3,597 research outputs found
A signature of anisotropic bubble collisions
Our universe may have formed via bubble nucleation in an eternally-inflating
background. Furthermore, the background may have a compact dimension---the
modulus of which tunnels out of a metastable minimum during bubble
nucleation---which subsequently grows to become one of our three large spatial
dimensions. When in this scenario our bubble universe collides with other ones
like it, the collision geometry is constrained by the reduced symmetry of the
tunneling instanton. While the regions affected by such bubble collisions still
appear (to leading order) as disks in an observer's sky, the centers of these
disks all lie on a single great circle, providing a distinct signature of
anisotropic bubble nucleation.Comment: 10 pages, 5 figures; v2: crucial error corrected, conclusions revise
Non-centro-symmetric superconductors Li2Pd3B and Li2(Pd0.8Pt0.2)3B: amplitude and phase fluctuations analysis of the experimental magnetization data
We report on magnetization data obtained as a function of temperature and
magnetic field in Li2 (Pd0.8Pt0.2)3B and Li2Pd3B non-centro-symmetric
superconductors. Reversible magnetization curves were plotted as M1/2 vs. T.
This allows study of the asymptotic behavior of the averaged order parameter
amplitude (gap) near the superconducting transition. Results of the analysis
show, as expected, a mean field superconducting transition for Li2Pd3B. On
contrary, a large deviation from the mean field behavior is revealed for
Li2(Pd0.8Pt0.2)3B. This is interpreted as due to the strength of the non s-wave
spin-triplet pairing in this Pt-containing compound which produces nodes in the
order parameter and consequently, phase fluctuations. The diamagnetic signal
above Tc(H) in Li2Pd3B is well explained by superconducting Gaussian
fluctuations, which agrees with the observed mean field transition. For
Li2(Pd0.8Pt0.2)3B the diamagnetic signal above Tc(H) is much higher than the
expected Gaussian values and appears to be well explained by three dimensional
critical fluctuations of the lowest-Landau-level type, which somehow agrees
with the scenario of a phase mediated transition.Comment: 7 pages (1 column) 3 figure
Onset of phase correlations in YBa2Cu3O{7-x} as determined from reversible magnetization measurements
Isofield magnetization curves are obtained and analyzed for three single
crystals of YBa2Cu3O{7-x}, ranging from optimally doped to very underdoped, as
well as the BCS superconductor Nb, in the presence of magnetic fields applied
both parallel and perpendicular to the planes. Near Tc, the magnetization
exhibits a temperature dependence \sqrt{M} [Ta(H)-T]^m. In accordance with
recent theories, we associated Ta(H) with the onset of coherent phase
fluctuations of the superconducting order parameter. For Nb and optimally doped
YBaCuO, Ta(H) is essentially identical to the mean-field transition line Tc(H).
The fitting exponent m=0.5 takes its mean-field value for Nb, and varies just
slightly from 0.5 for optimally doped YBaCuO. However, underdoped YBCO samples
exhibit anomalous behavior, with Ta(H)>Tc for H applied parallel to the c axis,
suggesting that the magnetization is probing a region of temperatures above Tc
where phase correlations persist. In this region, the fitting exponent falls in
the range 0.5 < m < 0.8 for H\parallel c, compared with m~0. for $H\parallel ab
planes. The results are interpreted in terms of an anisotropic pairing symmetry
of the order parameter: d-wave along the ab planes and s-wave along the c axis.Comment: 5 pages, 4 figure
Negative vacuum energy densities and the causal diamond measure
Arguably a major success of the landscape picture is the prediction of a
small, non-zero vacuum energy density. The details of this prediction depends
in part on how the diverging spacetime volume of the multiverse is regulated, a
question that remains unresolved. One proposal, the causal diamond measure, has
demonstrated many phenomenological successes, including predicting a
distribution of positive vacuum energy densities in good agreement with
observation. In the string landscape, however, the vacuum energy density is
expected to take positive and negative values. We find the causal diamond
measure gives a poor fit to observation in such a landscape -- in particular,
99.6% of observers in galaxies seemingly just like ours measure a vacuum energy
density smaller than we do, most of them measuring it to be negative.Comment: 9 pages, 3 figures; v2: minor error fixed (results essentially
unchanged), reference added; v3: published version, includes a few
clarification
A comparative study of high-field diamagnetic fluctuations in deoxygenated YBa2Cu3O(7-x) and polycrystalline (Bi-Pb)2Sr2Ca3O(10)
We studied three single crystals of YBa2Cu3O{7-x} with Tc= 62.5, 52, and 41
K, and a textured specimen of (Bi-Pb)2Sr2Ca2Cu3O10 with Tc=108 K, for H//c
axis. The reversible data were interpreted in terms of 2D lowest-Landau-level
fluctuation theory. The data were fit well by the 2D LLL expression for
magnetization obtained by Tesanovic etal., producing reasonable values of kappa
but larger values of dHc2/dT. Universality was studied by obtaining a
simultaneous scaling of Y123 data and Bi2223. An expression for the 2D x-axis
LLL scaling factor used to obtain the simultaneous scaling was extracted from
theory, and compared with the experimental values. The comparison between the
values of the x-axis produced a deviation of 40% which suggests that the
hypothesis of universality of the 2D-LLL fluctuations is not supported by the
studied samples. We finaly observe that Y123 magnetization data for
temperatures above obbey a universal scaling obtained for the diamagnetic
fluctuation magnetization from a theory considering non-local field effects.
The same scaling was not obbeyed by the corresponding magnetization calculated
from the two-dimensional lowest-Landau-level theory.Comment: 7 pages 5 figures, accept in Journ. Low Temp. Phy
Experimental observation of high field diamagnetic fluctuations in Niobium
We have performed a magnetic study of a bulk metallic sample of Nb with
critical temperature K. Magnetization versus temperature (M {\it
vs} T) data obtained for fixed magnetic fields above 1 kOe show a
superconducting transition which becomes broader as the field is increased. The
data are interpreted in terms of the diamagnetic lowest Landau level (LLL)
fluctuation theory. The scaling analysis gives values of the superconducting
transition temperature consistent with % . We search for
universal 3D LLL behavior by comparing scaling results for Nb and YBaCuO, but
obtain no evidence for universality.Comment: 5 pages, 6 figures, Accepted for publication in Phys.Rev.
Predicting the cosmological constant with the scale-factor cutoff measure
It is well known that anthropic selection from a landscape with a flat prior
distribution of cosmological constant Lambda gives a reasonable fit to
observation. However, a realistic model of the multiverse has a physical volume
that diverges with time, and the predicted distribution of Lambda depends on
how the spacetime volume is regulated. We study a simple model of the
multiverse with probabilities regulated by a scale-factor cutoff, and calculate
the resulting distribution, considering both positive and negative values of
Lambda. The results are in good agreement with observation. In particular, the
scale-factor cutoff strongly suppresses the probability for values of Lambda
that are more than about ten times the observed value. We also discuss several
qualitative features of the scale-factor cutoff, including aspects of the
distributions of the curvature parameter Omega and the primordial density
contrast Q.Comment: 16 pages, 6 figures, 2 appendice
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