36,033 research outputs found
High-energy emissions from neutron star mergers
In 2017, LIGO-Virgo collaborations reported detection of the first neutron
star merger event, GW170817, which is accompanied by electromagnetic
counterparts from radio to gamma rays. Although high-energy neutrinos were not
detected from this event, mergers of neutron stars are expected to produce such
high-energy particles. Relativistic jets are launched when neutron stars merge.
If the jets contain protons, they can emit high-energy neutrinos through
photomeson production. In addition, neutron star mergers produce massive and
fast ejecta, which can be a source of Galactic high-energy cosmic rays above
the knee. We briefly review what we learned from the multi-messenger event,
GW170817, and discuss prospects for multi-messenger detections and hadronic
cosmic-ray production related to the neutron star mergers.Comment: 9 pages, 4 figures, 5 tables, conference proceedings of UHECR 201
Anisotropic Electronic Structure of the Kondo Semiconductor CeFe2Al10 Studied by Optical Conductivity
We report temperature-dependent polarized optical conductivity
[] spectra of CeFeAl, which is a reference material
for CeRuAl and CeOsAl with an anomalous magnetic
transition at 28 K. The spectrum along the b-axis differs
greatly from that in the -plane, indicating that this material has an
anisotropic electronic structure. At low temperatures, in all axes, a shoulder
structure due to the optical transition across the hybridization gap between
the conduction band and the localized states, namely -
hybridization, appears at 55 meV. However, the gap opening temperature and the
temperature of appearance of the quasiparticle Drude weight are strongly
anisotropic indicating the anisotropic Kondo temperature. The strong
anisotropic nature in both electronic structure and Kondo temperature is
considered to be relevant the anomalous magnetic phase transition in
CeRuAl and CeOsAl.Comment: 5 pages, 4 figure
Nonlocal spin Hall effect and spin-orbit interaction in nonmagnetic metals
Spin Hall effect in a nonlocal spin-injection device is theoretically
studied. Using a nonlocal spin-injection technique, a pure spin current is
created in a nonmagnetic metal (N). The spin current flowing in N is deflected
by spin-orbit scattering to induce the Hall current in the transverse direction
and accumulate charge at the edges of N, yielding the spin-current induced Hall
effect. We propose a method for extracting information for spin-orbit
scattering in nonmagnetic metals.Comment: 2 pages, 1 figures; accepted in JMMM (Proceedings of ICM2006, Kyoto
Infrared spectroscopy under multi-extreme conditions: Direct observation of pseudo gap formation and collapse in CeSb
Infrared reflectivity measurements of CeSb under multi-extreme conditions
(low temperatures, high pressures and high magnetic fields) were performed. A
pseudo gap structure, which originates from the magnetic band folding effect,
responsible for the large enhancement in the electrical resistivity in the
single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure
of 4 GPa and at temperatures of 35 - 50 K. The optical spectrum of the pseudo
gap changes to that of a metallic structure with increasing magnetic field
strength and increasing temperature. This change is the result of the magnetic
phase transition from the AF-1 phase to other phases as a function of the
magnetic field strength and temperature. This result is the first optical
observation of the formation and collapse of a pseudo gap under multi-extreme
conditions.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
Effect of lift force on the aerodynamics of dust grains in the protoplanetary disk
We newly introduce lift force into the aerodynamics of dust grains in the
protoplanetary disk. Although many authors have so far investigated the effects
of the drag force, gravitational force and electric force on the dust grains,
the lift force has never been considered as a force exerted on the dust grains
in the gas disk. If the grains are spinning and moving in the fluid, then the
lift force is exerted on them. We show in this paper that the dust grains can
be continuously spinning due to the frequent collisions so that the lift force
continues to be exerted on them, which is valid in a certain parameter space
where the grain size is larger than ~ 1 m and where the distance from the
central star is larger than 1 AU for the minimum mass solar nebula. In
addition, we estimate the effects of the force on the grain motion and obtain
the result that the mean relative velocity between the grains due to the lift
force is comparable to the gas velocity in the Kepler rotational frame when the
Stokes number and lift-drag ratio are both ~ 1. This estimation is performed
under the assumptions of the steady state and the isotropic spin angular
momentum. We also estimate the mean relative velocity when the grains keep
spinning and conclude that the lift force marginally affects the mean relative
velocity in the minimum mass solar nebula. If there is a grain-concentrated
part in the disk, the relative velocity due to the lift force may dominate
there because of high collision rate.Comment: 9 pages, 4 figures. Accepted for publication in Earth, Planets and
Spac
Conditions for Gravitational Instability in Protoplanetary Disks
Gravitational instability is one of considerable mechanisms to explain the
formation of giant planets. We study the gravitational stability for the
protoplanetary disks around a protostar. The temperature and Toomre's Q-value
are calculated by assuming local equilibrium between viscous heating and
radiative cooling (local thermal equilibrium). We assume constant
viscosity and use a cooling function with realistic opacity. Then, we derive
the critical surface density that is necessary for a disk to
become gravitationally unstable as a function of . This critical surface
density is strongly affected by the temperature dependence of
the opacity. At the radius AU, where ices form, the value of
changes discontinuously by one order of magnitude. This
is determined only by local thermal process and criterion of
gravitational instability. By comparing a given surface density profile to
, one can discuss the gravitational instability of
protoplanetary disks. As an example, we discuss the gravitational instability
of two semi-analytic models for protoplanetary disks. One is the steady state
accretion disk, which is realized after the viscous evolution. The other is the
disk that has the same angular momentum distribution with its parent cloud
core, which corresponds to the disk that has just formed. As a result, it is
found that the disks tend to become gravitationally unstable for because ices enable the disks to become low temperature. In the region
closer to the protostar than , it is difficult for a typical
protoplanetary disk to fragment because of the high temperature and the large
Coriolis force. From this result, we conclude that the fragmentation near the
central star is possible but difficult.Comment: accepted for publication in PASJ. Draft version with 26 pages, 8
figures, 1 tabl
Quasi-particle scattering and protected nature of topological states in a parent topological insulator BiSe
We report on angle resolved photoemission spectroscopic studies on a parent
topological insulator (TI), BiSe. The line width of the spectral
function (inverse of the quasi-particle lifetime) of the topological metallic
(TM) states shows an anomalous behavior. This behavior can be reasonably
accounted for by assuming decay of the quasi-particles predominantly into bulk
electronic states through electron-electron interaction and defect scattering.
Studies on aged surfaces reveal that topological metallic states are very much
unaffected by the potentials created by adsorbed atoms or molecules on the
surface, indicating that topological states could be indeed protected against
weak perturbations.Comment: accepted for publication in Phys. Rev. B(R
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