16,888 research outputs found
Quasi-degenerate neutrinos and tri-bi-maximal mixing
We consider how, for quasi-degenerate neutrinos with tri-bi-maximal mixing at
a high-energy scale, the mixing angles are affected by radiative running from
high to low-energy scales in a supersymmetric theory. The limits on the
high-energy scale that follow from consistency with the observed mixing are
determined. We construct a model in which a non-Abelian discrete family
symmetry leads both to a quasi-degenerate neutrino mass spectrum and to near
tri-bi-maximal mixing.Comment: 8 pages, 3 figure
Synthetic considerations in the self-assembly of coordination polymers of pyridine-functionalised hybrid Mn-Anderson polyoxometalates
The incorporation of polyoxometalates (POMs) as structural units into ordered porous constructs such as metal-organic frameworks (MOFs) is desirable for a range of applications where intrinsic properties inherited from both the MOF and POM are utilised, including catalysis and magnetic data storage. The controlled self-assembly of targeted MOF topologies containing POM units is hampered by the wide range of oxo and hydroxo units on the peripheries of POMs that can act as coordinating groups towards linking metal cations leading to a diverse range of structures, but incorporation of organic donor units into hybrid POMs offers an alternative methodology to programmably synthesise POM/MOF conjugates. Herein, we report six coordination polymers obtained serendipitously wherein Zn2+ and Cu2+ link pyridine-appended Mn-Anderson clusters into two- and three-dimensional network solids with complex connectivities and topologies. Careful inspection of their solid-state structures has allowed us to identify common structure-directing features across these coordination polymers, including a square motif where two Zn2+ cations bridge two POMs. By correlating certain structural motifs with synthetic conditions we have formulated a series of design considerations for the self-assembly of coordination polymers of hybrid POMs, encompassing the selection of reaction conditions, co-ligands and linking metal cations. We anticipate that these synthetic guidelines will inform the future assembly of hybrid POMs into functional MOF materials
Encapsulation of a {Cu16} cluster containing four [Cu4O4] cubanes within an isopolyoxometalate {W44} cluster
We report a {Cu16} embedded within a {W44} cluster containing four cubane-like [Cu4O4] units within an isopolyoxotungstate (isoPOT) in a {Na4Cu4[(H2W11O38) (CH3COO)(OH)3]}4·88H2O (1) and a polyanion Cu-linked {W11} chain Na6Cu2[(H2W11O38)(CH3COO)(OH)]·26H2O (2). Electronically, the redox properties show that both compounds 1 and 2 undergo irreversible reductions resulting in the demetalation of the compounds, whilst the magnetic behavior of 1 and 2 shows a weak antiferromagnetic and a stronger ferromagnetic coupling, respectively
On the apparent absence of broad iron lines in Seyfert galaxies
We present an analysis of XMM-Newton observations of eleven Seyfert galaxies
that appear to be missing a broad iron K alpha line. These objects represent a
challenge to the established paradigm for active galactic nuclei, where a
relatively cold accretion disc feeds the central black hole. In that paradigm,
X-ray illumination of the accretion disc should lead to continuum and
fluorescence emission from iron which is broadened and shifted by relativistic
effects close the hole. We extend the work of Nandra et al. (2007), who found
no evidence for such a component in an earlier analysis of these objects, by
testing a variety of more complex relativistic reflection models. Specifically,
we consider the possibility that the disc is highly ionised, and/or that the
the reflection is heavily blurred by strong relativistic effects in a Kerr
geometry. We find that in 8/11 of the observations with no apparent broad iron
line, the fit is significantly improved when an ionised or strongly blurred
reflector is included, and that all 11 observations allow for such a component.
