10,791 research outputs found
Berry's phase contribution to the anomalous Hall effect of gadolinium
When conduction electrons are forced to follow the local spin texture, the
resulting Berry phase can induce an anomalous Hall effect (AHE). In gadolinium,
as in double-exchange magnets, the exchange interaction is mediated by the
conduction electrons and the AHE may therefore resemble that of chromium
dioxide and other metallic double-exchange ferromagnets. The Hall resistivity,
magnetoresistance, and magnetization of single crystal gadolinium were measured
in fields up to 30 T. Measurements between 2 K and 400 K are consistent with
previously reported data. A scaling analysis for the Hall resistivity as a
function of the magnetization suggests the presence of a Berry's-phase
contribution to the anomalous Hall effect.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Coercive Field and Magnetization Deficit in Ga(1-x)Mn(x)As Epilayers
We have studied the field dependence of the magnetization in epilayers of the
diluted magnetic semiconductor Ga(1-x)Mn(x)As for 0.0135 < x < 0.083.
Measurements of the low temperature magnetization in fields up to 3 T show a
significant deficit in the total moment below that expected for full saturation
of all the Mn spins. These results suggest that the spin state of the
non-ferromagnetic Mn spins is energetically well separated from the
ferromagnetism of the bulk of the spins. We have also studied the coercive
field (Hc) as a function of temperature and Mn concentration, finding that Hc
decreases with increasing Mn concentration as predicted theoretically.Comment: 15 total pages -- 5 text, 1 table, 4 figues. Accepted for publication
in MMM 2002 conference proceedings (APL
The Discovery of Extended Thermal X-ray Emission from PKS 2152-699: Evidence for a `Jet-cloud' Interaction
A Chandra ACIS-S observation of PKS 2152-699 reveals thermal emission from a
diffuse region around the core and a hotspot located 10" northeast from the
core. This is the first detection of thermal X-ray radiation on kiloparsec
scales from an extragalactic radio source. Two other hotspots located 47"
north-northeast and 26" southwest from the core were also detected. Using a
Raymond-Smith model, the first hotspot can be characterized with a thermal
plasma temperature of 2.6 K and an electron number density of 0.17
cm. These values correspond to a cooling time of about 1.6
yr. In addition, an emission line from the hotspot, possibly Fe xxv, was
detected at rest wavelength 10.04\AA.
The thermal X-ray emission from the first hotspot is offset from the radio
emission but is coincident with optical filaments detected with broadband
filters of HST/WFPC2. The best explanation for the X-ray, radio, and optical
emission is that of a `jet-cloud' interaction.
The diffuse emission around the nucleus of PKS 2152-699 can be modeled as a
thermal plasma with a temperature of 1.2 K and a luminosity of
1.8 erg s. This emission appears to be asymmetric with a
small extension toward Hotspot A, similar to a jet. An optical hotspot (EELR)
is seen less than an arcsecond away from this extension in the direction of the
core. This indicates that the extension may be caused by the jet interacting
with an inner ISM cloud, but entrainment of hot gas is unavoidable. Future
observations are discussed.Comment: To appear in the Astrophysical Journal 21 pages, 5 Postscript
figures, 1 table, AASTeX v. 5.
Breakdown of the lattice polaron picture in La0.7Ca0.3MnO3 single crystals
When heated through the magnetic transition at Tc, La0.7Ca0.3MnO3 changes
from a band metal to a polaronic insulator. The Hall constant R_H, through its
activated behavior and sign anomaly, provides key evidence for polaronic
behavior. We use R_H and the Hall mobility to demonstrate the breakdown of the
polaron phase. Above 1.4Tc, the polaron picture holds in detail, while below,
the activation energies of both R_H and the mobility deviate strongly from
their polaronic values. These changes reflect the presence of metallic,
ferromagnetic fluctuations, in the volume of which the Hall effect develops
additional contributions tied to quantal phases.Comment: 11 pages, 3 figures, final version to appear in Phys. Rev. B Rapi
{BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters
We have initiated a long-term project, the BOAO photometric survey of open
clusters, to enlarge our understanding of galactic structure using UBVI CCD
photometry of open clusters which have been little studied before. This is the
second paper of the project in which we present the photometry of 12 open
clusters. We have determined the cluster parameters by fitting the Padova
isochrones to the color-magnitude diagrams of the clusters. All the clusters
except for Be 0 and NGC 1348 are found to be intermediate-age to old (0.2 - 4.0
Gyrs) open clusters with a mean metallicity of [Fe/H] = 0.0.Comment: 11 page
Molecular dynamics study of the fragmentation of silicon doped fullerenes
Tight binding molecular dynamics simulations, with a non orthogonal basis
set, are performed to study the fragmentation of carbon fullerenes doped with
up to six silicon atoms. Both substitutional and adsorbed cases are considered.
