210 research outputs found
Complete Break Up of Ortho Positronium (Ps)- Hydrogenic ion System
The dynamics of the complete breakup process in an Ortho Ps - He+ system
including electron loss to the continuum (ELC) is studied where both the
projectile and the target get ionized. The process is essentially a four body
problem and the present model takes account of the two centre effect on the
electron ejected from the Ps atom which is crucial for a proper description of
the ELC phenomena. The calculations are performed in the framework of Coulomb
Distorted Eikonal Approximation. The exchange effect between the target and the
projectile electron is taken into account in a consistent manner. The proper
asymptotic 3-body boundary condition for this ionization process is also
satisfied in the present model. A distinct broad ELC peak is noted in the fully
differential cross sections (5DCS) for the Ps electron corroborating
qualitatively the experiment for the Ps - He system. Both the dynamics of the
ELC from the Ps and the ejected electron from the target He+ in the FDCS are
studied using coplanar geometry. Interesting features are noted in the FDCS for
both the electrons belonging to the target and the projectile.Comment: 14 pages,7 figure
Nernst Effect in Electron-Doped PrCeCuO
The Nernst effect of PrCeCuO (x=0.13, 0.15, and 0.17) has
been measured on thin film samples between 5-120 K and 0-14 T. In comparison to
recent measurements on hole-doped cuprates that showed an anomalously large
Nernst effect above the resistive T and H
\cite{xu,wang1,wang2,capan}, we find a normal Nernst effect above T and
H for all dopings. The lack of an anomalous Nernst effect in the
electron-doped compounds supports the models that explain this effect in terms
of amplitude and phase fluctuations in the hole-doped cuprates. In addition,
the H(T) determined from the Nernst effect shows a conventional behavior
for all dopings. The energy gap determined from H(0) decreases as the
system goes from under-doping to over-dopingin agreement with the recent
tunnelling experiments
Dune ages in the sand deserts of the southern Sahara and Sahel
In this paper we aim to document the history of aeolian processes within the southern Sahara as part of the INQUA Dune Atlas. We review available luminescence ages for sand dunes across the southern Sahara to develop an improved understanding of the dune chronology on a regional basis and attempt to correlate periods of sand accumulation. This was achieved by analysing dune age by country, as well as by latitude and longitude. The results show a very patchy spatial distribution of dune ages with large gaps that encompass some of the largest sand seas. Despite these gaps, some related patterns in dune morphology and stratigraphy appear to be consistent between northern Nigeria and southern Mali where older linear dunes are distinct from younger Late Holocene transverse and barchanoid dunes. Elsewhere in Mauretania linear dunes with different orientations appear to have accumulated at different times, most likely in response to changes in atmospheric circulation. Regional climatic changes are identified where dunes are transgressed by lake deposits within endorehic basins. We identify four locations where dune accumulation is terminated by lacustrine transgressions, two of which, in Lake Chad and the Bodélé Depression, occur shortly after the last glacial maximum (LGM). The third example at Gobiero in Niger occurred later, in the early Holocene, around 8.4 ka and a fourth marks a later transgression of Palaeolake MegaChad after 4.7 ka. Larger-scale latitudinal and longitudinal distributions in dune ages across the southern Sahara do not show any consistent patterns, though this may be due to the small sample size relative to the study area. In addition, local variations in external controls such as wind regime, rainfall, vegetation and sand supply need to be considered, sometimes on a site by site basis. Limiting the analysis to dune ages determined using the single-aliquot regenerative-dose (SAR) protocol indicates a lack of dune preservation during the LGM and the Younger Dryas, times associated with increased dust input to the oceans which is assumed to indicate increased aeolian activity. The SAR dune dates suggest that preservation of dunes at the onset of succeeding humid intervals is an important component of the dune record. The most striking examples of this phenomenon occur where dunes are preserved within endorehic basins by lacustrine transgressions
Electron Dynamics in NdCeCuO: Evidence for the Pseudogap State and Unconventional c-axis Response
Infrared reflectance measurements were made with light polarized along the a-
and c-axis of both superconducting and antiferromagnetic phases of electron
doped NdCeCuO. The results are compared to
characteristic features of the electromagnetic response in hole doped cuprates.
