21,154 research outputs found
Two 3-Branes in Randall-Sundrum Setup and Current Acceleration of the Universe
Five-dimensional spacetimes of two orbifold 3-branes are studied, by assuming
that {\em the two 3-branes are spatially homogeneous, isotropic, and
independent of time}, following the so-called "bulk-based" approach. The most
general form of the metric is obtained, and the corresponding field equations
are divided into three groups, one is valid on each of the two 3-branes, and
the third is valid in the bulk. The Einstein tensor on the 3-branes is
expressed in terms of the discontinuities of the first-order derivatives of the
metric coefficients. Thus, once the metric is known in the bulk, the
distribution of the Einstein tensor on the two 3-branes is uniquely determined.
As applications, we consider two different cases, one is in which the bulk is
locally , and the other is where it is vacuum. In some cases, it is
shown that the universe is first decelerating and then accelerating. The global
structure of the bulk as well as the 3-branes is also studied, and found that
in some cases the solutions may represent the collision of two orbifold
3-branes. The applications of the formulas to the studies of the cyclic
universe and the cosmological constant problem are also pointed out.Comment: revtex4, 14 figures, published in Nucl. Phys. B797 (2008) 395 - 43
Thermodynamic Geometry and Critical Behavior of Black Holes
Based on the observations that there exists an analogy between the
Reissner-Nordstr\"om-anti-de Sitter (RN-AdS) black holes and the van der
Waals-Maxwell liquid-gas system, in which a correspondence of variables is
, we study the Ruppeiner geometry, defined as
Hessian matrix of black hole entropy with respect to the internal energy (not
the mass) of black hole and electric potential (angular velocity), for the RN,
Kerr and RN-AdS black holes. It is found that the geometry is curved and the
scalar curvature goes to negative infinity at the Davies' phase transition
point for the RN and Kerr black holes.
Our result for the RN-AdS black holes is also in good agreement with the one
about phase transition and its critical behavior in the literature.Comment: Revtex, 18 pages including 4 figure
Dryland maize yields and water use efficiency in response to tillage and nutrient management practices in China
Rainfed crop production in northern China is constrained by low and variable rainfall. This study explored the effects of tillage and nutrient management practices on maize (Zea mays L.) yield and water use efficiency (WUE), at Shouyang Dryland Farming Experimental Station in northern China during 2003-2008. The experiment was set-up using a split-plot design with 3 tillage methods as main treatments: conventional, reduced (till with crop residue incoperated in fall but no-till in spring), and no-till. Sub-treatments were 3 NP fertilizer rates: 105-46, 179-78 and 210-92 kg N and P ha. -1 Maize grain yields were greatly influenced by the amount of growing season rainfall, and by soil water contents at sowing. Mean grain yields over the 6-year period in response to tillage treatments were 5604, 5347 and 5185 kg ha, under reduced, no-till and conventional tillage, respectively. Mean WUE was 13.7, 13.6 and 12.6 kg ha mm under reduced, no-till, and conventional tillage, respectively. Mean soil water contents at sowing and at harvest were significantly influenced by tillage treatments. At harvest time, the no-till treatment had ~8-12% more water in the soil than the conventional and reduced tillage treatments. Under conventional tillage, grain yields increased with NP fertilizer application rates. However, under reduced tillage, grain yields were highest with lowest NP fertilizer application rate. In conclusion, grain yields and WUE were highest under reduced tillage at modest NP fertilizer application rates of 105 kg N and 46 kg P per ha. No-till increased soil water storage by 8-12% and improved WUE compared to conventional tillage
A Constrained L1 Minimization Approach to Sparse Precision Matrix Estimation
A constrained L1 minimization method is proposed for estimating a sparse
inverse covariance matrix based on a sample of iid -variate random
variables. The resulting estimator is shown to enjoy a number of desirable
properties. In particular, it is shown that the rate of convergence between the
estimator and the true -sparse precision matrix under the spectral norm is
when the population distribution has either exponential-type
tails or polynomial-type tails. Convergence rates under the elementwise
norm and Frobenius norm are also presented. In addition, graphical
model selection is considered. The procedure is easily implementable by linear
programming. Numerical performance of the estimator is investigated using both
simulated and real data. In particular, the procedure is applied to analyze a
breast cancer dataset. The procedure performs favorably in comparison to
existing methods.Comment: To appear in Journal of the American Statistical Associatio
The Cardy-Verlinde formula and entropy of Topological Reissner-Nordstr\"om black holes in de Sitter spaces
In this paper we discuss the question of whether the entropy of cosmological
horizon in Topological Reissner-Nordstr\"om- de Sitter spaces can be described
by the Cardy-Verlinde formula, which is supposed to be an entropy formula of
conformal field theory in any dimension. Furthermore, we find that the entropy
of black hole horizon can also be rewritten in terms of the Cardy-Verlinde
formula for these black holes in de Sitter spaces, if we use the definition due
to Abbott and Deser for conserved charges in asymptotically de Sitter spaces.
Our result is in favour of the dS/CFT correspondence.Comment: 6 pages, accepted for publication in IJMP
Light-like noncommutativity and duality from open strings/branes
In this paper we perform some non-trivial tests for the recently obtained
open membrane/D-brane metrics and `generalized' noncommutativity parameters
using Dp/NS5/M5-branes which have been deformed by light-like fields. The
results obtained give further evidence that these open membrane/D-brane metrics
and `generalized' noncommutativity parameters are correct. Further, we use the
open brane data and supergravity duals to obtain more information about
non-gravitational theories with light-like noncommutativity, or `generalized'
light-like noncommutativity. In particular, we investigate various duality
relations (strong coupling limits). In the light-like case we also comment on
the relation between open membrane data (open membrane metric etc.) in six
dimensions and open string data in five dimensions. Finally, we investigate the
strong coupling limit (high energy limit) of five dimensional NCYM with
\Theta^{12}=\Theta^{34}. In particular, we find that this NCYM theory can be UV
completed by a DLCQ compactification of M-theory.Comment: 24 pages, Latex. v2:Comments and references added. v3:Version
published in JHE
Interaction of a Nanomagnet with a Weak Superconducting Link
We study electromagnetic interaction of a nanomagnet with a weak
superconducting link. Equations that govern coupled dynamics of the two systems
are derived and investigated numerically. We show that the presence of a small
magnet in the proximity of a weak link may be detected through Shapiro-like
steps caused by the precession of the magnetic moment. Despite very weak
magnetic field generated by the weak link, a time-dependent bias voltage
applied to the link can initiate a non-linear dynamics of the nanomagnet that
leads to the reversal of its magnetic moment. We also consider quantum problem
in which a nanomagnet interacting with a weak link is treated as a two-state
spin system due to quantum tunneling between spin-up and spin-down states.Comment: 7 pages, 4 figure
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