The fragmentation process is simulated starting from the equilibrium
configuration in each case and imposing a high initial temperature to the
atoms. Kinetic energy quickly converts into potential energy, so that the
system oscillates for some picoseconds and eventually breaks up. The most
probable first event for substituted fullerenes is the ejection of a C2
molecule, another very frequent event being that one Si atom goes to an
adsorbed position. Adsorbed Si clusters tend to desorb as a whole when they
have four or more atoms, while the smaller ones tend to dissociate and
sometimes interchange positions with the C atoms. These results are compared
with experimental information from mass abundance spectroscopy and the products
of photofragmentation.Comment: Seven two-column pages, six postscript figures. To be published in
Physical Review
Galactic Center Radio Constraints on Gamma-Ray Lines from Dark Matter Annihilation
Recent evidence for one or more gamma-ray lines at ~ 130 GeV in the Fermi-LAT
data from the Galactic Center has been interpreted as a hint for dark matter
annihilation to Z{\gamma} or H{\gamma} with an annihilation cross section,
~ 10^{-27} cm^3 s^{-1} . We test this hypothesis by comparing
synchrotron fluxes due to the electrons and positrons from the decay of the Z
or the H boson only in the Galactic Center against radio data from the same
region in the Galactic Center. We find that the radio data from single dish
telescopes marginally constrain this interpretation of the claimed gamma lines
for a contracted NFW profile. Already-operational radio telescopes such as LWA,
VLA-Low and LOFAR, and future radio telescopes like SKA, which are sensitive to
annihilation cross sections as small as 10^{-28} cm^3 s^{-1}, can confirm or
rule out this scenario very soon. We discuss the assumptions on the dark matter
profile, magnetic fields, and background radiation density profiles, and show
that the constraints are relatively robust for any reasonable assumptions.
Independent of the above said recent developments, we emphasize that our radio
constraints apply to all models where dark matter annihilates to Z{\gamma} or
H{\gamma}.Comment: v3: 18 pages, 7 figures. Minor changes. Published in Phys. Rev.
Collider Signatures of Neutrino Masses and Mixing from R-parity Violation
R-parity violation in the supersymmetric standard model can be the source of
neutrino masses and mixing. We analyze the neutrino mass matrix coming from
either bilinear or trilinear R-parity violation and its collider signatures,
assuming that the atmospheric and solar neutrino data are explained by three
active neutrino oscillations. Taking the gauge mediated supersymmetry breaking
mechanism, we show that the lightest neutralino decays well inside the detector
and the model could be tested by observing its branching ratios in the future
colliders. In the bilinear model where only the small solar neutrino mixing
angle can be accommodated, the relation, BR()
BR() BR(), serves as a
robust test of the model. The large mixing angle solution can be realized in
the trilinear model which predicts BR() BR() BR(). In either case, the
relation, BR() BR() BR(), should hold to be
consistent with the atmospheric neutrino and CHOOZ experiments.Comment: 24pages, Late
Neutrino Anomalies in Gauge Mediated Model with Trilinear R violation
The structure of neutrino masses and mixing resulting from trilinear
violating interactions is studied in the presence of the gauge mediated
supersymmetry breaking. Neutrino masses arise in this model at tree level
through the RG-induced vacuum expectation values of the sneutrinos and also
through direct contribution at 1-loop. The relative importance of these
contributions is determined by the values of the strong and weak coupling
constants. In case of purely couplings, the tree contribution
dominates over the 1-loop diagram. In this case, one simultaneously obtains
atmospheric neutrino oscillations and quasi-vacuum oscillations of the solar
neutrinos if all the \l' couplings are assumed to be of similar magnitudes.
If R parity violation arises from the trilinear \l couplings, then the loop
induced contribution dominates over the tree level. One cannot simultaneously
explain the solar and atmospheric deficit in this case if all the \l
couplings are of similar magnitude. This however becomes possible with
hierarchical \l and we give a specific example of this.Comment: 26 pages Latex, 2 figures, certain sections rewritten, improved
discussion about derivations added. To appear in Physical Review
- …