359 research outputs found
Comment: Superconducting transition in Nb nanowires fabricated using focused ion beam
In a recent paper Tettamanzi et al (2009 Nanotechnology \bf{20} 465302)
describe the fabrication of superconducting Nb nanowires using a focused ion
beam. They interpret their conductivity data in the framework of thermal and
quantum phase slips below . In the following we will argue that their
analysis is inappropriate and incomplete, leading to contradictory results.
Instead, we propose an interpretation of the data within a SN proximity model.Comment: 3 pages, 1 figure accepted in Nanotechnolog
Detection of OD towards the low-mass protostar IRAS16293-2422
Although water is an essential and widespread molecule in star-forming
regions, its chemical formation pathways are still not very well constrained.
Observing the level of deuterium fractionation of OH, a radical involved in the
water chemical network, is a promising way to infer its chemical origin. We aim
at understanding the formation mechanisms of water by investigating the origin
of its deuterium fractionation. This can be achieved by observing the abundance
of OD towards the low-mass protostar IRAS16293-2422, where the HDO distribution
is already known. Using the GREAT receiver on board SOFIA, we observed the
ground-state OD transition at 1391.5 GHz towards the low-mass protostar
IRAS16293-2422. We also present the detection of the HDO 111-000 line using the
APEX telescope. We compare the OD/HDO abundance ratio inferred from these
observations with the predictions of chemical models. The OD line is detected
in absorption towards the source continuum. This is the first detection of OD
outside the solar system. The SOFIA observation, coupled to the observation of
the HDO 111-000 line, provides an estimate of the abundance ratio OD/HDO ~
17-90 in the gas where the absorption takes place. This value is fairly high
compared with model predictions. This may be reconciled if reprocessing in the
gas by means of the dissociative recombination of H2DO+ further fractionates OH
with respect to water. The present observation demonstrates the capability of
the SOFIA/GREAT instrument to detect the ground transition of OD towards
star-forming regions in a frequency range that was not accessible before.
Dissociative recombination of H2DO+ may play an important role in setting a
high OD abundance. Measuring the branching ratios of this reaction in the
laboratory will be of great value for chemical models.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in A&A
SOFIA/GREAT special issu
Probability of the resistive state formation caused by absorption of a single-photon in current-carrying superconducting nano-strips
We have studied supercurrent-assisted formation of the resistive state in
nano-structured Nb and NbN superconducting films after absorption of a single
photon. In amorphous narrow NbN strips the probability of the resistive state
formation has a pronounced spectral cut-off. The corresponding threshold photon
energy decreases with the bias current. Analysis of the experimental data in
the framework of the generalized hot-spot model suggests that the quantum yield
for near-infrared photons increases faster than the photon nergy. Relaxation of
the resistive state depends on the photon energy making the phenomenon feasible
for the development of energy resolving single-photon detectors.Comment: 9 pages, 9 figures, submitted to Eur. Phys. Journa
Terahertz optically pumped silicon lasers
Stimulated terahertz (THz) emission from silicon single crystals doped by group-V donors has been obtained by optical excitation with pulsed infrared lasers. Pumping by a conventional TEA CO2 laser results in lasing on discrete lines between 1.3 and 7 THz (see figure). Laser thresholds can be as low as 10 kW/cm2. They depend on the donors species and the laser mechanism. Intracentre population inversion is realized between particular excited states which are large-spaced due to the chemical shift of the donor binding energy. The lifetime of an electron in an excited state (up to ~70 ps) is determined by the efficiency of phonon-assisted nonradiative relaxation. Optical excitation by the emission of a frequency-tunable free electron laser results in two different types of lasing. At relatively low pump intensities (~1 kW/cm2) the intracentre mechanism of lasing is dominating. At pump intensities above ~100 kW/cm2 stimulated scattering of pump photons on transverse acoustic intervalley phonons can occur in the vicinity of an impurity atom. This results in laser emission in the frequency range from 4.6 to 5.8 THz. In this case the laser frequency can be tuned proportionally to the pump frequency
Magnetoconductance and photoresponse properties of disordered NbTiN films
We report on the experimental study of phonon properties and electron-phonon scattering in thin superconducting NbTiN films, which are intensively exploited in various applications. Studied NbTiN films with sub-10-nm thicknesses are disordered with respect to electron transport, the Ioffe-Regel parameter of kFle=2.5–3.0 (kF is the Fermi wave vector, and le is the electron mean free path), the inelastic electron-phonon interaction, and the product qTle≪1 (qT is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derive the inelastic electron-phonon scattering rate 1/τe-ph and determine sound velocities and phonon heat capacities. In the temperature range from 12 to 20 K, the scattering rate varies with temperature as 1/τe-ph∝T3.45±0.05; its value extrapolated to 10 K amounts to approximately 1/16 ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we find a systematic reduction of the sound velocity in all these films by about 50% compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology
SARS-CoV-2 and Guillain-Barré syndrome: AIDP variant with a favourable outcome.
