545 research outputs found
Single domain magnetic helicity and triangular chirality in structurally enantiopure Ba3NbFe3Si2O14
A novel doubly chiral magnetic order is found out in the structurally chiral
langasite compound BaNbFeSiO. The magnetic moments are
distributed over planar frustrated triangular lattices of triangle units. On
each of these they form the same triangular configuration. This ferro-chiral
arrangement is helically modulated from plane to plane. Unpolarized neutron
scattering on a single crystal associated with spherical neutron polarimetry
proved that a single triangular chirality together with a single helicity is
stabilized in an enantiopure crystal. A mean field analysis allows discerning
the relevance on this selection of a twist in the plane to plane
supersuperexchange paths
Spatially Anisotropic Heisenberg Kagome Antiferromagnet
We study the quasi-one-dimensional limit of the spin-1/2 quantum Heisenberg
antiferromagnet on the kagome lattice. The lattice is divided into
antiferromagnetic spin-chains (exchange J) that are weakly coupled via
intermediate "dangling" spins (exchange J'). Using one-dimensional
bosonization, renormalization group methods, and current algebra techniques the
ground state is determined in the limit J'<<J. We find that the dangling spins
and chain spins form a spiral with O(1) and O(J'/J) static moments,
respectively, atop of which the chain spins exhibit a smaller O[(J'/J)^2]
antiferromagnetically ordered component along the axis perpendicular to the
spiral plane.Comment: 17 pages, 3 figures, corrected sign error, corrected typos, updated
reference
Gamma-Ray Spectra & Variability of the Crab Nebula Emission Observed by BATSE
We report ~ 600 days of BATSE earth-occultation observations of the total
gamma-ray (30 keV to 1.7 MeV) emission from the Crab nebula, between 1991 May
24 (TJD 8400) and 1994 October 2 (TJD 9627). Lightcurves from 35-100, 100-200,
200-300, 300-400, 400-700, and 700-1000 keV, show that positive fluxes were
detected by BATSE in each of these six energy bands at significances of
approximately 31, 20, 9.2, 4.5, 2.6, and 1.3 sigma respectively per day. We
also observed significant flux and spectral variations in the 35-300 keV energy
region, with time scales of days to weeks. The spectra below 300 keV, averaged
over typical CGRO viewing periods of 6-13 days, can be well described by a
broken power law with average indices of ~ 2.1 and ~ 2.4 varying around a
spectral break at ~ 100 keV. Above 300 keV, the long-term averaged spectra,
averaged over three 400 d periods (TJD 8400-8800, 8800-9200, and 9200-9628,
respectively) are well represented by the same power law with index of ~ 2.34
up to ~ 670 keV, plus a hard spectral component extending from ~ 670 keV to ~
1.7 MeV, with a spectral index of ~ 1.75. The latter component could be related
to a complex structure observed by COMPTEL in the 0.7-3 MeV range. Above 3 MeV,
the extrapolation of the power-law continuum determined by the low-energy BATSE
spectrum is consistent with fluxes measured by COMPTEL in the 3-25 MeV range,
and by EGRET from 30-50 MeV. We interpret these results as synchrotron emission
produced by the interaction of particles ejected from the pulsar with the field
in different dynamical regions of the nebula system, as observed recently by
HST, XMM-Newton, and Chandra.Comment: To be published in the November 20, 2003, Vol 598 issue of the
Astrophysical Journa
Gamma-ray lines and neutrons from solar flares
The energy spectrum of accelerated protons and nuclei at the site of a limb flare was derived by a technique, using observations of the time dependent flux of high energy neutrons at the Earth. This energy spectrum is very similar to the energy spectra of 7 disk flares for which the accelerated particle spectra was previously derived using observations of 4 to 7 MeV to 2.223 MeV fluence ratios. The implied spectra for all of these flares are too steep to produce any significant amount of radiation from pi meson decay. It is suggested that the observed 10 MeV gamma rays from the flare are bremsstrahlung of relativistic electrons
Vertex Corrections and the Korringa Ratio in Strongly Correlated Electron Materials
We show that the Korringa ratio, associated with nuclear magnetic resonance
in metals, is unity if vertex corrections for the dynamic spin susceptibility
are negligible and the hyperfine coupling is momentum independent. In the
absence of vertex corrections we also find a Korringa behaviour for , the
nuclear spin relaxation rate, i.e., , and a temperature
independent Knight shift. These results are independent of the form and
magnitude of the self-energy (so far as is consistent with neglecting vertex
