724 research outputs found
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
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
Chemically gated electronic structure of a superconducting doped topological insulator system
Angle resolved photoemission spectroscopy is used to observe changes in the
electronic structure of bulk-doped topological insulator CuBiSe as
additional copper atoms are deposited onto the cleaved crystal surface. Carrier
density and surface-normal electrical field strength near the crystal surface
are estimated to consider the effect of chemical surface gating on atypical
superconducting properties associated with topological insulator order, such as
the dynamics of theoretically predicted Majorana Fermion vortices
Localization of fermionic fields on braneworlds with bulk tachyon matter
Recently, Pal and Skar in [arXiv:hep-th/0701266] proposed a mechanism to
arise the warped braneworld models from bulk tachyon matter, which are endowed
with a thin brane and a thick brane. In this framework, we investigate
localization of fermionic fields on these branes. As in the 1/2 spin case, the
field can be localized on both the thin and thick branes with inclusion of
scalar background. In the 3/2 spin extension, the general supergravity action
coupled to chiral supermultiplets is considered to produce the localization on
both the branes as a result.Comment: 9 pages, no figure
Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe1-xCox)2As2 iron pnictides
An extensive calorimetric study of the normal- and superconducting-state
properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state
Sommerfeld coefficient increases (decreases) with Co doping for x
0.06), which illustrates the strong competition between magnetism and
superconductivity to monopolize the Fermi surface in the underdoped region and
the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All
superconducting samples exhibit a residual electronic density of states of
unknown origin in the zero-temperature limit, which is minimal at optimal
doping but increases to the normal-state value in the strongly under- and
over-doped regions. The remaining specific heat in the superconducting state is
well described using a two-band model with isotropic s-wave superconducting
gaps.Comment: Submitted to Europhysics Letter
Shear viscosity, instability and the upper bound of the Gauss-Bonnet coupling constant
We compute the dimensionality dependence of for charged black branes
with Gauss-Bonnet correction. We find that both causality and stability
constrain the value of Gauss-Bonnet coupling constant to be bounded by 1/4 in
the infinite dimensionality limit. We further show that higher dimensionality
stabilize the gravitational perturbation. The stabilization of the perturbation
in higher dimensional space-time is a straightforward consequence of the
Gauss-Bonnet coupling constant bound.Comment: 16 pages,3 figures+3 tables,typos corrected, published versio
Topological Surface States and Dirac point tuning in ternary Bi2Te2Se class of topological insulators
Using angle-resolved photoemission spectroscopy, we report electronic
structure for representative members of ternary topological insulators. We show
that several members of this family, such as Bi2Se2Te, Bi2Te2Se, and GeBi2Te4,
exhibit a singly degenerate Dirac-like surface state, while Bi2Se2S is a fully
gapped insulator with no measurable surface state. One of these compounds,
Bi2Se2Te, shows tunable surface state dispersion upon its electronic alloying
with Sb (SbxBi2-xSe2Te series). Other members of the ternary family such as
GeBi2Te4 and BiTe1.5S1.5 show an in-gap surface Dirac point, the former of
which has been predicted to show nonzero weak topological invariants such as
(1;111); thus belonging to a different topological class than BiTe1.5S1.5. The
measured band structure presented here will be a valuable guide for
interpreting transport, thermoelectric, and thermopower measurements on these
compounds. The unique surface band topology observed in these compounds
contributes towards identifying designer materials with desired flexibility
needed for thermoelectric and spintronic device fabrication.Comment: 9 pages, 6 figures; Related results at
http://online.kitp.ucsb.edu/online/topomat11/hasan
Dark energy from scalar field with Gauss Bonnet and non-minimal kinetic coupling
We study a model of scalar field with a general non-minimal kinetic coupling
to itself and to the curvature, and additional coupling to the Gauss Bonnet
4-dimensional invariant. The model presents rich cosmological dynamics and some
of its solutions are analyzed. A variety of scalar fields and potentials giving
rise to power-law expansion have been found. The dynamical equation of state is
studied for two cases, with and without free kinetic term . In both cases
phenomenologically acceptable solutions have been found. Some solutions
describe essentially dark energy behavior, and and some solutions contain the
decelerated and accelerated phases.Comment: 21 page
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