518 research outputs found
Ultra-low phase noise all-optical microwave generation setup based on commercial devices
In this paper, we present a very simple design based on commercial devices
for the all-optical generation of ultra-low phase noise microwave signals. A
commercial, fibered femtosecond laser is locked to a laser that is stabilized
to a commercial ULE Fabry-Perot cavity. The 10 GHz microwave signal extracted
from the femtosecond laser output exhibits a single sideband phase noise
at 1 Hz Fourier frequency, at
the level of the best value obtained with such "microwave photonics" laboratory
experiments \cite{Fortier2011}. Close-to-the-carrier ultra-low phase noise
microwave signals will now be available in laboratories outside the frequency
metrology field, opening up new possibilities in various domains.Comment: 8 pages, 3 figures. To be published in Applied Optics, early posting
version available at
http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=23114
Effects of Delivering Guanidinoacetic Acid or Its Prodrug to the Neural Tissue: Possible Relevance for Creatine Transporter Deficiency
The creatine precursor guanidinoacetate (GAA) was used as a dietary supplement in humans with no adverse events. Nevertheless, it has been suggested that GAA is epileptogenic or toxic to the nervous system. However, increased GAA content in rodents affected by guanidinoacetate methyltransferase (GAMT) deficiency might be responsible for their spared muscle function. Given these conflicting data, and lacking experimental evidence, we investigated whether GAA affected synaptic transmission in brain hippocampal slices. Incubation with 11.5 \ub5M GAA (the highest concentration in the cerebrospinal fluid of GAMT-deficient patients) did not change the postsynaptic compound action potential. Even 1 or 2 mM had no effect, while 4 mM caused a reversible decrease in the potential. Guanidinoacetate increased creatine and phosphocreatine, but not after blocking the creatine transporter (also used by GAA). In an attempt to allow the brain delivery of GAA when there was a creatine transporter deficiency, we synthesized diacetyl guanidinoacetic acid ethyl ester (diacetyl-GAAE), a lipophilic derivative. In brain slices, 0.1 mM did not cause electrophysiological changes and improved tissue viability after blockage of the creatine transporter. However, diacetyl-GAAE did not increase creatine nor phosphocreatine in brain slices after blockage of the creatine transporter. We conclude that: (1) upon acute administration, GAA is neither epileptogenic nor neurotoxic; (2) Diacetyl-GAAE improves tissue viability after blockage of the creatine transporter but not through an increase in creatine or phosphocreatine. Diacetyl-GAAE might give rise to a GAA\u2013phosphoGAA system that vicariates the missing creatine\u2013phosphocreatine system. Our in vitro data show that GAA supplementation may be safe in the short term, and that a lipophilic GAA prodrug may be useful in creatine transporter deficiency
Creatine salts provide neuroprotection even after partial impairment of the creatine transporter
open6Creatine, a compound that is critical for energy metabolism of nervous cells, crosses the blood-brain barrier (BBB) and the neuronal plasma membrane with difficulty, and only using its specific transporter. In the hereditary condition where the creatine transporter is defective (creatine transporter deficiency) there is no creatine in the brain, and administration of creatine is useless lacking the transporter. The disease is severe and incurable. Creatine-derived molecules that could cross BBB and plasma membrane independently of the transporter might be useful to cure this condition. Moreover, such molecules could be useful also in stroke and other brain ischemic conditions. In this paper, we investigated three creatine salts, creatine ascorbate, creatine gluconate and creatine glucose. Of these, creatine glucose was ineffective after transporter block with guanidine acetic acid (GPA) administration. Creatine ascorbate was not superior to creatine in increasing tissue creatine and phosphocreatine content after transporter impairment, however even after such impairment it delayed synaptic failure during anoxia. Finally, creatine gluconate was superior to creatine in increasing tissue content of creatine after transporter block and slowed down PS disappearance during anoxia, an effect that creatine did not have. These findings suggest that coupling creatine to molecules having a specific transporter may be a useful strategy in creatine transporter deficiency. In particular, creatine ascorbate has effects comparable to those of creatine in normal conditions, while being superior to it under conditions of missing or impaired creatine transporter.openAdriano, E; Garbati, P; Salis, A; Damonte, G; Millo, E; Balestrino, MAdriano, ENRICO GIOVANNI; Garbati, Patrizia; Salis, Annalisa; Damonte, Gianluca; Millo, Enrico; Balestrino, Maurizi
