20,659 research outputs found
Langmuir dark solitons in dense ultrarelativistic electron-positron gravito-plasma in pulsar magnetosphere
Nonlinear propagation of electrostatic modes in ultrarelativistic dense
elelectron-positron gravito-plasma at the polar cap region of pulsar
magnetosphere is considered. A nonlinear Schr\"{o}dinger equation is obtained
from the reductive perturbation method which predicts the existence of Langmuir
dark solitons. Relevance of the propagating dark solitons to the pulsar radio
emission is discussed.Comment: 6 pages, 7 figures. Accepted for publication in Astrophysics and
Space Science. arXiv admin note: text overlap with arXiv:astro-ph/9808047 by
other authors without attributio
Mesoscopic multiterminal Josephson structures: I. Effects of nonlocal weak coupling
We investigate nonlocal coherent transport in ballistic four-terminal
Josephson structures (where bulk superconductors (terminals) are connected
through a clean normal layer, e.g., a two-dimensional electron gas).
Coherent anisotropic superposition of macroscopic wave functions of the
superconductors in the normal region produces phase slip lines (2D analogs to
phase slip centres) and time-reversal symmetry breaking 2D vortex states in it,
as well as such effects as phase dragging and magnetic flux transfer. The
tunneling density of local Andreev states in the normal layer was shown to
contain peaks at the positions controlled by the phase differences between the
terminals.
We have obtained general dependence of these effects on the controlling
supercurrent/phase differences between the terminals of the ballistic
mesoscopic four-terminal SQUID.Comment: 18 pages, 11 figure
Towards Chemical Constraints on Hot Jupiter Migration
The origin of hot Jupiters -- gas giant exoplanets orbiting very close to
their host stars -- is a long-standing puzzle. Planet formation theories
suggest that such planets are unlikely to have formed in-situ but instead may
have formed at large orbital separations beyond the snow line and migrated
inward to their present orbits. Two competing hypotheses suggest that the
planets migrated either through interaction with the protoplanetary disk during
their formation, or by disk-free mechanisms such as gravitational interactions
with a third body. Observations of eccentricities and spin-orbit misalignments
of hot Jupiter systems have been unable to differentiate between the two
hypotheses. In the present work, we suggest that chemical depletions in hot
Jupiter atmospheres might be able to constrain their migration mechanisms. We
find that sub-solar carbon and oxygen abundances in Jovian-mass hot Jupiters
around Sun-like stars are hard to explain by disk migration. Instead, such
abundances are more readily explained by giant planets forming at large orbital
separations, either by core accretion or gravitational instability, and
migrating to close-in orbits via disk-free mechanisms involving dynamical
encounters. Such planets also contain solar or super-solar C/O ratios. On the
contrary, hot Jupiters with super-solar O and C abundances can be explained by
a variety of formation-migration pathways which, however, lead to solar or
sub-solar C/O ratios. Current estimates of low oxygen abundances in hot Jupiter
atmospheres may be indicative of disk-free migration mechanisms. We discuss
open questions in this area which future studies will need to investigate.Comment: Accepted for publication in ApJ Letter
Quasiclassical calculation of spontaneous current in restricted geometries
Calculation of current and order parameter distribution in inhomogeneous
superconductors is often based on a self-consistent solution of Eilenberger
equations for quasiclassical Green's functions. Compared to the original Gorkov
equations, the problem is much simplified due to the fact that the values of
Green's functions at a given point are connected to the bulk ones at infinity
(boundary values) by ``dragging'' along the classical trajectories of
quasiparticles. In finite size systems, where classical trajectories undergo
multiple reflections from surfaces and interfaces, the usefulness of the
approach is no longer obvious, since there is no simple criterion to determine
what boundary value a trajectory corresponds to, and whether it reaches
infinity at all. Here, we demonstrate the modification of the approach based on
the Schophol-Maki transformation, which provides the basis for stable numerical
calculations in 2D. We apply it to two examples: generation of spontaneous
currents and magnetic moments in isolated islands of d-wave superconductor with
subdominant order-parameters s and d_{xy}, and in a grain boundary junction
between two arbitrarily oriented d-wave superconductors. Both examples are
relevant to the discussion of time-reversal symmetry breaking in unconventional
superconductors, as well as for application in quantum computing.Comment: 6 pages, Submitted for publication in the proceedings of MS+S2002
conference, Japa
Decoherence in adiabatic quantum computation
We have studied the decoherence properties of adiabatic quantum computation
(AQC) in the presence of in general non-Markovian, e.g., low-frequency, noise.
The developed description of the incoherent Landau-Zener transitions shows that
the global AQC maintains its properties even for decoherence larger than the
minimum gap at the anticrossing of the two lowest energy levels. The more
efficient local AQC, however, does not improve scaling of the computation time
with the number of qubits as in the decoherence-free case. The scaling
improvement requires phase coherence throughout the computation, limiting the
computation time and the problem size n.Comment: 4 pages, 2 figures, published versio
Weighted ℓ_1 minimization for sparse recovery with prior information
In this paper we study the compressed sensing problem of recovering a sparse signal from a system of underdetermined linear equations when we have prior information about the probability of each entry of the unknown signal being nonzero. In particular, we focus on a model where the entries of the unknown vector fall into two sets, each with a different probability of being nonzero. We propose a weighted ℓ_1 minimization recovery algorithm and analyze its performance using a Grassman angle approach. We compute explicitly the relationship between the system parameters (the weights, the number of measurements, the size of the two sets, the probabilities of being non-zero) so that an iid random Gaussian measurement matrix along with weighted ℓ_1 minimization recovers almost all such sparse signals with overwhelming probability as the problem dimension increases. This allows us to compute the optimal weights. We also provide simulations to demonstrate the advantages of the method over conventional ℓ_1 optimization
Divide-and-conquer: Approaching the capacity of the two-pair bidirectional Gaussian relay network
The capacity region of multi-pair bidirectional relay networks, in which a
relay node facilitates the communication between multiple pairs of users, is
studied. This problem is first examined in the context of the linear shift
deterministic channel model. The capacity region of this network when the relay
is operating at either full-duplex mode or half-duplex mode for arbitrary
number of pairs is characterized. It is shown that the cut-set upper-bound is
tight and the capacity region is achieved by a so called divide-and-conquer
relaying strategy. The insights gained from the deterministic network are then
used for the Gaussian bidirectional relay network. The strategy in the
deterministic channel translates to a specific superposition of lattice codes
and random Gaussian codes at the source nodes and successive interference
cancelation at the receiving nodes for the Gaussian network. The achievable
rate of this scheme with two pairs is analyzed and it is shown that for all
channel gains it achieves to within 3 bits/sec/Hz per user of the cut-set
upper-bound. Hence, the capacity region of the two-pair bidirectional Gaussian
relay network to within 3 bits/sec/Hz per user is characterized.Comment: IEEE Trans. on Information Theory, accepte
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