3,338 research outputs found
Stress relief as the driving force for self-assembled Bi nanolines
Stress resulting from mismatch between a substrate and an adsorbed material
has often been thought to be the driving force for the self-assembly of
nanoscale structures. Bi nanolines self-assemble on Si(001), and are remarkable
for their straightness and length -- they are often more than 400 nm long, and
a kink in a nanoline has never been observed. Through electronic structure
calculations, we have found an energetically favourable structure for these
nanolines that agrees with our scanning tunneling microscopy and photoemission
experiments; the structure has an extremely unusual subsurface structure,
comprising a double core of 7-membered rings of silicon. Our proposed structure
explains all the observed features of the nanolines, and shows that surface
stress resulting from the mismatch between the Bi and the Si substrate are
responsible for their self-assembly. This has wider implications for the
controlled growth of nanostructures on semiconductor surfaces.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Origin of intrinsic dark count in superconducting nanowire single-photon detectors
The origin of the decoherence in superconducting nanowire single-photon
detectors, the so-called dark count, was investigated. We measured the
direct-current characteristics and bias-current dependencies of the dark count
rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the
results by theoretical models of thermal fluctuations of vortices. Our results
indicate that the current-assisted unbinding of vortex-antivortex pairs is the
dominant origin of the dark count.Comment: 10 pages, 2 figure
Proton Spin Relaxation Induced by Quantum Tunneling in Fe8 Molecular Nanomagnet
The spin-lattice relaxation rate and NMR spectra of H in
single crystal molecular magnets of Fe8 have been measured down to 15 mK. The
relaxation rate shows a strong temperature dependence down to 400
mK. The relaxation is well explained in terms of the thermal transition of the
iron state between the discreet energy levels of the total spin S=10. The
relaxation time becomes temperature independent below 300 mK and is
longer than 100 s. In this temperature region stepwise recovery of the
H-NMR signal after saturation was observed depending on the return field of
the sweep field. This phenomenon is attributed to resonant quantum tunneling at
the fields where levels cross and is discussed in terms of the Landau-Zener
transition.Comment: 13 pages, 5 figure
Picosecond timing of Microwave Cherenkov Impulses from High-Energy Particle Showers Using Dielectric-loaded Waveguides
We report on the first measurements of coherent microwave impulses from
high-energy particle-induced electromagnetic showers generated via the Askaryan
effect in a dielectric-loaded waveguide. Bunches of 12.16 GeV electrons with
total bunch energy of GeV were pre-showered in tungsten, and
then measured with WR-51 rectangular (12.6 mm by 6.3 mm) waveguide elements
loaded with solid alumina () bars. In the 5-8 GHz
single-mode band determined by the presence of the dielectric in the waveguide,
we observed band-limited microwave impulses with amplitude proportional to
bunch energy. Signals in different waveguide elements measuring the same shower
were used to estimate relative time differences with 2.3 picosecond precision.
These measurements establish a basis for using arrays of alumina-loaded
waveguide elements, with exceptional radiation hardness, as very high precision
timing planes for high-energy physics detectors.Comment: 16 pages, 15 figure
The Emerging QCD Frontier: The Electron Ion Collider
The self-interactions of gluons determine all the unique features of QCD and
lead to a dominant abundance of gluons inside matter already at moderate .
