709 research outputs found
Reverse geometric engineering of singularities
One can geometrically engineer supersymmetric field theories theories by
placing D-branes at or near singularities. The opposite process is described,
where one can reconstruct the singularities from quiver theories. The
description is in terms of a noncommutative quiver algebra which is constructed
from the quiver diagram and the superpotential. The center of this
noncommutative algebra is a commutative algebra, which is the ring of
holomorphic functions on a variety V. If certain algebraic conditions are met,
then the reverse geometric engineering produces V as the geometry that D-branes
probe. It is also argued that the identification of V is invariant under
Seiberg dualities.Comment: 17 pages, Latex. v2: updates reference
Can black holes be torn up by phantom dark energy in cyclic cosmology?
Infinitely cyclic cosmology is often frustrated by the black hole problem. It
has been speculated that this obstacle in cyclic cosmology can be removed by
taking into account a peculiar cyclic model derived from loop quantum cosmology
or the braneworld scenario, in which phantom dark energy plays a crucial role.
In this peculiar cyclic model, the mechanism of solving the black hole problem
is through tearing up black holes by phantom. However, using the theory of
fluid accretion onto black holes, we show in this paper that there exists
another possibility: that black holes cannot be torn up by phantom in this
cyclic model. We discussed this possibility and showed that the masses of black
holes might first decrease and then increase, through phantom accretion onto
black holes in the expanding stage of the cyclic universe.Comment: 6 pages, 2 figures; discussions adde
Determining Ratios of WIMP-Nucleon Cross Sections from Direct Dark Matter Detection Data
Weakly Interacting Massive Particles (WIMPs) are one of the leading
candidates for Dark Matter. So far the usual procedure for constraining the
WIMP-nucleon cross sections in direct Dark Matter detection experiments have
been to fit the predicted event rate based on some model(s) of the Galactic
halo and of WIMPs to experimental data. One has to assume whether the
spin-independent (SI) or the spin-dependent (SD) WIMP-nucleus interaction
dominates, and results of such data analyses are also expressed as functions of
the as yet unknown WIMP mass. In this article, I introduce methods for
extracting information on the WIMP-nucleon cross sections by considering a
general combination of the SI and SD interactions. Neither prior knowledge
about the local density and the velocity distribution of halo WIMPs nor about
their mass is needed. Assuming that an exponential-like shape of the recoil
spectrum is confirmed from experimental data, the required information are only
the measured recoil energies (in low energy ranges) and the number of events in
the first energy bin from two or more experiments.Comment: 33 pages, 20 eps figures; v2: typos fixed, references added and
updated, revised version for publicatio
Statistical Mechanical Theory of a Closed Oscillating Universe
Based on Newton's laws reformulated in the Hamiltonian dynamics combined with
statistical mechanics, we formulate a statistical mechanical theory supporting
the hypothesis of a closed oscillating universe. We find that the behaviour of
the universe as a whole can be represented by a free entropic oscillator whose
lifespan is nonhomogeneous, thus implying that time is shorter or longer
according to the state of the universe itself given through its entropy. We
conclude that time reduces to the entropy production of the universe and that a
nonzero entropy production means that local fluctuations could exist giving
rise to the appearance of masses and to the curvature of the space
Supergravity Null Scissors and Super-Crosses
In this paper we construct the supergravity solutions for the orthogonally
intersecting null scissors and the fluxed D-strings. We name the latter as the
super-crosses according to their shape. It turns out that the smeared solutions
are U-dual related to the intersecting -strings. Their open string
properties are also studied. As a by-product, we clarify the supersymmetry
conditions of D2-D2 pairs with most generic fluxes.Comment: 16 pages, 1 figure;v2. minor comment revised;v3. references added,
final to JHE
Improved Method of Fatigue Life Assessment for TiAl Alloys
For rapid fatigue life assessment of TiAl alloys,
the new method was proposed based on qualitative
and quantitative analyses. The qualitative
analysis was employed to illustrate the microstructure
effect for TiAl alloys on their fatigue
life. The new formula is derived for estimation
of the interaction forces of dislocations, which
yields quite satisfactory results. The results of
qualitative and quantitative analyses were used
to predict the fatigue life improvement by the
addition of trace elements producing grain refinement.Предложен новый метод оценки усталостной долговечности TiAl сплавов по результатам
количественного и качественного анализа. С помощью количественного анализа определено
влияние микроструктуры TiAl сплавов на их усталостную долговечность. Расчет усилия
между дислокациями проведен по стандартной формуле, с помощью которой можно получить точный результат. Количественный и качественный анализ показал, что усталостную
долговечность можно повысить путем введения микроэлементов, которые способствуют измельчению зерен.Запропоновано новий метод оцінки довговічності від утомленості ТіАl сплавів за
результатами кількісного та якісного аналізу. За допомогою кількісного аналізу
визначено вплив мікроструктури TiAl сплавів на їх довговічність від утомленості.
