3,480 research outputs found
High-sensitivity tool for studying phonon related mechanical losses in low loss materials
Fundamental mechanical loss mechanisms exist even in very pure materials, for
instance, due to the interactions of excited acoustic waves with thermal
phonons. A reduction of these losses in a certain frequency range is desired in
high precision instruments like gravitational wave detectors. Systematic
analyses of the mechanical losses in those low loss materials are essential for
this aim, performed in a highly sensitive experimental set-up. Our novel method
of mechanical spectroscopy, cryogenic resonant acoustic spectroscopy of bulk
materials (CRA spectroscopy), is well suited to systematically determine losses
at the resonant frequencies of the samples of less than 10^(-9) in the wide
temperature range from 5 to 300 K. A high precision set-up in a specially built
cryostat allows contactless excitation and readout of the oscillations of the
sample. The experimental set-up and measuring procedure are described.
Limitations to our experiment due to external loss mechanisms are analysed. The
influence of the suspension system as well as the sample preparation is
explained.Comment: 4 pages, 3 figures, proceedings of PHONONS07, submitted to Journal of
Physics: Conference Serie
High transport currents in mechanically reinforced MgB2 wires
We prepared and characterized monofilamentary MgB2 wires with a mechanically
reinforced composite sheath of Ta(Nb)/Cu/steel, which leads to dense filaments
and correspondingly high transport currents up to Jc = 10^5 A/cm^2 at 4.2 K,
self field. The reproducibility of the measured transport currents was
excellent and not depending on the wire diameter. Using different precursors,
commercial reacted powder or an unreacted Mg/B powder mixture, a strong
influence on the pinning behaviour and the irreversibility field was observed.
The critical transport current density showed a nearly linear temperature
dependency for all wires being still 52 kA/cm^2 at 20 K and 23 kA/cm^2 at 30 K.
Detailed data for Jc(B,T) and Tc(B) were measured.Comment: 21 pages, 13 figures, revised version, to be published in Supercond.
Sci. Techno
The Biot-Savart operator and electrodynamics on subdomains of the three-sphere
We study steady-state magnetic fields in the geometric setting of positive
curvature on subdomains of the three-dimensional sphere. By generalizing the
Biot-Savart law to an integral operator BS acting on all vector fields, we show
that electrodynamics in such a setting behaves rather similarly to Euclidean
electrodynamics. For instance, for current J and magnetic field BS(J), we show
that Maxwell's equations naturally hold. In all instances, the formulas we give
are geometrically meaningful: they are preserved by orientation-preserving
isometries of the three-sphere.
This article describes several properties of BS: we show it is self-adjoint,
bounded, and extends to a compact operator on a Hilbert space. For vector
fields that act like currents, we prove the curl operator is a left inverse to
BS; thus the Biot-Savart operator is important in the study of curl
eigenvalues, with applications to energy-minimization problems in geometry and
physics. We conclude with two examples, which indicate our bounds are typically
within an order of magnitude of being sharp.Comment: 24 pages (was 28 pages) Revised to include a new introduction, a
detailed example, and results about helicity; other changes for readabilit
Is the Unitarity of the quark-mixing-CKM-matrix violated in neutron -decay?
We report on a new measurement of neutron -decay asymmetry. From the
result \linebreak = -0.1189(7), we derive the ratio of the axial vector
to the vector coupling constant = = -1.2739(19). When
included in the world average for the neutron lifetime = 885.7(7)s, this
gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix . With this value and the Particle Data Group values for and
, we find a deviation from the unitarity condition for the first row of
the CKM matrix of = 0.0083(28), which is 3.0 times the stated error
Portable inhalation systemfor a dosed insulin supply
Интенсивная инсулинотерапия необходима для контроля состояния пациентов с диабетом.Несмотря на постоянное усовершенствование инсулинотерапии, все ещ? существует проблема неудобства режимов многократных инъекций инсулина. Целью данной работы является создание системы, позволяющей осуществлять ингаляцию инсулина.Intensive insulin therapy is necessary for the control of a condition diabetic patients. Despite the constant improvement of insulin therapy, there is still the problem of discomfort repeated regimes of insulin injections. The objective of this work is to create a system that allows the inhalation of insulin
Insensitivity of visual short-term memory to irrelevant visual information
Several authors have hypothesised that visuo-spatial working memory is functionally analogous to verbal working memory. Irrelevant background speech impairs verbal short-term memory. We investigated whether irrelevant visual information has an analogous effect on visual short-term memory, using a dynamic visual noise (DVN) technique known to disrupt visual imagery (Quinn & McConnell, 1996a). Experiment 1 replicated the effect of DVN on pegword imagery. Experiments 2 and 3 showed no effect of DVN on recall of static matrix patterns, despite a significant effect of a concurrent spatial tapping task. Experiment 4 showed no effect of DVN on encoding or maintenance of arrays of matrix patterns, despite testing memory by a recognition procedure to encourage visual rather than spatial processing. Serial position curves showed a one-item recency effect typical of visual short-term memory. Experiment 5 showed no effect of DVN on short-term recognition of Chinese characters, despite effects of visual similarity and a concurrent colour memory task that confirmed visual processing of the characters. We conclude that irrelevant visual noise does not impair visual short-term memory. Visual working memory may not be functionally analogous to verbal working memory, and different cognitive processes may underlie visual short-term memory and visual imagery
Experimental analysis of sample-based maps for long-term SLAM
This paper presents a system for long-term SLAM (simultaneous localization and mapping) by mobile service robots and its experimental evaluation in a real dynamic environment. To deal with the stability-plasticity dilemma (the trade-off between adaptation to new patterns and preservation of old patterns), the environment is represented at multiple timescales simultaneously (5 in our experiments). A sample-based representation is
