217 research outputs found
Electron Correlations and Two-Photon States in Polycyclic Aromatic Hydrocarbon Molecules: A Peculiar Role of Geometry
We present numerical studies of one- and two-photon excited states ordering
in a number of polycyclic aromatic hydrocarbon molecules: coronene,
hexa-peri-hexabenzocoronene and circumcoronene, all possessing point
group symmetry versus ovalene with symmetry, within the
Pariser-Parr-Pople model of interacting -electrons. The calculated
energies of the two-photon states as well as their relative two-photon
absorption cross-sections within the interacting model are qualitatively
different from single-particle descriptions. More remarkably, a peculiar role
of molecular geometry is found. The consequence of electron correlations is far
stronger for ovalene, where the lowest spin-singlet two-photon state is a
quantum superposition of pairs of lowest spin triplet states, as in the linear
polyenes. The same is not true for group hydrocarbons. Our work
indicates significant covalent character, in valence bond language, of the
ground state, the lowest spin triplet state and a few of the lowest two-photon
states in ovalene but not in those with symmetry.Comment: 11 pages, 3 figures, 3 table
An interoperability framework for security policy languages
A thesis submitted to the University of Bedfordshire in partial fulfilment of the requirements for the degree of Doctor of PhilosophySecurity policies are widely used across the IT industry in order to secure environments. Firewalls, routers, enterprise application or even operating systems like Windows and Unix are all using security policies to some extent in order to secure certain components. In order to automate enforcement of security policies, security policy languages
have been introduced. Security policy languages that are classified as computer software, like many other programming languages have been revolutionised during
the last decade. A number of security policy languages have been introduced in the industry in order to tackle a specific business requirements. Not to mention each of these security policy languages themselves evolved and enhanced during the last few years.
Having said that, a quick research on security policy languages shows that the industry suffers from the lack of a framework for security policy languages. Such a framework would facilitate the management of security policies from an abstract point. In order to achieve that specific goal, the framework utilises an abstract security policy language that is independent of existing security policy languages yet capable of expressing policies written in those languages.
Usage of interoperability framework for security policy languages as described above comes with major benefits that are categorised into two levels: short and long-term benefits. In short-term, industry and in particular multi-dimensional organisations that make use of multiple domains for different purposes would lower their security related costs by managing their security policies that are stretched across their environment and often managed locally. In the long term, usage of abstract security policy language that is independent of any existing security policy
languages, gradually paves the way for standardising security policy languages. A goal that seems unreachable at this moment of time. Taking the above facts into account, the aim of this research is to introduce and develop a novel framework for security policy languages. Using such a framework would allow multi-dimensional organisations to use an abstract policy language to orchestrate all security policies from a single point, which could then be propagated
across their environment. In addition, using such a framework would help security administrators to learn and use only one single, common abstract language to describe and model their environment(s)
Theory of interfacial charge-transfer complex photophysics in -conjugated polymer-fullerene blends
We present a theory of the electronic structure and photophysics of 1:1
blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the
same Coulomb-correlated Hamiltonian applied previously to interacting chains of
single-component -conjugated polymers, we find an exciplex state that
occurs below the polymer's optical exciton. Weak absorption from the ground
state occurs to the exciplex. We explain transient photoinduced absorptions in
the blend, observed for both above-gap and below-gap photoexcitations, within
our theory. Photoinduced absorptions for above-gap photoexcitation are from the
optical exciton as well as the exciplex, while for below-gap photoexcitation
induced absorptions are from the exciplex alone. In neither case are free
polarons generated in the time scale of the experiment. Importantly, the
photophysics of films of single-component -conjugated polymers and blends
can both be understood by extending Mulliken's theory of ground-state charge
transfer to the case of excited-state charge transfer.Comment: 9 pages, 8 figure
The Dynamical Cluster Approximation (DCA) versus the Cellular Dynamical Mean Field Theory (CDMFT) in strongly correlated electrons systems
We are commenting on the article Phys. Rev. {\bf B 65}, 155112 (2002) by G.
Biroli and G. Kotliar in which they make a comparison between two cluster
techniques, the {\it Cellular Dynamical Mean Field Theory} (CDMFT) and the {\it
Dynamical Cluster Approximation} (DCA). Based upon an incorrect implementation
of the DCA technique in their work, they conclude that the CDMFT is a faster
converging technique than the DCA. We present the correct DCA prescription for
the particular model Hamiltonian studied in their article and conclude that the
DCA, once implemented correctly, is a faster converging technique for the
quantities averaged over the cluster. We also refer to their latest response to
our comment where they argue that instead of averaging over the cluster, local
observables should be calculated in the bulk of the cluster which indeed makes
them converge much faster in the CDMFT than in the DCA. We however show that in
their original work, the authors themselves use the cluster averaged quantities
to draw their conclusions in favor of using the CDMFT over the DCA.Comment: Comment on Phys. Rev. B 65, 155112 (2002). 3 pages, 2 figure
Subgap Two-Photon States in Polycyclic Aromatic Hydrocarbons: Evidence for Strong Electron Correlations
Strong electron correlation effects in the photophysics of
quasi-one-dimensional -conjugated organic systems such as polyenes,
polyacetylenes, polydiacetylenes, etc., have been extensively studied. Far less
is known on correlation effects in two-dimensional -conjugated systems.
Here we present theoretical and experimental evidence for moderate repulsive
electron-electron interactions in a number of finite polycyclic aromatic
hydrocarbon molecules with symmetry. We show that the excited state
orderings in these molecules are reversed relative to that expected within
one-electron and mean-field theories. Our results reflect similarities as well
as differences in the role and magnitude of electron correlation effects in
these two-dimensional molecules compared to those in polyenes.Comment: 11 pages, 5 figures, 2 table
Ferromagnetic Spin Coupling as the Origin of 0.7 Anomaly in Quantum Point Contacts
We study one-dimensional itinerant electron models with ferromagnetic
coupling to investigate the origin of 0.7 anomaly in quantum point contacts.
Linear conductance calculations from the quantum Monte Carlo technique for spin
interactions of different spatial range suggest that anomaly
results from a strong interaction of low-density conduction electrons to
ferromagnetic fluctuations formed across the potential barrier. The conductance
plateau appears due to the strong incoherent scattering at high temperature
when the electron traversal time matches the time scale of dynamic
ferromagnetic excitations.Comment: 5 pages, 4 figure
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