218,512 research outputs found
Ratcheting Heat Flux against a Thermal Bias
Merely rocking the temperature in one heat bath can direct a steady heat flux
from cold to hot against a non-zero thermal bias in stylized nonlinear lattice
junctions that are sandwiched between two heat baths. Likewise, for an average
zero-temperature difference between the two contacts a net, ratchet-like heat
flux emerges. Computer simulations show that this very heat flux can be
controlled and reversed by suitably tailoring the frequency ( 100
MHz) of the alternating temperature field.Comment: 5 pages, 6 figure
Factorization theorems, effective field theory, and nonleptonic heavy meson decays
The nonleptonic heavy meson decays
and are studied based on the three-scale perturbative QCD
factorization theorem developed recently. In this formalism the
Bauer-Stech-Wirbel parameters a_1 and a_2 are treated as the Wilson
coefficients, whose evolution from the W boson mass down to the characteristic
scale of the decay processes is determined by effective field theory. The
evolution from the characteristic scale to a lower hadronic scale is formulated
by the Sudakov resummation. The scale-setting ambiguity, which exists in the
conventional approach to nonleptonic heavy meson decays, is moderated.
Nonfactorizable and nonspectator contributions are taken into account as part
of the hard decay subamplitudes. Our formalism is applicable to both bottom and
charm decays, and predictions, including those for the ratios R and R_L
associated with the decays, are consistent with
experimental data.Comment: 39 pages, latex, 5 figures, revised version with some correction
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Tuning magnetic anisotropy of epitaxial Ag/Fe/Fe0.5Co0.5/MgO(001) films
Single crystalline Ag/Fe/Fe0.5Co0.5/MgO(001) films were grown by Molecular Beam Epitaxy and investigated by Magneto-Optic Kerr Effect (MOKE). We find that even though the 4-fold magnetic anisotropies of Ag/Fe/MgO(001) and Ag/Fe0.5Co0.5/MgO(001) films are different from the corresponding bulk values, their opposite signs allow a fine tuning of the 4-fold magnetic anisotropy in Ag/Fe/Fe0.5Co0.5/MgO(001) films by varying the Fe and Fe0.5Co0.5 film thicknesses. In particular, the critical point of zero anisotropy can be achieved in a wide range of film thicknesses. Using Rotational MOKE, we determined and constructed the anisotropy phase diagram in the Fe and Fe0.5Co0.5 thickness plane from which the zero anisotropy exhibits a linear relation between the Fe and Fe0.5Co0.5 thickness
The impact and penetration of location-based services
Since the invention of digital technology, its development has followed an entrenched path ofminiaturisation and decentralisation with increasing focus on individual and niche applications. Computerhardware has moved from remote centres to desktop and hand held devices whilst being embedded invarious material infrastructures. Software has followed the same course. The entire process has convergedon a path where various analogue devices have become digital and are increasingly being embedded inmachines at the smallest scale. In a parallel but essential development, there has been a convergence ofcomputers with communications ensuring that the delivery and interaction mechanisms for computersoftware is now focused on networks of individuals, not simply through the desktop, but in mobilecontexts. Various inert media such as fixed television is becoming more flexible as computers and visualmedia are becoming one.With such massive convergence and miniaturisation, new software and new applications define the cuttingedge. As computers are being increasingly tailored to individual niches, then new digital services areemerging, many of which represent applications which hitherto did not exist or at best were rarely focusedon a mass market. Location based services form one such application and in this paper, we will bothspeculate on and make some initial predictions of the geographical extent to which such services willpenetrate different markets. We define such services in detail below but suffice it to say at this stage thatsuch functions involve the delivery of traditional services using digital media and telecommunications.High profile applications are now being focused on hand held devices, typically involving information onproduct location and entertainment but wider applications involve fixed installations on the desktop whereservices are delivered through traditional fixed infrastructure. Both wire and wireless applications definethis domain. The market for such services is inevitably volatile and unpredictable at this early stage but wewill attempt here to provide some rudimentary estimates of what might happen in the next five to tenyears.The ?network society? which has developed through this convergence, is, according to Castells (1989,2000) changing and re-structuring the material basis of society such that information has come todominate wealth creation in a way that information is both a raw material of production and an outcome ofproduction as a tradable commodity. This has been fuelled by the way technology has expanded followingMoore?s Law and by fundamental changes in the way telecommunications, finance, insurance, utilitiesand so on is being regulated. Location based services are becoming an integral part of this fabric and thesereflect yet another convergence between geographic information systems, global positioning systems, andsatellite remote sensing. The first geographical information system, CGIS, was developed as part of theCanada Land Inventory in 1965 and the acronym ?GIS? was introduced in 1970. 