1,362 research outputs found
Magnetic and Structural Studies of the Quasi-Two-Dimensional Spin-Gap System (CuCl)LaNb2O7
We report magnetization, nuclear magnetic resonance (NMR), nuclear quadrupole
resonance (NQR), and transmission electron microscopy (TEM) studies on the
quasi-two-dimensional spin-gap system (CuCl)LaNb2O7, a possible candidate for
the J1-J2 model on a square lattice. A sharp single NQR line is observed at the
Cu and Cl sites, indicating that both Cu and Cl atoms occupy a unique site.
However, the electric field gradient tensors at the Cu, Cl, and La sites do not
have axial symmetry. This is incompatible with the reported crystal structure.
Thus the J1-J2 model has to be modified. We propose alternative two-dimensional
dimer models based on the NMR, NQR, and TEM results. The value of the hyperfine
coupling constant at the Cu sites indicates that the spin density is mainly on
the d(3z2-r2) orbital (z parallel c). At 1.5 K, Cu- and Nb-NMR signals
disappear above the critical field Bc1 = 10.3 T determined from the onset of
the magnetization, indicating a field-induced magnetic phase transition at Bc1.Comment: 9 pages, 16 figure
Accelerated cosmological expansion due to a scalar field whose potential has a positive lower bound
In many cases a nonlinear scalar field with potential can lead to
accelerated expansion in cosmological models. This paper contains mathematical
results on this subject for homogeneous spacetimes. It is shown that, under the
assumption that has a strictly positive minimum, Wald's theorem on
spacetimes with positive cosmological constant can be generalized to a wide
class of potentials. In some cases detailed information on late-time
asymptotics is obtained. Results on the behaviour in the past time direction
are also presented.Comment: 16 page
Intermediate inflation and the slow-roll approximation
It is shown that spatially homogeneous solutions of the Einstein equations
coupled to a nonlinear scalar field and other matter exhibit accelerated
expansion at late times for a wide variety of potentials . These potentials
are strictly positive but tend to zero at infinity. They satisfy restrictions
on and related to the slow-roll approximation. These results
generalize Wald's theorem for spacetimes with positive cosmological constant to
those with accelerated expansion driven by potentials belonging to a large
class.Comment: 19 pages, results unchanged, additional backgroun
Can Gravitational Waves Prevent Inflation?
To investigate the cosmic no hair conjecture, we analyze numerically
1-dimensional plane symmetrical inhomogeneities due to gravitational waves in
vacuum spacetimes with a positive cosmological constant. Assuming periodic
gravitational pulse waves initially, we study the time evolution of those waves
and the nature of their collisions. As measures of inhomogeneity on each
hypersurface, we use the 3-dimensional Riemann invariant and the electric and magnetic parts of
the Weyl tensor. We find a temporal growth of the curvature in the waves'
collision region, but the overall expansion of the universe later overcomes
this effect. No singularity appears and the result is a ``no hair" de Sitter
spacetime. The waves we study have amplitudes between and widths between ,
where , the horizon scale of de Sitter spacetime. This
supports the cosmic no hair conjecture.Comment: LaTeX, 11 pages, 3 figures are available on request <To
[email protected] (Hisa-aki SHINKAI)>, WU-AP/29/9
Closed cosmologies with a perfect fluid and a scalar field
Closed, spatially homogeneous cosmological models with a perfect fluid and a
scalar field with exponential potential are investigated, using dynamical
systems methods. First, we consider the closed Friedmann-Robertson-Walker
models, discussing the global dynamics in detail. Next, we investigate
Kantowski-Sachs models, for which the future and past attractors are
determined. The global asymptotic behaviour of both the
Friedmann-Robertson-Walker and the Kantowski-Sachs models is that they either
expand from an initial singularity, reach a maximum expansion and thereafter
recollapse to a final singularity (for all values of the potential parameter
kappa), or else they expand forever towards a flat power-law inflationary
solution (when kappa^2<2). As an illustration of the intermediate dynamical
behaviour of the Kantowski-Sachs models, we examine the cases of no barotropic
fluid, and of a massless scalar field in detail. We also briefly discuss
Bianchi type IX models.Comment: 15 pages, 10 figure
Energy Density of Non-Minimally Coupled Scalar Field Cosmologies
Scalar fields coupled to gravity via in arbitrary
Friedmann-Robertson-Walker backgrounds can be represented by an effective flat
space field theory. We derive an expression for the scalar energy density where
the effective scalar mass becomes an explicit function of and the scale
factor. The scalar quartic self-coupling gets shifted and can vanish for a
particular choice of . Gravitationally induced symmetry breaking and
de-stabilization are possible in this theory.Comment: 18 pages in standard Late
Late-time oscillatory behaviour for self-gravitating scalar fields
This paper investigates the late-time behaviour of certain cosmological
models where oscillations play an essential role. Rigorous results are proved
on the asymptotics of homogeneous and isotropic spacetimes with a linear
massive scalar field as source. Various generalizations are obtained for
nonlinear massive scalar fields, -essence models and gravity. The
effect of adding ordinary matter is discussed as is the case of nonlinear
scalar fields whose potential has a degenerate zero.Comment: 17 pages, additional reference
Study of blind thrust faults underlying Tokyo and Osaka urban areas using a combination of high-resolution seismic reflection profiling and continuous coring
We acquired high-resolution seismic reflection profiles and continuously cored boreholes to evaluate active flexures
produced by major blind thrust fault systems within two densely populated Neogene-Quaternary sedimentary
basins in Japan: the Fukaya Fault System near Tokyo in the Kanto Basin and the Uemachi Fault System in
the Osaka Basin. The high-resolution seismic reflection survey made clear the length, geometry and growth history
of fault-related folds, or flexures formed above the two blind thrusts. Continuously cored boreholes linked
with high-resolution seismic profiles enabled us to estimate the uplift rate as defined by shallow stratigraphic
horizons and constrain the age of the most recent growth of the flexures during earthquakes on the Fukaya and
Uemachi fault systems. Even with the high quality of the data we collected, it is still not possible to exactly constrain
the age of the most recent blind thrust earthquake recorded by flexure of these fault-related folds. Data
presented in this paper form the basis for future efforts aimed at mechanical and kinematic models for fault
growth to evaluate the activity of blind thrusts underlying urban areas
No Chaos in Brane-World Cosmology
We discuss the asymptotic dynamical evolution of spatially homogeneous
brane-world cosmological models close to the initial singularity. We find that
generically the cosmological singularity is isotropic in Bianchi type IX
brane-world models and consequently these models do not exhibit Mixmaster or
chaotic-like behaviour close to the initial singularity. We argue that this is
typical of more general cosmological models in the brane-world scenario. In
particular, we show that an isotropic singularity is a past-attractor in all
orthogonal Bianchi models and is a local past-attractor in a class of
inhomogeneous brane-world models.Comment: Updated Refs. Final version to appear in Class. Quantum Gra
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