37,648 research outputs found
Peak Energy-Isotropic Energy Relation in the Off-Axis Gamma-Ray Burst Model
Using a simple uniform jet model of prompt emissions of gamma-ray bursts
(GRBs), we reproduce the observed peak energy--isotropic energy relation. A
Monte Carlo simulation shows that the low-isotropic energy part of the relation
is dominated by events viewed from off-axis directions, and the number of the
off-axis events is about one-third of the on-axis emissions. We also compute
the observed event rates of the GRBs, the X-ray-rich GRBs, and the X-ray
flashes detected by HETE-2, and we find that they are similar.Comment: 11 pages, 2 eps figure
Nonlinear Evolution of Very Small Scale Cosmological Baryon Perturbations at Recombination
The evolution of baryon density perturbations on very small scales is
investigated. In particular, the nonlinear growth induced by the radiation drag
force from the shear velocity field on larger scales during the recombination
epoch, which is originally proposed by Shaviv in 1998, is studied in detail. It
is found that inclusion of the diffusion term which Shaviv neglected in his
analysis results in rather mild growth whose growth rate is instead
of enormous amplification of Shaviv's original claim since the
diffusion suppresses the growth. The growth factor strongly depends on the
amplitude of the large scale velocity field. The nonlinear growth mechanism is
applied to density perturbations of general adiabatic cold dark matter (CDM)
models. In these models, it has been found in the previous works that the
baryon density perturbations are not completely erased by diffusion damping if
there exists gravitational potential of CDM. With employing the perturbed rate
equation which is derived in this paper, the nonlinear evolution of baryon
density perturbations is investigated. It is found that: (1) The nonlinear
growth is larger for smaller scales. This mechanism only affects the
perturbations whose scales are smaller than , which are
coincident with the stellar scales. (2) The maximum growth factors of baryon
density fluctuations for various COBE normalized CDM models are typically less
than factor 10 for large scale velocity peaks. (3) The growth factor
depends on .Comment: 24 pages, 9 figures, submitted to Ap
I=2 Pion Scattering Length from Two-Pion Wave Functions
We calculate the two-pion wave function in the ground state of the I=2
-wave system and find the interaction range between two pions, which allows
us to examine the validity of the necessary condition for the finite-volume
method for the scattering length proposed by L\"uscher. We work in the quenched
approximation employing a renormalization group improved gauge action for
gluons and an improved Wilson action for quarks at on
, and lattices. We conclude
that the necessary condition is satisfied within the statistical errors for the
lattice sizes () when the quark mass is in the range
that corresponds to . We obtain the
scattering length with a smaller statistical error from the wave function than
from the two-pion time correlator.Comment: LaTeX2e, 34 pages, 11 eps figures, uses revtex4 and graphic
A lattice NRQCD calculation of the mixing parameter B_B
We present a lattice calculation of the B meson B-parameter B_B using the
NRQCD action. The heavy quark mass dependence is explicitly studied over a mass
range between m_b and 4m_b with the and actions. We
find that the ratios of lattice matrix elements and
, which contribute to B_B through mixing, have
significant dependence while that of the leading operator
has little effect. The combined result for
B_B(m_b) has small but non-zero mass dependence, and the B_B(m_b) becomes
smaller by 10% with the 1/m_Q correction compared to the static result. Our
result in the quenched approximation at \beta=5.9 is B_{B_d}(5 GeV) =
0.75(3)(12), where the first error is statistical and the second is a
systematic uncertainty.Comment: 20 pages, 11 figures, uses REVTeX, typos correcte
Differential decay rate for semileptonic decays
We present our study on semileptonic decay form factors
with NRQCD action for heavy quark from a quenched lattice QCD simulation at
=5.9 on a lattice. We obtain form factors defined in the
context of heavy quark effective theory by Burdman et al. and find that their
correction is small. The limit of physical heavy and light quark masses
can be performed without introducing any model function, and we obtain a
prediction for the differential decay rate . We also discuss the
soft pion limit of the form factors.Comment: Lattice 2000, 4 pages, 4 figures, Late
Multilayer graphene shows intrinsic resistance peaks in the carrier density dependence
Since the advent of graphene, a variety of studies have been performed to
elucidate its fundamental physics, or to explore its practical applications.
Gate-tunable resistance is one of the most important properties of graphene and
has been studied in 1-3 layer graphene in a number of efforts to control the
band gap to obtain a large on-off ratio. On the other hand, the transport
property of multilayer graphene with more than three layers is less well
understood. Here we show a new aspect of multilayer graphene. We found that
four-layer graphene shows intrinsic peak structures in the gate voltage
dependence of its resistance at zero magnetic field. Measurement of quantum
oscillations in magnetic field confirmed that the peaks originate from the
specific band structure of graphene and appear at the carrier density for the
bottoms of conduction bands and valence bands. The intrinsic peak structures
should generally be observed in AB-stacked multilayer graphene. The present
results would be significant for understanding the physics of graphene and
making graphene FET devices
Arbitrary Choice of Basic Variables in Density Functional Theory. II. Illustrative Applications
Our recent theory (Ref. 1) enables us to choose arbitrary quantities as the
basic variables of the density functional theory. In this paper we apply it to
several cases. In the case where the occupation matrix of localized orbitals is
chosen as a basic variable, we can obtain the single-particle equation which is
equivalent to that of the LDA+U method. The theory also leads to the
Hartree-Fock-Kohn-Sham equation by letting the exchange energy be a basic
variable. Furthermore, if the quantity associated with the density of states
near the Fermi level is chosen as a basic variable, the resulting
single-particle equation includes the additional potential which could mainly
modify the energy-band structures near the Fermi level.Comment: 27 page
The spin state transition in LaCoO; revising a revision
Using soft x-ray absorption spectroscopy and magnetic circular dichroism at
the Co- edge we reveal that the spin state transition in LaCoO
can be well described by a low-spin ground state and a triply-degenerate
high-spin first excited state. From the temperature dependence of the spectral
lineshapes we find that LaCoO at finite temperatures is an inhomogeneous
mixed-spin-state system. Crucial is that the magnetic circular dichroism signal
in the paramagnetic state carries a large orbital momentum. This directly shows
that the currently accepted low-/intermediate-spin picture is at variance.
Parameters derived from these spectroscopies fully explain existing magnetic
susceptibility, electron spin resonance and inelastic neutron data
Equation of state in finite-temperature QCD with improved Wilson quarks
We study finite-temperature phase transition and equation of state for
two-flavor QCD at using an RG-improved gauge action and a
meanfield-improved clover quark action. The pressure is computed using the
integral method. The O(4) scaling of chiral order parameter is also examined.Comment: LATTICE99(finite temperature), 3 page
Atomic displacements and lattice distortion in the magnetic-field-induced charge ordered state of SmRuP
Structural properties of SmRuP in the anomalous magnetic ordered
phase between K and K in magnetic fields has
been studied by x-ray diffraction. Atomic displacements of Ru and P, reflecting
the field-induced charge order of the electrons, have been deduced by
analyzing the intensities of the forbidden Bragg peaks, assuming a cubic space
group . Also, by utilizing high-resolution x-ray diffraction
experiment, we observed a splitting of fundamental Bragg peaks, clarifying that
the unit cell in the magnetic ordered phase is rhombohedral elongated along the
axis. Responses of the rhombohedral domains to the magnetic
field, which reflects the direction of the magnetic moment, is studied in
detail.Comment: 11 pages, 11 figures, accepted for publication in PR
- …