3,020 research outputs found
Competing interactions and symmetry breaking in the Hubbard-Holstein model
Competing interactions are often responsible for intriguing phase diagrams in
correlated electron systems. Here we analyze the competition of instantaneous
short range Coulomb interaction with the retarded electron-electron
interaction induced by an electron-phonon coupling as described by the
Hubbard-Holstein model. The ground state phase diagram of this model in the
limit of large dimensions at half filling is established. The study is based on
dynamical mean field theory combined with the numerical renormalization group.
Depending on , , and the phonon frequency , the ground state is
antiferromagnetically (AFM) or charge ordered (CO). We find quantum phase
transitions from the AFM to CO state to occur when , where
characterizes the phonon induced effective attraction. The transition
is continuous for small couplings and large phonon frequencies and
becomes discontinuous for large couplings and small values of . We
comment on the possible relevance of this work for
BaKBiO.Comment: 6 pages,5 figure
The BaBar Electromagnetic Calorimeter: Status and Performance Improvements
The electromagnetic calorimeter at the BaBar detector, part of the asymmetric
B Factory at SLAC, measures photons in the energy range from 20 MeV to 8 GeV
with high resolution. The current status of the calorimeter, now in its seventh
year of operation, is being presented, as well as details on improvements made
to the analysis code during the last years.Comment: 6 pages, 15 figures, presented at the 2005 IEEE Nuclear Science
Symposium and submitted to the Conference Proceedings of the 2005 IEEE
Nuclear Science Symposiu
Unbundling Policy in the United States Players, Outcomes and Effects
Building on attempts during the 1980s to establish principles of Open Network Architecture (ONA), unbundling obligations became a cornerstone of the framework for local competition devised by the Telecommunications Act of 1996. Several of the regulations developed by the Federal Communications Commission (FCC), including the impairment test to assess whether a network element had to be unbundled, the TELRIC pricing method, the obligation to re-bundle network elements to service platforms and the unbundling provisions for broadband networks were challenged repeatedly in court. In response to multiple defeats of earlier rules, the FCC had to refine its approach and define unbundling obligations more narrowly. Effective as of March 11th, 2005, unbundling obligations will essentially be limited to the local copper loop, dedicated interoffice transportation on routes connecting small markets, and high-capacity loops in small markets. Carriers presently using unbundled network elements that do not qualify under the new rules will have to transition to alternative solutions within 12-18 months. During this period, the FCC has set higher ceiling prices for these unbundled network elements. The Commission affirmed the elimination in 2003 of its unbundling obligations in broadband markets.Unbundling; voice; broadband
Bundling, Differentiation, Alliances and Mergers: Convergence Strategies in U.S. Communication Markets
Convergence is a multi-facetted phenomenon affecting the technological basis of information and communication industries, the boundaries of existing and new markets, and the organization of service providers. Convergence in substitutes will tend to increase the intensity of competition but convergence in complements may have the opposite effect. Given the economics of advanced communication industries, convergence necessitates strategies to overcome the risk of commodification at the level of networks, applications, and services. The paper examines bundling, differentiation, alliances, and merger strategies adopted by North American service providers in response to convergence. Service providers'opportunities and risks in the emerging environment differ considerably, with cable and telephone service providers presently in stronger positions than wireless service providers, broadcasters, and satellite service providers. New entrants such as Vonage, Skype, Google, and Yahoo have high disruptive potential but remain disadvantaged without their own access networks.convergence; bundling; differentiation; alliances; mergers
Governing the Networks of the Information Society. Prospects and limits of policy in a complex technical system
This paper examines the prospects and limits of policies towards information and communications technologies (ICTs). The co-evolution of technological, economic, and political factors that has affected the information network infrastructure during the past three decades has transformed it from a relatively closed to more open system. As a consequence, the degree of complexity of the ICT infrastructure has increased with far-reaching implications for its governance. Paradoxically, policy was better able to control important performance characteristics, such as prices or investment levels, during the past monopoly era. However, the ability to control came at the high price of the inefficiencies associated with monopoly organization. In the present more competitive framework, many feasible policy instruments only work indirectly. Sector performance is an emergent property resulting from decentralized decisions in markets. It is influenced but not fully determined by policy choices. These changes need to be recognized more explicitly in the theoretical foundations, the formation and the implementation of policy. Applying concepts from the theory of complex evolving systems, the paper develops lessons for the design of effective information and communications policy.Information and communication technology, governance, complexity, incomplete information, institutions, feasible policy
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