3,663 research outputs found
Stochastic Approach to Flat Direction during Inflation
We revisit the time evolution of a flat and non-flat direction system during
inflation. In order to take into account quantum noises in the analysis, we
base on stochastic formalism and solve coupled Langevin equations numerically.
We focus on a class of models in which tree-level Hubble-induced mass is not
generated. Although the non-flat directions can block the growth of the flat
direction's variance in principle, the blocking effects are suppressed by the
effective masses of the non-flat directions. We find that the fate of the flat
direction during inflation is determined by one-loop radiative corrections and
non-renormalizable terms as usually considered, if we remove the zero-point
fluctuation from the noise terms.Comment: 17 pages, 4 figures, v2: minor corrections made, published in JCA
511 keV line and diffuse gamma rays from moduli
We obtain the spectrum of gamma ray emissions from the moduli whose decay
into accounts for the 511 keV line observed by SPI/INTERGRAL. The
moduli emit gamma rays through internal bremsstrahlung, and also decay directly
into two gammas via tree and/or one-loop diagrams. We show that the internal
bremsstahlung constrains the mass of the moduli below MeV
model-independently. On the other hand, the flux of two gammas directly decayed
from the moduli through one loop diagrams will exceed the observed galactic
diffuse gamma-ray background if the moduli mass exceeds MeV in the
typical situation. Moreover, forthcoming analysis of SPI data in the range of
1-8 MeV may detect the line emisson with the energy half the moduli mass in the
near future, which confirms the decaying moduli scenario.Comment: 6 pages, 5 figures, published versio
Spin-Nematic and Spin-Density-Wave Orders in Spatially Anisotropic Frustrated Magnets in a Magnetic Field
We develop a microscopic theory of finite-temperature spin-nematic orderings
in three-dimensional spatially anisotropic magnets consisting of weakly-coupled
frustrated spin-1/2 chains with nearest-neighbor and next-nearest-neighbor
couplings in a magnetic field. Combining a field theoretical technique with
density-matrix renormalization group results, we complete finite-temperature
phase diagrams in a wide magnetic-field range that possess spin-bond-nematic
and incommensurate spin-density-wave ordered phases. The effects of a four-spin
interaction are also studied. The relevance of our results to
quasi-one-dimensional edge-shared cuprate magnets such as LiCuVO4 is discussed.Comment: 5 pages (2 column version), 4 figures, Revtex, published versio
Constraints on the mass of a habitable planet with water of nebular origin
From an astrobiological point of view, special attention has been paid to the
probability of habitable planets in extrasolar systems. The purpose of this
study is to constrain a possible range of the mass of a terrestrial planet that
can get water. We focus on the process of water production through oxidation of
the atmospheric hydrogen--the nebular gas having been attracted
gravitationally--by oxide available at the planetary surface. For the water
production to work well on a planet, a sufficient amount of hydrogen and enough
high temperature to melt the planetary surface are needed. We have simulated
the structure of the atmosphere that connects with the protoplanetary nebula
for wide ranges of heat flux, opacity, and density of the nebular gas. We have
found both requirements are fulfilled for an Earth-mass planet for wide ranges
of the parameters. We have also found the surface temperature of planets of <=
0.3 Earth masses is lower than the melting temperature of silicate (~ 1500K).
On the other hand, a planet of more than several Earth masses becomes a gas
giant planet through runaway accretion of the nebular gas.Comment: 25 pages, 8 figures, to appear in the 01 September 2006 issue of Ap
Formation of terrestrial planets in disks evolving via disk winds and implications for the origin of the solar system's terrestrial planets
Recent three-dimensional magnetohydrodynamical simulations have identified a
disk wind by which gas materials are lost from the surface of a protoplanetary
disk, which can significantly alter the evolution of the inner disk and the
formation of terrestrial planets. A simultaneous description of the realistic
evolution of the gaseous and solid components in a disk may provide a clue for
solving the problem of the mass concentration of the terrestrial planets in the
solar system. We simulate the formation of terrestrial planets from planetary
embryos in a disk that evolves via magnetorotational instability and a disk
wind. The aim is to examine the effects of a disk wind on the orbital evolution
and final configuration of planetary systems. We perform N-body simulations of
sixty 0.1 Earth-mass embryos in an evolving disk. The evolution of the gas
surface density of the disk is tracked by solving a one-dimensional diffusion
equation with a sink term that accounts for the disk wind. We find that even in
the case of a weak disk wind, the radial slope of the gas surface density of
the inner disk becomes shallower, which slows or halts the type I migration of
embryos. If the effect of the disk wind is strong, the disk profile is
significantly altered (e.g., positive surface density gradient, inside-out
evacuation), leading to outward migration of embryos inside ~ 1 AU. Disk winds
play an essential role in terrestrial planet formation inside a few AU by
changing the disk profile. In addition, embryos can undergo convergent
migration to ~ 1 AU in certainly probable conditions. In such a case, the
characteristic features of the solar system's terrestrial planets (e.g., mass
concentration around 1 AU, late giant impact) may be reproduced.Comment: 8 pages, 4 figures, accepted for publication in A&
Spatially Inhomogeneous Superconducting State near in UPdAl
We have performed Al-NMR measurements on single-crystalline
UPdAl with the field parallel to the axis to investigate the
superconducting (SC) properties near the upper critical field of
superconductivity . The broadening of the NMR linewidth below 14~K
indicates the appearance of the internal field at the Al site, which originates
from the antiferromagnetically ordered moments of U 5 electrons. In the SC
state well below = 3.4~T, the broadening of the NMR linewidth
due to the SC diamagnetism and a decrease in the Knight shift are observed,
which are well-understood by the framework of spin-singlet superconductivity.
