1,188 research outputs found
Gyroscopic control of a rigid body constrained to rotate about a fixed axis
Gyroscopic control of spin rate or orientation of rigid bodies constrained to rotate about fixed axi
On the CSFT approach to localized closed string tachyons
We compute the potential for localized closed string tachyons in bosonic
string theory on the orbifold C/Z_4 using level-truncated closed string field
theory. The critical points of the potential exhibit features which agree with
their conjectured identification as lower-order orbifolds. However this case
also raises some questions regarding the quantitative predictions associated
with these conjectures.Comment: 20 pages, 3 figures, v2: The relation between the flat space and
orbifold gravitational constants has been corrected. This resolves the puzzle
of multiple predictions, but worsens the agreement between the depth of the
potential and the change in the deficit angl
A holographic proof of the strong subadditivity of entanglement entropy
When a quantum system is divided into subsystems, their entanglement
entropies are subject to an inequality known as "strong subadditivity". For a
field theory this inequality can be stated as follows: given any two regions of
space and , . Recently, a
method has been found for computing entanglement entropies in any field theory
for which there is a holographically dual gravity theory. In this note we give
a simple geometrical proof of strong subadditivity employing this holographic
prescription.Comment: 9 pages, 3 figure
Strong subadditivity and the covariant holographic entanglement entropy formula
Headrick and Takayanagi showed that the Ryu-Takayanagi holographic
entanglement entropy formula generally obeys the strong subadditivity (SSA)
inequality, a fundamental property of entropy. However, the Ryu-Takayanagi
formula only applies when the bulk spacetime is static. It is not known whether
the covariant generalization proposed by Hubeny, Rangamani, and Takayanagi
(HRT) also obeys SSA. We investigate this question in three-dimensional
AdS-Vaidya spacetimes, finding that SSA is obeyed as long as the bulk spacetime
satisfies the null energy condition. This provides strong support for the
validity of the HRT formula.Comment: 38 page
Interfacial Studies in Semiconductor Heterostructures by X-Ray Diffraction Techniques
X-ray radiation is a non-destructive probe well suited to assess structural perfection of semiconductor material. Three techniques are used to study the interfacial roughness, period fluctuations and annealing-induced interdiffusion in various superlattice structures. Reflectivity of long period Si/Si1-xGex multiple quantum wells reveals an asymmetry oriented along the direction of miscut in the interface roughness with the Si1-xGex to Si interfaces being about twice as rough (0.5 versus 0.3 nm) as the Si to Si1-xGex interfaces. For Si-Si0.65Ge0.35 multiple quantum wells, diffuse scattering is minimal for a growth temperature of 550°C and increases substantially at very low (250°C) or high (750°C) growth temperatures. In (SimGen)p short period superlattices, the X-ray reflectivity data are consistent with interfacial mixing over about two monolayers and thickness fluctuations of about 5% vertically in the structures. For superlattices grown on vicinal surfaces, the roughness spectrum is correlated with the surface miscut orientation. Double-crystal X-ray diffraction using symmetrical and asymmetrical reflections has been used to study epitaxial lattice distortion and strain relaxation in InGaAs/GaAs heterostructures grown on (100) on-orientation and 2° off (100) GaAs surfaces. It is shown that thick InGaAs films retain an appreciable fraction of their initial strain and that their crystal lattice is triclinically distorted. The magnitude of the deformation is larger when growth is carried out on a vicinal surface
Comparative Biologies of the Cryptic, Sympatric Species, Trupanea bisetosa and T. nigricornis (Diptera: Tephritidae) in Southern California
The biologies of the sympatric, cryptic species, Trupanea nigricornis (Coquillett), a flower head-infesting fruit fly attacking a wide range of hosts in 8 tribes, 33 genera, and at least 71 species of Asteraceae, and T. bisetosa (Coquillett), an oligophage attacking only 6 hosts in 4 genera of the tribe Heliantheae, are described and compared. A major biological distinction between these species was their ovipositional behavior, whereby females oviposited different numbers of eggs at different sites in different developmental stages of flower heads of their hosts. The larvae of these species showed minor differences in their feeding behaviors, and their puparia were formed and located similarly in host flower heads. Development from egg to adult under field conditions lasted up to 35 d for each species. These species showed subtle differences in their courtship and mating behaviors, and substantial differences in the daily timing of courtship
A numerical approach to finding general stationary vacuum black holes
The Harmonic Einstein equation is the vacuum Einstein equation supplemented
by a gauge fixing term which we take to be that of DeTurck. For static black
holes analytically continued to Riemannian manifolds without boundary at the
horizon this equation has previously been shown to be elliptic, and Ricci flow
and Newton's method provide good numerical algorithms to solve it. Here we
extend these techniques to the arbitrary cohomogeneity stationary case which
must be treated in Lorentzian signature. For stationary spacetimes with
globally timelike Killing vector the Harmonic Einstein equation is elliptic. In
the presence of horizons and ergo-regions it is less obviously so. Motivated by
the Rigidity theorem we study a class of stationary black hole spacetimes,
considered previously by Harmark, general enough to include the asymptotically
flat case in higher dimensions. We argue the Harmonic Einstein equation
consistently truncates to this class of spacetimes giving an elliptic problem.
The Killing horizons and axes of rotational symmetry are boundaries for this
problem and we determine boundary conditions there. As a simple example we
numerically construct 4D rotating black holes in a cavity using Anderson's
boundary conditions. We demonstrate both Newton's method and Ricci flow to find
these Lorentzian solutions.Comment: 43 pages, 7 figure
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