2,792 research outputs found
Highly Dispersive Spin Excitations in the Chain Cuprate Li2CuO2
We present an inelastic neutron scattering investigation of Li2CuO2 detecting
the long sought quasi-1D magnetic excitations with a large dispersion along the
CuO2-chains studied up to 25 meV. The total dispersion is governed by a
surprisingly large ferromagnetic (FM) nearest-neighbor exchange integral
J1=-228 K. An anomalous quartic dispersion near the zone center and a
pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral
excitation with a pitch angle about 41 degree point to the vicinity of a 3D
FM-spiral critical point. The leading exchange couplings are obtained applying
standard linear spin-wave theory. The 2nd neighbor inter-chain interaction
suppresses a spiral state and drives the FM in-chain ordering below the Ne'el
temperature. The obtained exchange parameters are in agreement with the results
for a realistic five-band extended Hubbard Cu 3d O 2p model and L(S)DA+U
predictions.Comment: 6 pages, 4 figures, submitted to Europhys. Let
Elastic scattering of 11,8 MEV deuterons from several elements
The angular distributions of 11.8 MeV deuterons elastically scattered from C, Mg, Al, Ti, Fe, Ni, Cu, Zn, Zr, Nb, Rh, Pd, Ag, Cd, In, Sn, Ta and Au have been measured. The detector is capable of electronically separating deuterons from other ions which may enter the detector. The data has been taken in 2 degree steps between 20° and 165°. The structure in the angular distribution observed with the light target elements vanishes with increasing atomic weight. An exception is observed between A=90 and A=120
Towards Probing Conformational States of Y2 Receptor Using Hyperpolarized 129Xe NMR
G protein-coupled receptors can adopt many different conformational states, each of them exhibiting different restraints towards downstream signaling pathways. One promising strategy to identify and quantify this conformational landscape is to introduce a cysteine at a receptor site sensitive to different states and label this cysteine with a probe for detection. Here, the application of NMR of hyperpolarized 129Xe for the detection of the conformational states of human neuropeptide Y2 receptor is introduced. The xenon trapping cage molecule cryptophane-A attached to a cysteine in extracellular loop 2 of the receptor facilitates chemical exchange saturation transfer experiments without and in the presence of native ligand neuropeptide Y. High-quality spectra indicative of structural states of the receptor–cage conjugate were obtained. Specifically, five signals could be assigned to the conjugate in the apo form. After the addition of NPY, one additional signal and subtle modifications in the persisting signals could be detected. The correlation of the spectroscopic signals and structural states was achieved with molecular dynamics simulations, suggesting frequent contact between the xenon trapping cage and the receptor surface but a preferred interaction with the bound ligand
Correlation between the residual resistance ratio and magnetoresistance in MgB2
The resistivity and magnetoresistance in the normal state for bulk and
thin-film MgB2 with different nominal compositions have been studied
systematically. These samples show different temperature dependences of normal
state resistivity and residual resistance ratios although their superconducting
transition temperatures are nearly the same, except for the thin-film sample.
The correlation between the residual resistance ratio (RRR) and the power law
dependence of the low temperature resistivity, rho vs. T^c, indicates that the
electron-phonon interaction is important. It is found that the
magnetoresistance (MR) in the normal state scales well with the RRR, a0(MR)
proportional to (RRR)^2.2 +/- 0.1 at 50 K. This accounts for the large
difference in magnetoresistance reported by various groups, due to different
defect scatterings in the samples.Comment: 10 pages, 3 figures, submitted to Phys. Rev. B (July 6, 2001; revised
September 27, 2001); discussion of the need for excess Mg in processing and
of the power law dependence of the low temperature resistivity added in
response to referee's comment
Ferromagnetic transition metal implanted ZnO: a diluted magnetic semiconductor?
