4,590 research outputs found
Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors
We present a comparative study of ultrafast photo-conversion dynamics in
tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical
pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is
activated and temperature-independent respectively, demonstrating dominant
singlet-triplet exciton fission. In Pc films (as well as C-doped films)
this decay channel is suppressed by electron trapping. These results
demonstrate the central role of crystallinity and purity in photogeneration
processes and will constrain the design of future photovoltaic devices.Comment:
Phase diagram for morphological transitions of wetting films on chemically structured substrates
Using an interface displacement model we calculate the shapes of thin
liquidlike films adsorbed on flat substrates containing a chemical stripe. We
determine the entire phase diagram of morphological phase transitions in these
films as function of temperature, undersaturation, and stripe widthComment: 15 pages, RevTeX, 7 Figure
Soft Phonon Anomalies in the Relaxor Ferroelectric Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3
Neutron inelastic scattering measurements of the polar TO phonon mode
dispersion in the cubic relaxor Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3 at 500K reveal
anomalous behavior in which the optic branch appears to drop precipitously into
the acoustic branch at a finite value of the momentum transfer q=0.2 inverse
Angstroms, measured from the zone center. We speculate this behavior is the
result of nanometer-sized polar regions in the crystal.Comment: 4 pages, 4 figure
Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O_3
The intrinsic linewidth of the transverse acoustic (TA) phonon
observed in the relaxor ferroelectric compound
Pb(MgNbTiO (PMN-20%PT) begins to broaden
with decreasing temperature around 650 K, nearly 300 K above the ferroelectric
transition temperature ( K). We speculate that this anomalous
behavior is directly related to the condensation of polarized, nanometer-sized,
regions at the Burns temperature . We also observe the ``waterfall''
anomaly previously seen in pure PMN, in which the transverse optic (TO) branch
appears to drop precipitously into the TA branch at a finite momentum transfer
\AA. The waterfall feature is seen even at
temperatures above . This latter result suggests that the PNR exist as
dynamic entities above .Comment: 6 pages, 4 figure
Electrostatic model of atomic ordering in complex perovskite alloys
We present a simple ionic model which successfully reproduces the various
types of compositional long-range order observed in a large class of complex
insulating perovskite alloys. The model assumes that the driving mechanism
responsible for the ordering is simply the electrostatic interaction between
the different ionic species. A possible new explanation for the anomalous
long-range order observed in some Pb relaxor alloys, involving the proposed
existence of a small amount of Pb^4+ on the B sublattice, is suggested by an
analysis of the model.Comment: 4 pages, two-column style with 1 postscript figure embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#lb_orde
Crystal Structures and Electronic Properties of Haloform-Intercalated C60
Using density functional methods we calculated structural and electronic
properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3
(X=Cl,Br). Both compounds are narrow band insulator materials with a gap
between valence and conduction bands larger than 1 eV. The calculated widths of
the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively.
The orbitals of the haloform molecules overlap with the orbitals of the
fullerene molecules and the p-type orbitals of halogen atoms significantly
contribute to the valence and conduction bands of C60 2CHX3. Charging with
electrons and holes turns the systems to metals. Contrary to expectation, 10 to
20 % of the charge is on the haloform molecules and is thus not completely
localized on the fullerene molecules. Calculations on different crystal
structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at
the Fermi energy are sensitive to the orientation of the haloform and C60
molecules. At a charging of three holes, which corresponds to the
superconducting phase of pure C60 and C60 2CHX3, the calculated density of
states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60
2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table
The Hamiltonian of the V Spin System from first-principles Density-Functional Calculations
We report first-principles all-electron density-functional based studies of
the electronic structure, magnetic ordering and anisotropy for the V
molecular magnet. From these calculations, we determine a Heisenberg
Hamiltonian with four antiferromagnetic and one {\em ferromagnetic} coupling.
We perform direct diagonalization to determine the temperature dependence of
the susceptibility. This Hamiltonian reproduces the experimentally observed
spin =1/2 ground state and low-lying =3/2 excited state. A small
anisotropy term is necessary to account for the temperature independent part of
the magnetization curve.Comment: 4 pages in RevTeX format + 2 ps-figures, accepted by PRL Feb. 2001
(previous version was an older version of the paper
'Prove me the bam!': victimization and agency in the lives of young women who commit violent offences
This article reviews the evidence regarding young womenâs involvement in violent crime and, drawing on recent research carried out in HMPYOI Cornton Vale in Scotland, provides an overview of the characteristics, needs and deeds of young women sentenced to imprisonment for violent offending. Through the use of direct quotations, the article suggests that young womenâs anger and aggression is often related to their experiences of family violence and abuse, and the acquisition of a negative worldview in which other people are considered as being 'out to get you' or ready to 'put one over on you'. The young women survived in these circumstances, not by adopting discourses that cast them as exploited victims, but by drawing on (sub)cultural norms and values which promote pre-emptive violence and the defence of respect. The implications of these findings for those who work with such young women are also discussed
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