5,639 research outputs found
Disorder Induced Ferromagnetism in CaRuO3
The magnetic ground state of perovskite structure CaRuO3 has been enigmatic
for decades. Here we show that paramagnetic CaRuO3 can be made ferromagnetic by
very small amounts of partial substitution of Ru by Ti. Magnetic hysteresis
loops are observed at 5 K for as little as 2% Ti substitution. Ti is
non-magnetic and isovalent with Ru, indicating that the primary effect of the
substitution is the disruption of the magnetic ground state of CaRuO3 through
disorder. The data suggest that CaRuO3 is poised at a critical point between
ferromagnetic and paramagnetic ground states
Search for Direct CP Violation in Non-Leptonic Decays of Charged and Hyperons
A search for direct CP violation in the non-leptonic decays of hyperons has
been performed. In comparing the product of the decay parameters,
, in terms of an asymmetry parameter,
, between hyperons and anti-hyperons in the charged and decay sequence, we found no evidence of
direct CP violations. The parameter was measured to be .Comment: Submitted for publication; RevTex, 13 pages, 4 figure
On the High Energy Emission of the Short GRB 090510
Long-lived high-energy (>100MeV) emission, a common feature of most Fermi-LAT
detected gamma-ray burst, is detected up to \sim 10^2 s in the short GRB
090510. We study the origin of this long-lived high-energy emission, using
broad-band observations including X-ray and optical data. We confirm that the
late > 100 MeV, X-ray and optical emission can be naturally explained via
synchrotron emission from an adiabatic forward shock propagating into a
homogeneous ambient medium with low number density. The Klein-Nishina effects
are found to be significant, and effects due to jet spreading and magnetic
field amplification in the shock appear to be required. Under the constraints
from the low-energy observations, the adiabatic forward shock synchrotron
emission is consistent with the later-time (t>2s) high-energy emission, but
falls below the early-time (t < 2s) high energy emission. Thus we argue that an
extra high energy component is needed at early times. A standard reverse shock
origin is found to be inconsistent with this extra component. Therefore, we
attribute the early part of the high-energy emission (t< 2s) to the prompt
component, and the long-lived high energy emission (t>2s) to the adiabatic
forward shock synchrotron afterglow radiation. This avoids the requirement for
an extremely high initial Lorentz factor.Comment: 29 pages, 2 figures; Accepted for publication in Ap
Novel critical exponent of magnetization curves near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3 (A = Ca, La0.5Na0.5, and La)
We report a novel critical exponent delta=3/2 of magnetization curves
M=H^{1/delta} near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3
(A = Ca, La0.5Na0.5, and La), which the mean field theory of the
Ginzburg-Landau-Wilson type fails to reproduce. The effect of dirty
ferromagnetic spin fluctuations might be a key.Comment: 4 pages, 5 figure
Synchrotron signature of a relativistic blast wave with decaying microturbulence
Microphysics of weakly magnetized relativistic collisionless shock waves,
corroborated by recent high performance numerical simulations, indicate the
presence of a microturbulent layer of large magnetic field strength behind the
shock front, which must decay beyond some hundreds of skin depths. The present
paper discusses the dynamics of such microturbulence, borrowing from these same
numerical simulations, and calculates the synchrotron signature of a powerlaw
of shock accelerated particles. The decaying microturbulent layer is found to
leave distinct signatures in the spectro-temporal evolution of the spectrum
of a decelerating blast wave, which are
potentially visible in early multi-wavelength follow-up observations of
gamma-ray bursts. This paper also discusses the influence of the evolving
microturbulence on the acceleration process, with particular emphasis on the
maximal energy of synchrotron afterglow photons, which falls in the GeV range
for standard gamma-ray burst parameters. Finally, this paper argues that the
evolving microturbulence plays a key role in shaping the spectra of recently
observed gamma-ray bursts with extended GeV emission, such as GRB090510.Comment: version to appear in MNRAS -- minor modifications + added discussion
of synchrotron self-absorption; 23 pages, 8 figure
HyperCP: A high-rate spectrometer for the study of charged hyperon and kaon decays
The HyperCP experiment (Fermilab E871) was designed to search for rare
phenomena in the decays of charged strange particles, in particular CP
violation in and hyperon decays with a sensitivity of
. Intense charged secondary beams were produced by 800 GeV/c protons
and momentum-selected by a magnetic channel. Decay products were detected in a
large-acceptance, high-rate magnetic spectrometer using multiwire proportional
chambers, trigger hodoscopes, a hadronic calorimeter, and a muon-detection
system. Nearly identical acceptances and efficiencies for hyperons and
antihyperons decaying within an evacuated volume were achieved by reversing the
polarities of the channel and spectrometer magnets. A high-rate
data-acquisition system enabled 231 billion events to be recorded in twelve
months of data-taking.Comment: 107 pages, 45 Postscript figures, 14 tables, Elsevier LaTeX,
submitted to Nucl. Instrum. Meth.
Investigation of the ferromagnetic transition in the correlated 4d perovskites SrRuRhO
The solid-solution SrRuRhO () is a
variable-electron-configuration system forming in the nearly-cubic-perovskite
basis, ranging from the ferromagnetic 4 to the enhanced paramagnetic
4. Polycrystalline single-phase samples were obtained over the whole
composition range by a high-pressure-heating technique, followed by
measurements of magnetic susceptibility, magnetization, specific heat,
thermopower, and electrical resistivity. The ferromagnetic order in long range
is gradually suppressed by the Rh substitution and vanishes at .
The electronic term of specific-heat shows unusual behavior near the critical
Rh concentration; the feature does not match even qualitatively with what was
reported for the related perovskites (Sr,Ca)RuO. Furthermore, another
anomaly in the specific heat was observed at .Comment: Accepted for publication in PR
Hamiltonian Description of Composite Fermions: Magnetoexciton Dispersions
A microscopic Hamiltonian theory of the FQHE, developed by Shankar and myself
based on the fermionic Chern-Simons approach, has recently been quite
successful in calculating gaps in Fractional Quantum Hall states, and in
predicting approximate scaling relations between the gaps of different
fractions. I now apply this formalism towards computing magnetoexciton
dispersions (including spin-flip dispersions) in the , 2/5, and 3/7
gapped fractions, and find approximate agreement with numerical results. I also
analyse the evolution of these dispersions with increasing sample thickness,
modelled by a potential soft at high momenta. New results are obtained for
instabilities as a function of thickness for 2/5 and 3/7, and it is shown that
the spin-polarized 2/5 state, in contrast to the spin-polarized 1/3 state,
cannot be described as a simple quantum ferromagnet.Comment: 18 pages, 18 encapsulated ps figure
Inelastic neutron scattering study of magnetic excitations in SrRuO
Magnetic excitations in \srruo ~ have been studied by inelastic neutron
scattering. The magnetic fluctuations are dominated by incommensurate peaks
related to the Fermi surface nesting of the quasi-one-dimensional - and
-bands. The shape of the incommensurate signal agrees well with RPA
calculations. At the incommensurate {\bf Q}-positions the energy spectrum
considerably softens upon cooling pointing to a close magnetic instability :
\srruo ~does not exhibit quantum criticality but is very close to it. -scaling may be fitted to the data for temperatures above 30 K. Below the
superconducting transition, the magnetic response at the nesting signal is not
found to change in the energy range down to 0.4meV.Comment: 11 pages 9 figure
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