257 research outputs found
Suppression of \bbox{T_c} in superconducting amorphous wires
The suppression of the mean field temperature of the superconducting
transition, , in homogeneous amorphous wires is studied. We develop a
theory that gives in situations when the dynamically enhanced Coulomb
repulsion competes with the contact attraction. The theory accurately describes
recent experiments on --suppression in superconducting wires, after a
procedure that minimizes the role of nonuniversal mechanisms influencing
is applied.Comment: RevTeX, 4 pages, 3 figure
The unusual thickness dependence of superconductivity in -MoGe thin films
Thin films of -MoGe show progressively reduced 's as the
thickness is decreased below 30 nm and the sheet resistance exceeds 100
. We have performed far-infrared transmission and reflection
measurements for a set of -MoGe films to characterize this weakened
superconducting state. Our results show the presence of an energy gap with
ratio in all films studied, slightly higher
than the BCS value, even though the transition temperatures decrease
significantly as film thickness is reduced. The material properties follow
BCS-Eliashberg theory with a large residual scattering rate except that the
coherence peak seen in the optical scattering rate is found to be strongly
smeared out in the thinner superconducting samples. A peak in the optical mass
renormalization at is predicted and observed for the first time
Evaluation of Automated Anthropometrics Produced By Smartphone-Based Machine Learning: A Comparison With Traditional Anthropometric Assessments
Automated visual anthropometrics produced by mobile applications are accessible and cost-effective with the potential to assess clinically relevant anthropometrics without a trained technician present. Thus, the aim of this study was to evaluate the precision and agreement of smartphone-based automated anthropometrics against reference tape measurements. Waist and hip circumference (WC; HC), waist-to-hip ratio (WHR), and waist-to-height ratio (W:HT), were collected from 115 participants (69 F) using a tape measure and two smartphone applications (MeThreeSixty®, myBVI®) across multiple smartphone types. Precision metrics were used to assess test-retest precision of the automated measures. Agreement between the circumferences produced by each mobile application and the reference were assessed using equivalence testing and other validity metrics. All mobile applications across smartphone types produced reliable estimates for each variable with ICCs ≥0.93 (all
Time-restricted Feeding Plus Resistance Training in Active Females: A Randomized Trial
Background A very limited amount of research has examined intermittent fasting (IF) programs, such as time-restricted feeding (TRF), in active populations. Objective Our objective was to examine the effects of TRF, with or without β-hydroxy β-methylbutyrate (HMB) supplementation, during resistance training (RT). Methods This study employed a randomized, placebo-controlled, reduced factorial design and was double-blind with respect to supplementation in TRF groups. Resistance-trained females were randomly assigned to a control diet (CD), TRF, or TRF plus 3 g/d HMB (TRFHMB). TRF groups consumed all calories between 1200 h and 2000 h, whereas the CD group ate regularly from breakfast until the end of the day. All groups completed 8 wk of supervised RT and consumed supplemental whey protein. Body composition, muscular performance, dietary intake, physical activity, and physiological variables were assessed. Data were analyzed prior to unblinding using mixed models and both intention-to-treat (ITT) and per protocol (PP) frameworks. Results Forty participants were included in ITT, and 24 were included in PP. Energy and protein intake (1.6 g/kg/d) did not differ between groups despite different feeding durations (TRF and TRFHMB: ∼7.5 h/d; CD: ∼13 h/d). Comparable fat-free mass (FFM) accretion (+2% to 3% relative to baseline) and skeletal muscle hypertrophy occurred in all groups. Differential effects on fat mass (CD: +2%; TRF: −2% to −4%; TRFHMB: −4% to −7%) were statistically significant in the PP analysis, but not ITT. Muscular performance improved without differences between groups. No changes in physiological variables occurred in any group, and minimal side effects were reported. Conclusions IF, in the form of TRF, did not attenuate RT adaptations in resistance-trained females. Similar FFM accretion, skeletal muscle hypertrophy, and muscular performance improvements can be achieved with dramatically different feeding programs that contain similar energy and protein content during RT. Supplemental HMB during fasting periods of TRF did not definitively improve outcomes. This study was prospectively registered at clinicaltrials.gov as NCT03404271
Weak Localization Effect in Superconductors
We study the effect of weak localization on the transition temperatures of
superconductors using time-reversed scattered state pairs, and find that the
weak localization effect weakens electron-phonon interactions. With solving the
BCS equation, the calculated values for are in good agreement
with experimental data for various two- and three-dimensional disordered
superconductors. We also find that the critical sheet resistance for the
suppression of superconductivity in thin films does not satisfy the universal
behavior but depends on sample, in good agreement with experiments. but depends
on sample, in good agreement with experiments.Comment: 14 pages, Revtex, 5 ps figure
Effect of thermal phase fluctuations on the superfluid density of two-dimensional superconducting films
High precision measurements of the complex sheet conductivity of
superconducting Mo77Ge23 thin films have been made from 0.4 K through Tc. A
sharp drop in the inverse sheet inductance, 1/L(T), is observed at a
temperature, Tc, which lies below the mean-field transition temperature, Tco.