Despite their dominant role, the properties of gluons remain largely
unexplored. Tantalizing hints of saturated gluon densities have been found in
+p collisions at HERA, and in d+Au and Au+Au collisions at RHIC. Saturation
physics will have a profound influence on heavy-ion collisions at the LHC. But
unveiling the collective behavior of dense assemblies of gluons under
conditions where their self-interactions dominate will require an Electron-Ion
Collider (EIC): a new facility with capabilities well beyond those In this
paper I outline the compelling physics case for +A collisions at an EIC and
discuss briefly the status of machine design concepts. of any existing
accelerator.Comment: 11 pages, 9 figures, prepared for 20th International Conference on
Ultra-Relativistic Nucleus-Nucleus Collisions: Quark Matter 2008 (QM2008),
Jaipur, India, 4-10 Feb. 200
Long-distance entanglement-based quantum key distribution over optical fiber
We report the first entanglement-based quantum key distribution (QKD) experiment over a 100-km optical fiber. We used superconducting single photon detectors based on NbN nanowires that provide high-speed single photon detection for the 1.5-µm telecom band, an efficient entangled photon pair source that consists of a fiber coupled periodically poled lithium niobate waveguide and ultra low loss filters, and planar lightwave circuit Mach-Zehnder interferometers (MZIs) with ultra stable operation. These characteristics enabled us to perform an entanglement-based QKD experiment over a 100-km optical fiber. In the experiment, which lasted approximately 8 hours, we successfully generated a 16 kbit sifted key with a quantum bit error rate of 6.9 % at a rate of 0.59 bits per second, from which we were able to distill a 3.9 kbit secure key
'We have dealt with this situation randomly': A peer ethnographic approach with teachers in refugee settings in the age of COVID-19
This study examines the challenges faced by English language teachers working in non-formal tertiary education programs in Jordan’s refugee settings. It focuses specifically on their experiences as they transferred their teaching online during the COVID-19 pandemic. It draws from a post-digital theoretical perspective in which the digital, physical, and social are all interconnected within complex educational ecologies. Working closely with five teachers as peer researchers between April and July 2021, the study utilises the peer ethnographic evaluation research methodology. This paper draws from interview data generated during the study and uses four vignettes to synthesise key findings. The vignettes illustrate the amplified disadvantage experienced by teachers in refugee settings during the pandemic due to pre-existing disparities and emerging digital inequalities. The paper directs attention to human-technology relationships and the ways in which digital technologies are embedded in socio-technical networks, and generate, and potentially worsen, various disadvantages
Development of High Precision Timing Counter Based on Plastic Scintillator with SiPM Readout
High-time-resolution counters based on plastic scintillator with silicon
photomultiplier (SiPM) readout have been developed for applications to high
energy physics experiments for which relatively large-sized counters are
required. We have studied counter sizes up to mm^3 with
series connection of multiple SiPMs to increase the sensitive area and thus
achieve better time resolution. A readout scheme with analog shaping and
digital waveform analysis is optimized to achieve the highest time resolution.
The timing performance is measured using electrons from a Sr-90 radioactive
source, comparing different scintillators, counter dimensions, and types of
near-ultraviolet sensitive SiPMs. As a result, a resolution of ps at 1 MeV energy deposition is obtained for counter size mm^3 with three SiPMs ( mm^2 each) at each end of the
scintillator. The time resolution improves with the number of photons detected
by the SiPMs. The SiPMs from Hamamatsu Photonics give the best time resolution
because of their high photon detection efficiency in the near-ultraviolet
region. Further improvement is possible by increasing the number of SiPMs
attached to the scintillator.Comment: 11 pages, 17 figures, accepted by IEEE Trans. Nucl. Sc
Implantation of a Cushioning Injectable Implant Using Needle Arthroscopy in the Foot and Ankle and First Carpometacarpal Joint
Injectable implants constitute a newly developed treatment class in the battle against osteoarthritis. They consist of water-formulated supramolecular polymer, coming from a new class of resorbable biomedical materials, and are implanted in encapsulated joints in a liquid form, where they solidify to form a tough, elastic, and cushioning layer between the joint surfaces. To resort any effect, intra-articular delivery should be guaranteed, and the implant should be distributed throughout the entire joint space. Traditional implantation techniques do not seem to suffice for this new implant class, being either imprecise (traditional injection) or overly invasive (open procedures and traditional arthroscopic surgery). We describe a needle arthroscopic implantation technique to reap the benefits of both worlds, ensuring precise implant delivery while avoiding unnecessarily invasive procedures. This study depicts our needle arthroscopic technique for implantation of injectable implants in the ankle, first metatarsophalangeal joint, and first carpometacarpal joint.</p
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