348 research outputs found
AdS/CFT for Four-Point Amplitudes involving Gravitino Exchange
In this paper we compute the tree-level four-point scattering amplitude of
two dilatini and two axion-dilaton fields in type IIB supergravity in AdS5 x
S5. A special feature of this process is that there is an "exotic" channel in
which there are no singleparticle poles. Another novelty is that this process
involves the exchange of a bulk gravitino. The amplitude is interpreted in
terms of N = 4 supersymmetric Yang-Mills theory at large 't Hooft coupling.
Properties of the Operator Product Expansion are used to analyze the various
contributions from single- and double-trace operators in the weak and strongly
coupled regimes, and to determine the anomalous dimensions of semi-short
operators. The analysis is particularly clear in the exotic channel, given the
absence of BPS states.Comment: 32 pages, 1 figure. Published Version. Minor change
Boundary multi-trace deformations and OPEs in AdS/CFT correspondence
We argue that multi-trace deformations of the boundary CFT in AdS/CFT
correspondence can arise through the OPE of single-trace operators. We work out
the example of a scalar field in AdS_5 with cubic self interaction. By an
appropriate reparametrization of the boundary data we are able to deform the
boundary CFT by a marginal operator that couples to the conformal anomaly. Our
method can be used in the analysis of multi-trace deformations in N=4 SYM where
the OPEs of various single-trace operators are known.Comment: 18 pages, v2 refinements and acknowledgements adde
Tunable resonators for quantum circuits
We have designed, fabricated and measured high-Q coplanar
waveguide microwave resonators whose resonance frequency is made tunable with
magnetic field by inserting a DC-SQUID array (including 1 or 7 SQUIDs) inside.
Their tunability range is 30% of the zero field frequency. Their quality factor
reaches up to 3. We present a model based on thermal fluctuations
that accounts for the dependance of the quality factor with magnetic field.Comment: subm. to JLTP (Proc. of LTD12 conference
Dielectric functions and collective excitations in MgB_2
The frequency- and momentum-dependent dielectric function as well as the energy loss function Im[-\protect{]} are calculated for intermetallic superconductor
by using two {\it ab initio} methods: the plane-wave pseudopotential method and
the tight-binding version of the LMTO method. We find two plasmon modes
dispersing at energies -8 eV and -22 eV. The high energy
plasmon results from a free electron like plasmon mode while the low energy
collective excitation has its origin in a peculiar character of the band
structure. Both plasmon modes demonstrate clearly anisotropic behaviour of both
the peak position and the peak width. In particular, the low energy collective
excitation has practically zero width in the direction perpendicular to boron
layers and broadens in other directions.Comment: 3 pages with 10 postscript figures. Submitted to PRB on May 14 200
Thermoelectric power of MgBBe
We investigated thermoelectric power of MgBBe (,
0.2, 0.3, 0.4, and 0.6). decreases systematically with , suggesting
that the hole density increases. Our band calculation shows that the increase
occurs in the -band. With the hole-doping, decreases.
Implication of this phenomenon is discussed within the BCS framework. While the
Mott formula explains only the linear part of at low temperature,
incorporation of electron-phonon interaction enables us to explain over
wide temperature range including the anomalous behavior at high temperature.Comment: 4 pages, 4 figure
Effects of C, Cu and Be substitutions in superconducting MgB2
Density functional calculations are used to investigate the effects of
partial substitutional alloying of the B site in MgB2 with C and Be alone and
combined with alloying of the Mg site with Cu. The effect of such substitutions
on the electronic structure, electron phonon coupling and superconductivity are
discussed. We find that Be substitution for B is unfavorable for
superconductivity as it leads to a softer lattice and weaker electron-phonon
couplings. Replacement of Mg by Cu leads to an increase in the stiffness and
doping level at the same time, while the carrier concentration can be
controlled by partial replacement of B by C. We estimate that with full
replacement of Mg by Cu and fractional substitution of B by C, Tc values of 50K
may be attainable.Comment: 5 pages, 4 figure
Surface and Image-Potential States on the MgB_2(0001) Surfaces
We present a self-consistent pseudopotential calculation of surface and
image-potential states on for both -terminated () and
-terminated () surfaces. We find a variety of very clear surface and
subsurface states as well as resonance image-potential states n=1,2 on both
surfaces. The surface layer DOS at is increased by 55% at and by
90% at the surface compared to DOS in the corresponding bulk layers.Comment: 3 pages, 6 figure
On Low-Energy Effective Actions in N = 2, 4 Superconformal Theories in Four Dimensions
We study some aspects of low-energy effective actions in 4-d superconformal
gauge theories on the Coulomb branch. We describe superconformal invariants
constructed in terms of N=2 abelian vector multiplet which play the role of
building blocks for the N=2,4 supersymmetric low-energy effective actions. We
compute the one-loop effective actions in constant N=2 field strength
background in N=4 SYM theory and in N=2 SU(2) SYM theory with four
hypermultiplets in fundamental representation. Using the classification of
superconformal invariants we then find the manifestly N=2 superconformal form
of these effective actions. While our explicit computations are done in the
one-loop approximation, our conclusions about the structure of the effective
actions in N=2 superconformal theories are general. We comment on some
applications to supergravity - gauge theory duality in the description of
D-brane interactions.Comment: 18 pages, latex, comments/reference adde
FERMION ZERO MODES AND BLACK-HOLE HYPERMULTIPLETS WITH RIGID SUPERSYMMETRY
The gravitini zero modes riding on top of the extreme Reissner-Nordstrom
black-hole solution of N=2 supergravity are shown to be normalizable. The
gravitini and dilatini zero modes of axion-dilaton extreme black-hole solutions
of N=4 supergravity are also given and found to have finite norms. These norms
are duality invariant. The finiteness and positivity of the norms in both cases
are found to be correlated with the Witten-Israel-Nester construction; however,
we have replaced the Witten condition by the pure-spin-3/2 constraint on the
gravitini. We compare our calculation of the norms with the calculations which
provide the moduli space metric for extreme black holes.
The action of the N=2 hypermultiplet with an off-shell central charge
describes the solitons of N=2 supergravity. This action, in the
Majumdar-Papapetrou multi-black-hole background, is shown to be N=2 rigidly
supersymmetric.Comment: 18 pages, LaTe
Decoherence and CPT Violation in a Stringy Model of Space-Time Foam
I discuss a model inspired from the string/brane framework, in which our
Universe is represented as a three brane, propagating in a bulk space time
punctured by D0-brane (D-particle) defects. As the D3-brane world moves in the
bulk, the D-particles cross it, and from an effective observer on D3 the
situation looks like a ``space-time foam'' with the defects ``flashing'' on and
off (``D-particle foam''). The open strings, with their ends attached on the
brane, which represent matter in this scenario, can interact with the
D-particles on the D3-brane universe in a topologically non-trivial manner,
involving splitting and capture of the strings by the D0-brane defects. Such
processes are described by logarithmic conformal field theories on the
world-sheet. Physically, they result in effective decoherence of the string
matter on the D3 brane, and as a result, of CPT Violation, but of a type that
implies an ill-defined nature of the effective CPT operator. Due to electric
charge conservation, only electrically neutral (string) matter can exhibit such
interactions with the D-particle foam. This may have unique, experimentally
detectable, consequences for electrically-neutral entangled quantum matter
states on the brane world, in particular the modification of the pertinent EPR
Correlation of neutral mesons in a meson factory.Comment: 41 pages Latex, five eps figures incorporated. Uses special macro
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