73,751 research outputs found
Low frequency electrical noise across contacts between a normal conductor and superconducting bulk YBa2Cu3O7
Virtually every device that makes use of the new ceramic superconductors will need normal conductor to supercondutor contacts. The current-voltage and electrical noise characteristics of these contacts could be become important design considerations. I-V and low frequency electrical noise measurements are presented on contacts between a normal conductor and superconducting polycrystalline YBa2Cu3O7. The contacts were formed by first sputtering gold palladium pads onto the surface of the bulk superconductor and then using silver epoxy to attach a wire(s) to each pad. Voltage across the contacts was found for small current densities. The voltage spectral density, S sub v(f), a quanity often used to characterize electrical noise, very closely followed an empirical relationship given by, S sub v(f) = C(VR)sq/f, where V is the DC voltage across the contact, R is the contact resistance, F is frequency, and C is a contant found to be 2 x 10(exp -10)/Omega sq at 78 K. This relationship was found to be independent of contact area, contact geometry, sample fabrication technique, and sample density
Low frequency electrical noise across contacts between a normal conductor and superconducting bulk YBa2Cu3O7
Virtually every device that makes use of the new ceramic superconductors will need normal conductor to superconductor contacts. The current-voltage and electrical noise characteristics of these contacts could become important design considerations. I-V and low frequency electrical noise measurements are presented on contacts between a normal conductor and superconducting polycrystalline YBa2Cu3O7. The contacts were formed by first sputtering gold palladium pads onto the surface of the bulk superconductor and then using silver epoxy to attach a wire(s) to each pad. Voltage across the contacts was found for small current densities. The voltage spectral density, S sub v(f), a quantity often used to characterize electrical noise, very closely followed an empirical relationship given by S sub v(f) = C(VR)sq/f, where V is the DC voltage across the contact, R is the contact resistance, F is frequency, and C is a contant found to be 2 x 10(exp -10)/Omega sq at 78 K. This relationship was found to be independent of contact area, contact geometry, sample fabrication technique, and sample density
Gravitino or Axino Dark Matter with Reheat Temperature as high as GeV
A new scheme for lightest supersymmetric particle (LSP) dark matter is
introduced and studied in theories of TeV supersymmetry with a QCD axion, ,
and a high reheat temperature after inflation, . A large overproduction of
axinos () and gravitinos () from scattering at , and
from freeze-in at the TeV scale, is diluted by the late decay of a saxion
condensate that arises from inflation. The two lightest superpartners are
, with mass of order the TeV scale, and with mass
anywhere between the keV and TeV scales, depending on the mediation
scale of supersymmetry breaking. Dark matter contains both warm and cold
components: for LSP the warm component arises from , while for LSP the warm component arises
from . The free-streaming scale for the warm
component is predicted to be of order 1 Mpc (and independent of in
the case of LSP). can be as high as GeV, for any
value of , solving the gravitino problem. The PQ symmetry breaking
scale depends on and and can be anywhere in the range
GeV. Detailed predictions are made for the lifetime of
the neutralino LOSP decaying to and ,
which is in the range of m over much of parameter space. For an
axion misalignment angle of order unity, the axion contribution to dark matter
is sub-dominant, except when approaches GeV.Comment: 43 pages, 16 figure
The Weak Scale from BBN
The measured values of the weak scale, , and the first generation masses,
, are simultaneously explained in the multiverse, with all these
parameters scanning independently. At the same time, several remarkable
coincidences are understood. Small variations in these parameters away from
their measured values lead to the instability of hydrogen, the instability of
heavy nuclei, and either a hydrogen or a helium dominated universe from Big
Bang Nucleosynthesis. In the 4d parameter space of ,
catastrophic boundaries are reached by separately increasing each parameter
above its measured value by a factor of , respectively.
The fine-tuning problem of the weak scale in the Standard Model is solved: as
is increased beyond the observed value, it is impossible to maintain a
significant cosmological hydrogen abundance for any values of that
yield both hydrogen and heavy nuclei stability.
For very large values of a new regime is entered where weak interactions
freeze out before the QCD phase transition. The helium abundance becomes
independent of and is determined by the cosmic baryon and lepton
asymmetries. To maintain our explanation of from the anthropic cost of
helium dominance then requires universes with such large to be rare in the
multiverse. Implications of this are explored, including the possibility that
new physics below 10 TeV cuts off the fine-tuning in .Comment: 26 pages plus appendix, 13 figure
Cost-effective aperture arrays for SKA Phase 1: single or dual-band?
