702 research outputs found
Non-Abelian Duality for Open Strings
We examine non-abelian duality transformations in the open string case. After
gauging the isometries of the target space and developing the general
formalism, we study in details the duals oftarget spaces with SO(N) isometries
which, for the SO(2) case, reduces to the known abelian T-duals. We apply the
formalism to electrically and magnetically charged 4D black hole solutions and,
as in the abelian case, dual coordinates satisfy Dirichlet conditions.Comment: 18 pages, Latex. Some formulas are added. Final version to appear in
Nucl. Phys.
Coefficient of Restitution for Viscoelastic Spheres: The Effect of Delayed Recovery
The coefficient of normal restitution of colliding viscoelastic spheres is
computed as a function of the material properties and the impact velocity. From
simple arguments it becomes clear that in a collision of purely repulsively
interacting particles, the particles loose contact slightly before the distance
of the centers of the spheres reaches the sum of the radii, that is, the
particles recover their shape only after they lose contact with their collision
partner. This effect was neglected in earlier calculations which leads
erroneously to attractive forces and, thus, to an underestimation of the
coefficient of restitution. As a result we find a novel dependence of the
coefficient of restitution on the impact rate.Comment: 11 pages, 2 figure
Coefficient of tangential restitution for the linear dashpot model
The linear dashpot model for the inelastic normal force between colliding
spheres leads to a constant coefficient of normal restitution,
const., which makes this model very popular for the investigation
of dilute and moderately dense granular systems. For two frequently used models
for the tangential interaction force we determine the coefficient of tangential
restitution , both analytically and by numerical integration of
Newton's equation. Although const. for the linear-dashpot model,
we obtain pronounced and characteristic dependencies of the tangential
coefficient on the impact velocity . The
results may be used for event-driven simulations of granular systems of
frictional particles.Comment: 12 pages, 12 figure
Chains of Viscoelastic Spheres
Given a chain of viscoelastic spheres with fixed masses of the first and last
particles. We raise the question: How to chose the masses of the other
particles of the chain to assure maximal energy transfer? The results are
compared with a chain of particles for which a constant coefficient of
restitution is assumed. Our simple example shows that the assumption of
viscoelastic particle properties has not only important consequences for very
large systems (see [1]) but leads also to qualitative changes in small systems
as compared with particles interacting via a constant restitution coefficient.Comment: 11 pages, 6 figure
Profile of Corporate Social Media Consumer Segments
The trade and academic literature is replete with commentary about the need for companies to develop promotional strategies and to adopt media platforms that are more engaging and conversational with customers than the traditional top-down company directed one-way communication strategies of the past (Thomas, Peters, Howell and Robbins, 2012; Foster, West and Francescucci 2011; Deighton and Kornfeld, 2009). This viewpoint is supported by Christodoulides (2008) who reported that many customers view information about a company or brand that they obtained from blogs, social networking sites and the like as being more relevant, believable and important to them in their interactions with the company than similar company provided information. Social media savvy customers are by all counts a highly diverse lot and reaching them effectively requires the use of different messages for different groups (Heo and Cho, 2009; Stern, Rao and Gould 1990). For example, some social media users follow a wide variety of posts and enjoy the benefits of special offers while others have mixed feelings about being marketed to on social media websites. Interestingly, this new brand of customer also expects businesses to really know them, their life cycles, their personalities and the goods and services that they desire (Beauchamp 2013)
Asymmetric optical nuclear spin pumping in a single uncharged quantum dot
A highly asymmetric dynamic nuclear spin pumping is observed in a single self
assembled InGaAs quantum dot subject to resonant optical pumping of the neutral
exciton transition leading to a large maximum polarization of 54%. This dynamic
nuclear polarization is found to be much stronger following pumping of the
higher energy Zeeman state. Time-resolved measurements allow us to directly
monitor the buildup of the nuclear spin polarization in real time and to
quantitatively study the dynamics of the process. A strong dependence of the
observed dynamic nuclear polarization on the applied magnetic field is found,
with resonances in the pumping efficiency being observed for particular
magnetic fields. We develop a model that fully accounts for the observed
behaviour, where the pumping of the nuclear spin system is due to
hyperfine-mediated spin flip transitions between the states of the neutral
exciton manifold.Comment: published version; 4+ pages, 3 figures (eps
String Kaluza-Klein cosmologies with RR-fields
We construct 4-dimensional cosmological FRW--models by compactifying a black
5-brane solution of type IIB supergravity, which carries both magnetic
NS-NS-charge and RR-charge. The influence of nontrivial RR-fields on the
dynamics of the cosmological models is investigated.Comment: 15 pages, LaTeX, psfig.sty, 5 figures. References added, version to
appear in Phys.Lett.
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