525 research outputs found
Characterization of the Positivity of the Density Matrix in Terms of the Coherence Vector Representation
A parameterization of the density operator, a coherence vector
representation, which uses a basis of orthogonal, traceless, Hermitian matrices
is discussed. Using this parameterization we find the region of permissible
vectors which represent a density operator. The inequalities which specify the
region are shown to involve the Casimir invariants of the group. In particular
cases, this allows the determination of degeneracies in the spectrum of the
operator. The identification of the Casimir invariants also provides a method
of constructing quantities which are invariant under {\it local} unitary
operations. Several examples are given which illustrate the constraints
provided by the positivity requirements and the utility of the coherence vector
parameterization.Comment: significantly rewritten and submitted for publicatio
Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate
We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The
large number of observed vibrational modes is related to the low symmetry of
the molecule, and they are grouped together in clusters. Analysis of the mode
character based on molecular dynamics simulations and model compound studies
shows that all vibrations are complex; motion from a majority of atoms in the
molecule contribute to most modes. Three features involving intramolecular
vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm displays the
largest deviation with field and is mainly intensity related. A comparison
between the temperature dependent absorption difference spectra, the gradual
low-temperature cluster framework distortion as assessed by neutron diffraction
data, and field dependent absorption difference spectra suggests that this mode
may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte
Modeling magnetospheric fields in the Jupiter system
The various processes which generate magnetic fields within the Jupiter
system are exemplary for a large class of similar processes occurring at other
planets in the solar system, but also around extrasolar planets. Jupiter's
large internal dynamo magnetic field generates a gigantic magnetosphere, which
is strongly rotational driven and possesses large plasma sources located deeply
within the magnetosphere. The combination of the latter two effects is the
primary reason for Jupiter's main auroral ovals. Jupiter's moon Ganymede is the
only known moon with an intrinsic dynamo magnetic field, which generates a
mini-magnetosphere located within Jupiter's larger magnetosphere including two
auroral ovals. Ganymede's magnetosphere is qualitatively different compared to
the one from Jupiter. It possesses no bow shock but develops Alfv\'en wings
similar to most of the extrasolar planets which orbit their host stars within
0.1 AU. New numerical models of Jupiter's and Ganymede's magnetospheres
presented here provide quantitative insight into the processes that maintain
these magnetospheres. Jupiter's magnetospheric field is approximately
time-periodic at the locations of Jupiter's moons and induces secondary
magnetic fields in electrically conductive layers such as subsurface oceans. In
the case of Ganymede, these secondary magnetic fields influence the oscillation
of the location of its auroral ovals. Based on dedicated Hubble Space Telescope
observations, an analysis of the amplitudes of the auroral oscillations
provides evidence that Ganymede harbors a subsurface ocean. Callisto in
contrast does not possess a mini-magnetosphere, but still shows a perturbed
magnetic field environment. Callisto's ionosphere and atmospheric UV emission
is different compared to the other Galilean satellites as it is primarily been
generated by solar photons compared to magnetospheric electrons.Comment: Chapter for Book: Planetary Magnetis
The Intentional Use of Service Recovery Strategies to Influence Consumer Emotion, Cognition and Behaviour
Service recovery strategies have been identified as a critical factor in the success of. service organizations. This study develops a conceptual frame work to investigate how specific service recovery strategies influence the emotional, cognitive and negative behavioural responses of . consumers., as well as how emotion and cognition influence negative behavior. Understanding the impact of specific service recovery strategies will allow service providers' to more deliberately and intentionally engage in strategies that result in positive organizational outcomes. This study was conducted using a 2 x 2 between-subjects quasi-experimental design. The results suggest that service recovery has a significant impact on emotion, cognition and negative behavior. Similarly, satisfaction, negative emotion and positive emotion all influence negative behavior but distributive justice has no effect
Fibroblast-adapted human CMV vaccines elicit predominantly conventional CD8 T cell responses in humans
Transplantation and autoimmunit
Recent experimental results in sub- and near-barrier heavy ion fusion reactions
Recent advances obtained in the field of near and sub-barrier heavy-ion
fusion reactions are reviewed. Emphasis is given to the results obtained in the
last decade, and focus will be mainly on the experimental work performed
concerning the influence of transfer channels on fusion cross sections and the
hindrance phenomenon far below the barrier. Indeed, early data of sub-barrier
fusion taught us that cross sections may strongly depend on the low-energy
collective modes of the colliding nuclei, and, possibly, on couplings to
transfer channels. The coupled-channels (CC) model has been quite successful in
the interpretation of the experimental evidences. Fusion barrier distributions
often yield the fingerprint of the relevant coupled channels. Recent results
obtained by using radioactive beams are reported. At deep sub-barrier energies,
the slope of the excitation function in a semi-logarithmic plot keeps
increasing in many cases and standard CC calculations over-predict the cross
sections. This was named a hindrance phenomenon, and its physical origin is
still a matter of debate. Recent theoretical developments suggest that this
effect, at least partially, may be a consequence of the Pauli exclusion
principle. The hindrance may have far-reaching consequences in astrophysics
where fusion of light systems determines stellar evolution during the carbon
and oxygen burning stages, and yields important information for exotic
reactions that take place in the inner crust of accreting neutron stars.Comment: 40 pages, 63 figures, review paper accepted for EPJ
Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding
Within the broad class of multiferroics (compounds showing a coexistence of
magnetism and ferroelectricity), we focus on the subclass of "improper
electronic ferroelectrics", i.e. correlated materials where electronic degrees
of freedom (such as spin, charge or orbital) drive ferroelectricity. In
particular, in spin-induced ferroelectrics, there is not only a {\em
coexistence} of the two intriguing magnetic and dipolar orders; rather, there
is such an intimate link that one drives the other, suggesting a giant
magnetoelectric coupling. Via first-principles approaches based on density
functional theory, we review the microscopic mechanisms at the basis of
multiferroicity in several compounds, ranging from transition metal oxides to
organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic
frameworks, MOFs)Comment: 22 pages, 9 figure
Climate change research and action must look beyond 2100
Anthropogenic activity is changing Earth's climate and ecosystems in ways that are potentially dangerous and disruptive to humans. Greenhouse gas concentrations in the atmosphere continue to rise, ensuring that these changes will be felt for centuries beyond 2100, the current benchmark for projection. Estimating the effects of past, current, and potential future emissions to only 2100 is therefore short-sighted. Critical problems for food production and climate-forced human migration are projected to arise well before 2100, raising questions regarding the habitability of some regions of the Earth after the turn of the century. To highlight the need for more distant horizon scanning, we model climate change to 2500 under a suite of emission scenarios and quantify associated projections of crop viability and heat stress. Together, our projections show global climate impacts increase significantly after 2100 without rapid mitigation. As a result, we argue that projections of climate and its effects on human well-being and associated governance and policy must be framed beyond 2100
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