12,512 research outputs found
Self-steepening of light pulses
Self-steepening of light pulses due to propagation in medium with intensity-dependent index of refractio
Density of bulk trap states in organic semiconductor crystals: discrete levels induced by oxygen in rubrene
The density of trap states in the bandgap of semiconducting organic single
crystals has been measured quantitatively and with high energy resolution by
means of the experimental method of temperature-dependent
space-charge-limited-current spectroscopy (TD-SCLC). This spectroscopy has been
applied to study bulk rubrene single crystals, which are shown by this
technique to be of high chemical and structural quality. A density of deep trap
states as low as ~ 10^{15} cm^{-3} is measured in the purest crystals, and the
exponentially varying shallow trap density near the band edge could be
identified (1 decade in the density of states per ~25 meV). Furthermore, we
have induced and spectroscopically identified an oxygen related sharp hole bulk
trap state at 0.27 eV above the valence band.Comment: published in Phys. Rev. B, high quality figures:
http://www.cpfs.mpg.de/~krellner
Milli-second Oscillations in the Persistent and Bursting Flux of Aql X-1 During an Outburst
The Rossi X-Ray Timing Explorer observed the soft X-Ray transient Aql X-1
during its outburst in February and March 1997. We report the discovery of
quasi-periodic oscillations (QPOs) in its persistent flux with frequencies in
the range of 740 to 830 Hz, Q-value of over 100, and a fractional RMS amplitude
of (6.8 +- 0.6)%, and nearly coherent oscillations (NCOs) during a Type-I burst
with a frequency of 549 Hz. The frequency of the QPOs in the persistent flux is
correlated with the mass accretion rate on time scale of hours, but not on time
scale of days. This is most likely the manifestation in a single source of the
kHz QPO puzzle observed among many sources, i.e., on the one hand, individual
sources show a correlation between the QPO frequency and the inferred mass
accretion rate, on the other hand, the dozen or so sources with luminosities
spanning two decades have essentially the same QPO frequencies. We propose that
this multi-valued QPO frequency and mass accretion rate correlation indicates
the existence of many similar regimes of the accretion disk. These regimes,
with a very similar energy spectrum and QPO frequency, are distinguished from
each other by the mass accretion rate or the total X-ray flux. The NCOs during
the burst can be made almost perfectly coherent by taking into account a large
frequency derivative. This strongly suggests that this frequency is related to
the neutron star spin frequency. The large frequency derivative is attributable
to the expansion or contraction of the neutron star photosphere during the
burst.Comment: 6 pages, LaTex (aas2pp4), Accepted for publication in ApJ Let
Recommended from our members
Colin Marsh (1996) Handbook for Beginning Teachers. Addison Wesley Longman: Australia. 406 pp
This study examines the relationship between corporate irresponsibility (CI) and social evaluations of the firm. Utilising a large and unique corporate irresponsibility dataset, we explore how, when, and for whom corporate irresponsibility alters stakeholder perceptions of organisational reputation. By combining attribution theory and expectancy violations theory, our study elucidates the conditions under which organisational assessors attribute hypocrisy. Results generally indicate that organisational reputation, derived from the assessments of managers and market analysts, is infrequently influenced by acts of corporate irresponsibility alone. Contrary to previous studies, we find that stakeholder impressions of the firm are largely path dependent. In turn, our findings reveal that firms previously believed to be most socially responsible are perceived as hypocritical and sanctioned when causality for acts of CI can be externally validated. Conversely, our study reveals risks associated with firms believed to be least socially responsible, in that stakeholders may expect firms with reduced social performance to be culpable for corporate irresponsibility when accused of highly egregious behaviour
Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PNs and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 μm IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the "cometary knots," which have been previously studied in this PN. The "tails" of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H_2 models, with a small (~10%) component from photodissociation regions. In the northeast arc, the H_2 emission is located in a shell outside the Hα emission
Higher anisotropic d-wave symmetry in cuprate superconductors
We derive a pair potential from tight binding further neighbours attraction
that leads to superconducting gap symmetry similar to that of the
phenomenological spin fluctuation theory of high temperature superconductors
(Monthoux, Balatsky, Pines, Phys. Rev. Lett. {\bf 67}, 3448). We show that
higher anisotropic d-wave than the simpliest d-wave symmetry is one of the
important ingredients responsible for higher BCS characteristic ratio.Comment: Latex 5 pages, 3 figures attached, Journal Ref. : Journal of Physics
C, Vol. 11, issue 30, L371-L377 (1999
Evaluation of automated decisionmaking methodologies and development of an integrated robotic system simulation. Appendix A: ROBSIM user's guide
The purpose of the Robotics Simulation Program is to provide a broad range of computer capabilities to assist in the design, verification, simulation, and study of robotics systems. ROBSIM is program in FORTRAN 77 for use on a VAX 11/750 computer under the VMS operating system. This user's guide describes the capabilities of the ROBSIM programs, including the system definition function, the analysis tools function and the postprocessor function. The options a user may encounter with each of these executables are explained in detail and the different program prompts appearing to the user are included. Some useful suggestions concerning the appropriate answers to be given by the user are provided. An example user interactive run in enclosed for each of the main program services, and some of the capabilities are illustrated
Evaluation of automated decision making methodologies and development of an integrated robotic system simulation: Study results
The implementation of a generic computer simulation for manipulator systems (ROBSIM) is described. The program is written in FORTRAN, and allows the user to: (1) Interactively define a manipulator system consisting of multiple arms, load objects, targets, and an environment; (2) Request graphic display or replay of manipulator motion; (3) Investigate and simulate various control methods including manual force/torque and active compliance control; and (4) Perform kinematic analysis, requirements analysis, and response simulation of manipulamotion. Previous reports have described the algorithms and procedures for using ROBSIM. These reports are superseded and additional features which were added are described. They are: (1) The ability to define motion profiles and compute loads on a common base to which manipulator arms are attached; (2) Capability to accept data describing manipulator geometry from a Computer Aided Design data base using the Initial Graphics exchange Specification format; (3) A manipulator control algorithm derived from processing the TV image of known reference points on a target; and (4) A vocabulary of simple high level task commands which can be used to define task scenarios
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