An important design decision for the first phase of the Square Kilometre
Array is whether the low frequency component (SKA1-low) should be implemented
as a single or dual-band aperture array; that is, using one or two antenna
element designs to observe the 70-450 MHz frequency band. This memo uses an
elementary parametric analysis to make a quantitative, first-order cost
comparison of representative implementations of a single and dual-band system,
chosen for comparable performance characteristics. A direct comparison of the
SKA1-low station costs reveals that those costs are similar, although the
uncertainties are high. The cost impact on the broader telescope system varies:
the deployment and site preparation costs are higher for the dual-band array,
but the digital signal processing costs are higher for the single-band array.
This parametric analysis also shows that a first stage of analogue tile
beamforming, as opposed to only station-level, all-digital beamforming, has the
potential to significantly reduce the cost of the SKA1-low stations. However,
tile beamforming can limit flexibility and performance, principally in terms of
reducing accessible field of view. We examine the cost impacts in the context
of scientific performance, for which the spacing and intra-station layout of
the antenna elements are important derived parameters. We discuss the
implications of the many possible intra-station signal transport and processing
architectures and consider areas where future work could improve the accuracy
of SKA1-low costing.Comment: 64 pages, 23 figures, submitted to the SKA Memo serie
The strong nonlinear interaction of Tollmien-Schlichting waves and Taylor-Goertler vortices in curved channel flow
Viscous fluid flows with curved streamlines can support both centrifugal and viscous traveling wave instabilities. Here the interaction of these instabilities in the context of the fully developed flow in a curved channel is discussed. The viscous (Tollmein-Schlichting) instability is described asymptotically at high Reynolds numbers and it is found that it can induce a Taylor-Goertler flow even at extremely small amplitudes. In this interaction, the Tollmein-Schlichting wave can drive a vortex state with wavelength either comparable with the channel width or the wavelength of lower branch viscous modes. The nonlinear equations which describe these interactions are solved for nonlinear equilibrium states
Comparison of Material Properties and Microstructure of Specimens Built Using the 3D Systems Vanguard HS and Vanguard HiQ+HSSLS Systems
The HiQ upgrade to the 3D Systems Vanguard selective laser sintering (SLS) machine incorporates a revised thermal calibration system and new control software. The paper compares
the tensile modulus, tensile strength, elongation at break, flexural modulus, Izod impact resistance and microstructure of two batteries of standard specimens built from recycled Duraform
PA (Nylon 12). The first set is built on a Vanguard HS system and the second on the same
system with the HiQ upgrade installed. The upgrade reduces user intervention, decreases total build time and improves surface finish. However, using the default processing parameters,
tensile, flexure and impact properties are all found to decline after the upgrade is installed.Mechanical Engineerin
Recurrence rates for SIDS - the importance of risk stratification
Objective:
To investigate the importance of stratification by risk factors in computing the probability of a second SIDS in a family.
Design: Simulation Study
Background:
The fact that a baby dies suddenly and unexpectedly means that there is a raised probability that the baby’s family have risk factors associated with Sudden Infant Death Syndrome (SIDS). Thus one cannot consider the risk of a subsequent death to be that of the general population. The Confidential Enquiry into Stillbirths and Deaths in Infancy (CESDI)6 identified three major social risk factors: smoking, age1, and unemployed/unwaged as major risk factors. It gave estimates of risk for families with different numbers of these risk factors. We investigate whether it is reasonable to assume that, conditional on these risk factors, the risk of a second event is independent of the risk of the first and as a consequence one can square the risks to get the risk of two SIDS in a family. We have used CESDI data to estimate the probability of a second SID in a family under different plausible scenarios of the prevalence of the risk factors. We have applied the model to make predictions in the Care of Next Infant (CONI) study7.
Results:
The model gave plausible predictions. The CONI study observed 18 second SIDS. Our model predicted 14 (95% prediction interval 7 to 21).
Conclusion:
When considering the risk of a subsequent SIDS in a family one should always take into account the known risk factors. If all risks have been identified, then conditional on these risks, the risk of two events is the product of the individual risks However for a given family we cannot quantify the magnitude of the increased risk because of other possible risk factors not accounted for in the model
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