84 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
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
Dynamics of Impurity and Valence Bands in GaMnAs within the Dynamical Mean Field Approximation
We calculate the density-of-states and the spectral function of GaMnAs within
the dynamical mean-field approximation. Our model includes the competing
effects of the strong spin-orbit coupling on the J=3/2 GaAs hole bands and the
exchange interaction between the magnetic ions and the itinerant holes. We
study the quasi-particle and impurity bands in the paramagnetic and
ferromagnetic phases for different values of impurity-hole coupling at the Mn
doping of x=0.05. By analyzing the anisotropic angular distribution of the
impurity band carriers at T=0, we conclude that the carrier polarization is
optimal when the carriers move along the direction parallel to the average
magnetization.Comment: 6 pages, 4 figure
Triplet excitations in carbon nanostructures
We show that the energy differences between the lowest optical singlet
exciton and the lowest triplet exciton in semiconducting single-walled carbon
nanotubes with diameter nm and graphene nanoribbons with widths nm are an order of magnitude smaller than in the -conjugated polymer
poly(para-phenylenevinylene). Our calculated energy gaps between the singlet
and triplet excitons are in excellent agreement with the measured values in
three different nanotubes with diameters close to 1 nm. The spatial extent of
the triplet exciton is nearly the same as that of the singlet exciton in wide
nanotubes and nanoribbons, in contrast to that in -conjugated polymers, in
which the triplet exciton exhibits strong spatial confinement. Weakly confined
behavior of the triplet state begins in nanoribbons with widths as narrow as
2.5 times the graphene unit lattice vector. We discuss possible consequences of
the small singlet-triplet energy difference in the carbon nanostructures on
device applications.Comment: 9 pages, 2 tables, 4 figure
Magnetism in semiconductors: A dynamical mean-field study of ferromagnetism in Ga1-xMnxAs
We employ the dynamical mean-field approximation to perform a systematic study of magnetism in Ga1-xMnxAs. Our model incorporates the effects of the strong spin-orbit coupling on the J=32 GaAs valence bands and of the exchange interaction between the randomly distributed magnetic ions and the itinerant holes. The ferromagnetic phase transition temperature Tc is obtained for different values of the impurity-hole coupling Jc and of the hole concentration nh at the Mn doping of x=0.05. We also investigate the temperature dependence of the local magnetization and spin polarization of the holes. By comparing our results with those for a single band Hamiltonian, we conclude that the spin-orbit coupling in Ga1-xMnxAs gives rise to frustration in the ferromagnetic order, strengthening recent findings by Zaránd and Jankó [Phys. Rev. Lett. 89, 047201 (2002)]. © 2005 The American Physical Society
Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film
We report pressure-dependent transient picosecond and continuous-wave
photomodulation studies of disordered and ordered films of
2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced
absorption (PA) bands in the disordered film exhibit very weak pressure
dependence and are assigned to intrachain excitons and polarons. In contrast,
the ordered film exhibits two additional transient PA bands in the midinfrared
that blueshift dramatically with pressure. Based on high-order configuration
interaction calculations we ascribe the PA bands in the ordered film to
excimers. Our work brings insight to the exciton binding energy in ordered
films versus disordered films and solutions. The reduced exciton binding energy
in ordered films is due to new energy states appearing below the continuum band
threshold of the single strand.Comment: 5.5 pages, 5 figure
- …