208 research outputs found
Impurity and spin effects on the magneto-spectroscopy of a THz-modulated nanostructure
We present a grid-free DFT model appropriate to explore the time evolution of
electronic states in a semiconductor nanostructure. The model can be used to
investigate both the linear and the nonlinear response of the system to an
external short-time perturbation in the THz regime. We use the model to study
the effects of impurities on the magneto-spectroscopy of a two-dimensional
electron gas in a nanostructure excited by an intense THz radiation. We do
observe a reduction in the binding energy of the impurity with increasing
excitation strength, and at a finite magnetic field we find a slow onset of
collective spin-oscillations that can be made to vanish with a stronger
excitation.Comment: LaTeX,10 pages with 11 embedded postscript figure
Non-Perturbative Corrections and Modularity in N=1 Type IIB Compactifications
Non-perturbative corrections and modular properties of four-dimensional type
IIB Calabi-Yau orientifolds are discussed. It is shown that certain
non-perturbative alpha' corrections survive in the large volume limit of the
orientifold and periodically correct the Kahler potential. These corrections
depend on the NS-NS two form and have to be completed by D-instanton
contributions to transform covariantely under symmetries of the type IIB
orientifold background. It is shown that generically also the D-instanton
superpotential depends on the two-form moduli as well as on the complex
dilaton. These contributions can arise through theta-functions with the dilaton
as modular parameter. An orientifold of the Enriques Calabi-Yau allows to
illustrate these general considerations. It is shown that this compactification
leads to a controlled four-dimensional N=1 effective theory due to the absence
of various quantum corrections. Making contact to the underlying topological
string theory the D-instanton superpotential is proposed to be related to a
specific modular form counting D3, D1, D(-1) degeneracies on the Enriques
Calabi-Yau.Comment: 35 page
A functorial construction of moduli of sheaves
We show how natural functors from the category of coherent sheaves on a
projective scheme to categories of Kronecker modules can be used to construct
moduli spaces of semistable sheaves. This construction simplifies or clarifies
technical aspects of existing constructions and yields new simpler definitions
of theta functions, about which more complete results can be proved.Comment: 52 pp. Dedicated to the memory of Joseph Le Potier. To appear in
Inventiones Mathematicae. Slight change in the definition of the Kronecker
algebra in Secs 1 (p3) and 2.2 (p6), with corresponding small alterations
elsewhere, to make the constructions work for non-reduced schemes. Section
6.5 rewritten. Remark 2.6 and new references adde
Response function analysis of excited-state kinetic energy functional constructed by splitting k-space
Over the past decade, fundamentals of time independent density functional
theory for excited state have been established. However, construction of the
corresponding energy functionals for excited states remains a challenging
problem. We have developed a method for constructing functionals for excited
states by splitting k-space according to the occupation of orbitals. In this
paper we first show the accuracy of kinetic energy functional thus obtained. We
then perform a response function analysis of the kinetic energy functional
proposed by us and show why method of splitting the k-space could be the method
of choice for construction of energy functionals for excited states.Comment: 11 page
The spectrum of BPS branes on a noncompact Calabi-Yau
We begin the study of the spectrum of BPS branes and its variation on lines
of marginal stability on O_P^2(-3), a Calabi-Yau ALE space asymptotic to
C^3/Z_3. We show how to get the complete spectrum near the large volume limit
and near the orbifold point, and find a striking similarity between the
descriptions of holomorphic bundles and BPS branes in these two limits. We use
these results to develop a general picture of the spectrum. We also suggest a
generalization of some of the ideas to the quintic Calabi-Yau.Comment: harvmac, 45 pp. (v2: added references
Regular black holes in quadratic gravity
The first-order correction of the perturbative solution of the coupled
equations of the quadratic gravity and nonlinear electrodynamics is
constructed, with the zeroth-order solution coinciding with the ones given by
Ay\'on-Beato and Garc{\'\i}a and by Bronnikov. It is shown that a simple
generalization of the Bronnikov's electromagnetic Lagrangian leads to the
solution expressible in terms of the polylogarithm functions. The solution is
parametrized by two integration constants and depends on two free parameters.
By the boundary conditions the integration constants are related to the charge
and total mass of the system as seen by a distant observer, whereas the free
parameters are adjusted to make the resultant line element regular at the
center. It is argued that various curvature invariants are also regular there
that strongly suggests the regularity of the spacetime. Despite the complexity
of the problem the obtained solution can be studied analytically. The location
of the event horizon of the black hole, its asymptotics and temperature are
calculated. Special emphasis is put on the extremal configuration
Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters
Empirical evidence for both stellar mass black holes M_bh<10^2 M_sun) and
supermassive black holes (SMBHs, M_bh>10^5 M_sun) is well established.
Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is
correlated with the bulge mass, and even more strongly with the central stellar
velocity dispersion sigma_c, the `M-sigma' relation. On the other hand,
evidence for "intermediate-mass" black holes (IMBHs, with masses in the range
1^2 - 10^5 M_sun) is relatively sparse, with only a few mass measurements
reported in globular clusters (GCs), dwarf galaxies and low-mass AGNs. We
explore the question of whether globular clusters extend the M-sigma
relationship for galaxies to lower black hole masses and find that available
data for globular clusters are consistent with the extrapolation of this
relationship. We use this extrapolated M-sigma relationship to predict the
putative black hole masses of those globular clusters where existence of
central IMBH was proposed. We discuss how globular clusters can be used as a
constraint on theories making specific predictions for the low-mass end of the
M-sigma relation.Comment: 14 pages, 3 figures, accepted for publication in Astrophysics and
Space Science; fixed typos and a quote in Sec.
Tomographic Representation of Minisuperspace Quantum Cosmology and Noether Symmetries
The probability representation, in which cosmological quantum states are
described by a standard positive probability distribution, is constructed for
minisuperspace models selected by Noether symmetries. In such a case, the
tomographic probability distribution provides the classical evolution for the
models and can be considered an approach to select "observable" universes. Some
specific examples, derived from Extended Theories of Gravity, are worked out.
We discuss also how to connect tomograms, symmetries and cosmological
parameters.Comment: 15 page
InfluĂȘncia da estação do ano nas caracterĂsticas do sĂȘmen e na concentração de hormĂŽnios em touros Nelore e Simental
A bull breeding soundness evaluation system with greater emphasis on scrotal circumference
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