proposed, where older memories fade at different rates depending on the timescale, and robust statistics are used to interpret the samples. The dynamics of this representation are analysed in a five week experiment, measuring the relative influence of short- and long-term memories over time, and further demonstrating the robustness of the approach
The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
We use the Gaussian-fit results of Paper I to investigate the properties of
interstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNM
and CNM) are physically distinct components. The CNM spin temperature histogram
peaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volume
filling fraction of about 0.50 for the WNM; this value is very rough. The
upper-limit WNM temperatures determined from line width range upward from about
500 K; a minimum of about 48% of the WNM lies in the thermally unstable region
500 to 5000 K. The WNM is a prominent constituent of the interstellar medium
and its properties depend on many factors, requiring global models that include
all relevant energy sources, of which there are many. We use Principal
Components Analysis, together with a form of least squares fitting that
accounts for errors in both the independent and dependent parameters, to
discuss the relationships among the four CNM Gaussian parameters. The spin
temperature T_s and column density N(HI) are, approximately, the two most
important eigenvectors; as such, they are sufficient, convenient, and
physically meaningful primary parameters for describing CNM clouds. The Mach
number of internal macroscopic motions for CNM clouds is typically 2.5, but
there are wide variations. We discuss the historical tau-T_s relationship in
some detail and show that it has little physical meaning. We discuss CNM
morphology using the CNM pressure known from UV stellar absorption lines.
Knowing the pressure allows us to show that CNM structures cannot be isotropic
but instead are sheetlike, with length-to-thickness aspect ratios ranging up to
about 280. We present large-scale maps of two regions where CNM lies in very
large ``blobby sheets''.Comment: Revised submission to Ap.J. Changes include: (1) correction of
turbulent Mach number in equation 16 and figure 12; the new typical value is
1.3 versus the old, incorrect value 2.5. (2) smaller typeface for the
astro-ph version to conserve paper. 60 pages, 16 figure
Mechanical losses in low loss materials studied by Cryogenic Resonant Acoustic spectroscopy of bulk materials (CRA spectroscopy)
Mechanical losses of crystalline silicon and calcium fluoride have been
analyzed in the temperature range from 5 to 300 K by our novel mechanical
spectroscopy method, cryogenic resonant acoustic spectroscopy of bulk materials
(CRA spectrocopy). The focus lies on the interpretation of the measured data
according to phonon-phonon interactions and defect induced losses in
consideration of the excited mode shape.Comment: 4 pages, 4 figures, proceedings of the PHONONS 2007, submitted to
Journal of Physics: Conference Serie
Effects and Moderators of Acute Aerobic Exercise on Subsequent Interference Control: A Systematic Review and Meta-Analysis
Background: Acute aerobic exercise leads to positive physiological adaptations within the central nervous system. These findings inspired research on potential cognitive benefits following acute aerobic exercise. The effects of acute aerobic exercise on subsequent cognitive performance, by far, have been the most researched for interference control, a subcomponent of executive function. The results of primary studies on the effects of acute aerobic exercise on subsequent interference control performance are inconsistent. Therefore, we used meta-analytic methods to pool available effect sizes, and to identify covariates that determine the magnitude of exercise-induced interference control benefits. Methods: Medline, PsycINFO, and SPORTDiscus were searched for eligible records. Hedges' g corrected standardized mean difference values (SMDs) were used for analyses. Random-effects weights were used to pool effect sizes. Moderator analyses were conducted using meta-regressions and subgroups analyses. Covariates that were here tested for moderation included parameters of the applied exercise regimen (exercise intensity and exercise duration), characteristics of examined participants (age and fitness), and methodological features of existing research (type of control group, familiarization with test procedure, type of test variable, delay between exercise cessation, and testing). Results: Fifty studies, with data from 2,366 participants, were included in qualitative and quantitative synthesis. A small, significant beneficial effect of acute aerobic exercise on time-dependent measures of interference control was revealed (k = 49, Hedges' g = −0.26, 95%CI: −34 to −0.18). Effect sizes from time-dependent measures of interference control varied widely and heterogeneity reached statistical significance (T2 = 0.0557, I2 = 28.8%). Moderator analyses revealed that higher exercise intensities (vigorous intensity and high-intensity interval training), also participants at younger or older age, and participants who are familiar with the testing procedure prior to the experiment, benefitted most from acute aerobic exercise. However, noticeable heterogeneity remained unexplained within specific subgroups (high-intensity interval training, preadolescent children, and active and supervised control group). Conclusion: Acute aerobic exercise improves subsequent interference control performance. However, the covariates exercise intensity, participants' age, and familiarization with testing procedure determine the magnitude of that effect. Methodological features were not found to influence the magnitude of effects. This dismisses some doubts that exercise induced benefits for interference control performance are scientific artifacts. The fact that large heterogeneity remained unexplained in some subgroups indicates the need for further research on covariates within these subgroups. It should be noted that effect sizes for all analyses were small. © Copyright © 2019 Oberste, Javelle, Sharma, Joisten, Walzik, Bloch and Zimmer
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