661 research outputs found
Space Charge Transfer in Hybrid Inorganic/Organic Systems
We discuss density functional theory calculations of hybrid inorganic/organic
systems (HIOS) that explicitly include the global effects of doping (i.e.
position of the Fermi level) and the formation of a space-charge layer. For the
example of tetrafluoro-tetracyanoquinodimethane (F4TCNQ) on the
ZnO(000) surface we show that the adsorption energy and electron
transfer depend strongly on the ZnO doping. The associated work function
changes are large, for which the formation of space-charge layers is the main
driving force. The prominent doping effects are expected to be quite general
for charge-transfer interfaces in HIOS and important for device design
Exploring the Bonding of Large Hydrocarbons on Noble Metals: Diindoperylene on Cu(111), Ag(111), and Au(111)
We present a benchmark study for the adsorption of a large pi-conjugated
organic molecule on different noble metal surfaces, which is based on X-ray
standing wave (XSW) measurements and density functional theory calculations
with van der Waals (vdW) interactions. The bonding distances of
diindenoperylene on Cu(111), Ag(111), and Au(111) surfaces (2.51 A, 3.01 A, and
3.10 A, respectively) determined with the normal incidence XSW technique are
compared with calculations. Excellent agreement with the experimental data,
i.e. deviations less than 0.1 A, is achieved using the Perdew-Burke-Ernzerhof
functional with vdW interactions that include the collective response of
substrate electrons (PBE+vdW^{surf} method). Noteworthy, the calculations show
that the vdW contribution to the adsorption energy increases in the order
Au(111) < Ag(111) < Cu(111).Comment: 6 pages, 4 figures, accepted by Phys. Rev.
A First Search for Cosmogenic Neutrinos with the ARIANNA Hexagonal Radio Array
The ARIANNA experiment seeks to observe the diffuse flux of neutrinos in the
10^8 - 10^10 GeV energy range using a grid of radio detectors at the surface of
the Ross Ice Shelf of Antarctica. The detector measures the coherent Cherenkov
radiation produced at radio frequencies, from about 100 MHz to 1 GHz, by
charged particle showers generated by neutrino interactions in the ice. The
ARIANNA Hexagonal Radio Array (HRA) is being constructed as a prototype for the
full array. During the 2013-14 austral summer, three HRA stations collected
radio data which was wirelessly transmitted off site in nearly real-time. The
performance of these stations is described and a simple analysis to search for
neutrino signals is presented. The analysis employs a set of three cuts that
reject background triggers while preserving 90% of simulated cosmogenic
neutrino triggers. No neutrino candidates are found in the data and a
model-independent 90% confidence level Neyman upper limit is placed on the all
flavor neutrino+antineutrino flux in a sliding decade-wide energy bin. The
limit reaches a minimum of 1.9x10^-23 GeV^-1 cm^-2 s^-1 sr^-1 in the 10^8.5 -
10^9.5 GeV energy bin. Simulations of the performance of the full detector are
also described. The sensitivity of the full ARIANNA experiment is presented and
compared with current neutrino flux models.Comment: 22 pages, 22 figures. Published in Astroparticle Physic
Search for transient optical counterparts to high-energy IceCube neutrinos with Pan-STARRS1
In order to identify the sources of the observed diffuse high-energy neutrino
flux, it is crucial to discover their electromagnetic counterparts. IceCube
began releasing alerts for single high-energy ( TeV) neutrino
detections with sky localisation regions of order 1 deg radius in 2016. We used
Pan-STARRS1 to follow-up five of these alerts during 2016-2017 to search for
any optical transients that may be related to the neutrinos. Typically 10-20
faint ( mag) extragalactic transients are found within the
Pan-STARRS1 footprints and are generally consistent with being unrelated field
supernovae (SNe) and AGN. We looked for unusual properties of the detected
transients, such as temporal coincidence of explosion epoch with the IceCube
timestamp. We found only one transient that had properties worthy of a specific
follow-up. In the Pan-STARRS1 imaging for IceCube-160427A (probability to be of
astrophysical origin of 50 %), we found a SN PS16cgx, located at 10.0'
from the nominal IceCube direction. Spectroscopic observations of PS16cgx
showed that it was an H-poor SN at z = 0.2895. The spectra and light curve
resemble some high-energy Type Ic SNe, raising the possibility of a jet driven
SN with an explosion epoch temporally coincident with the neutrino detection.
However, distinguishing Type Ia and Type Ic SNe at this redshift is notoriously
difficult. Based on all available data we conclude that the transient is more
likely to be a Type Ia with relatively weak SiII absorption and a fairly normal
rest-frame r-band light curve. If, as predicted, there is no high-energy
neutrino emission from Type Ia SNe, then PS16cgx must be a random coincidence,
and unrelated to the IceCube-160427A. We find no other plausible optical
transient for any of the five IceCube events observed down to a 5
limiting magnitude of mag, between 1 day and 25 days after
detection.Comment: 20 pages, 6 figures, accepted to A&
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