853 research outputs found
Anisotropic In-Plane Strain and Transport in Epitaxial Nd(0.2)Sr(0.8)MnO(3) Thin Films
The structure, morphology, and electrical properties of epitaxial a-axis
oriented thin films of Nd(0.2)Sr(0.8)MnO(3) are reported for thicknesses 10 nm
<= t <= 150 nm. Films were grown with both tensile and compressive strain on
various substrates. It is found that the elongated crystallographic c-axes of
the films remain fully strained to the substrates for all thicknesses in both
strain states. Relaxation of the a and b axes is observed for t>= 65 nm with
films grown under tensile strain developing uniaxial crack arrays (running
along the c axis) due to a highly anisotropic thermal expansion. For the latter
films, the room-temperature in-plane electrical resistivity anisotropy,
rho_b/rho_c, increases approximately exponentially with increasing film
thickness to values of ~1000 in the thickest films studied. Films under tension
have their Neel temperatures enhanced by ~25 K independent of thickness,
consistent with an enhancement of ferromagnetic exchange along their expanded c
axes.Comment: 14 pp., 9 Fig.
Warped compactification on curved manifolds
The characterization of a six- (or seven)-dimensional internal manifold with
metric as having positive, zero or negative curvature is expected to be an
important aspect of warped compactifications in supergravity. In this context,
Douglas and Kallosh recently pointed out that a compact internal space with
negative curvature could help to construct four-dimensional de Sitter solutions
only if the extra dimensions are strongly warped or there are large stringy
corrections. That is, the problem of finding 4-dimensional de Sitter solutions
is well posed, if all extra dimensions are physically compact, which is called
a no-go theorem. Here, we show that the above conclusion does not extend to a
general class of warped compactifications in classical supergravity that allow
a non-compact direction or cosmological solutions for which the internal space
is asymptotic to a cone over a product of compact Einstein spaces or spheres.
For clarity, we present classical solutions that compactify higher-dimensional
spacetime to produce a Robertson--Walker universe with de Sitter-type expansion
plus one extra non-compact direction. Such models are found to admit both an
effective four-dimensional Newton constant that remains finite and a
normalizable zero-mode graviton wavefunction. We also exhibit the possibility
of obtaining 4D de Sitter solutions by including the effect of fluxes (p-form
field strengths).Comment: 24 pages, 1 figure; v5 significant changes in the presentation,
published (journal) versio
Late-time Cosmic Dynamics from M-theory
We consider the behaviour of the cosmological acceleration for time-dependent
hyperbolic and flux compactifications of M-theory, with an exponential
potential. For flat and closed cosmologies it is seen that a positive
acceleration is always transient for both compactifications. For open
cosmologies, both compactifications can give at late times periods of positive
acceleration. As a function of proper time this acceleration has a power law
decay and can be either positive, negative or oscillatory.Comment: 10 pages, LaTeX, 2 figure
Completely localized gravity with higher curvature terms
In the intersecting braneworld models, higher curvature corrections to the
Einstein action are necessary to provide a non-trivial geometry (brane tension)
at the brane junctions. By introducing such terms in a Gauss-Bonnet form, we
give an effective description of localized gravity on the singular
delta-function branes. There exists a non-vanishing brane tension at the
four-dimensional brane intersection of two 4-branes. Importantly, we give
explicit expressions of the graviton propagator and show that the
Randall-Sundrum single-brane model with a Gauss-Bonnet term in the bulk
correctly gives a massless graviton on the brane as for the RS model. We
explore some crucial features of completely localized gravity in the solitonic
braneworld solutions obtained with a choice (\xi=1) of solutions. The no-go
theorem known for Einstein's theory may not apply to the \xi=1 solution. As
complementary discussions, we provide an effective description of the power-law
corrections to Newtonian gravity on the branes or at the common intersection
thereof.Comment: 19 pages, LaTeX, Revised/Published Versio
Inflation and Quintessence: Theoretical Approach of Cosmological Reconstruction
In the first part of this paper, we outline the construction of an
inflationary cosmology in the framework where inflation is described by a
universally evolving scalar field, with the Lagrangian . By considering a generic situation
