3,639 research outputs found
Quantum Limitations on the Storage and Transmission of Information
Information must take up space, must weigh, and its flux must be limited.
Quantum limits on communication and information storage leading to these
conclusions are here described. Quantum channel capacity theory is reviewed for
both steady state and burst communication. An analytic approximation is given
for the maximum signal information possible with occupation number signal
states as a function of mean signal energy. A theorem guaranteeing that these
states are optimal for communication is proved. A heuristic "proof" of the
linear bound on communication is given, followed by rigorous proofs for signals
with specified mean energy, and for signals with given energy budget. And
systems of many parallel quantum channels are shown to obey the linear bound
for a natural channel architecture. The time--energy uncertainty principle is
reformulated in information language by means of the linear bound. The quantum
bound on information storage capacity of quantum mechanical and quantum field
devices is reviewed. A simplified version of the analytic proof for the bound
is given for the latter case. Solitons as information caches are discussed, as
is information storage in one dimensional systems. The influence of signal
self--gravitation on communication is considerd. Finally, it is shown that
acceleration of a receiver acts to block information transfer.Comment: Published relatively inaccessible review on a perennially interesting
subject. Plain TeX, 47 pages, 5 jpg figures (not embedded
Structural and Magnetic Characteristics of MnAs Nanoclusters Embedded in Be-doped GaAs
We describe a systematic study of the synthesis, microstructure and
magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of
MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created
during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by
molecular beam epitaxy. Transmission electron microscopy and magnetometry
studies reveal two distinct classes of nanoclustered samples whose structural
and magnetic properties depend on the Mn content of the initial (Ga,Mn)As
layer. For Mn content in the range 5% - 7.5%, annealing creates a
superparamagnetic material with a uniform distribution of small clusters
(diameter around 6 nm) and with a low blocking temperature (T_B approximately
10 K). While transmission electron microscopy cannot definitively identify the
composition and crystalline phase of these small clusters, our experimental
data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich
regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing
results in an inhomogeneous distribution of both small clusters as well as much
larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also
exhibit supermagnetism, albeit with substantially larger magnetic moments and
coercive fields, and blocking temperatures well above room temperature
Bound states due to an accelerated mirror
We discuss an effect of accelerated mirrors which remained hitherto
unnoticed, the formation of a field condensate near its surface for massive
fields. From the view point of an observer attached to the mirror, this is
effect is rather natural because a gravitational field is felt there. The
novelty here is that since the effect is not observer dependent even inertial
observers will detect the formation of this condensate. We further show that
this localization is in agreement with Bekenstein's entropy bound.Comment: Final version to appear in PR
Boundary conditions and the entropy bound
The entropy-to-energy bound is examined for a quantum scalar field confined
to a cavity and satisfying Robin condition on the boundary of the cavity. It is
found that near certain points in the space of the parameter defining the
boundary condition the lowest eigenfrequency (while non-zero) becomes
arbitrarily small. Estimating, according to Bekenstein and Schiffer, the ratio
by the -function, , we compute
explicitly and find that it is not bounded near those points that signals
violation of the bound. We interpret our results as imposing certain
constraints on the value of the boundary interaction and estimate the forbidden
region in the parameter space of the boundary conditions.Comment: 16 pages, latex, v2: typos corrected, to appear in Phys.Rev.
Low Velocity Granular Drag in Reduced Gravity
We probe the dependence of the low velocity drag force in granular materials
on the effective gravitational acceleration (geff) through studies of spherical
granular materials saturated within fluids of varying density. We vary geff by
a factor of 20, and we find that the granular drag is proportional to geff,
i.e., that the granular drag follows the expected relation Fprobe = {\eta}
{\rho}grain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical
cylinder with depth of insertion, hprobe, diameter dprobe, moving through
grains of density {\rho}grain, and where {\eta} is a dimensionless constant.
This dimensionless constant shows no systematic variation over four orders of
magnitude in effective grain weight, demonstrating that the relation holds over
that entire range to within the precision of our data
Measurements of Nanoscale Domain Wall Flexing in a Ferromagnetic Thin Film
We use the high spatial sensitivity of the anomalous Hall effect in the
ferromagnetic semiconductor Ga1-xMnxAs, combined with the magneto-optical Kerr
effect, to probe the nanoscale elastic flexing behavior of a single magnetic
domain wall in a ferromagnetic thin film. Our technique allows position
sensitive characterization of the pinning site density, which we estimate to be
around 10^14 cm^{-3}. Analysis of single site depinning events and their
temperature dependence yields estimates of pinning site forces (10 pN range) as
well as the thermal deactivation energy. Finally, our data hints at a much
higher intrinsic domain wall mobility for flexing than previously observed in
optically-probed micron scale measurements
High temperature onset of field-induced transitions in the spin-ice compound Dy2Ti2O7
We have studied the field-dependent ac magnetic susceptibility of single
crystals of Dy2Ti2O7 spin ice along the [111] direction in the temperature
range 1.8 K - 7 K. Our data reflect the onset of local spin ice order in the
appearance of different field regimes. In particular, we observe a prominent
feature at approximately 1.0 T that is a precursor of the low-temperature
metamagnetic transition out of field-induced kagome ice, below which the
kinetic constraints imposed by the ice rules manifest themselves in a
substantial frequency-dependence of the susceptibility. Despite the relatively
high temperatures, our results are consistent with a monopole picture, and they
demonstrate that such a picture can give physical insight to the spin ice
systems even outside the low-temperature, low-density limit where monopole
excitations are well-defined quasiparticles
A Tool for Stability and Power-Sharing Analysis of a Generalized Class of Droop Controllers for High-Voltage Direct-Current Transmission Systems
The problem of primary control of high-voltage direct-current transmission systems is addressed in this paper, which contains four main contributions. First, a new nonlinear, more realistic model for the system suitable for primary control design, which takes into account nonlinearities introduced by conventional inner controllers, is proposed. Second, necessary conditions—dependent on some free controller tuning para-meters—for the existence of equilibria is determined. Third, additional (necessary) conditions are formulated for these equilibria to satisfy the power-sharing constraints. Fourth, establish conditions for the stability of a given equilibrium point. The usefulness of the theoretical results is illustrated via numerical calculations on a four-terminal example
Orientational Ordering and Dynamics of Rodlike Polyelectrolytes
The interplay between electrostatic interactions and orientational
correlations is studied for a model system of charged rods positioned on a
chain, using Monte Carlo simulation techniques. It is shown that the coupling
brings about the notion of {\em electrostatic frustration}, which in turn
results in: (i) a rich variety of novel orientational orderings such as chiral
phases, and (ii) an inherently slow dynamics characterized by
stretched-exponential behavior in the relaxation functions of the system.Comment: 7 pages, 10 figure
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