10,307 research outputs found
Different Behavior of Magnetic Impurities in Crystalline and Ammorphous States of Superconductors
It has been observed that the effect of magnetic impurities in a
superconductor is drastically different depending on whether the host
superconductor is in a crystalline or an amorphous state. Based on the recent
theory of Kim and Overhauser (KO), it is shown that as the system is getting
disordered, the initial slope of the depression is decreasing by a
factor , when the mean free path becomes smaller
than the BCS coherence length , which is in agreement with
experimental findings. In addition, for a superconductor in a crystalline state
in the presence of magnetic impurities the superconducting transition
temperature drops sharply from about 50% of (for a pure
system) to zero near the critical impurity concentration. This {\sl pure limit
behavior} was indeed found by Roden and Zimmermeyer in crystalline Cd.
Recently, Porto and Parpia have also found the same {\sl pure limit behavior}
in superfluid He-3 in aerogel, which may be understood within the framework of
the KO theory.Comment: 7 figures, 20 pages, latex, to appear in Superconductor Science and
Technolog
Impurity scattering in a d-wave superconductor
The influence of (non-magnetic and magnetic) impurities on the transition
temperature of a d-wave superconductor is studied anew within the framework of
BCS theory. Pairing interaction decreases linearly with the impurity
concentration. Accordingly suppression is proportional to the
(potential or exchange) scattering rate, , due to impurities. The
initial slope versus is found to depend on the superconductor contrary
to Abrikosov-Gor'kov type theory. Near the critical impurity concentration
drops abruptly to zero. Because the potential scattering rate is
generally much larger than the exchange scattering rate, magnetic impurities
will also act as non-magnetic impurities as far as the decrease is
concerned. The implication for the impurity doping effect in high
superconductors is also discussed.Comment: 12 pages and 1 figure, PlainTex, submitted to Mod. Phys. Lett. B, For
more information, please see "http://taesan.kaist.ac.kr/~yjkim
Stochastic Analysis Of Response Functions Of Nitrogen In Stream Water
Thesis (M.S.) University of Alaska Fairbanks, 1992In the present study, a stochastic model of nitrogen in streams is created using a new mathematical technique, Probability Density Function/Moment technique (PDF/M). The model is based on a set of four highly non-linear second order equations for nitrogen species in streams (NH\sb3, NO\sb2, NO\sb3, and organic-N). The purpose of the PDF/M technique is to include occurrence of natural variability. The first step is to separate the stochastic terms from the non-stochastic terms and solve the resulting set of equations simultaneously. The moments of the output variables then are obtained using expectation mathematics. The moments are used in a solution of the Fokker-Planck equation to produce an analytical solution for the probability density functions of the dependent variables. Comparison of the present study to the results of the Monte Carlo method showed the application of PDF/M technique to nitrogen cycle simulation valid. <p
Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn
We perform hybrid functional and quasi-particle band structure calculations
with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge,
and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the
characteristics of band edge states, i.e., band and valley degeneracies, and
orbital characters, are found to be conserved, independent of the computational
schemes such as density functional generalized gradient approximation, hybrid
functionals, or quasi-particle calculations. However, the magnitude of the
calculated band gap varies significantly with the computational schemes. Within
density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge,
and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely
underestimated compared to the experimental gaps, due to the band gap error in
the density functional theory and the significant relativistic effect on the
low-energy band structures. By employing hybrid-functional calculations with a
35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame
the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations
on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and
0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the
experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.Comment: 23 pages, including 84 references, 5 tables, 3 figure
Simulating Problem Difficulty in Arithmetic Cognition Through Dynamic Connectionist Models
The present study aims to investigate similarities between how humans and
connectionist models experience difficulty in arithmetic problems. Problem
difficulty was operationalized by the number of carries involved in solving a
given problem. Problem difficulty was measured in humans by response time, and
in models by computational steps. The present study found that both humans and
connectionist models experience difficulty similarly when solving binary
addition and subtraction. Specifically, both agents found difficulty to be
strictly increasing with respect to the number of carries. Another notable
similarity is that problem difficulty increases more steeply in subtraction
than in addition, for both humans and connectionist models. Further
investigation on two model hyperparameters --- confidence threshold and hidden
dimension --- shows higher confidence thresholds cause the model to take more
computational steps to arrive at the correct answer. Likewise, larger hidden
dimensions cause the model to take more computational steps to correctly answer
arithmetic problems; however, this effect by hidden dimensions is negligible.Comment: 7 pages; 15 figures; 5 tables; Published in the proceedings of the
17th International Conference on Cognitive Modelling (ICCM 2019
Counterintuitive example on relation between ZT and thermoelectric efficiency
The thermoelectric figure of merit ZT, which is defined using electrical
conductivity, Seebeck coefficient, thermal conductivity, and absolute
temperature T, has been widely used as a simple estimator of the conversion
efficiency of a thermoelectric heat engine. When material properties are
constant or slowly varying with T, a higher ZT ensures a higher maximum
conversion efficiency of thermoelectric materials. However, as material
properties can vary strongly with T, efficiency predictions based on ZT can be
inaccurate, especially for wide-temperature applications. Moreover, although ZT
values continue to increase, there has been no investigation of the
relationship between ZT and the efficiency in the higher ZT regime. In this
paper, we report a counterintuitive situation by comparing two materials:
although one material has a higher ZT value over the whole operational
temperature range, its maximum conversion efficiency is smaller than that of
the other. This indicates that, for material comparisons, the evaluation of
exact efficiencies as opposed to a simple comparison of the ZTs is necessary in
certain cases.Comment: 12 pages, 2 tables, 2 figure
Development and Evaluation of a Smartphone Application for Managing Gestational Diabetes Mellitus
Objectives The purpose of this study was to develop and evaluate an application (app) that provides tailored recommendations based on lifestyle and clinical data entered by the user. Methods Knowledge and functions required for the gestational diabetes mellitus (GDM) management app were extracted from clinical practice guidelines and evaluated through an online survey. Common and tailored recommendations were developed and evaluated with a content validity index. Algorithms to link tailored recommendations with a patient\u27s data were developed and evaluated by experts. An Android-based app was developed and evaluated by comparing the process of data entry and recommendation retrieval and the usability of the app. After the app was revised, the user acceptance of the app was evaluated. Results Six domains of knowledge and 14 functions were extracted. Seven common and 49 tailored recommendations were developed. Nine lifestyle and clinical data elements were modeled. Eight algorithms with 18 decision nodes presenting tailored recommendations based on patient\u27s data and 12 user interface screens were developed. All recommendations obtained from the use of app concurred with recommendations derived by algorithms. The average usability score was 69.5 out of 100. The user acceptance score with behavioral intention to use was 5.5, intrinsic motivation 4.3, the perceived ease of use score was 4.6, and the perceived usefulness score was 5.0 out of 7, respectively. Conclusions The GDM management knowledge and tailored recommendations obtained in this study could be of help in managing GDM
Fertility decline in South Korea
노트 : Paper presented for presentation at the XXV IUSSP International Population Conference, 18-23 July 2005, Tours, Franc
Stability‐induced modification of sea surface winds over Gulf Stream rings
Satellite‐borne scatterometer and infrared data collected over Gulf Stream warm and cold core rings are used to study the effect of the sea‐air temperature difference on the wind speed over rings. The observed acceleration of the wind over warm core rings and deceleration over cold core rings is found to be consistent with that predicted by the planetary boundary layer model of Brown and Foster [1994]. In addition it is shown that the distance over which the winds respond to an ocean surface temperature step is short (≤25km) while the distance over which the marine boundary layer responds to a surface temperature step is long (≥175km)
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