17,703 research outputs found
Effects of electron coupling to intra- and inter-molecular vibrational modes on the transport properties of single crystal organic semiconductors
Electron coupling to intra- and inter-molecular vibrational modes is
investigated in models appropriate to single crystal organic semiconductors,
such as oligoacenes. Focus is on spectral and transport properties of these
systems beyond perturbative approaches. The interplay between different
couplings strongly affects the temperature band renormalization that is the
result of a subtle equilibrium between opposite tendencies: band narrowing due
to interaction with local modes, band widening due to electron coupling to non
local modes. The model provides an accurate description of the mobility as
function of temperature: indeed, it has the correct order of magnitude, at low
temperatures, it scales as a power-law with the exponent
larger than unity, and, at high temperatures, shows an hopping behavior with a
small activation energy.Comment: 3 Figures, Submitte
Nonlinear Realizations of Supersymmetry and Other Symmetries
Simultaneous nonlinear realizations of spontaneously broken supersymmetry in
conjunction with other spontaneous and/or explicitly broken symmetries
including R symmetry, global chiral symmetry, dilatations and the
superconformal symmetries is reviewed.Comment: 15 pages, invited brief review for Mod. Phys. Lett.
Spin-dependent recombination in Czochralski silicon containing oxide precipitates
Electrically detected magnetic resonance is used to identify recombination
centers in a set of Czochralski grown silicon samples processed to contain
strained oxide precipitates with a wide range of densities (~ 1e9 cm-3 to ~
7e10 cm-3). Measurements reveal that photo-excited charge carriers recombine
through Pb0 and Pb1 dangling bonds and comparison to precipitate-free material
indicates that these are present at both the sample surface and the oxide
precipitates. The electronic recombination rates vary approximately linearly
with precipitate density. Additional resonance lines arising from iron-boron
and interstitial iron are observed and discussed. Our observations are
inconsistent with bolometric heating and interpreted in terms of spin-dependent
recombination. Electrically detected magnetic resonance is thus a very powerful
and sensitive spectroscopic technique to selectively probe recombination
centers in modern photovoltaic device materials.Comment: 8 pages, 8 figure
Persistent X-Ray Photoconductivity and Percolation of Metallic Clusters in Charge-Ordered Manganites
Charge-ordered manganites of composition exhibit persistent photoconductivity upon
exposure to x-rays. This is not always accompanied by a significant increase in
the {\it number} of conduction electrons as predicted by conventional models of
persistent photoconductivity. An analysis of the x-ray diffraction patterns and
current-voltage characteristics shows that x-ray illumination results in a
microscopically phase separated state in which charge-ordered insulating
regions provide barriers against charge transport between metallic clusters.
The dominant effect of x-ray illumination is to enhance the electron {\it
mobility} by lowering or removing these barriers. A mechanism based on magnetic
degrees of freedom is proposed.Comment: 8 pages, 4 figure
Interplay between electron-phonon couplings and disorder strength on the transport properties of organic semiconductors
The combined effect of bulk and interface electron-phonon couplings on the
transport properties is investigated in a model for organic semiconductors
gated with polarizable dielectrics. While the bulk electron-phonon interaction
affects the behavior of mobility in the coherent regime below room temperature,
the interface coupling is dominant for the activated high contribution of
localized polarons. In order to improve the description of the transport
properties, the presence of disorder is needed in addition to electron-phonon
couplings. The effects of a weak disorder largely enhance the activation
energies of mobility and induce the small polaron formation at lower values of
electron-phonon couplings in the experimentally relevant window . The results are discussed in connection with experimental data of rubrene
organic field-effect transistors.Comment: 4 pages, 3 figure
X-ray fluoresced high-Z (up to Z = 82) K-x-rays produced by LiNbO3 and LiTaO3 pyroelectric crystal electron accelerators
High-energy bremsstrahlung and K X-rays were used to produce nearly
background-free K X-ray spectra of up to 87 keV (Pb) via X-ray fluorescence.
The fluorescing radiation was produced by electron accelerators, consisting of
heated and cooled cylindrical LiTaO3 and LiNbO3 crystals at mTorr pressures.
The newly discovered process of gas amplification whereby the ambient gas
pressure is optimized to maximize the electron energy was used to produce
energetic electrons which when incident on a W/Bi target gave rise to a
radiation field consisting of high-energy bremsstrahlung as well as W and Bi K
X-rays. These photons were used to fluoresce Ta and Pb K X-rays.Comment: 6 pages, 6 figures, PD
Theory of the Franck-Condon blockade regime
Strong coupling of electronic and vibrational degrees of freedom entails a
low-bias suppression of the current through single-molecule devices, termed
Franck-Condon blockade. In the limit of slow vibrational relaxation, transport
in the Franck-Condon-blockade regime proceeds via avalanches of large numbers
of electrons, which are interrupted by long waiting times without electron
transfer. The avalanches consist of smaller avalanches, leading to a
self-similar hierarchy which terminates once the number of transferred
electrons per avalanche becomes of the order of unity. Experimental signatures
of self-similar avalanche transport are strongly enhanced current (shot) noise,
as expressed by giant Fano factors, and a power-law noise spectrum. We develop
a theory of the Franck-Condon-blockade regime with particular emphasis on
effects of electron cotunneling through highly excited vibrational states. As
opposed to the exponential suppression of sequential tunneling rates for
low-lying vibrational states, cotunneling rates suffer only a power-law
suppression. This leads to a regime where cotunneling dominates the current for
any gate voltage. Including cotunneling within a rate-equation approach to
transport, we find that both the Franck-Condon blockade and self-similar
avalanche transport remain intact in this regime. We predict that cotunneling
leads to absorption-induced vibrational sidebands in the Coulomb-blockaded
regime as well as intrinsic telegraph noise near the charge degeneracy point.Comment: 20 pages, 10 figures; minor changes, version published in Phys. Rev.
Coherent optical transfer of Feshbach molecules to a lower vibrational state
Using the technique of stimulated Raman adiabatic passage (STIRAP) we have
coherently transferred ultracold 87Rb2 Feshbach molecules into a more deeply
bound vibrational quantum level. Our measurements indicate a high transfer
efficiency of up to 87%. As the molecules are held in an optical lattice with
not more than a single molecule per lattice site, inelastic collisions between
the molecules are suppressed and we observe long molecular lifetimes of about 1
s. Using STIRAP we have created quantum superpositions of the two molecular
states and tested their coherence interferometrically. These results represent
an important step towards Bose-Einstein condensation (BEC) of molecules in the
vibrational ground state.Comment: 4 pages, 5 figure
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