6,195 research outputs found
Accretion physics of AM Herculis binaries, I. Results from one-dimensional stationary radiation hydrodynamics
We have solved the one-dimensional stationary two-fluid hydrodynamic
equations for post-shock flows on accreting magnetic white dwarfs simultaneous
with the fully frequency and angle-dependent radiative transfer for cyclotron
radiation and bremsstrahlung. Magnetic field strengths B = 10 to 100 MG are
considered. At given B, this theory relates the properties of the emission
region to a single physical parameter, the mass flow density (or accretion rate
per unit area). We present the normalized temperature profiles and fit formulae
for the peak electron temperature, the geometrical shock height, and the column
density of the post-shock flow. The results apply to pillbox-shaped emission
regions. With a first-order temperature correction they can also be used for
narrower columns provided they are not too tall.Comment: 10 pages with 10 Postscript figures, accepted for publication in
Astronomy & Astrophysics. The source file contains Table 1a/b in ASCII forma
Ricci-flat deformation of orbifolds and localized tachyonic modes
We study Ricci-flat deformations of orbifolds in type II theory. We obtain a
simple formula for mass corrections to the twisted modes due to the
deformations, and apply it to originally tachyonic and massless states in
several examples. In the case of supersymmetric orbifolds, we find that
tachyonic states appear when the deformation breaks all the supersymmetries. We
also study nonsupersymmetric orbifolds C^2/Z_{2N(2N+1)}, which is T-dual to N
type 0 NS5-branes. For N>=2, we compute mass corrections for states, which have
string scale tachyonic masses. We find that the corrected masses coincide to
ones obtained by solving the wave equation for the tachyon field in the smeared
type 0 NS5-brane background geometry. For N=1, we show that the unstable mode
representing the bubble creation is the unique tachyonic mode.Comment: 20 pages, minor collection
Global symmetries and 't Hooft anomalies in brane tilings
We investigate the relation between gauge theories and brane configurations
described by brane tilings. We identify U(1)_B (baryonic), U(1)_M (mesonic),
and U(1)_R global symmetries in gauge theories with gauge symmetries in the
brane configurations. We also show that U(1)_MU(1)_B^2 and U(1)_RU(1)_B^2 't
Hooft anomalies are reproduced as gauge transformations of the classical brane
action.Comment: 41 pages, 6 figure
Enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems
We analyzed the enhancement of the Gilbert damping constant due to spin
pumping in non-collinear ferromagnet / non-magnet / ferromagnet trilayer
systems. We show that the Gilbert damping constant depends both on the
precession angle of the magnetization of the free layer and on the direction of
the magntization of the fixed layer. We find the condition to be satisfied to
realize strong enhancement of the Gilbert damping constant.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
Symmetry of `molecular' configurations of interacting electrons in a quantum dot in strong magnetic fields
A molecular description for magic-number configurations of interacting
electrons in a quantum dot in high magnetic fields developed by one of the
authors has been elaborated for four, five and six electron dots. For four
electrons, the magic spin-singlet states are found to alternate between two
different resonating valence bond (RVB)-like states. For the five-electron
spin-polarized case, the molecular description is shown to work for the known
phenomenon of magic-number sequences that correspond to both the N-fold
symmetric ring configuration and a -fold symmetric one with a center
electron. A six-electron dot is shown here to have an additional feature in
which inclusion of quantum mechanical mixing between classical configurations,
which are deformed and degenerate, restores the N-fold symmetry and reproduces
the ground-state energy accurately.Comment: 4 pages, to be published in Physisca
Decoherence of localized spins interacting via RKKY interaction
We theoretically study decoherence of two localized spins interacting via the
RKKY interaction in one-, two-, and three-dimensional electron gas. We derive
the kinetic equation for the reduced density matrix of the localized spins and
show that energy relaxation caused by singlet-triplet transition is suppressed
when the RKKY interaction is ferromagnetic. We also estimate the decoherence
time of the system consisting of two quantum dots embedded in a two dimensional
electron gas.Comment: 4pages, 2figure
Vertically coupled double quantum dots in magnetic fields
