1,182 research outputs found
Fluctuation conductivity in superconductors in strong electric fields
We study the effect of a strong electric field on the fluctuation
conductivity within the time-dependent Ginzburg-Landau theory for the case of
arbitrary dimension. Our results are based on the analytical derivation of the
velocity distribution law for the fluctuation Cooper pairs, from the Boltzmann
equation. Special attention is drawn to the case of small nonlinearity of
conductivity, which can be investigated experimentally. We obtain a general
relation between the nonlinear conductivity and the temperature derivative of
the linear Aslamazov-Larkin conductivity, applicable to any superconductor. For
the important case of layered superconductors we derive an analogous relation
between the small nonlinear correction for the conductivity and the
fluctuational magnetoconductivity. On the basis of these relations we provide
new experimental methods for determining both the lifetime constant of
metastable Cooper pairs above T_c and the coherence length. A systematic
investigation of the 3rd harmonic of the electric field generated by a harmonic
current can serve as an alternative method for the examination of the
metastable Cooper-pair relaxation time.Comment: 18 pages, REVTeX, submitted to Phys. Rev.
Single domain transport measurements of C60 films
Thin films of potassium doped C60, an organic semiconductor, have been grown
on silicon. The films were grown in ultra-high vacuum by thermal evaporation of
C60 onto oxide-terminated silicon as well as reconstructed Si(111). The
substrate termination had a drastic influence on the C60 growth mode which is
directly reflected in the electrical properties of the films. Measured on the
single domain length scale, these films revealed resistivities comparable to
bulk single crystals. In situ electrical transport properties were correlated
to the morphology of the film determined by scanning tunneling microscopy. The
observed excess conductivity above the superconducting transition can be
attributed to two-dimensional fluctuations.Comment: 4 pages, 4 figure
Nonlocal Conductivity in the Vortex-Liquid Regime of a Two-Dimensional Superconductor
We have simulated the time-dependent Ginzburg-Landau equation with thermal
fluctuations, to study the nonlocal dc conductivity of a superconducting film.
Having examined points in the phase diagram at a wide range of temperatures and
fields below the mean-field upper critical field, we find a portion of the
vortex-liquid regime in which the nonlocal ohmic conductivity in real space is
negative over a distance several times the spacing between vortices. The effect
is suppressed when driven beyond linear response. Earlier work had predicted
the existence of such a regime, due to the high viscosity of a
strongly-correlated vortex liquid. This behavior is clearly distinguishable
from the monotonic spatial fall-off of the conductivity in the higher
temperature or field regimes approaching the normal state. The possibilities
for experimental study of the nonlocal transport properties are discussed.Comment: 18 pages, revtex, 6 postscript figure
High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc
The advent of high transverse-field muon spin rotation (TF-muSR) has led to
recent muSR investigations of the magnetic-field response of cuprates above the
superconducting transition temperature T_c. Here the results of such
experiments on hole-doped cuprates are reviewed. Although these investigations
are currently ongoing, it is clear that the effects of high field on the
internal magnetic field distribution of these materials is dependent upon a
competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the
response to the external field above Tc is dominated by heterogeneous spin
magnetism. However, the magnetism that dominates the observed inhomogeneous
line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely
different kind of magnetism in the heavily overdoped regime. The origin of the
magnetism above x ~ 0.19 is currently unknown, but its presence hints at a
competition between superconductivity and magnetism that is reminiscent of the
underdoped regime. In contrast, the width of the internal field distribution of
underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of
superconducting carriers. This observation suggests that the magnetic response
above Tc is not dominated by electronic moments, but rather inhomogeneous
fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference
A superconducting-nanowire 3-terminal electronic device
In existing superconducting electronic systems, Josephson junctions play a
central role in processing and transmitting small-amplitude electrical signals.
However, Josephson-junction-based devices have a number of limitations
including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability
to drive large impedances, and (4) difficulty in controlling the junction
critical current (which depends sensitively on sub-Angstrom-scale thickness
variation of the tunneling barrier). Here we present a nanowire-based
superconducting electronic device, which we call the nanocryotron (nTron), that
does not rely on Josephson junctions and can be patterned from a single thin
film of superconducting material with conventional electron-beam lithography.
The nTron is a 3-terminal, T-shaped planar device with a gain of ~20 that is
capable of driving impedances of more than 100 k{\Omega}, and operates in
typical ambient magnetic fields at temperatures of 4.2K. The device uses a
localized, Joule-heated hotspot formed in the gate to modulate current flow in
a perpendicular superconducting channel. We have characterized the nTron,
matched it to a theoretical framework, and applied it both as a digital logic
element in a half-adder circuit, and as a digital amplifier for superconducting
nanowire single-photon detectors pulses. The nTron has immediate applications
in classical and quantum communications, photon sensing and astronomy, and its
performance characteristics make it compatible with existing superconducting
technologies. Furthermore, because the hotspot effect occurs in all known
superconductors, we expect the design to be extensible to other materials,
providing a path to digital logic, switching, and amplification in
high-temperature superconductors
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NGO Legitimacy: Four Models
The aim of this paper is to examine NGOsâ legitimacy in the context of global politics. In order to yield a better understanding of NGOsâ legitimacy at the international level it is important to examine how their legitimacy claims are evaluated. This paper proposes dividing the literature into four models based on the theoretical and analytical approaches to their legitimacy claims: the market model, social change model, new institutionalism model and the critical model. The legitimacy criteria generated by the models are significantly different in their analytical scope of how one is to assess the role of NGOs operating as political actors contributing to democracy. The paper argues that the models present incomplete, and sometimes conflicting, views of NGOsâ legitimacy and that this poses a legitimacy dilemma for those assessing the political agency of NGOs in world politics. The paper concludes that only by approaching their legitimacy holistically can the democratic role of NGOs be explored and analysed in the context of world politics
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