3,050 research outputs found
Kelvin Probe Spectroscopy of a Two-Dimensional Electron Gas Below 300 mK
A scanning force microscope with a base temperature below 300 mK is used for
measuring the local electron density of a two-dimensional electron gas embedded
in an Ga[Al]As heterostructure. At different separations between AFM tip and
sample, a dc-voltage is applied between the tip and the electron gas while
simultaneously recording the frequency shift of the oscillating tip. Using a
plate capacitor model the local electron density can be extracted from the
data. The result coincides within 10% with the data obtained from transport
measurements.Comment: 3 pages, 3 figure
On the reliability of inflaton potential reconstruction
If primordial scalar and tensor perturbation spectra can be inferred from
observations of the cosmic background radiation and large-scale structure, then
one might hope to reconstruct a unique single-field inflaton potential capable
of generating the observed spectra. In this paper we examine conditions under
which such a potential can be reliably reconstructed. For it to be possible at
all, the spectra must be well fit by a Taylor series expansion. A complete
reconstruction requires a statistically-significant tensor mode to be measured
in the microwave background. We find that the observational uncertainties
dominate the theoretical error from use of the slow-roll approximation, and
conclude that the reconstruction procedure will never insidiously lead to an
irrelevant potential.Comment: 16 page LaTeX file with eight postscript figures embedded with epsf;
no special macros neede
Modeling Thermal Fluctuations: Phase Mixing and Percolation
We consider the nonequilibrium dynamics of a a real scalar field in a
degenerate double-well potential. The system is prepared in the lowest free
energy state in one of the wells and the dynamics is driven by the coupling of
the field to a thermal bath. Using a simple analytical model, based on the
subcritical bubbles method, we compute the fraction of the total volume which
fluctuates to the opposite phase as a function of the parameters of the
potential. Furthermore, we show how complete phase mixing, {\em i.e.} symmetry
restoration, is related to percolation, which is dynamically driven by domain
instability. Our method describes quantitatively recent results obtained by
numerical simulations, and is applicable to systems in the Ising universality
class.Comment: Latex, 7 pages, 2 postscript figures, submitted to PRL. Also
available at http://fnas08.fnal.gov
Exact inflationary solutions
We present a new class of exact inflationary solutions for the evolution of a
universe with spatial curvature, filled with a perfect fluid, a scalar field
with potential and a cosmological
constant . With the potential and a negative cosmological
constant, the scale factor experiments a graceful exit.
We give a brief discussion about the physical meaning of the solutions.Comment: 10 pages, revtex file, 6 figures included with epsf. To be published
in IJMP-
Cosmology of Axions and Moduli: A Dynamical Systems Approach
This paper is concerned with string cosmology and the dynamics of multiple
scalar fields in potentials that can become negative, and their features as
(Early) Dark Energy models. Our point of departure is the "String Axiverse", a
scenario that motivates the existence of cosmologically light axion fields as a
generic consequence of string theory. We couple such an axion to its
corresponding modulus. We give a detailed presentation of the rich cosmology of
such a model, ranging from the setting of initial conditions on the fields
during inflation, to the asymptotic future. We present some simplifying
assumptions based on the fixing of the axion decay constant , and on the
effective field theory when the modulus trajectory is adiabatic, and find the
conditions under which these assumptions break down. As a by-product of our
analysis, we find that relaxing the assumption of fixed leads to the
appearance of a new meta-stable de-Sitter region for the modulus without the
need for uplifting by an additional constant. A dynamical systems analysis
reveals the existence of many fixed point attractors, repellers and saddle
points, which we analyse in detail. We also provide geometric interpretations
of the phase space. The fixed points can be used to bound the couplings in the
model. A systematic scan of certain regions of parameter space reveals that the
future evolution of the universe in this model can be rich, containing multiple
epochs of accelerated expansion.Comment: 27 pages, 12 figures, comments welcome, v2 minor correction
Slow Quenches Produce Fuzzy, Transient Vortices
We examine the Zurek scenario for the production of vortices in quenches of
liquid in the light of recent experiments. Extending our previous
results to later times, we argue that short wavelength thermal fluctuations
make vortices poorly defined until after the transition has occurred. Further,
if and when vortices appear, it is plausible that that they will decay faster
than anticipated from turbulence experiments, irrespective of quench rates.Comment: 4 pages, Revtex file, no figures Apart from a more appropriate title,
this paper differs from its predecessor by including temperature, as well as
pressure, quenche
Some approximate analytical methods in the study of the self-avoiding loop model with variable bending rigidity and the critical behaviour of the strong coupling lattice Schwinger model with Wilson fermions
Some time ago Salmhofer demonstrated the equivalence of the strong coupling
lattice Schwinger model with Wilson fermions to a certain 8-vertex model which
can be understood as a self-avoiding loop model on the square lattice with
bending rigidity and monomer weight . The
present paper applies two approximate analytical methods to the investigation
of critical properties of the self-avoiding loop model with variable bending
rigidity, discusses their validity and makes comparison with known MC results.
One method is based on the independent loop approximation used in the
literature for studying phase transitions in polymers, liquid helium and cosmic
strings. The second method relies on the known exact solution of the
self-avoiding loop model with bending rigidity . The present
investigation confirms recent findings that the strong coupling lattice
Schwinger model becomes critical for . The phase
transition is of second order and lies in the Ising model universality class.
Finally, the central charge of the strong coupling Schwinger model at
criticality is discussed and predicted to be .Comment: 22 pages LaTeX, 6 Postscript figure
A Hamilton-Jacobi approach to non-slow-roll inflation
I describe a general approach to characterizing cosmological inflation
outside the standard slow-roll approximation, based on the Hamilton-Jacobi
formulation of scalar field dynamics. The basic idea is to view the equation of
state of the scalar field matter as the fundamental dynamical variable, as
opposed to the field value or the expansion rate. I discuss how to formulate
the equations of motion for scalar and tensor fluctuations in situations where
the assumption of slow roll is not valid. I apply the general results to the
simple case of inflation from an ``inverted'' polynomial potential, and to the
more complicated case of hybrid inflation.Comment: 21 pages, RevTeX (minor revisions to match published version
Assessing the Accuracy of National Land Cover Dataset Area Estimates at Multiple Spatial Extents
Site-specific accuracy assessments evaluate fine-scale accuracy of land-use/land-cover (LULC) datasets but provide little insight into accuracy of area estimates of LULC classes derived from sampling units of varying size. Additionally, accuracy of landscape structure metrics calculated from area estimates cannot be determined solely from site-specific assessments. We used LULC data from Rhode Island and Massachusetts as reference to determine the accuracy of area measurements from the National Land Cover Dataset (NLCD) within spatial units ranging from 0.1 to 200 km2. When regressed on reference area, NLCD area of developed land, agriculture, forest, and water had positive linear relationships with high r2, suggesting acceptable accuracy. However, many of these classes also displayed mean differences (NLCD â REFERENCE), and linear relationships between the NLCD and reference were not one-to-one (i.e., low r2, ÎČ0 â 0, âÎČ1 â 1), suggesting mapped area is different from true area. Rangeland, wetland, and barren were consistently, poorly classified
Interactions between Cosmic Strings: An Analytical Study
We derive analytic expressions for the interaction energy between two general
cosmic strings as the function of their relative orientation and the
ratio of the coupling constants in the model. The results are relevant to the
statistic description of strings away from critical coupling and shed some
light on the mechanisms involved in string formation and the evolution of
string networks.Comment: 31 pages,REVTEX, Imperial/TP/93-94/3
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