1971 saw the firstcommercial satellite, LANDSAT-1. The 1970s also saw prototypes of ISDN and mobile telephone and theintroduction of TCP/IP as the dominant network protocol. The 1980s saw the IBM XT (1982) and thebeginning of de-regulation in the US, Europe and Japan of key sectors within the economy. Finally in the 1990s, we saw the introduction of the World Wide Web and the ubiquitous pervasion of business andrecreation of networked PC?s, the Internet, mobile communications and the growing use of GPS forlocational positioning and GIS for the organisation and visualisation of spatial data. By the end of the 20thcentury, the number of mobile telephone users had reached 700 million worldwide. The increasingmobility of individuals, the anticipated availability of broadband communications for mobile devices andthe growing volumes of location specific information available in databases will inevitably lead to thedemand for services that will deliver location related information to individuals on the move. Suchlocation based services (LBS) although in a very early stage of development, are likely to play anincreasingly important part in the development of social structures and business in the coming decades.In this paper we begin by defining location based services within the context we have just sketched. Wethen develop a simple model of the market for location-based services developing the standard non-linearsaturation model of market penetration. We illustrate this for mobile devices, namely mobile phones in thefollowing sections and then we develop an analysis of different geographical regimes which arecharacterised by different growth rates and income levels worldwide. This leads us to speculate on theextent to which location based services are beginning to take off and penetrate the market. We concludewith scenarios for future growth through the analogy of GIS and mobile penetration
Cosmic Needles versus Cosmic Microwave Background Radiation
It has been suggested by a number of authors that the 2.7K cosmic microwave
background (CMB) radiation might have arisen from the radiation from Population
III objects thermalized by conducting cosmic graphite/iron needle-shaped dust.
Due to lack of an accurate solution to the absorption properties of exceedingly
elongated grains, in existing literature which studies the CMB thermalizing
process they are generally modelled as (1) needle-like spheroids in terms of
the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory.
We show here that the Rayleigh approximation is not valid since the Rayleigh
criterion is not satisfied for highly conducting needles. We also show that the
available intergalactic iron dust, if modelled as infinite cylinders, is not
sufficient to supply the required opacity at long wavelengths to obtain the
observed isotropy and Planckian nature of the CMB. If appealing to the antenna
theory, conducting iron needles with exceedingly large elongations (10^4)
appear able to provide sufficient opacity to thermalize the CMB within the iron
density limit. But the applicability of the antenna theory to exceedingly thin
needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ
Shot noise spectrum of superradiant entangled excitons
The shot noise produced by tunneling of electrons and holes into a double dot
system incorporated inside a p-i-n junction is investigated theoretically. The
enhancement of the shot noise is shown to originate from the entangled
electron-hole pair created by superradiance. The analogy to the superconducting
cooper pair box is pointed out. A series of Zeno-like measurements is shown to
destroy the entanglement, except for the case of maximum entanglement.Comment: 5 pages, 3 figures, to appear in Phys. Rev. B (2004
Applicability of Perturbative QCD to Decays
We examine the applicability of perturbative QCD to meson decays into
mesons. We find that the perturbative QCD formalism, which includes Sudakov
effects at intermediate energy scales, is applicable to the semi-leptonic decay
, when the meson recoils fast. Following this conclusion, we
analyze the two-body non-leptonic decays and . By
comparing our predictions with experimental data, we extract the matrix element
.Comment: 18 pages in Latex, figures are available upon reques
Ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting laser devices
Spin-polarized lasers offer new encouraging possibilities for future devices. We investigate the polarization dynamics of electrically pumped vertical-cavity surface-emitting lasers after additional spin injection at room temperature. We find that the circular polarization degree exhibits faster dynamics than the emitted light. Moreover the experimental results demonstrate a strongly damped ultrafast circular polarization oscillation due to spin injection with an oscillation frequency of approximately 11GHz depending on the birefringence in the VCSEL device. We compare our experimental results with theoretical calculations based on rate-equations. This allows us to predict undamped long persisting ultrafast polarization oscillations, which reveal the potential of spin-VCSELs for ultrafast modulation applications
Extraction of from the Decay
We develop the perturbative QCD formalism including Sudakov effects for
semi-leptonic meson decays. We evaluate the differential decay rate of
, and find that the perturbative calculation is reliable for
the energy fraction of the pion above 0.3. Combining predictions from the soft
pion theorems, we extract the value of the matrix element which is
roughly .Comment: 10 pages, CCUTH-94-05, IP-ASTP-13-9
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