that inflaton attains a nearly constant velocity, during inflation, (where is the e-folding time), we find
the conditions that have to satisfied by the (reconstructed) scalar potential
to be consistent with the WMAP inflationary data. In the second part of this
paper, we introduce a novel approach of constructing dark energy within the
context of the standard scalar-tensor gravity. The assumption that a scalar
field might roll with a nearly constant velocity, during inflation, can also be
applied to {\it quintessence} or dark energy models. For the minimally coupled
quintessence, (where is the
standard matter-quintessence coupling), the dark energy equation of state in
the range can be obtained for . For
, the model allows for only modest evolution of dark energy density
with redshift. The effect of the matter-quintessence coupling can be
significant only if , while a small coupling
will have almost no effect on cosmological parameters. The
best fit value of in our model is found to be , but it may contain significant numerical errors, viz , which thereby implies the consistency of our model with general
relativity (for which ) at level.Comment: 33 pages, several figures; significant extension (models confronted
with data
Village Baseline Study: Site Analysis Report for Rupandehi, Madhuri Village, Nepal
This is the report of the village baseline study of Madhuri Village in the CCAFS benchmark site of Rupandehi, Nepal conducted from June 5-9, 2011 to complement an earlier household baseline survey done in the same village.
Madhuri is located in the fertile area of the Indo-Gangetic Plain in Nepal yet faces challenges due to increasing populations, encroachment on forests, decreasing soil fertility, limited agriculture and animal productivity, lack of opportunities, and variable climatic conditions. Its circumstances present manageable opportunities to prevent an increase in food insecurity and further degradation natural resources. Madhuri has yet to incur any food or environmental crises
Effective Lagrangian from Higher Curvature Terms: Absence of vDVZ Discontinuity in AdS Space
We argue that the van Dam-Veltman-Zakharov discontinuity arising in the limit of the massive graviton through an explicit Pauli-Fierz mass term
could be absent in anti de Sitter space. This is possible if the graviton can
acquire mass spontaneously from the higher curvature terms or/and the massless
limit is attained faster than the cosmological constant . We discuss the effects of higher-curvature couplings and of an explicit
cosmological term () on stability of such continuity and of massive
excitations.Comment: 23 pages, Latex, the version to appear in Class. Quant. Gra
Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)
Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and
dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15).
Static dielectric constants are determined from the low-T limiting behavior of
the permittivity. The estimated polarizability for bound holes ~ 10^{-22}
cm^{-3} implies a radius comparable to the interatomic spacing, consistent with
the small polaron picture established from prior transport studies near room
temperature and above on nearby compositions. Relaxation peaks in the
dielectric loss associated with charge-carrier hopping yield activation
energies in good agreement with low-T hopping energies determined from
variable-range hopping fits of the dc resistivity. The doping dependence of
these energies suggests that the orthorhombic, canted antiferromagnetic ground
state tends toward an insulator-metal transition that is not realized due to
the formation of the ferromagnetic insulating state near Mn(4+) concentration ~
0.13.Comment: PRB in press, 5 pages, 6 figure
Accelerating Cosmologies from Exponential Potentials
An exponential potential of the form arising from
the hyperbolic or flux compactification of higher-dimensional theories is of
interest for getting short periods of accelerated cosmological expansions.
Using a similar potential but derived for the combined case of hyperbolic-flux
compactification, we study the four-dimensional flat (and open) FLRW
cosmologies and give analytic (and numerical) solutions with exponential
behavior of scale factors. We show that, for the M-theory motivated potentials,
the cosmic acceleration of the universe can be eternal if the spatial curvature
of the 4d spacetime is negative, while the acceleration is only transient for a
spatially flat universe. We also comment on the size of the internal space and
its associated geometric bounds on massive Kaluza-Klein excitations.Comment: 17 pages, 6 figures; minor typos fixe
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