67,409 research outputs found
Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors
Based on a two-dimensional model of coupled two-leg spin ladders, we derive a
unified picture of recent neutron scattering data of stripe-ordered
La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the
superstructure satellites and of the triplon excitations at higher energies.
The resonance peak at the antiferromagnetic wave vector Q_AF in the
stripe-ordered phase corresponds to a saddle point in the dispersion of the
magnetic excitations. Quantitative agreement with the neutron data is obtained
for J= 130-160 meV and J_cyc/J = 0.2-0.25.Comment: 4 pages, 4 figures included updated version taking new data into
account; factor in spectral weight corrected; Figs. 2 and 4 change
Moment screening in the correlated Kondo lattice model
The magnetic correlations, local moments and the susceptibility in the
correlated 2D Kondo lattice model at half filling are investigated. We
calculate their systematic dependence on the control parameters J_K/t and U/t.
An unbiased and reliable exact diagonalization (ED) approach for ground state
properties as well as the finite temperature Lanczos method (FTLM) for specific
heat and the uniform susceptibility are employed for small tiles on the square
lattice. They lead to two major results: Firstly we show that the screened
local moment exhibits non-monotonic behavior as a function of U for weak Kondo
coupling J_K. Secondly the temperature dependence of the susceptibility
obtained from FTLM allows to extract the dependence of the characteristic Kondo
temperature scale T* on the correlation strength U. A monotonic increase of T*
for small U is found resolving the ambiguity from earlier investigations. In
the large U limit the model is equivalent to the 2D Kondo necklace model with
two types of localized spins. In this limit the numerical results can be
compared to those of the analytical bond operator method in mean field
treatment and excellent agreement for the total paramagnetic moment is found,
supporting the reliability of both methods.Comment: 19 pages, 9 figure
Kondo Lattice Model with Finite Temperature Lanczos Method
We investigate the Kondo Lattice Model on 2D clusters using the Finite
Temperature Lanczos Method. The temperature dependence of thermodynamic and
correlations functions are systematically studied for various Kondo couplings
JK. The ground state value of the total local moment is presented as well.
Finally, the phase diagrams of the finite clusters are constructed for periodic
and open boundary conditions. For the two boundary conditions, two different
regimes are found for small JK/t, depending on the distribution of
non-interacting conduction electron states. If there are states within JK
around the Fermi level, two energy scales, linear and quadratic in JK, exist.
The former is associated with the onsite screening and the latter with the RKKY
interaction. If there are no states within JK around the Fermi level, the only
energy scale is that of the RKKY interaction. Our results imply that the form
of the electron density of states (DOS) plays an important role in the
competition between the Kondo screening and the RKKY interaction. The former is
stronger if the DOS is larger around the Fermi level, while the latter is less
sensitive to the form of the DOS.Comment: 7 pages, 7 figures; corrected typo
Magnetic susceptibility in quasi one-dimensional Ba2V3O9: chain segmentation versus the staggered field effect
A pronounced Curie-like upturn of the magnetic susceptibility chi(T) of the
quasi one-dimensional spin chain compound Ba2V3O9 has been found recently.
Frequently this is taken as a signature for a staggered field mechanism due to
the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We
calculate this contribution within a realistic structure of vanadium 3d- and
oxygen 2p-orbitals and conclude that this mechanism is far too small to explain
experimental results. We propose that the Curie term is rather due to a
segmentation of spin chains caused by broken magnetic bonds which leads to
uncompensated S=1/2 spins of segments with odd numbers of spins. Using a
finite-temperature Lanczos method we calculate their effective moment and show
that ~1% of broken magnetic bonds is sufficient to reproduce the anomalous
low-T behavior of chi(T) in Ba2V3O9.Comment: 5 pages, 5 figures, REVTeX 4, minor corrections to the text,
references adde
Transient and steady-state shear banding in a lamellar phase as studied by Rheo-NMR
Flow fields and shear-induced structures in the lamellar (L-alpha) phase of the system triethylene glycol mono n-decyl ether (C10E3)/water were investigated by NMR velocimetry, diffusometry, and H-2 NMR spectroscopy. The transformation from multilamellar vesicles (MLVs) to aligned planar lamellae is accompanied by a transient gradient shear banding. A high-shear-rate band of aligned lamellae forms next to the moving inner wall of the cylindrical Couette shear cell while a low-shear-rate band of the initial MLV structure remains close to the outer stationary wall. The band of layers grows at the expense of the band of MLVs until the transformation is completed. This process scales with the applied strain. Wall slip is a characteristic of the MLV state, while aligned layers show no deviation from Newtonian flow. The homogeneous nature of the opposite transformation from well aligned layers to MLVs via an intermediate structure resembling undulated multilamellar cylinders is confirmed. The strain dependence of this transformation appears to be independent of temperature. The shear diagram, which represents the shear-induced structures as a function of temperature and shear rate, contains a transition region between stable layers and stable MLVs. The steady-state structures in the transition region show a continuous change from layer-like at high temperature to MLV-like at lower temperature. These structures are homogeneous on a length scale above a few micrometers
Type Ia supernova diversity: Standardizing the candles
Future use of type Ia supernovae for cosmology aims not only to determine the
equation of state of dark energy, but also to constrain possible variations in
its value. To achieve this goal, supernovae need to become better calibrated
standard candles - not only to improve the precision of the measurement, but
more importantly to gain better control over systematic uncertainties in order
to ensure the accuracy of the result.
Here we report on a project to quantify the diversity in type Ia supernovae,
and to look for trends and/or sub-types that can be used to improve their
calibration as standard candles. We implement a version of principal component
analysis on type Ia supernova spectra. Although the quantity of data is not
sufficient to draw any firm conclusions we show that this method holds promise
for, at the very least, effectively separating peculiar supernovae. Whether it
can be further used to improve the calibration of normal type Ia's remains a
project for future study.Comment: Conference Proceedings. Cefalu 2006, The multicoloured landscape of
compact objects and their explosive origins. Six pages, three figure
Entangled states of trapped ions allow measuring the magnetic field gradient of a single atomic spin
Using trapped ions in an entangled state we propose detecting a magnetic
dipole of a single atom at distance of a few m. This requires a
measurement of the magnetic field gradient at a level of about 10
Tesla/m. We discuss applications e.g. in determining a wide variation of
ionic magnetic moments, for investigating the magnetic substructure of ions
with a level structure not accessible for optical cooling and detection,and for
studying exotic or rare ions, and molecular ions. The scheme may also be used
for measureing spin imbalances of neutral atoms or atomic ensembles trapped by
optical dipole forces. As the proposed method relies on techniques well
established in ion trap quantum information processing it is within reach of
current technology.Comment: 4 pages, 2 fi
SkyMapper and the Southern Sky Survey - a resource for the southern sky
SkyMapper is amongst the first of a new generation of dedicated, wide-field
survey telescopes. The 1.3m SkyMapper telescope features a 5.7 square degree
field-of-view Cassegrain imager and will see first light in late 2007. The
primary goal of the facility is to conduct the Southern Sky Survey a six
colour, six epoch survey of the southern sky. The survey will provide
photometry for objects between 8th and 23rd magnitude with global photometric
accuracy of 0.03 magnitudes and astrometry to 50 mas. This will represent a
valuable scientific resource for the southern sky and in addition provide a
basis for photometric and astrometric calibration of imaging data.Comment: 6 pages, 4 figures, proceedings of ESO Calibration Workshop 200
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