1,331 research outputs found
Modeling Space-Charge Limited Currents in Organic Semiconductors: Extracting Trap Density and Mobility
We have developed and applied a mobility edge model that takes into account
drift and diffusion currents to characterize the space charge limited current
in organic semiconductors. The numerical solution of the drift-diffusion
equation allows the utilization of asymmetric contacts to describe the built-in
potential within the device. The model has been applied to extract information
of the distribution of traps from experimental current-voltage measurements of
a rubrene single crystal from Krellner et al. [Phys. Rev. B, 75(24), 245115]
showing excellent agreement across several orders of magnitude of current.
Although the two contacts are made of the same metal, an energy offset of 580
meV between them, ascribed to differences in the deposition techniques
(lamination vs. evaporation) was essential to correctly interpret the shape of
the current-voltage characteristics at low voltage. A band mobility 0.13 cm2/Vs
for holes was estimated, which is consistent with transport along the long axis
of the orthorhombic unit cell. The total density of traps deeper than 0.1 eV
was 2.2\times1016 cm-3. The sensitivity analysis and error estimation in the
obtained parameters shows that it is not possible to accurately resolve the
shape of the trap distribution for energies deeper than 0.3 eV or shallower
than 0.1 eV above the valence band edge. The total number of traps deeper than
0.3 eV however can be estimated. Contact asymmetry and the diffusion component
of the current play an important role in the description of the device at low
bias, and are required to obtain reliable information about the distribution of
deep traps
The Precision Determination of Invisible-Particle Masses at the LHC
We develop techniques to determine the mass scale of invisible particles
pair-produced at hadron colliders. We employ the constrained mass variable
m_2C, which provides an event-by-event lower-bound to the mass scale given a
mass difference. We complement this variable with a new variable m_2C,UB which
provides an additional upper bound to the mass scale, and demonstrate its
utility with a realistic case study of a supersymmetry model. These variables
together effectively quantify the `kink' in the function Max m_T2 which has
been proposed as a mass-determination technique for collider-produced dark
matter. An important advantage of the m_2C method is that it does not rely
simply on the position at the endpoint, but it uses the additional information
contained in events which lie far from the endpoint. We found the mass by
comparing the HERWIG generated m_2C distribution to ideal distributions for
different masses. We find that for the case studied, with 100 fb^-1 of
integrated luminosity (about 400 signal events), the invisible particle's mass
can be measured to a precision of 4.1 GeV. We conclude that this technique's
precision and accuracy is as good as, if not better than, the best known
techniques for invisible-particle mass-determination at hadron colliders.Comment: 20 pages, 11 figures, minor correction
A new Skyrme interaction with improved spin-isospin properties
A correct determination of the spin-isospin properties of the nuclear
effective interaction should lead, among other improvements, to an accurate
description of the Gamow-Teller Resonance (GTR). These nuclear excitations
impact on a variety of physical processes: from the response in charge-exchange
reactions of nuclei naturally present in the Earth, to the description of the
stellar nucleosynthesis, and of the pre-supernova explosion core-collapse
evolution of massive stars in the Universe. A reliable description of the GTR
provides also stringent tests for neutrinoless double- decay
calculations. We present a new Skyrme interaction as accurate as previous
forces in the description of finite nuclei and of uniform matter properties
around saturation density, and that account well for the GTR in Ca,
Zr and Pb, the Isobaric Analog Resonance and the Spin Dipole
Resonance in Zr and Pb.Comment: Predictions on the IAR and SDR and comparison with the SGII
interaction for the GTRs where adde
Enhanced material defect imaging with a radio-frequency atomic magnetometer
Imaging of structural defects in a material can be realized with a radio-frequency atomic magnetometer by monitoring the material’s response to a radio-frequency excitation field. We demonstrate two measurement configurations that enable the increase of the amplitude and phase contrast in images that represent a structural defect in electrically conductive and magnetically permeable samples. Both concepts involve the elimination of the excitation field component, orthogonal to the sample surface, from the atomic magnetometer signal. The first method relies on the implementation of a set of coils that directly compensates the excitation field component in the magnetometer signal. The second takes advantage of the fact that the radio-frequency magnetometer is not sensitive to the magnetic field oscillating along one of its axes. Results from simple modelling confirm the experimental observation and are discussed in detail
Inductive imaging of the concealed defects with radio-frequency atomic magnetometers
We explore the capabilities of the radio-frequency atomic magnetometers in the non-destructive detection of concealed defects. We present results from the systematic magnetic inductive measurement of various defect types in an electrically conductive object at different rf field frequencies (0.4–12 kHz) that indicate the presence of an optimum operational frequency of the sensor. The optimum in the frequency dependence of the amplitude/phase contrast for defects under a 0.5–1.5 mm conductive barrier was observed within the 1–2 kHz frequency range. The experiments are performed in the self-compensated configuration that automatically removes the background signal created by the rf field producing object response
Generation of atomic spin orientation with a linearly polarized beam in room-temperature alkali-metal vapor
Traditionally, atomic spin orientation is achieved by the transfer of angular momentum from polarized light to an atomic system. We demonstrate the mechanism of orientation generation in room-temperature caesium vapors that combines three elements: optical pumping, nonlinear spin dynamics, and spin-exchange collisions. Through the variation of the spin-exchange relaxation rate, the transition between an aligned and an oriented atomic sample is presented. The observation is performed by monitoring the atomic radio-frequency spectra. The measurement configuration discussed paves the way to simple and robust radio-frequency atomic magnetometers that are based on a single low-power laser diode that approach the performance of multilaser pump-probe systems
Global Analysis of Nucleon Strange Form Factors at Low
We perform a global analysis of all recent experimental data from elastic
parity-violating electron scattering at low . The values of the electric
and magnetic strange form factors of the nucleon are determined at
GeV/ to be and .Comment: 8 pages, 1 figur
Energy distribution and cooling of a single atom in an optical tweezer
We investigate experimentally the energy distribution of a single rubidium
atom trapped in a strongly focused dipole trap under various cooling regimes.
Using two different methods to measure the mean energy of the atom, we show
that the energy distribution of the radiatively cooled atom is close to
thermal. We then demonstrate how to reduce the energy of the single atom, first
by adiabatic cooling, and then by truncating the Boltzmann distribution of the
single atom. This provides a non-deterministic way to prepare atoms at low
microKelvin temperatures, close to the ground state of the trapping potential.Comment: 9 pages, 6 figures, published in PR
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