corrections) and of the dimensionality of the system.Comment: 5 pages. accepted for publication in J. Phys.: Condens. Matte
The Heisenberg antiferromagnet on an anisotropic triangular lattice: linear spin-wave theory
We consider the effect of quantum spin fluctuations on the ground state
properties of the Heisenberg antiferromagnet on an anisotropic triangular
lattice using linear spin-wave theory. This model should describe the magnetic
properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The ground state energy, the staggered
magnetization, magnon excitation spectra and spin-wave velocities are computed
as a function of the ratio between the second and first neighbours, J2/J1. We
find that near J2/J1 = 0.5, i.e., in the region where the classical spin
configuration changes from a Neel ordered phase to a spiral phase, the
staggered magnetization vanishes, suggesting the possibility of a quantum
disordered state. In this region, the quantum correction to the magnetization
is large but finite. This is in contrast to the frustrated Heisenberg model on
a square lattice, for which the quantum correction diverges logarithmically at
the transition from the Neel to the collinear phase. For large J2/J1, the model
becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the
quantum correction to the magnetization, within LSW, becomes comparable to the
classical magnetization, suggesting the possibility of a quantum disordered
state. We show that, in this regime, quantum fluctuations are much larger than
for a set of weakly coupled chains with non-frustated interchain coupling.Comment: 10 pages, RevTeX + epsf, 5 figures Replaced with published version.
Comparison to series expansions energies include
Vacuolating cytotoxin and variants in Atg16L1 that disrupt autophagy promote Helicobacter pylori infection in humans
Peer reviewedPreprin
Effect of Disorder on Fermi surface in Heavy Electron Systems
The Kondo lattice model with substitutional disorder is studied with
attention to the size of the Fermi surface and the associated Dingle
temperature. The model serves for understanding heavy-fermion Ce compounds
alloyed with La according to substitution Ce{x}La{1-x}. The Fermi surface is
identified from the steepest change of the momentum distribution of conduction
electrons, and is derived at low enough temperature by the dynamical mean-field
theory (DMFT) combined with the coherent potential approximation (CPA). The
Fermi surface without magnetic field increases in size with decreasing x from
x=1 (Ce end), and disappears at such x that gives the same number of localized
spins as that of conduction electrons. From the opposite limit of x=0 (La end),
the Fermi surface broadens quickly as x increases, but stays at the same
position as that of the La end. With increasing magnetic field, a metamagnetic
transition occurs, and the Fermi surface above the critical field changes
continuously across the whole range of x. The Dingle temperature takes a
maximum around x=0.5. Implication of the results to experimental observation is
discussed.Comment: 5 pages, 5 figure
On the state dependency of fast feedback processes in (palaeo) climate sensitivity
Palaeo data have been frequently used to determine the equilibrium (Charney)
climate sensitivity , and - if slow feedback processes (e.g. land
ice-albedo) are adequately taken into account - they indicate a similar range
as estimates based on instrumental data and climate model results. Most studies
implicitly assume the (fast) feedback processes to be independent of the
background climate state, e.g., equally strong during warm and cold periods.
Here we assess the dependency of the fast feedback processes on the background
climate state using data of the last 800 kyr and a conceptual climate model for
interpretation. Applying a new method to account for background state
dependency, we find K(Wm) using the latest LGM
temperature reconstruction and significantly lower climate sensitivity during
glacial climates. Due to uncertainties in reconstructing the LGM temperature
anomaly, is estimated in the range K(Wm).Comment: submitted to Geophysical Research Letter
Orbital ferromagnetism and anomalous Hall effect in antiferromagnets on distorted fcc lattice
The Berry phase due to the spin wavefunction gives rise to the orbital
ferromagnetism and anomalous Hall effect in the non-coplanar antiferromagnetic
ordered state on face centered cubic (fcc) lattice once the crystal is
distorted perpendicular to (1,1,1) or (1,1,0)- plane. The relevance to the real
systems -FeMn and NiS is also discussed.Comment: 4 pages, 3 figure
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