7,929 research outputs found
Lone Pair Effect, Structural Distortions and Potential for Superconductivity in Tl Perovskites
Drawing the analogy to BaBiO3, we investigate via ab-initio electronic
structure calculations potential new superconductors of the type ATlX3 with A =
Rb, Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. Based on
chemical reasoning, supported by the calculations, we show that Tl-based
perovskites have structural and charge instabilities driven by the lone pair
effect, similar to the case of BaBiO3, effectively becoming A2Tl1+Tl3+X6. We
find that upon hole doping of RbTlCl3, structures without Tl1+, Tl3+ charge
disproportionation become more stable, although the ideal cubic perovskite,
often viewed as the best host for superconductivity, should not be the most
stable phase in the system. The known superconductor (Sr,K)BiO3 and hole doped
RbTlCl3, predicted to be most stable in the same tetragonal structure, display
highly analogous calculated electronic band structures.Comment: 5 pages, 5 figure
Five-Branes in Heterotic Brane-World Theories
The effective action for five-dimensional heterotic M-theory in the presence
of five-branes is systematically derived from Horava-Witten theory coupled to
an M5-brane world-volume theory. This leads to a five-dimensional N=1 gauged
supergravity theory on S^1/Z_2 coupled to four-dimensional N=1 theories
residing on the two orbifold fixed planes and an additional bulk three-brane.
We analyse the properties of this action, particularly the four-dimensional
effective theory associated with the domain-wall vacuum state. The moduli
Kahler potential and the gauge-kinetic functions are determined along with the
explicit relations between four-dimensional superfields and five-dimensional
component fields.Comment: 19 pages, Latex, typos corrected, reference adde
A technique for automatic real time scoring of several simultaneous sleep electroencephalograms
Automatic real-time scoring of simultaneous sleep electroencephalogram
Heterotic M-Theory Cosmology in Four and Five Dimensions
We study rolling radii solutions in the context of the four- and
five-dimensional effective actions of heterotic M-theory. For the standard
four-dimensional solutions with varying dilaton and T-modulus, we find
approximate five-dimensional counterparts. These are new, generically
non-separating solutions corresponding to a pair of five-dimensional domain
walls evolving in time. Loop corrections in the four-dimensional theory are
described by certain excitations of fields in the fifth dimension. We point out
that the two exact separable solutions previously discovered are precisely the
special cases for which the loop corrections are time-independent. Generically,
loop corrections vary with time. Moreover, for a subset of solutions they
increase in time, evolving into complicated, non-separating solutions. In this
paper we compute these solutions to leading, non-trivial order. Using the
equations for the induced brane metric, we present a general argument showing
that the accelerating backgrounds of this type cannot evolve smoothly into
decelerating backgrounds.Comment: 15 pages, Latex, 1 eps figur
Interesting consequences of brane cosmology
We discuss cosmology in four dimensions within a context of brane-world
scenario.Such models can predict chaotic inflation with very low reheat
temperature depending on the brane tension. We notice that the gravitino
abundance is different in the brane-world cosmology and by tuning the brane
tension it is possible to get extremely low abundance. We also study
Affleck-Dine baryogenesis in our toy model.Comment: 5 pages, Trivial changes to match the published versio
Yukawa Textures From Heterotic Stability Walls
A holomorphic vector bundle on a Calabi-Yau threefold, X, with h^{1,1}(X)>1
can have regions of its Kahler cone where it is slope-stable, that is, where
the four-dimensional theory is N=1 supersymmetric, bounded by "walls of
stability". On these walls the bundle becomes poly-stable, decomposing into a
direct sum, and the low energy gauge group is enhanced by at least one
anomalous U(1) gauge factor. In this paper, we show that these additional
symmetries can strongly constrain the superpotential in the stable region,
leading to non-trivial textures of Yukawa interactions and restrictions on
allowed masses for vector-like pairs of matter multiplets. The Yukawa textures
exhibit a hierarchy; large couplings arise on the stability wall and some
suppressed interactions "grow back" off the wall, where the extended U(1)
symmetries are spontaneously broken. A number of explicit examples are
presented involving both one and two stability walls, with different
decompositions of the bundle structure group. A three family standard-like
model with no vector-like pairs is given as an example of a class of SU(4)
bundles that has a naturally heavy third quark/lepton family. Finally, we
present the complete set of Yukawa textures that can arise for any holomorphic
bundle with one stability wall where the structure group breaks into two
factors.Comment: 53 pages, 4 figures and 13 table
P-values for high-dimensional regression
Assigning significance in high-dimensional regression is challenging. Most
computationally efficient selection algorithms cannot guard against inclusion
of noise variables. Asymptotically valid p-values are not available. An
exception is a recent proposal by Wasserman and Roeder (2008) which splits the
data into two parts. The number of variables is then reduced to a manageable
size using the first split, while classical variable selection techniques can
be applied to the remaining variables, using the data from the second split.
This yields asymptotic error control under minimal conditions. It involves,
however, a one-time random split of the data. Results are sensitive to this
arbitrary choice: it amounts to a `p-value lottery' and makes it difficult to
reproduce results. Here, we show that inference across multiple random splits
can be aggregated, while keeping asymptotic control over the inclusion of noise
variables. We show that the resulting p-values can be used for control of both
family-wise error (FWER) and false discovery rate (FDR). In addition, the
proposed aggregation is shown to improve power while reducing the number of
falsely selected variables substantially.Comment: 25 pages, 4 figure
BPS Branes in Cosmology
The possibility to study the M/string theory cosmology via 5d bulk & brane
action is investigated. The role of the 4-form field in the theory of BPS
branes in 5d is clarified. We describe arguments suggesting that the effective
4d description of the universe in the ekpyrotic scenario (hep-th/0103239)
should lead to contraction rather than expansion of the universe. To verify
these arguments, we study the full 5d action prior to its integration over the
5th dimension. We show that if one adds the potential V(Y) to the action of the
bulk brane, then the metric ansatz used in the ekpyrotic scenario does not
solve the dilaton and gravitational equations. To find a consistent
cosmological solution one must use a more general metric ansatz and a complete
5d description of the brane interaction instead of simply adding an effective
4d bulk brane potential V(Y).Comment: 18 pages, revte
Cosmological Perturbations in Brane-World Theories: Formalism
We develop a gauge-invariant formalism to describe metric perturbations in
five-dimensional brane-world theories. In particular, this formalism applies to
models originating from heterotic M-theory. We introduce a generalized
longitudinal gauge for scalar perturbations. As an application, we discuss some
aspects of the evolution of fluctuations on the brane. Moreover, we show how
the five-dimensional formalism can be matched to the known four-dimensional one
in the limit where an effective four-dimensional description is appropriate.Comment: 16 pages, no figure, matches version to appear in PR
Vector Bundle Moduli Superpotentials in Heterotic Superstrings and M-Theory
The non-perturbative superpotential generated by a heterotic superstring
wrapped once around a genus-zero holomorphic curve is proportional to the
Pfaffian involving the determinant of a Dirac operator on this curve. We show
that the space of zero modes of this Dirac operator is the kernel of a linear
mapping that is dependent on the associated vector bundle moduli. By explicitly
computing the determinant of this map, one can deduce whether or not the
dimension of the space of zero modes vanishes. It is shown that this
information is sufficient to completely determine the Pfaffian and, hence, the
non-perturbative superpotential as explicit holomorphic functions of the vector
bundle moduli. This method is illustrated by a number of non-trivial examples.Comment: 81 pages, LaTeX, corrected typo
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