327 research outputs found
Developing the repository manager community
This paper describes activities which have taken place
within the UK institutional repository (IR) sector
focusing on developing a community of practice through
the sharing of experiences and best practice. This
includes work done by the UK Council of Research
Repositories (UKCoRR) and other bodies, together with
informal activities, such as sharing the experience of
organising Open Access Week events. The paper also
considers future work to be undertaken by UKCoRR to
continue developing the community
Invariant imbedding theory of mode conversion in inhomogeneous plasmas. II. Mode conversion in cold, magnetized plasmas with perpendicular inhomogeneity
A new version of the invariant imbedding theory for the propagation of
coupled waves in inhomogeneous media is applied to the mode conversion of high
frequency electromagnetic waves into electrostatic modes in cold, magnetized
and stratified plasmas. The cases where the external magnetic field is applied
perpendicularly to the direction of inhomogeneity and the electron density
profile is linear are considered. Extensive and numerically exact results for
the mode conversion coefficients, the reflectances and the wave electric and
magnetic field profiles inside the inhomogeneous plasma are obtained. The
dependences of mode conversion phenomena on the magnitude of the external
magnetic field, the incident angle and the wave frequency are explored in
detail.Comment: 11 figures, to be published in Physics of Plasma
Understanding the Geometry of Astrophysical Magnetic Fields
Faraday rotation measurements have provided an invaluable technique with
which to measure the properties of astrophysical magnetized plasmas.
Unfortunately, typical observations provide information only about the
density-weighted average of the magnetic field component parallel to the line
of sight. As a result, the magnetic field geometry along the line of sight, and
in many cases even the location of the rotating material, is poorly
constrained. Frequently, interpretations of Faraday rotation observations are
dependent upon underlying models of the magnetic field being probed (e.g.,
uniform, turbulent, equipartition). However, we show that at sufficiently low
frequencies, specifically below roughly 13 (RM/rad m^-2)^(1/4) (B/G)^(1/2) MHz,
the character of Faraday rotation changes, entering what we term the
``super-adiabatic regime'' in which the rotation measure is proportional to the
integrated absolute value of the line-of-sight component of the field. As a
consequence, comparing rotation measures at high frequencies with those in this
new regime provides direct information about the geometry of the magnetic field
along the line of sight. Furthermore, the frequency defining the transition to
this new regime, nu_SA, depends directly upon the local electron density and
magnetic field strength where the magnetic field is perpendicular to the line
of sight, allowing the unambiguous distinction between Faraday rotation within
and in front of the emission region. Typical values of nu_SA range from 10 kHz
to 10 GHz, depending upon the details of the Faraday rotating environment. In
particular, for resolved AGN, including the black holes at the center of the
Milky Way (Sgr A*) and M81, nu_SA ranges from roughly 10 MHz to 10 GHz, and
thus can be probed via existing and up-coming ground-based radio observatories.Comment: 13 pages, 5 figures, submitted to Ap
Relativity principles in 1+1 dimensions and differential aging reversal
We study the behavior of clocks in 1+1 spacetime assuming the relativity
principle, the principle of constancy of the speed of light and the clock
hypothesis. These requirements are satisfied by a class of Finslerian theories
parametrized by a real coefficient , special relativity being recovered
for . The effect of differential aging is studied for the different
values of . Below the critical values the differential
aging has the usual direction - after a round trip the accelerated observer
returns younger than the twin at rest in the inertial frame - while above the
critical values the differential aging changes sign. The non-relativistic case
is treated by introducing a formal analogy with thermodynamics.Comment: 12 pages, no figures. Previous title "Parity violating terms in
clocks' behavior and differential aging reversal". v2: shortened
introduction, some sections removed, pointed out the relation with Finsler
metrics. Submitted to Found. Phys. Let
Deoxyribonucleic Acid Encoded and Size-Defined π-Stacking of Perylene Diimides
Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of π-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a “toolbox” for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic–hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular π wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin–orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision
Photogeneration of Spin Quintet Triplet–Triplet Excitations in DNA-Assembled Pentacene Stacks
Singlet fission (SF), an exciton-doubling process observed in certain molecular semiconductors where two triplet excitons are generated from one singlet exciton, requires correctly tuned intermolecular coupling to allow separation of the two triplets to different molecular units. We explore this using DNA-encoded assembly of SF-capable pentacenes into discrete π-stacked constructs of defined size and geometry. Precise structural control is achieved via a combination of the DNA duplex formation between complementary single-stranded DNA and the local molecular geometry that directs the SF chromophores into a stable and predictable slip-stacked configuration, as confirmed by molecular dynamics (MD) modeling. Transient electron spin resonance spectroscopy revealed that within these DNA-assembled pentacene stacks, SF evolves via a bound triplet pair quintet state, which subsequently converts into free triplets. SF evolution via a long-lived quintet state sets specific requirements on intermolecular coupling, rendering the quintet spectrum and its zero-field-splitting parameters highly sensitive to intermolecular geometry. We have found that the experimental spectra and zero-field-splitting parameters are consistent with a slight systematic strain relative to the MD-optimized geometry. Thus, the transient electron spin resonance analysis is a powerful tool to test and refine the MD-derived structure models. DNA-encoded assembly of coupled semiconductor molecules allows controlled construction of electronically functional structures, but brings with it significant dynamic and polar disorders. Our findings here of efficient SF through quintet states demonstrate that these conditions still allow efficient and controlled semiconductor operation and point toward future opportunities for constructing functional optoelectronic systems
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