73,189 research outputs found
On the spectrum of a matrix model for the D=11 supermembrane compactified on a torus with non-trivial winding
The spectrum of the Hamiltonian of the double compactified D=11 supermembrane
with non-trivial central charge or equivalently the non-commutative symplectic
super Maxwell theory is analyzed. In distinction to what occurs for the D=11
supermembrane in Minkowski target space where the bosonic potential presents
string-like spikes which render the spectrum of the supersymmetric model
continuous, we prove that the potential of the bosonic compactified membrane
with non-trivial central charge is strictly positive definite and becomes
infinity in all directions when the norm of the configuration space goes to
infinity. This ensures that the resolvent of the bosonic Hamiltonian is
compact. We find an upper bound for the asymptotic distribution of the
eigenvalues.Comment: 11 pages, LaTe
Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping
We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that “seed” InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of “seeds” on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth
The spread of the gluon k_t-distribution and the determination of the saturation scale at hadron colliders in resummed NLL BFKL
The transverse momentum distribution of soft hadrons and jets that accompany
central hard-scattering production at hadron colliders is of great importance,
since it has a direct bearing on the ability to separate new physics signals
from Standard Model backgrounds. We compare the predictions for the gluonic
k_t-distribution using two different approaches: resummed NLL BFKL and DGLAP
evolution. We find that as long as the initial and final virtualities (k_t)
along the emission chain are not too close to each other, the NLL resummed BFKL
results do not differ significantly from those obtained using standard DGLAP
evolution. The saturation momentum Q_s(x), calculated within the resummed BFKL
approach, grows with 1/x even slower than in the leading-order DGLAP case.Comment: 24 pages, 8 figures, An improved, slightly more precise NLL
resummation is used and the figures are updated accordingly. The conclusions
are unchange
A note on the improvement ambiguity of the stress tensor and the critical limits of correlation functions
I study various properties of the critical limits of correlators containing
insertions of conserved and anomalous currents. In particular, I show that the
improvement term of the stress tensor can be fixed unambiguously, studying the
RG interpolation between the UV and IR limits. The removal of the improvement
ambiguity is encoded in a variational principle, which makes use of sum rules
for the trace anomalies a and a'. Compatible results follow from the analysis
of the RG equations. I perform a number of self-consistency checks and discuss
the issues in a large set of theories.Comment: 15 page
Gaugino mass non-universality in an SO(10) supersymmetric Grand Unified Theory: low-energy spectra and collider signals
We derive the non-universal gaugino mass ratios in a supergravity (SUGRA)
framework where the Higgs superfields belong to the non-singlet representations
{\bf 54} and {\bf 770} in a SO(10) Grand Unified Theory (GUT). We evaluate the
ratios for the phenomenologically viable intermediate breaking chain . After a full calculation of the
gaugino mass ratios, noting some errors in the earlier calculation for {\bf
54}, we obtain, using the renormalisation group equations (RGE), interesting
low scale phenomenology of such breaking patterns. Here, we assume the breaking
of the SO(10) GUT group to the intermediate gauge group and that to the
Standard Model (SM) take place at the GUT scale itself. We also study the
collider signatures in multilepton channels at the Large Hadron Collider (LHC)
for some selected benchmark points allowed by the cold dark matter relic
density constraint provided by the WMAP data and compare these results with the
minimal supergravity (mSUGRA) framework with similar gluino masses indicating
their distinguishability in this regard.Comment: 35 pages, 5 figures, 9 tables. Part of the calculations modifie
The chiral and flavour projection of Dirac-Kahler fermions in the geometric discretization
It is shown that an exact chiral symmetry can be described for Dirac-Kahler
fermions using the two complexes of the geometric discretization. This
principle is extended to describe exact flavour projection and it is shown that
this necessitates the introduction of a new operator and two new structures of
complex. To describe simultaneous chiral and flavour projection, eight
complexes are needed in all and it is shown that projection leaves a single
flavour of chiral field on each.Comment: v2: 17 pages, Latex. 5 images eps. Added references, reformatted and
