6,287 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
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 ubiquitous 1100 charge ordering in organic charge-transfer solids
Charge and spin-orderings in the 1/4-filled organic CT solids are of strong
interest, especially in view of their possible relations to organic
superconductivity. We show that the charge order (CO) in both 1D and 2D CT
solids is of the ...1100... type, in contradiction to mean field prediction of
>...1010... CO. We present detailed computations for metal-insulator and
magnetic insulator-insulator transitions in the theta-ET materials. Complete
agreement with experiments in several theta systems is found. Similar
comparisons between theory and experiments in TCNQ, TMTTF, TMTSF, and ET
materials prove the ubiquity of this phenomenon.Comment: 3 pages, 4 eps figures; ICSM 200
Longevity of supersymmetric flat directions
We examine the fate of supersymmetric flat directions. We argue that the
non-perturbative decay of the flat direction via preheating is an unlikely
event. In order to address this issue, first we identify the physical degrees
of freedom and their masses in presence of a large flat direction VEV (Vacuum
Expectation Value). We explicitly show that the (complex) flat direction and
its fermionic partner are the only light {\it physical} fields in the spectrum.
If the flat direction VEV is much larger than the weak scale, and it has a
rotational motion, there will be no resonant particle production at all. The
case of multiple flat directions is more involved. We illustrate that in many
cases of physical interest, the situation becomes effectively the same as that
of a single flat direction, or collection of independent single directions. In
such cases preheating is not relevant. In an absence of a fast non-perturbative
decay, the flat direction survives long enough to affect thermalization in
supersymmetric models as described in hep-ph/0505050 and hep-ph/0512227. It can
also ``terminate'' an early stage of non-perturbative inflaton decay as
discussed in hep-ph/0603244.Comment: 9 revtex pages, v3: expanded discussion on two flat directions, minor
modifications, conclusions unchange
Identifying the curvaton within MSSM
We consider inflaton couplings to MSSM flat directions and the thermalization
of the inflaton decay products, taking into account gauge symmetry breaking due
to flat direction condensates. We then search for a suitable curvaton candidate
among the flat directions, requiring an early thermally induced start for the
flat direction oscillations to facilitate the necessary curvaton energy density
dominance. We demonstrate that the supersymmetry breaking -term is crucial
for achieving a successful curvaton scenario. Among the many possible
candidates, we identify the flat direction as a viable MSSM
curvaton.Comment: 9 pages. Discussion on the evaporation of condensate added, final
version published in JCA
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
Stable isotopic study of late Neoproterozoic-early Cambrian (?) sediments from Nagaur-Ganganagar basin, western India: possible signatures of global and regional C-isotopic events
Carbonate rocks of late Neoproterozoic-early Cambrian age occur in two extremities of the Nagaur-Ganganagar basin in western India: in east as part of Bilara hills (type locality) and in west as part of Baghewala oil field. These rocks have close stratigraphic correlation with the Ara Formation (Huqf Group, south Oman salt basin) which transgresses the late Neoproterozoic-early Cambrian boundary. Sedimentological and stable isotopic studies of these rocks show prevalence of arid and evaporitic conditions during their depostion. Close resemblance of C-isotopic profiles of Hanseran evaporites and Bilara carbonates allows intra-basinal correlation and suggests that they are coeval facies variants. There are several characteristic carbon isotopic excursions one of which can be tentatively correlated with the globally recorded excursion close to the late Neoproterozoic-early Cambrian boundary. In contrast other peaks are possibly of regional significance and can be attributed to fluctuations in bioproductivity, correlatable with variation in nutrient supply on a basinal scale. Highly depleted carbon isotopic composition of the organic matter in subsurface organic-rich carbonates (Upper carbonate Formation, Baghewala-II core) has been ascribed to incorporation of biomass synthesized by chemoautotrophic bacteria under anoxic or eutrophic basinal condition caused by salinity stratification
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