1,666 research outputs found
Defect healing at room temperature in pentacene thin films and improved transistor performance
We report on a healing of defects at room temperature in the organic
semiconductor pentacene. This peculiar effect is a direct consequence of the
weak intermolecular interaction which is characteristic of organic
semiconductors. Pentacene thin-film transistors were fabricated and
characterized by in situ gated four-terminal measurements. Under high vacuum
conditions (base pressure of order 10E-8 mbar), the device performance is found
to improve with time. The effective field-effect mobility increases by as much
as a factor of two and mobilities up to 0.45 cm2/Vs were achieved. In addition,
the contact resistance decreases by more than an order of magnitude and there
is a significant reduction in current hysteresis. Oxygen/nitrogen exposure and
annealing experiments show the improvement of the electronic parameters to be
driven by a thermally promoted process and not by chemical doping. In order to
extract the spectral density of trap states from the transistor
characteristics, we have implemented a powerful scheme which allows for a
calculation of the trap densities with high accuracy in a straightforward
fashion. We show the performance improvement to be due to a reduction in the
density of shallow traps <0.15 eV from the valence band edge, while the
energetically deeper traps are essentially unaffected. This work contributes to
an understanding of the shallow traps in organic semiconductors and identifies
structural point defects within the grains of the polycrystalline thin films as
a major cause.Comment: 13 pages, 13 figures, to be published in Phys. Rev.
Direct numerical simulation of homogeneous nucleation and growth in a phase-field model using cell dynamics method
Homogeneous nucleation and growth in a simplest two-dimensional phase field
model is numerically studied using the cell dynamics method. Whole process from
nucleation to growth is simulated and is shown to follow closely the
Kolmogorov-Johnson-Mehl-Avrami (KJMA) scenario of phase transformation.
Specifically the time evolution of the volume fraction of new stable phase is
found to follow closely the KJMA formula. By fitting the KJMA formula directly
to the simulation data, not only the Avrami exponent but the magnitude of
nucleation rate and, in particular, of incubation time are quantitatively
studied. The modified Avrami plot is also used to verify the derived KJMA
parameters. It is found that the Avrami exponent is close to the ideal
theoretical value m=3. The temperature dependence of nucleation rate follows
the activation-type behavior expected from the classical nucleation theory. On
the other hand, the temperature dependence of incubation time does not follow
the exponential activation-type behavior. Rather the incubation time is
inversely proportional to the temperature predicted from the theory of
Shneidman and Weinberg [J. Non-Cryst. Solids {\bf 160}, 89 (1993)]. A need to
restrict thermal noise in simulation to deduce correct Avrami exponent is also
discussed.Comment: 9 pages, 8 figures, Journal of Chemical Physics to be publishe
Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance
We studied the influence of oxygen on the electronic trap states in a
pentacene thin film. This was done by carrying out gated four-terminal
measurements on thin-film transistors as a function of temperature and without
ever exposing the samples to ambient air. Photooxidation of pentacene is shown
to lead to a peak of trap states centered at 0.28 eV from the mobility edge,
with trap densities of the order of 10(18) cm(-3). These trap states need to be
occupied at first and cause a reduction in the number of free carriers, i.e. a
consistent shift of the density of free holes as a function of gate voltage.
Moreover, the exposure to oxygen reduces the mobility of the charge carriers
above the mobility edge. We correlate the change of these transport parameters
with the change of the essential device parameters, i.e. subthreshold
performance and effective field-effect mobility. This study supports the
assumption of a mobility edge for charge transport, and contributes to a
detailed understanding of an important degradation mechanism of organic
field-effect transistors. Deep traps in an organic field-effect transistor
reduce the effective field-effect mobility by reducing the number of free
carriers and their mobility above the mobility edge.Comment: 13 pages, 14 figures, to be published in Phys. Rev.
