2,540 research outputs found
Mapping the potential within a nanoscale undoped GaAs region using a scanning electron microscope
Semiconductor dopant profiling using secondary electron imaging in a scanning
electron microscope (SEM) has been developed in recent years. In this paper, we
show that the mechanism behind it also allows mapping of the electric potential
of undoped regions. By using an unbiased GaAs/AlGaAs heterostructure, this
article demonstrates the direct observation of the electrostatic potential
variation inside a 90nm wide undoped GaAs channel surrounded by ionized
dopants. The secondary electron emission intensities are compared with
two-dimensional numerical solutions of the electric potential.Comment: 7 pages, 3 figure
Cognitive consequences of clumsy automation on high workload, high consequence human performance
The growth of computational power has fueled attempts to automate more of the human role in complex problem solving domains, especially those where system faults have high consequences and where periods of high workload may saturate the performance capacity of human operators. Examples of these domains include flightdecks, space stations, air traffic control, nuclear power operation, ground satellite control rooms, and surgical operating rooms. Automation efforts may have unanticipated effects on human performance, particularly if they increase the workload at peak workload times or change the practitioners' strategies for coping with workload. Smooth and effective changes in automation requires detailed understanding of the congnitive tasks confronting the user: it has been called user centered automation. The introduction of a new computerized technology in a group of hospital operating rooms used for heart surgery was observed. The study revealed how automation, especially 'clumsy automation', effects practitioner work patterns and suggest that clumsy automation constrains users in specific and significant ways. Users tailor both the new system and their tasks in order to accommodate the needs of process and production. The study of this tailoring may prove a powerful tool for exposing previously hidden patterns of user data processing, integration, and decision making which may, in turn, be useful in the design of more effective human-machine systems
Largeness and SQ-universality of cyclically presented groups
Largeness, SQ-universality, and the existence of free subgroups of rank 2 are measures of the complexity of a finitely presented group. We obtain conditions under which a cyclically presented group possesses one or more of these properties. We apply our results to a class of groups introduced by Prishchepov which contains, amongst others, the various generalizations of Fibonacci groups introduced by Campbell and Robertson
A polychromatic 'greenbeard' locus determines patterns of cooperation in a social amoeba
Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs. Cheater impact can be diminished if cooperators display a tag (‘greenbeard’) and recognise and preferentially direct cooperation towards other tag carriers. Despite its popular appeal, the feasibility of such greenbeards has been questioned because the complex patterns of partner-specific cooperative behaviours seen in nature require greenbeards to come in different colours. Here we show that a locus (‘Tgr’) of a social amoeba represents a polychromatic greenbeard. Patterns of natural Tgr locus sequence polymorphisms predict partner-specific patterns of cooperation by underlying variation in partner-specific protein–protein binding strength and recognition specificity. Finally, Tgr locus polymorphisms increase fitness because they help avoid potential costs of cooperating with incompatible partners. These results suggest that a polychromatic greenbeard can provide a key mechanism for the evolutionary maintenance of cooperation
Multi-color Optical and NIR Light Curves of 64 Stripped-Envelope Core-Collapse Supernovae
We present a densely-sampled, homogeneous set of light curves of 64 low
redshift (z < 0.05) stripped-envelope supernovae (SN of type IIb, Ib, Ic and
Ic-bl). These data were obtained between 2001 and 2009 at the Fred L. Whipple
Observatory (FLWO) on Mt. Hopkins in Arizona, with the optical FLWO 1.2-m and
the near-infrared PAIRITEL 1.3-m telescopes. Our dataset consists of 4543
optical photometric measurements on 61 SN, including a combination of UBVRI,
UBVr'i', and u'BVr'i', and 2142 JHKs near-infrared measurements on 25 SN. This
sample constitutes the most extensive multi-color data set of stripped-envelope
SN to date. Our photometry is based on template-subtracted images to eliminate
any potential host galaxy light contamination. This work presents these
photometric data, compares them with data in the literature, and estimates
basic statistical quantities: date of maximum, color, and photometric
properties. We identify promising color trends that may permit the
identification of stripped-envelope SN subtypes from their photometry alone.
Many of these SN were observed spectroscopically by the CfA SN group, and the
spectra are presented in a companion paper (Modjaz et al. 2014). A thorough
exploration that combines the CfA photometry and spectroscopy of
stripped-envelope core-collapse SN will be presented in a follow-up paper.Comment: 26 pages, 17 figures, 8 tables. Revised version resubmitted to ApJ
Supplements after referee report. Additional online material is available
through http://cosmo.nyu.edu/SNYU
Optical Spectra of 73 Stripped-Envelope Core-Collapse Supernovae
We present 645 optical spectra of 73 supernovae (SNe) of Types IIb, Ib, Ic,
and broad-lined Ic. All of these types are attributed to the core collapse of
massive stars, with varying degrees of intact H and He envelopes before
explosion. The SNe in our sample have a mean redshift = 4200 km/s. Most of
these spectra were gathered at the Harvard-Smithsonian Center for Astrophysics
(CfA) between 2004 and 2009. For 53 SNe, these are the first published spectra.
The data coverage range from mere identification (1-3 spectra) for a few SNe to
extensive series of observations (10-30 spectra) that trace the spectral
evolution for others, with an average of 9 spectra per SN. For 44 SNe of the 73
SNe presented here, we have well-determined dates of maximum light to determine
the phase of each spectrum. Our sample constitutes the most extensive spectral
library of stripped-envelope SNe to date. We provide very early coverage (as
early as 30 days before V-band max) for photospheric spectra, as well as
late-time nebular coverage when the innermost regions of the SNe are visible
(as late as 2 years after explosion, while for SN1993J, we have data as late as
11.6 years). This data set has homogeneous observations and reductions that
allow us to study the spectroscopic diversity of these classes of stripped SNe
and to compare these to SNe associated with gamma-ray bursts. We undertake
these matters in follow-up papers.Comment: Published by the Astronomical Journal in May 2015. All spectra are
publicly available at the CfA SN archive:
http://www.cfa.harvard.edu/supernova/SNarchive.html . A companion paper on
constructing SNID templates based on these spectra is by Liu & Modjaz (2014)
and the resulting SNID templates are available from the NYU website:
http://cosmo.nyu.edu/SNYU/spectra
On covers of cyclic acts over monoids
In (Bull. Lond. Math. Soc. 33:385–390, 2001) Bican, Bashir and Enochs finally solved a long standing conjecture in module theory that all modules over a unitary ring have a flat cover. The only substantial work on covers of acts over monoids seems to be that of Isbell (Semigroup Forum 2:95–118, 1971), Fountain (Proc. Edinb. Math. Soc. (2) 20:87–93, 1976) and Kilp (Semigroup Forum 53:225–229, 1996) who only consider projective covers. To our knowledge the situation for flat covers of acts has not been addressed and this paper is an attempt to initiate such a study. We consider almost exclusively covers of cyclic acts and restrict our attention to strongly flat and condition (P) covers. We give a necessary and sufficient condition for the existence of such covers and for a monoid to have the property that all its cyclic right acts have a strongly flat cover (resp. (P)-cover). We give numerous classes of monoids that satisfy these conditions and we also show that there are monoids that do not satisfy this condition in the strongly flat case. We give a new necessary and sufficient condition for a cyclic act to have a projective cover and provide a new proof of one of Isbell’s classic results concerning projective covers. We show also that condition (P) covers are not unique, unlike the situation for projective covers
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