Recently theoretical works predict that some semiconductors (e.g. ZnO) doped
with magnetic ions are diluted magnetic semiconductors (DMS). In DMS magnetic
ions substitute cation sites of the host semiconductor and are coupled by free
carriers resulting in ferromagnetism. One of the main obstacles in creating DMS
materials is the formation of secondary phases because of the solid-solubility
limit of magnetic ions in semiconductor host. In our study transition metal
ions were implanted into ZnO single crystals with the peak concentrations of
0.5-10 at.%. We established a correlation between structural and magnetic
properties. By synchrotron radiation X-ray diffraction (XRD) secondary phases
(Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as
the source for ferromagnetism. Due to their different crystallographic
orientation with respect to the host crystal these nanocrystals in some cases
are very difficult to be detected by a simple Bragg-Brentano scan. This results
in the pitfall of using XRD to exclude secondary phase formation in DMS
materials. For comparison, the solubility of Co diluted in ZnO films ranges
between 10 and 40 at.% using different growth conditions pulsed laser
deposition. Such diluted, Co-doped ZnO films show paramagnetic behaviour.
However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d
exchange interaction as compared to Co-implanted ZnO single crystals.Comment: 27 pages, 8 figure
Superconducting properties and c-axis superstructure of Mg1-xAlxB2
The superconducting and structural properties of a series of Mg1-xAlxB2
samples have been investigated. X-ray diffraction results confirmed the
existence of a structural transition associated with the significant change in
inter-boron layer distance as reported previously by Slusky et al.
Moreover,transmission-electron-microscopy observations revealed the existence
of a superstructure with doubled lattice constant along the c-axis direction.
We propose that this superstructure is essentially related to the structural
transition. The modifications of superconducting transition temperature Tc, the
normal state resistivity, and the upper critical field Bc2 by Al doping are
discussed in terms of Al-substitution induced changes in the electronic
structure at the Fermi energy.Comment: 15 pages, 7 figure
Einstein and Brans-Dicke frames in multidimensional cosmology
Inhomogeneous multidimensional cosmological models with a higher dimensional
space-time manifold M= M_0 x M_1 ...x M_n are investigated under dimensional
reduction to a D_0-dimensional effective non-minimally coupled sigma-model
which generalizes the familiar Brans-Dicke model.
It is argued that the Einstein frame should be considered as the physical
one. The general prescription for the Einstein frame reformulation of known
solutions in the Brans-Dicke frame is given. As an example, the reformulation
is demonstrated explicitly for the generalized Kasner solutions where it is
shown that in the Einstein frame there are no solutions with inflation of the
external space.Comment: 27 pages, Revte
X-ray absorption and optical spectroscopy studies of (MgAl)B
X-ray absorption spectroscopy and optical reflectance measurements have been
carried out to elucidate the evolution of the electronic structure in
(MgAl)B for \emph{x} = 0.0,0.1, 0.2, 0.3, and 0.4. The
important role of B 2\emph{p} hole states to superconductivity has
been identified, and the decrease in the hole carrier number is
\emph{quantitatively} determined. The rate of the decrease in the hole
concentration agree well with the theoretical calculations. On the other
hand,while the evolution of the electronic structure is gradual through the
doping range, suppression is most significant at \emph{x} = 0.4. These
results suggest that the superstructure in (MgAl)B, in
addition to the holes, can affect the lattice dynamics and contributes
to the suppression effect. Other possible explanations like the
topological change of the band Fermi surface are also discussed.Comment: 17 pages, 5 figures. Phys. Rev. B, in pres
Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910
As a result of the variability survey in Chi Persei and NGC6910, the number
of Beta Cep stars that are members of these two open clusters is increased to
twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational
properties, in particular the frequency spectra, of Beta Cep stars in both
clusters and explain the differences in terms of the global parameters of the
clusters. We also indicate that the more complicated pattern of the variability
among B type stars in Chi Persei is very likely caused by higher rotational
velocities of stars in this cluster. We conclude that the sample of pulsating
stars in the two open clusters constitutes a very good starting point for the
ensemble asteroseismology of Beta Cep-type stars and maybe also for other
B-type pulsators.Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife,
Lanzarote, Feb 2010, submitte
Thermal conductivity via magnetic excitations in spin-chain materials
We discuss the recent progress and the current status of experimental
investigations of spin-mediated energy transport in spin-chain and spin-ladder
materials with antiferromagnetic coupling. We briefly outline the central
results of theoretical studies on the subject but focus mainly on recent
experimental results that were obtained on materials which may be regarded as
adequate physical realizations of the idealized theoretical model systems. Some
open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure
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