Just below Tc, the suppression of 1/L(T) below its mean-field value indicates
that longitudinal phase fluctuations have nearly their full classical
amplitude, but they disappear rapidly as T decreases. We argue that there is a
quantum crossover at about 0.94 Tco, below which classical phase fluctuations
are suppressed.Comment: 14 pages, 3 figures. Subm. to PR
Flux flow resistivity and vortex viscosity of high-Tc films
The flux flow regime of high-T samples of different normal state
resistivities is studied in the temperature range where the sign of the Hall
effect is reversed. The scaling of the vortex viscosity with normal state
resistivity is consistent with the Bardeen-Stephen theory. Estimates of the
influence of possible mechanisms suggested for the sign reversal of the Hall
effect are also given.Comment: 3 pages. 4 figures upon reques
Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition
The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and
one In/InOx film have been measured in zero field using a two-coil mutual
inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to
have three contributions: the inductive superfluid, renormalized by nonvortex
phase fluctuations; conventional vortex-antivortex pairs, whose contribution
turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii
unbinding temperature; and an anomalous contribution. The latter is
predominantly resistive, persists well below the KTB temperature, and is weakly
dependent on frequency down to remarkably low frequencies, at least 100 Hz. It
increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about
half the energy to create a vortex-antivortex pair, indicating that the
frequency dependence is that of individual excitations, rather than critical
behavior.Comment: 10 pages, 10 figs; subm PR
Associations Between Visceral Adipose Tissue Estimates Produced By Near-Infrared Spectroscopy, Mobile Anthropometrics, and Traditional Body Composition Assessments and Estimates Derived From Dual-Energy X-Ray Absorptiometry
Assessments of visceral adipose tissue (VAT) are critical in preventing metabolic disorders; however, there are limited measurement methods that are accurate and accessible for VAT. The purpose of this cross-sectional study was to evaluate the association between VAT estimates from consumer-grade devices and traditional anthropometrics and VAT and subcutaneous adipose tissue (SAT) from dual-energy X-ray absorptiometry (DXA). Data were collected from 182 participants (female = 114; White = 127; Black/African-American (BAA) = 48) which included anthropometrics and indices of VAT produced by near-infrared reactance spectroscopy (NIRS), visual body composition (VBC) and multifrequency BIA (MFBIA). VAT and SAT were collected using DXA. Bivariate and partial correlations were calculated between DXAVAT and DXASAT and other VAT estimates. All VAT indices had positive moderate–strong correlations with VAT (all P \u3c 0·001) and SAT (all P \u3c 0·001). Only waist:hip (r = 0·69), VATVBC (r = 0·84), and VATMFBIA (r = 0·86) had stronger associations with VAT than SAT (P \u3c 0·001). Partial associations between VATVBC and VATMFBIA were only stronger for VAT than SAT in White participants (r = 0·67, P \u3c 0·001) but not female, male, or BAA participants individually. Partial correlations for waist:hip were stronger for VAT than SAT, but only for male (r = 0·40, P \u3c 0·010) or White participants (r = 0·48, P \u3c 0·001). NIRS was amongst the weakest predictors of VAT which was highest in male participants (r = 0·39, P \u3c 0·010) but non-existent in BAA participants (r = –0·02, P \u3e 0·050) after adjusting for SAT. Both anthropometric and consumer-grade VAT indices are consistently better predictors of SAT than VAT. These data highlight the need for a standardised, but convenient, VAT estimation protocol that can account for the relationship between SAT and VAT that differs by sex/race
Effect of granularity on the insulator-superconductor transition in ultrathin Bi films
We have studied the insulator-superconductor transition (IST) by tuning the
thickness in quench-condensed films. The resistive transitions of the
superconducting films are smooth and can be considered to represent
"homogeneous" films. The observation of an IST very close to the quantum
resistance for pairs, on several substrates supports
this idea. The relevant length scales here are the localization length, and the
coherence length. However, at the transition, the localization length is much
higher than the superconducting coherence length, contrary to expectation for a
"homogeneous" transition. This suggests the invalidity of a purely fermionic
model for the transition. Furthermore, the current-voltage characteristics of
the superconducting films are hysteretic, and show the films to be granular.
The relevant energy scales here are the Josephson coupling energy and the
charging energy. However, Josephson coupling energies () and the charging
energies () at the IST, they are found to obey the relation .
This is again contrary to expectation, for the IST in a granular or
inhomogeneous, system. Hence, a purely bosonic picture of the transition is
also inconsistent with our observations. We conclude that the IST observed in
our experiments may be either an intermediate case between the fermioinc and
bosonic mechanisms, or in a regime of charge and vortex dynamics for which a
quantitative analysis has not yet been done.Comment: accepted in Physical Review
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