2,981 research outputs found
symmetries of the Ricci tensor of static space times with maximal symmetric transverse spaces
Static space times with maximal symmetric transverse spaces are classified
according to their Ricci collineations. These are investigated for
non-degenerate Ricci tensor (). It turns out that the
only collineations admitted by these spaces can be ten, seven, six or four.
Some new metrics admitting proper Ricci collineations are also investigated.Comment: 11 page
Holography and D3-branes in Melvin Universes
Recently,in a paper hep-th/0511197, it was found that non-commutative super
Yang-Mills (NCSYM) theory with space-dependent noncommutativity can be
formulated as a decoupling limit of open strings ending on D3-branes wrapping a
Melvin universe supported by a flux of the NSNS B-field. Under S-duality, we
show that this theory turns into a noncommutative open string (NCOS) theory
with space-dependent space-time noncommutativity and effective space-dependent
string scale. It is an NCOS theory with both space-dependent space-space and
space-time noncommutativities under more general
transformation. These space-dependent noncommutative theories (NCSYM and NCOS)
have completely the same thermodynamics as that of ordinary super YM theory,
NCSYM and NCOS theories with constant noncommutativity in the dual supergravity
description. Starting from black D3-brane solution in the Melvin universe and
making a Lorentz boost along one of spatial directions on the worldvolume of
D3-branes, we show that the decoupled theory is a light-like NCSYM theory with
space-dependent noncommutativity in a static frame or in an infinite-momentum
frame depending on whether there is a gravitational pp-wave on the worldvolume
of the D3-branes.Comment: Latex, 13 pages, v2: typos fixed, the version in PR
Notes on Entropy Force in General Spherically Symmetric Spacetimes
In a recent paper [arXiv:1001.0785], Verlinde has shown that the Newton
gravity appears as an entropy force. In this paper we show how gravity appears
as entropy force in Einstein's equation of gravitational field in a general
spherically symmetric spacetime. We mainly focus on the trapping horizon of the
spacetime. We find that when matter fields are absent, the change of entropy
associated with the trapping horizon indeed can be identified with an entropy
force. When matter fields are present, we see that heat flux of matter fields
also leads to the change of entropy. Applying arguments made by Verlinde and
Smolin, respectively, to the trapping horizon, we find that the entropy force
is given by the surface gravity of the horizon. The cases in the untrapped
region of the spacetime are also discussed.Comment: revtex4, 21 pages, no figures, one reference added, published
version, to appear in Phys.Rev.
Diffusive versus local spin currents in dynamic spin pumping systems
Using microscopic theory, we investigate the properties of a spin current
driven by magnetization dynamics. In the limit of smooth magnetization texture,
the dominant spin current induced by the spin pumping effect is shown to be the
diffusive spin current, i.e., the one arising from only a diffusion associated
with spin accumulation. That is to say, there is no effective field that
locally drives the spin current. We also investigate the conversion mechanism
of the pumped spin current into a charge current by spin-orbit interactions,
specifically the inverse spin Hall effect. We show that the spin-charge
conversion does not always occur and that it depends strongly on the type of
spin-orbit interaction. In a Rashba spin-orbit system, the local part of the
charge current is proportional to the spin relaxation torque, and the local
spin current, which does not arise from the spin accumulation, does not play
any role in the conversion. In contrast, the diffusive spin current contributes
to the diffusive charge current. Alternatively, for spin-orbit interactions
arising from random impurities, the local charge current is proportional to the
local spin current that constitutes only a small fraction of the total spin
current. Clearly, the dominant spin current (diffusive spin current) is not
converted into a charge current. Therefore, the nature of the spin current is
fundamentally different depending on its origin and thus the spin transport and
the spin-charge conversion behavior need to be discussed together along with
spin current generation
Holographic Superconductors with Ho\v{r}ava-Lifshitz Black Holes
We discuss the phase transition of planar black holes in Ho\v{r}ava-Lifshitz
gravity by introducing a Maxwell field and a complex scalar field. We calculate
the condensates of the charged operators in the dual CFTs when the mass square
of the complex scalar filed is and , respectively. We
compute the electrical conductivity of the \hl superconductor in the probe
approximation. In particular, it is found that there exists a spike in the
conductivity for the case of the operator with scaling dimension one. These
results are quite similar to those in the case of Schwarzschild-AdS black
holes, which demonstrates that the holographic superconductivity is a robust
phenomenon associated with asymptotic AdS black holes.Comment: 12 pages, 7 figures,refs adde
Spectrum from the warped compactifications with the de Sitter universe
We discuss the spectrum of the tensor metric perturbations and the stability
of warped compactifications with the de Sitter spacetime in the
higher-dimensional gravity. The spacetime structure is given in terms of the
warped product of the non-compact direction, the spherical internal dimensions
and the four-dimensional de Sitter spacetime. To realize a finite bulk volume,
we construct the brane world model, using the cut-copy-paste method. Then, we
compactify the spherical directions on the brane. In any case, we show the
existence of the massless zero mode and the mass gap of it with massive
Kaluza-Klein modes. Although the brane involves the spherical dimensions, no
light massive mode is excited. We also investigate the scalar perturbations,
and show that the model is unstable due to the existence of a tachyonic bound
state, which seems to have the universal negative mass square, irrespective of
the number of spacetime dimensions.Comment: Journal version (JHEP
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