Ground-state and excited-state properties of vertically coupled double
quantum dots are studied by exact diagonalization. Magic-number total angular
momenta that minimize the total energy are found to reflect a crossover between
electron configurations dominated by intra-layer correlation and ones dominated
by inter-layer correlation. The position of the crossover is governed by the
strength of the inter-layer electron tunneling and magnetic field. The magic
numbers should have an observable effect on the far infra-red optical
absorption spectrum, since Kohn's theorem does not hold when the confinement
potential is different for two dots. This is indeed confirmed here from a
numerical calculation that includes Landau level mixing. Our results take full
account of the effect of spin degrees of freedom. A key feature is that the
total spin, , of the system and the magic-number angular momentum are
intimately linked because of strong electron correlation. Thus jumps hand
in hand with the total angular momentum as the magnetic field is varied. One
important consequence of this is that the spin blockade (an inhibition of
single-electron tunneling) should occur in some magnetic field regions because
of a spin selection rule. Owing to the flexibility arising from the presence of
both intra-layer and inter-layer correlations, the spin blockade is easier to
realize in double dots than in single dots.Comment: to be published in Phys. Rev. B1
Supergiant Barocaloric Effects in Acetoxy Silicone Rubber over a Wide Temperature Range: Great Potential for Solid-state Cooling
Solid-state cooling based on caloric effects is considered a viable
alternative to replace the conventional vapor-compression refrigeration
systems. Regarding barocaloric materials, recent results show that elastomers
are promising candidates for cooling applications around room-temperature. In
the present paper, we report supergiant barocaloric effects observed in acetoxy
silicone rubber - a very popular, low-cost and environmentally friendly
elastomer. Huge values of adiabatic temperature change and reversible
isothermal entropy change were obtained upon moderate applied pressures and
relatively low strains. These huge barocaloric changes are associated both to
the polymer chains rearrangements induced by confined compression and to the
first-order structural transition. The results are comparable to the best
barocaloric materials reported so far, opening encouraging prospects for the
application of elastomers in near future solid-state cooling devices.Comment: 19 pages, 7 figures, 2 table
In-shock Cooling in Numerical Simulations
We model a one-dimensional shock-tube using smoothed particle hydrodynamics
and investigate the consequences of having finite shock-width in numerical
simulations. We investigate the cooling of gas during passage through the shock
for different cooling regimes. For a shock temperature of 10^5K, the maximum
temperature of the gas is much reduced and the cooling time was reduced by a
factor of 2. At lower temperatures, we are especially interested in the
production of molecular Hydrogen and so we follow the ionization level and H_2
abundance across the shock. This regime is particularly relevent to simulations
of primordial galaxy formation for halos in which the virial temperature of the
galaxy is sufficiently high to partially re-ionize the gas. The effect of
in-shock cooling is substantial: the maximum temperature the gas reaches
compared to the theoretical temperature was found to vary between 0.15 and 0.81
for the simulations performed. The downstream ionization level is reduced from
the theoretical level by a factor of between 2.4 and 12.5, and the resulting
H_2 abundance was found to be reduced to a fraction of 0.45 to 0.74 of its
theoretical value. At temperatures above 10^5K, radiative shocks are unstable
and will oscillate. We reproduce these oscillations and find good agreement
with the previous work of Chevalier and Imamura (1982), and Imamura, Wolff and
Durisen (1984). The effect of in-shock cooling in such shocks is difficult to
quantify, but is undoubtedly present.Comment: 8 pages, LaTeX, 7 figure
Exact solution for the stationary Kardar-Parisi-Zhang equation
We obtain the first exact solution for the stationary one-dimensional
Kardar-Parisi-Zhang equation. A formula for the distribution of the height is
given in terms of a Fredholm determinant, which is valid for any finite time
. The expression is explicit and compact enough so that it can be evaluated
numerically. Furthermore, by extending the same scheme, we find an exact
formula for the stationary two-point correlation function.Comment: 9 pages, 3 figure
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