7,275 research outputs found
Stress Tensor and Bulk Viscosity in Relativistic Nuclear Collisions
We discuss the influence of different initial conditions for the stress
tensor and the effect of bulk viscosity on the expansion and cooling of the
fireball created in relativistic heavy-ion collisions. In particular, we
explore the evolution of longitudinal and transverse components of the pressure
and the extent of dissipative entropy production in the one-dimensional,
boost-invariant hydrodynamic model. We find that a bulk viscosity consistent
with recent estimates from lattice QCD further slows the equilibration of the
system, however it does not significantly increase the entropy produced
Chiral structures of lander molecules on Cu(100)
Supramolecular assemblies of lander molecules (CH) on Cu(100)
are investigated with low-temperature scanning tunneling microscopy. The
energetically most favourable conformation of the adsorbed molecule is found to
exist in two mirror symmetric enantiomers or conformers. At low coverage, the
molecules align in enantiomerically pure chains along the chiral directions
and . The arrangement is proposed to be
mainly governed by intermolecular van-der-Waals interaction. At higher
coverages, the molecular chains arrange into chiral domains, for which a
structural model is presented.Comment: to appear in Nanotechnology vol. 15 (2004
Self-Screened Parton Cascades
The high density of scattered partons predicted in nuclear collisions at very
high energy makes color screening effects significant. We explain how these
screening mechanisms may suppress nonperturbative, soft QCD processes,
permitting a consistent calculation of quark-gluon plasma formation within the
framework of perturbative QCD. We present results of a model calculation of
these effects including predictions for the initial thermalized state for heavy
nuclei colliding at RHIC and LHC.Comment: Talk presented at the RHIC'96 Workshop, BNL, July 199
Costs, Institutional Mobility Barriers, and Market Structure: Advertising Agencies as Multiproduct Firms
What accounts for the diversity and limited concentration that has long characterized the organization of the advertising agency industry? This question is addressed by treating an advertising agency as a multiproduct firm. The firm's product line or service mix is defined in terms of the set of different media categories where an agency places the advertising messages which it creates on behalf of its clients. Evidence is presented indicating that the structure of demand and costs in the advertising agency industry conforms to the conditions that MacDonald and Slivinski (1987) showed were required for an industry to sustain an equilibrium with diversified firms. Building on this framework, we formulate a set of three hypotheses relating to the realization of product-specific scale and scope economies. The first two hypotheses posit that given low fixed costs and minimal entry barriers, both media-specific scale and scope economies are available and can be exploited by relatively small-size agencies. The third hypothesis suggests that large agencies may experience diseconomies of scope as a consequence of excessive diversification induced by two pervasive industry institutional phenomena: (i) 'bundling' of agency services to match client demand for a mix of media advertising; and (ii) 'conflict policy' which prohibits an agency from serving competing accounts and operates as a mobility constraint. Utilizing a multiproduct cost function, we estimate media-specific scale and scope economies for a cross-section of 401 U.S. agencies in 1987. The results obtained support the set of three hypotheses outlined above.
Phase coherence length and quantum interference patterns at step edges
The accepted approximation used to describe quantum interference patterns at
steps is shown to be incorrect. As a result, electron lifetimes determined
using it are in error by a factor 2.Comment: 3 pages, 1 figur
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