Розрахунок зусилля між дислокаціями проведено за стандартною формулою, за
допомогою якої можна отримати точний результат. Кількісний та якісний аналіз
показав, що довговічність від утомленості можна підвищити шляхом введення мікроелементів, які сприяють подрібненню зерен
Calculation of the γ-TiAl Lattice Resistance
The dislocation width and lattice resistance (Peierls stress) of a γ-TiAl alloy are calculated by the density ratio method. The lattice resistance is shown to decrease with the dislocation width. The relationship between the Peierls stress and dislocation width variation is defined by theoretical derivation. The yield stress is negatively correlated with the shear stress of the material. It can become a useful tool for choosing an appropriate shear stress under deformation.Методом отношения плотностей рассчитаны ширина дислокации и сопротивление решетки (напряжение Пайерлса). Результаты показывают, что сопротивление решетки уменьшается с увеличением ширины дислокации. Путем расчета получена зависимость между напряжением Пайерлса и дислокационным упрочнением, свидетельствующая о том, что предел текучести отрицательно коррелирует с напряжением сдвига материала. Поэтому она может дать полезную информацию для выбора соответствующего напряжения сдвига в процессе растяжения.Методом відносини щільностей розраховані ширина дислокації і опір решітки (напруження Пайерлса). Результати показують, що опір решітки зменшується зі збільшенням ширини дислокації. Шляхом розрахунку отримана залежність між напруженням Пайерлса і дислокаційним зміцненням, яка свідчить про те, що межа плинності негативно корелює з напруженням зсуву матеріалу. Тому вона може дати корисну інформацію для вибору відповідного напруження зсуву в процесі розтягування
Effect of bilayer coupling on tunneling conductance of double-layer high T_c cuprates
Physical effects of bilayer coupling on the tunneling spectroscopy of high
T cuprates are investigated. The bilayer coupling separates the bonding
and antibonding bands and leads to a splitting of the coherence peaks in the
tunneling differential conductance. However, the coherence peak of the bonding
band is strongly suppressed and broadened by the particle-hole asymmetry in the
density of states and finite quasiparticle life-time, and is difficult to
resolve by experiments. This gives a qualitative account why the bilayer
splitting of the coherence peaks was not clearly observed in tunneling
measurements of double-layer high-T oxides.Comment: 4 pages, 3 figures, to be published in PR
Deconstructing Noncommutativity with a Giant Fuzzy Moose
We argue that the worldvolume theories of D-branes probing orbifolds with
discrete torsion develop, in the large quiver limit, new non-commutative
directions. This provides an explicit `deconstruction' of a wide class of
noncommutative theories. This also provides insight into the physical meaning
of discrete torsion and its relation to the T-dual B field. We demonstrate that
the strict large quiver limit reproduces the matrix theory construction of
higher-dimensional D-branes, and argue that finite `fuzzy moose' theories
provide novel regularizations of non-commutative theories and explicit string
theory realizations of gauge theories on fuzzy tori. We also comment briefly on
the relation to NCOS, (2,0) and little string theories.Comment: 22 pages, 3 figures, typos caught and refs added; expanded
interpretation of discrete torsio
String Cosmology: A Review
We give an overview of the status of string cosmology. We explain the
motivation for the subject, outline the main problems, and assess some of the
proposed solutions. Our focus is on those aspects of cosmology that benefit
from the structure of an ultraviolet-complete theory.Comment: 55 pages. v2: references adde
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