In contrast, the Knight shift does not change below , and the NMR
spectrum is broadened symmetrically in the SC state in the field range of 3~T
. The unusual NMR spectrum near
suggests that a spatially inhomogeneous SC state such as the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state would be realized.Comment: 5 pages, 5 figure
Evolution of Protoplanetary Discs with Magnetically Driven Disc Winds
Aims: We investigate the evolution of protoplanetary discs (PPDs hereafter)
with magnetically driven disc winds and viscous heating. Methods: We consider
an initially massive disc with ~0.1 Msun to track the evolution from the early
stage of PPDs. We solve the time evolution of surface density and temperature
by taking into account viscous heating and the loss of the mass and the angular
momentum by the disc winds within the framework of a standard alpha model for
accretion discs. Our model parameters, turbulent viscosity, disc wind mass
loss, and disc wind torque, which are adopted from local magnetohydrodynamical
simulations and constrained by the global energetics of the gravitational
accretion, largely depends on the physical condition of PPDs, particularly on
the evolution of the vertical magnetic flux in weakly ionized PPDs. Results:
Although there are still uncertainties concerning the evolution of the vertical
magnetic flux remaining, surface densities show a large variety, depending on
the combination of these three parameters, some of which are very different
from the surface density expected from the standard accretion. When a PPD is in
a "wind-driven accretion" state with the preserved vertical magnetic field, the
radial dependence of the surface density can be positive in the inner region
<1-10 au. The mass accretion rates are consistent with observations, even in
the very low level of magnetohydrodynamical turbulence. Such a positive radial
slope of the surface density gives a great impact on planet formation because
(i)it inhibits the inward drift or even results in the outward drift of
pebble/boulder-sized solid bodies, and (ii) it also makes the inward type-I
migration of proto-planets slower or even reversed. Conclusions: The variety of
our calculated PPDs should yield a wide variety of exoplanet systems.Comment: 16 pages, 11 figures embedded, accepted by A&A (comments are welcome
A critical Mach number for electron injection in collisionless shocks
Electron acceleration in collisionless shocks with arbitrary magnetic field
orientations is discussed. It is shown that the injection of thermal electrons
into diffusive shock acceleration process is achieved by an electron beam with
a loss-cone in velocity space that is reflected back upstream from the shock
through shock drift acceleration mechanism. The electron beam is able to excite
whistler waves which can scatter the energetic electrons themselves when the
Alfven Mach number of the shock is sufficiently high. A critical Mach number
for the electron injection is obtained as a function of upstream parameters.
The application to supernova remnant shocks is discussed.Comment: 4 pages, 2 figure, accepted for publication in Physical Review
Letter
Analytic Approach to the Cloud-in-cloud Problem for Non-Gaussian Density Fluctuations
We revisit the cloud-in-cloud problem for non-Gaussian density fluctuations.
We show that the extended Press-Schechter (EPS) formalism for non-Gaussian
fluctuations has a flaw in describing mass functions regardless of type of
filtering. As an example, we consider non-Gaussian models in which density
fluctuations at a point obeys a \chi^2 distribution with \nu degrees of
freedom. We find that mass functions predicted by using an integral formula
proposed by Jedamzik, and Yano, Nagashima and Gouda, properly taking into
account correlation between objects at different scales, deviate from those
predicted by using the EPS formalism, especially for strongly non-Gaussian
fluctuations. Our results for the mass function at large mass scales are
consistent with those by Avelino and Viana obtained from numerical simulations.Comment: 10 pages, 7 EPS files, submitted to Ap
Observation of the Fano-Kondo Anti-Resonance in a Quantum Wire with a Side-Coupled Quantum Dot
We have observed the Fano-Kondo anti-resonance in a quantum wire with a
side-coupled quantum dot. In a weak coupling regime, dips due to the Fano
effect appeared. As the coupling strength increased, conductance in the regions
between the dips decreased alternately. From the temperature dependence and the
response to the magnetic field, we conclude that the conductance reduction is
due to the Fano-Kondo anti-resonance. At a Kondo valley with the Fano parameter
, the phase shift is locked to against the gate voltage
when the system is close to the unitary limit in agreement with theoretical
predictions by Gerland {\it et al.} [Phys. Rev. Lett. {\bf 84}, 3710 (2000)].Comment: 4 pages, 4 figure
- …