Scanning tunneling spectroscopy of SmFeAsO0.85: Possible evidence for d-wave order parameter symmetry
We report a scanning tunneling spectroscopy investigation of polycrystalline
SmFeAsO0.85 having a superconducting transition at 52 K. On large regions of
the sample surface the tunneling spectra exhibited V-shaped gap structures with
no coherence peaks, indicating degraded surface properties. In some regions,
however, the coherence peaks were clearly observed, and the V-shaped gaps could
be fit to the theory of tunneling into a d-wave superconductor, yielding gap
values between 8 to 8.5 meV, corresponding to the ratio 2D/KTc ~ 3.55 - 3.8,
which is slightly above the BCS weak-coupling prediction. In other regions the
spectra exhibited zero-bias conductance peaks, consistent with a d-wave order
parameter symmetry
Enhancement of the superconducting transition temperature in La2-xSrxCuO4 bilayers: Role of pairing and phase stiffness
The superconducting transition temperature, Tc, of bilayers comprising
underdoped La2-xSrxCuO4 films capped by a thin heavily overdoped metallic
La1.65Sr0.35CuO4 layer, is found to increase with respect to Tc of the bare
underdoped films. The highest Tc is achieved for x = 0.12, close to the
'anomalous' 1/8 doping level, and exceeds that of the optimally-doped bare
film. Our data suggest that the enhanced superconductivity is confined to the
interface between the layers. We attribute the effect to a combination of the
high pairing scale in the underdoped layer with an enhanced phase stiffness
induced by the overdoped film.Comment: Published versio
Ultrastable lasers based on vibration insensitive cavities
We present two ultra-stable lasers based on two vibration insensitive cavity
designs, one with vertical optical axis geometry, the other horizontal.
Ultra-stable cavities are constructed with fused silica mirror substrates,
shown to decrease the thermal noise limit, in order to improve the frequency
stability over previous designs. Vibration sensitivity components measured are
equal to or better than 1.5e-11 per m.s^-2 for each spatial direction, which
shows significant improvement over previous studies. We have tested the very
low dependence on the position of the cavity support points, in order to
establish that our designs eliminate the need for fine tuning to achieve
extremely low vibration sensitivity. Relative frequency measurements show that
at least one of the stabilized lasers has a stability better than 5.6e-16 at 1
second, which is the best result obtained for this length of cavity.Comment: 8 pages 12 figure
Ultra-low noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock
We demonstrate the use of a fiber-based femtosecond laser locked onto an
ultra-stable optical cavity to generate a low-noise microwave reference signal.
Comparison with both a liquid Helium cryogenic sapphire oscillator (CSO) and a
Ti:Sapphire-based optical frequency comb system exhibit a stability about
between 1 s and 10 s. The microwave signal from the fiber
system is used to perform Ramsey spectroscopy in a state-of-the-art Cesium
fountain clock. The resulting clock system is compared to the CSO and exhibits
a stability of . Our continuously operated
fiber-based system therefore demonstrates its potential to replace the CSO for
atomic clocks with high stability in both the optical and microwave domain,
most particularly for operational primary frequency standards.Comment: 3 pages, 3 figure
Correlation of tunneling spectra with surface nano-morphology and doping in thin YBa2Cu3O7-delta films
Tunneling spectra measured on thin epitaxial YBa2Cu3O7-delta films are found
to exhibit strong spatial variations, showing U and V-shaped gaps as well as
zero bias conductance peaks typical of a d-wave superconductor. A full
correspondence is found between the tunneling spectra and the surface
morphology down to a level of a unit-cell step. Splitting of the zero bias
conductance peak is seen in optimally-doped and overdoped films, but not in the
underdoped ones, suggesting that there is no transition to a state of broken
time reversal symmetry in the underdoped regimeComment: accepted to ep
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