The disc inclinations are found generally to be around 60 degrees, which when
combined with a steep emissivity profile results in strong relativistic
blurring of the reflection, rendering the K alpha line difficult to distinguish
from the underlying continuum. Nevertheless, relativistic reflection does
appear to be present, and the strength of the smeared reflection is similar to
that expected from a flat disc illuminated by a point source. Such blurred
reflection and the associated steep radial emissivity profiles are consistent
with the gravitational light bending of the continuum photons close to the
black hole.Comment: 9 pages, 2 figures. Accepted for publication in MNRA
Evocative computing â creating meaningful lasting experiences in connecting with the past
We present an approach â evocative computing â that demonstrates how âat handâ technologies can be âpicked upâ and used by people to create meaningful and lasting experiences, through connecting and interacting with the past. The approach is instantiated here through a suite of interactive technologies configured for an indoor-outdoor setting that enables groups to explore, discover and research the history and background of a public cemetery. We report on a two-part study where different groups visited the cemetery and interacted with the digital tools and resources. During their activities serendipitous uses of the technology led to connections being made between personal memo-ries and ongoing activities. Furthermore, these experiences were found to be long-lasting; a follow-up study, one year later, showed them to be highly memorable, and in some cases leading participants to take up new directions in their work. We discuss the value of evocative computing for enriching user experiences and engagement with heritage practices
Strings in extremal BTZ black holes
We study the spectrum of the worldsheet theory of the bosonic closed string
in the massless and extremal rotating BTZ black holes. We use a hyperbolic
Wakimoto representation of the SL(2,R) currents to construct vertex operators
for the string modes on these backgrounds. We argue that there are tachyons in
the twisted sector, but these are not localised near the horizon. We study the
relation to the null orbifold in the limit of vanishing cosmological constant.
We also discuss the problem of extending this analysis to the supersymmetric
case.Comment: 20 pages, no figure
Bit-String Models for Parasex
We present different bit-string models of haploid asexual populations in
which individuals may exchange part of their genome with other individuals
(parasex) according to a given probability. We study the advantages of this
parasex concerning population sizes, genetic fitness and diversity. We find
that the exchange of genomes always improves these features.Comment: 12 pages including 7 figure
Anomalous Breaking of Anisotropic Scaling Symmetry in the Quantum Lifshitz Model
In this note we investigate the anomalous breaking of anisotropic scaling
symmetry in a non-relativistic field theory with dynamical exponent z=2. On
general grounds, one can show that there exist two possible "central charges"
which characterize the breaking of scale invariance. Using heat kernel methods,
we compute these two central charges in the quantum Lifshitz model, a free
field theory which is second order in time and fourth order in spatial
derivatives. We find that one of the two central charges vanishes.
Interestingly, this is also true for strongly coupled non-relativistic field
theories with a geometric dual described by a metric and a massive vector
field.Comment: 26 pages; major revision (results were unaffected), published versio
Channel kets, entangled states, and the location of quantum information
The well-known duality relating entangled states and noisy quantum channels
is expressed in terms of a channel ket, a pure state on a suitable tripartite
system, which functions as a pre-probability allowing the calculation of
statistical correlations between, for example, the entrance and exit of a
channel, once a framework has been chosen so as to allow a consistent set of
probabilities. In each framework the standard notions of ordinary (classical)
information theory apply, and it makes sense to ask whether information of a
particular sort about one system is or is not present in another system.
Quantum effects arise when a single pre-probability is used to compute
statistical correlations in different incompatible frameworks, and various
constraints on the presence and absence of different kinds of information are
expressed in a set of all-or-nothing theorems which generalize or give a
precise meaning to the concept of ``no-cloning.'' These theorems are used to
discuss: the location of information in quantum channels modeled using a
mixed-state environment; the (classical-quantum) channels introduced by
Holevo; and the location of information in the physical carriers of a quantum
code. It is proposed that both channel and entanglement problems be classified
in terms of pure states (functioning as pre-probabilities) on systems of parts, with mixed bipartite entanglement and simple noisy channels belonging
to the category , a five-qubit code to the category , etc.; then by
the dimensions of the Hilbert spaces of the component parts, along with other
criteria yet to be determined.Comment: Latex 32 pages, 4 figures in text using PSTricks. Version 3: Minor
typographical errors correcte
Thermodynamics of Chemical Waves
Chemical waves constitute a known class of dissipative structures emerging in
reaction-diffusion systems. They play a crucial role in biology, spreading
information rapidly to synchronize and coordinate biological events. We develop
a rigorous thermodynamic theory of reaction-diffusion systems to characterize
chemical waves. Our main result is the definition of the proper thermodynamic
potential of the local dynamics as a nonequilibrium free energy density and
establishing its balance equation. This enables us to identify the dynamics of
the free energy, of the dissipation, and of the work spent to sustain the wave
propagation. Two prototypical classes of chemical waves are examined. From a
thermodynamic perspective, the first is sustained by relaxation towards
equilibrium and the second by nonconservative forces generated by chemostats.
We analytically study step-like waves, called wavefronts, using the
Fisher-Kolmogorov equation as representative of the first class and oscillating
waves in the Brusselator model as representative of the second. Given the
fundamental role of chemical waves as message carriers in biosystems, our
thermodynamic theory constitutes an important step toward an understanding of
information transfers and processing in biology.Comment: 12 pages, 2 figure
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