The spectrum of COVID-19, caused by severe acute respiratory syndrome coronavirus 2 infection (SARS-CoV-2), includes different neurologic manifestations of the central and peripheral nervous system.
From March through April 2020, in two university hospitals located in western Switzerland, we examined three patients with Guillain-Barré syndrome (GBS) following SARS-CoV-2.
These cases were characterized by a primary demyelinating electrophysiological pattern (Acute inflammatory demyelinating polyneuropathy or AIDP) and a less severe disease course compared to recently published case series. Clinical improvement was observed in all patients at week five. One patient was discharged from hospital after full recovery with persistence of minor neurological signs (areflexia). Two of the three patients remained hospitalized: one was able to walk and the other could stand up with assistance.
We report three cases of typical GBS (AIDP) occurring after SARS-CoV-2 infection and presenting with a favourable clinical course. Given the interval between COVID-19-related symptoms and neurological manifestations (mean of 15 days) we postulate a secondary immune-mediated mechanism rather than direct viral damage
Characteristics of superconducting tungsten silicide WxSi1-x for single photon detection
Superconducting properties of three series of amorphous WxSi1-x films with
different thickness and stoichiometry were investigated by dc transport
measurements in a magnetic field up to 9 T. These amorphous WxSi1-x films were
deposited by magnetron co-sputtering of the elemental source targets onto
silicon substrates at room temperature and patterned in form of bridges by
optical lithography and reactive ion etching. Analysis of the data on
magnetoconductivity allowed us to extract the critical temperature,
superconducting coherence length, magnetic penetration depth, and diffusion
coefficient of electrons in the normal state as a function of film thickness
for each stoichiometry. Two basic time constants were derived from transport
and time-resolving measurements. A dynamic process of the formation of a
hot-spot was analyzed in the framework of a diffusion-based vortex-entry model.
We used the two stage diffusion approach and defined a hotspot size by assuming
that the quasi-particles and normal-state electrons have the equal diffusion
constant. Our findings are consistent with the most recent results on a
hot-spot relaxation time in the WxSi1-x superconducting nanowire single-photon
detector. In the 5 nm thick W0.85Si0.15 film the hot-spot has a diameter of 105
nm at the peak of the number of non-equilibrium quasi-particles
Heterodyne receiver at 2.5 THz with quantum cascade laser and hot electron bolometric mixer
Quantum cascade lasers (QCLs) operating at 2.5 THz have been used for gas phase spectroscopy and as local oscillator in a heterodyne receiver. One QCL has a Fabry-Perot resonator while the other has a distributed feedback resonator. The linewidth and frequency tunability of both QCLs have been investigated by either mixing two modes of the QCL or by mixing the emission from the QCL with the emission from a 2.5 THz gas laser. The frequency tunability as well as the linewidth is sufficient for Doppler limited spectroscopy of methanol gas. The QCLs have been used successfully as local oscillators in a heterodyne receiver. Noise temperature measurements with a hot electron bolometer and a QCL yielded the same result as with a gas laser as local oscillator
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