5,216 research outputs found
Tracking the ultrafast motion of an antiferromagnetic order parameter
The unique functionalities of antiferromagnets offer promising routes to
advance information technology. Their compensated magnetic order leads to spin
resonances in the THz-regime, which suggest the possibility to coherently
control antiferromagnetic (AFM) devices orders of magnitude faster than
traditional electronics. However, the required time resolution, complex
sublattice interations and the relative inaccessibility of the AFM order
parameter pose serious challenges to studying AFM spin dynamics. Here, we
reveal the temporal evolution of an AFM order parameter directly in the time
domain. We modulate the AFM order in hexagonal YMnO by coherent
magnon excitation and track the ensuing motion of the AFM order parameter using
time-resolved optical second-harmonic generation (SHG). The dynamic symmetry
reduction by the moving order parameter allows us to separate electron dynamics
from spin dynamics. As transient symmetry reductions are common to coherent
excitations, we have a general tool for tracking the ultrafast motion of an AFM
order parameter.Comment: 5 pages, 4 figure
DOBBS: Towards a Comprehensive Dataset to Study the Browsing Behavior of Online Users
The investigation of the browsing behavior of users provides useful
information to optimize web site design, web browser design, search engines
offerings, and online advertisement. This has been a topic of active research
since the Web started and a large body of work exists. However, new online
services as well as advances in Web and mobile technologies clearly changed the
meaning behind "browsing the Web" and require a fresh look at the problem and
research, specifically in respect to whether the used models are still
appropriate. Platforms such as YouTube, Netflix or last.fm have started to
replace the traditional media channels (cinema, television, radio) and media
distribution formats (CD, DVD, Blu-ray). Social networks (e.g., Facebook) and
platforms for browser games attracted whole new, particularly less tech-savvy
audiences. Furthermore, advances in mobile technologies and devices made
browsing "on-the-move" the norm and changed the user behavior as in the mobile
case browsing is often being influenced by the user's location and context in
the physical world. Commonly used datasets, such as web server access logs or
search engines transaction logs, are inherently not capable of capturing the
browsing behavior of users in all these facets. DOBBS (DERI Online Behavior
Study) is an effort to create such a dataset in a non-intrusive, completely
anonymous and privacy-preserving way. To this end, DOBBS provides a browser
add-on that users can install, which keeps track of their browsing behavior
(e.g., how much time they spent on the Web, how long they stay on a website,
how often they visit a website, how they use their browser, etc.). In this
paper, we outline the motivation behind DOBBS, describe the add-on and captured
data in detail, and present some first results to highlight the strengths of
DOBBS
Production and Properties of the Liquid Scintillators used in the Stereo Reactor Neutrino Experiment
The electron antineutrino spectrum in the Stereo reactor experiment (ILL
Grenoble) is measured via the inverse beta decay signals in an organic liquid
scintillator. The six target cells of the Stereo detector are filled with about
1800 litres of Gd-loaded liquid scintillator optimised for the requirements of
the experiment. These target cells are surrounded by similar cells containing
liquid scintillator without the Gd-loading. The development and characteristics
of these scintillators are reported. In particular, the transparency, light
production and pulse shape discrimination capabilities of the organic liquids
are discussed.Comment: 10 pages, 4 figure
The Ultraluminous X-ray Source in Holmberg IX and its Environment
We present optical observations of an ultraluminous X-ray source (ULX) in
Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray luminosity
of some 10^{40} erg/s. It is located in a huge (400pc x 300pc) ionized nebula
being much larger than normal supernova remnants. From the observed emission
lines (widths and ratios) we find that the structure is due to collisional
excitation by shocks, rather than by photoionization.
We identify the optical counterpart to be a 22.8 mag blue star (M_V=-5.0)
belonging to a small stellar cluster. From isochrone fitting of our
multi-colour photometry we determine a cluster age of 20 to 50 Myr. We also
discovered strong stellar HeII4686 emission (equivalent width of 10 A) which
proves the identification with the X-ray source, and which suggests the
presence of an X-ray heated accretion disc around the putative black hole.Comment: 2 pages, 2 figures, to appear in the proceedings of the IAU Symposium
230, "Populations of High Energy Sources in Galaxies", Dublin, 15-19 Aug 200
Ultraluminous X-ray Sources: Bubbles and Optical Counterparts
Optical studies of ultraluminous X-ray sources (ULX) in nearby galaxies have
turned out to be instrumental in discriminating between various models
including the much advertised intermediate mass black hole hypothesis and
various beaming scenarios. Here we report on ESO VLT and SUBARU observations of
ULX that have revealed the parent stellar clusters with ages of some 60 million
years in two cases. Thus we are able to derive upper limits of about 8 M_sun
for the mass donors in these systems. The optical counterparts are dominated by
X-ray heated accretion disks, and the discovery of the HeII4686 emission line
now allows to derive dynamical masses in these systems. Apparent radial
velocity variations of 300 km/s have been detected in NGC 1313 X-2 which, if
confirmed by further observations, would exclude the presence of IMBH in these
systems.Comment: 4 pages, to appear in the proceedings of IAU Symposium 230,
"Populations of High Energy Sources in Galaxies", Dublin, 15-19 Aug 200
Virtual Location-Based Services: Merging the Physical and Virtual World
Location-based services gained much popularity through providing users with
helpful information with respect to their current location. The search and
recommendation of nearby locations or places, and the navigation to a specific
location are some of the most prominent location-based services. As a recent
trend, virtual location-based services consider webpages or sites associated
with a location as 'virtual locations' that online users can visit in spite of
not being physically present at the location. The presence of links between
virtual locations and the corresponding physical locations (e.g., geo-location
information of a restaurant linked to its website), allows for novel types of
services and applications which constitute virtual location-based services
(VLBS). The quality and potential benefits of such services largely depends on
the existence of websites referring to physical locations. In this paper, we
investigate the usefulness of linking virtual and physical locations. For this,
we analyze the presence and distribution of virtual locations, i.e., websites
referring to places, for two Irish cities. Using simulated tracks based on a
user movement model, we investigate how mobile users move through the Web as
virtual space. Our results show that virtual locations are omnipresent in urban
areas, and that the situation that a user is close to even several such
locations at any time is rather the normal case instead of the exception
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