons of the octet and decuplet is considered. These include,
in particular, electromagnetic, axial, pseudoscalar and pion nucleon form
factors and the related static properties like magnetic moments, radii and
coupling constants of the nucleon as well as the mass splittings and
electromagnetic form factors of hyperons. Predictions are given for the strange
form factors, the scalar form factor and the tensor charge of the nucleon.Comment: 104 pages, 27 figures as uuencoded and compressed postscript files ,
hardcopy available upon request; Prog.Part.Nucl.Phys. 37 (1996) (in print
Scheme dependence of NLO corrections to exclusive processes
We apply the so-called conformal subtraction scheme to predict perturbatively
exclusive processes beyond leading order. Taking into account evolution
effects, we study the scheme dependence for the photon-to-pion transition form
factor and the electromagnetic pion form factor at next-to-leading order for
different pion distribution amplitudes. Relying on the conformally covariant
operator product expansion and using the known higher order results for
polarized deep inelastic scattering, we are able to predict perturbative
corrections to the hard-scattering amplitude of the photon-to-pion transition
form factor beyond next-to-leading order in the conformal scheme restricted to
the conformal limit of the theory.Comment: RevTeX, 25 pages, 2 figures, 5 tables, minor changes, to be published
in Phys. Rev.
A 3′ UTR SNP in COL18A1 Is Associated with Susceptibility to HBV Related Hepatocellular Carcinoma in Chinese: Three Independent Case-Control Studies
BACKGROUND: Accumulated evidences indicate that single nucleotide polymorphisms (SNP) in angiogenesis and tumorigenesis related genes are associated with risk of Hepatocellular carcinoma (HCC). COL18A1 encodes the precursor of endostatin, which is a broad-spectrum angiogenesis inhibitor, and we speculate that SNPs in COL18A1 may be associated with susceptibility to HCC. METHODS AND FINDINGS: We carried out a 2-stage association study in 3 independent case-control groups in a total of 1067 chronic hepatitis B (CHB) patients and 808 hepatitis B virus (HBV) related HCC patients in Han Chinese. Four SNPs which can represent all potential functional SNPs with MAF>0.1 recorded in HapMap database were genotyped using TaqMan methods. Levels of total COL18A1 mRNA were also examined using quantitative real-time RT-PCR. We found that rs7499 located in 3'-UTR to be strongly associated with HBV related HCC (P(combined) = 0.0000005, OR = 0.72, 95%CI = 0.63-0.82). COL18A1 mRNA expression was significantly decreased as the disease progressed (P = 0.000026). CONCLUSION: These findings indicate that COL18A1 rs7499 may contribute to the risk of HCC in Han Chinese
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