Within the CuO planes the frequency dependent scattering rate,
1/, is depressed below 650 cm; this behavior is a
hallmark of the pseudogap state. While in several hole doped compounds the
energy scales associated with the pseudogap and superconducting states are
quite close, we are able to show that in NdCeCuO
the two scales differ by more than one order of magnitude. Another feature of
the in-plane charge response is a peak in the real part of the conductivity,
, at 50-110 cm which is in sharp contrast with the
Drude-like response where is centered at . This
latter effect is similar to what is found in disordered hole doped cuprates and
is discussed in the context of carrier localization. Examination of the c-axis
conductivity gives evidence for an anomalously broad frequency range from which
the interlayer superfluid is accumulated. Compelling evidence for the pseudogap
state as well as other characteristics of the charge dynamics in
NdCeCuO signal global similarities of the cuprate
phase diagram with respect to electron and hole doping.Comment: Submitted to PR
Angular momentum exchange during secular migration of two-planet systems
We investigate the secular dynamics of two-planet coplanar systems evolving
under mutual gravitational interactions and dissipative forces. We consider two
mechanisms responsible for the planetary migration: star-planet (or
planet-satellite) tidal interactions and interactions of a planet with a
gaseous disc. We show that each migration mechanism is characterized by a
specific law of orbital angular momentum exchange. Calculating stationary
solutions of the conservative secular problem and taking into account the
orbital angular momentum leakage, we trace the evolutionary routes followed by
the planet pairs during the migration process. This procedure allows us to
recover the dynamical history of two-planet systems and constrain parameters of
the involved physical processes.Comment: 20 pages, 9 figures, accepted for publication in Celestial Mechanics
and Dynamical Astronomy (special issue on Exoplanets
The Behavioral Roots of Information Systems Security:Exploring Key Factors Related to Unethical IT Use
Unethical information technology (IT) use, related to activities such as hacking, software piracy, phishing, and spoofing, has become a major security concern for individuals, organizations, and society in terms of the threat to information systems (IS) security. While there is a growing body of work on this phenomenon, we notice several gaps, limitations, and inconsistencies in the literature. In order to further understand this complex phenomenon and reconcile past findings, we conduct an exploratory study to uncover the nomological network of key constructs salient to this phenomenon, and the nature of their interrelationships. Using a scenario-based study of young adult participants, and both linear and nonlinear analyses, we uncover key nuances of this phenomenon of unethical IT use. We find that unethical IT use is a complex phenomenon, often characterized by nonlinear and idiosyncratic relationships between the constructs that capture it. Overall, ethical beliefs held by the individuals, along with economic, social, and technological considerations are found to be relevant to this phenomenon. In terms of practical implications, these results suggest that multiple interventions at various levels may be required to combat this growing threat to IS security
Photonic molecules and spectral engineering
This chapter reviews the fundamental optical properties and applications of
pho-tonic molecules (PMs) - photonic structures formed by electromagnetic
coupling of two or more optical microcavities (photonic atoms). Controllable
interaction between light and matter in photonic atoms can be further modified
and en-hanced by the manipulation of their mutual coupling. Mechanical and
optical tunability of PMs not only adds new functionalities to
microcavity-based optical components but also paves the way for their use as
testbeds for the exploration of novel physical regimes in atomic physics and
quantum optics. Theoretical studies carried on for over a decade yielded novel
PM designs that make possible lowering thresholds of semiconductor microlasers,
producing directional light emission, achieving optically-induced transparency,
and enhancing sensitivity of microcavity-based bio-, stress- and
rotation-sensors. Recent advances in material science and nano-fabrication
techniques make possible the realization of optimally-tuned PMs for cavity
quantum electrodynamic experiments, classical and quantum information
processing, and sensing.Comment: A review book chapter: 29 pages, 19 figure
History of clinical transplantation
How transplantation came to be a clinical discipline can be pieced together by perusing two volumes of reminiscences collected by Paul I. Terasaki in 1991-1992 from many of the persons who were directly involved. One volume was devoted to the discovery of the major histocompatibility complex (MHC), with particular reference to the human leukocyte antigens (HLAs) that are widely used today for tissue matching.1 The other focused on milestones in the development of clinical transplantation.2 All the contributions described in both volumes can be traced back in one way or other to the demonstration in the mid-1940s by Peter Brian Medawar that the rejection of allografts is an immunological phenomenon.3,4 © 2008 Springer New York
The Physics of Star Cluster Formation and Evolution
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe
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