clarification of some point
Atom-wave diffraction between the Raman-Nath and the Bragg regime: Effective Rabi frequency, losses, and phase shifts
We present an analytic theory of the diffraction of (matter) waves by a
lattice in the "quasi-Bragg" regime, by which we mean the transition region
between the long-interaction Bragg and "channelling" regimes and the
short-interaction Raman-Nath regime. The Schroedinger equation is solved by
adiabatic expansion, using the conventional adiabatic approximation as a
starting point, and re-inserting the result into the Schroedinger equation to
yield a second order correction. Closed expressions for arbitrary pulse shapes
and diffraction orders are obtained and the losses of the population to output
states otherwise forbidden by the Bragg condition are derived. We consider the
phase shift due to couplings of the desired output to these states that depends
on the interaction strength and duration and show how these can be kept
negligible by a choice of smooth (e.g., Gaussian) envelope functions even in
situations that substantially violate the adiabaticity condition. We also give
an efficient method for calculating the effective Rabi frequency (which is
related to the eigenvalues of Mathieu functions) in the quasi-Bragg regime.Comment: Minor additions, more concise text. To appear in Phys. Rev. A. 20
pages, 10 figure
Exact Green's Function of the reversible diffusion-influenced reaction for an isolated pair in 2D
We derive an exact Green's function of the diffusion equation for a pair of
spherical interacting particles in 2D subject to a back-reaction boundary
condition.Comment: 6 pages, 1 Figur
Making Ends Meet: String Unification and Low-Energy Data
A long-standing problem in string phenomenology has been the fact that the
string unification scale disagrees with the GUT scale obtained by extrapolating
low-energy data within the framework of the minimal supersymmetric standard
model (MSSM). In this paper we examine several effects that may modify the
minimal string predictions and thereby bring string-scale unification into
agreement with low-energy data. These include heavy string threshold
corrections, non-standard hypercharge normalizations, light SUSY thresholds,
intermediate gauge structure, and thresholds arising from extra matter beyond
the MSSM. We explicitly evaluate these contributions within a variety of
realistic free-fermionic string models, including the flipped SU(5), SO(6) x
SO(4), and various SU(3) x SU(2) x U(1) models, and find that most of these
sources do not substantially alter the minimal string predictions. Indeed, we
find that the only way to reconcile string unification with low-energy data is
through certain types of extra matter. Remarkably, however, many of the
realistic string models contain precisely this required matter in their
low-energy spectra.Comment: 10 pages, standard LaTeX, 1 figure (Encapsulated PostScript), version
published in Phys. Rev. Lett. 75 (1995) 264
Charge Localization and Ordering in AMnO Hollandite Group Oxides: Impact of Density Functional Theory Approaches
The phases of AMnO hollandite group oxides emerge from the
competition between ionic interactions, Jahn-Teller effects, charge ordering,
and magnetic interactions. Their balanced treatment with feasible computational
approaches can be challenging for commonly used approximations in Density
Functional Theory. Three examples (A = Ag, Li and K) are studied with a
sequence of different approximate exchange-correlation functionals. Starting
from a generalized gradient approximation (GGA), an extension to include van
der Waals interactions and a recently proposed meta-GGA are considered. Then
local Coulomb interactions for the Mn electrons are more explicitly
considered with the DFT+ approach. Finally selected results from a hybrid
functional approach provide a reference. Results for the binding energy of the
A species in the parent oxide highlight the role of van der Waals interactions.
Relatively accurate results for insertion energies can be achieved with a low
and a high approach. In the low case, the materials are described
as band metals with a high symmetry, tetragonal crystal structure. In the high
case, the electrons donated by A result in formation of local Mn
centers and corresponding Jahn-Teller distortions characterized by a local
order parameter. The resulting degree of monoclinic distortion depends on
charge ordering and magnetic interactions in the phase formed. The reference
hybrid functional results show charge localization and ordering. Comparison to
low temperature experiments of related compounds suggests that charge
localization is the physically correct result for the hollandite group oxides
studied here. . . .Comment: 16 pages, 8 figure
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