Ensemble of Vortex Loops in the Abelian-Projected SU(3)-Gluodynamics
Grand canonical ensemble of small vortex loops emerging in the London limit
of the effective Abelian-projected theory of the SU(3)-gluodynamics is
investigated in the dilute gas approximation. An essential difference of this
system from the SU(2)-case is the presence of two interacting gases of vortex
loops. Two alternative representations for the partition function of such a
grand canonical ensemble are derived, and one of them, which is a
representation in terms of the integrals over vortex loops, is employed for the
evaluation of the correlators of both kinds of loops in the low-energy limit.Comment: 10 pages, LaTeX2e, no figures, minor corrections, to appear in Mod.
Phys. Lett.
The Abelian Higgs Model as an Ensemble of Vortex Loops
In the London limit of the Ginzburg-Landau theory (Abelian Higgs model),
vortex dipoles (small vortex loops) are treated as a grand canonical ensemble
in the dilute gas approximation. The summation over these objects with the most
general rotation- and translation invariant measure of integration over their
shapes leads to effective sine-Gordon theories of the dual fields. The
representations of the partition functions of both grand canonical ensembles
are derived in the form of the integrals over the vortex dipoles and the small
vortex loops, respectively. By virtue of these representations, the bilocal
correlator of the vortex dipoles (loops) is calculated in the low-energy limit.
It is further demonstrated that once the vortex dipoles (loops) are considered
as such an ensemble rather than individual ones, the London limit of the
Ginzburg-Landau theory (Abelian Higgs model) with external monopoles is
equivalent up to the leading order in the inverse UV cutoff to the compact QED
in the corresponding dimension with the charge of Cooper pairs changed due to
the Debye screening.Comment: 17 pages, LaTeX2e, no figures, dedicated to Prof. Yu.A. Simonov on
the occasion of his 65-th birthday, final published version (minor
corrections, references added
One-second coherence for a single electron spin coupled to a multi-qubit nuclear-spin environment
Single electron spins coupled to multiple nuclear spins provide promising
multi-qubit registers for quantum sensing and quantum networks. The obtainable
level of control is determined by how well the electron spin can be selectively
coupled to, and decoupled from, the surrounding nuclear spins. Here we realize
a coherence time exceeding a second for a single electron spin through
decoupling sequences tailored to its microscopic nuclear-spin environment. We
first use the electron spin to probe the environment, which is accurately
described by seven individual and six pairs of coupled carbon-13 spins. We
develop initialization, control and readout of the carbon-13 pairs in order to
directly reveal their atomic structure. We then exploit this knowledge to store
quantum states for over a second by carefully avoiding unwanted interactions.
These results provide a proof-of-principle for quantum sensing of complex
multi-spin systems and an opportunity for multi-qubit quantum registers with
long coherence times
Do you need a job to find a job?
This paper investigates whether job offers arrive more frequently for those in employment than for those in unemployment. To this end, we take advantage of a unique Australian data set which contains information on both accepted and rejected job offers. Our estimation strategy takes account of the selectivity associated with the initial employment state and we allow for individual heterogeneity in the probability of obtaining jobs. Our results reveal that, across the wage range, individuals are about equally likely to obtain a job offer in employment as in unemployment. This implies that encouraging unemployed (rather than employed) search through the provision of unemployment benefits does not improve the speed of a job match
String Fields and the Standard Model
The Cremmer-Scherk mechanism is generalised in a non-Abelian context. In the
presence of the Higgs scalars of the standard model it is argued that fields
arising from the low energy effective string action may contribute to the mass
generation of the observed vector bosons that mediate the electroweak
interactions and that future analyses of experimental data should consider the
possibility of string induced radiative corrections to the Weinberg angle
coming from physics beyond the standard model.Comment: 4 pages, LATEX, no figure
Superconductors with Topological Order
We propose a mechanism of superconductivity in which the order of the ground
state does not arise from the usual Landau mechanism of spontaneous symmetry
breaking but is rather of topological origin. The low-energy effective theory
is formulated in terms of emerging gauge fields rather than a local order
parameter and the ground state is degenerate on topologically non-trivial
manifolds. The simplest example of this mechanism of superconductivty is
concretely realized as global superconductivty in Josephson junction arrays.Comment: 4 pages, no figure
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