7,423 research outputs found
A statistical model for the intrinsically broad superconducting to normal transition in quasi-two-dimensional crystalline organic metals
Although quasi-two-dimensional organic superconductors such as
-(BEDT-TTF)Cu(NCS) seem to be very clean systems, with apparent
quasiparticle mean-free paths of several thousand \AA, the superconducting
transition is intrinsically broad (e.g K wide for K).
We propose that this is due to the extreme anisotropy of these materials, which
greatly exacerbates the statistical effects of spatial variations in the
potential experienced by the quasiparticles. Using a statistical model, we are
able to account for the experimental observations. A parameter , which
characterises the spatial potential variations, may be derived from
Shubnikov-de Haas oscillation experiments. Using this value, we are able to
predict a transition width which is in good agreement with that observed in MHz
penetration-depth measurements on the same sample.Comment: 8 pages, 2 figures, submitted to J. Phys. Condens. Matte
Analysis of dynamic stall using unsteady boundary-layer theory
The unsteady turbulent boundary layer and potential flow about a pitching airfoil are analyzed using numerical methods to determine the effect of pitch rate on the delay in forward movement of the rear flow reversal point. An explicit finite difference scheme is used to integrate the unsteady boundary layer equations, which are coupled at each instant of time to a fully unsteady and nonlinear potential flow analysis. A substantial delay in forward movement of the reversal point is demonstrated with increasing pitch rate, and it is shown that the delay results partly from the alleviation of the gradients in the potential flow, and partly from the effects of unsteadiness in the boundary layer itself. The predicted delay in flow-reversal onset, and its variation with pitch rate, are shown to be in reasonable agreement with experimental data relating to the delay in dynamic stall. From the comparisons it can be concluded (a) that the effects of time-dependence are sufficient to explain the failure of the boundary layer to separate during the dynamic overshoot, and (b) that there may be some link between forward movement of the reversal point and dynamic stall
Inadequacies in the conventional treatment of the radiation field of moving sources
There is a fundamental difference between the classical expression for the
retarded electromagnetic potential and the corresponding retarded solution of
the wave equation that governs the electromagnetic field. While the boundary
contribution to the retarded solution for the {\em potential} can always be
rendered equal to zero by means of a gauge transformation that preserves the
Lorenz condition, the boundary contribution to the retarded solution of the
wave equation governing the {\em field} may be neglected only if it diminishes
with distance faster than the contribution of the source density in the far
zone. In the case of a source whose distribution pattern both rotates and
travels faster than light {\em in vacuo}, as realized in recent experiments,
the boundary term in the retarded solution governing the field is by a factor
of the order of {\em larger} than the source term of this solution in
the limit that the distance of the boundary from the source tends to
infinity. This result is consistent with the prediction of the retarded
potential that part of the radiation field generated by a rotating superluminal
source decays as , instead of , a prediction that is
confirmed experimentally. More importantly, it pinpoints the reason why an
argument based on a solution of the wave equation governing the field in which
the boundary term is neglected (such as appears in the published literature)
misses the nonspherical decay of the field
Inflation with a graceful exit and entrance driven by Hawking radiation
We present a model for cosmological inflation which has a natural "turn on"
and a natural "turn off" mechanism. In our model inflation is driven by the
Hawking-like radiation that occurs in Friedman-Robertson-Walker (FRW)
space-time. This Hawking-like radiation results in an effective negative
pressure "fluid" which leads to a rapid period of expansion in the very early
Universe. As the Universe expands the FRW Hawking temperature decreases and the
inflationary expansion turns off and makes a natural transition to the power
law expansion of a radiation dominated universe. The "turn on" mechanism is
more speculative, but is based on the common hypothesis that in a quantum
theory of gravity at very high temperatures/high densities Hawking radiation
will stop. Applying this speculation to the very early Universe implies that
the Hawking-like radiation of the FRW space-time will be turned off and
therefore the inflation driven by this radiation will turn off.Comment: 19 pages, 2 figures revtex, matches PRD published versio
Experimental observation of Frohlich superconductivity in high magnetic fields
Resistivity and irreversible magnetisation data taken within the
high-magnetic-field CDWx phase of the quasi-two-dimensional organic metal
alpha-(BEDT-TTF)2KHg(SCN)4 are shown to be consistent with a field-induced
inhomogeneous superconducting phase. In-plane skin-depth measurements show that
the resistive transition on entering the CDWx phase is both isotropic and
representative of the bulk.Comment: ten pages, four figure
Opening of DNA double strands by helicases. Active versus passive opening
Helicase opening of double-stranded nucleic acids may be "active" (the
helicase directly destabilizes the dsNA to promote opening) or "passive" (the
helicase binds ssNA available due to a thermal fluctuation which opens part of
the dsNA). We describe helicase opening of dsNA, based on helicases which bind
single NA strands and move towards the double-stranded region, using a discrete
``hopping'' model. The interaction between the helicase and the junction where
the double strand opens is characterized by an interaction potential. The form
of the potential determines whether the opening is active or passive. We
calculate the rate of passive opening for the helicase PcrA, and show that the
rate increases when the opening is active. Finally, we examine how to choose
the interaction potential to optimize the rate of strand separation. One
important result is our finding that active opening can increase the unwinding
rate by 7 fold compared to passive opening.Comment: 13 pages, 3 figure
Comparison of the Fermi-surface topologies of kappa-(BEDT-TTF)_2 Cu(NCS)_2 and its deuterated analogue
We have measured details of the quasi one-dimensional Fermi-surface sections
in the organic superconductor kappa-(BEDT-TTF)_2 Cu(NCS)_2 and its deuterated
analogue using angle-dependent millimetre-wave techniques. There are
significant differences in the corrugations of the Fermi surfaces in the
deuterated and undeuterated salts. We suggest that this is important in
understanding the inverse isotope effect, where the superconducting transition
temperature rises on deuteration. The data support models for superconductivity
which invoke electron-electron interactions depending on the topological
properties of the Fermi surface
Fermi Surface Study of Quasi-Two-Dimensional Organic Conductors by Magnetooptical Measurements
Magnetooptical measurements of several quasi-two-dimensional (q2D) organic
conductors, which have simple Fermi surface structure, have been performed by
using a cavity perturbation technique. Despite of the simple Fermi surface
structure, magnetooptical resonance results show a dramatic difference for each
sample. Cyclotron resonances (CR) were observed for q-(BEDT-TTF)2I3 and
(BEDT-TTF)3Br(pBIB), while periodic orbit resonances (POR) were observed for
(BEDT-TTF)2Br(DIA) and (BEDT-TTF)3Cl(DFBIB). The selection of the resonance
seems to correspond with the skin depth for each sample. The effective mass of
POR seems to have a mass enhancement due to the many-body effect, while
effective mass of CR is independent of the strength of the electron-electron
interaction. The scattering time deduced from each resonance's linewidth will
be also presented.Comment: 10 pages, 8 figures, to be published to J. Phys. Soc. Jpn Vol.72 No.1
(accepted
Angle Dependent Magnetoresistance of the Layered Organic Superconductor \kappa-(ET)2Cu(NCS)2: Simulation and Experiment
The angle-dependences of the magnetoresistance of two different isotopic
substitutions (deuterated and undeuterated) of the layered organic
superconductor \kappa-(ET)2Cu(NCS)2 are presented. The angle dependent
magnetoresistance oscillations (AMRO) arising from the quasi-one-dimensional
(Q1D) and quasi-two-dimensional (Q2D) Fermi surfaces in this material are often
confused. By using the Boltzman transport equation extensive simulations of the
AMRO are made that reveal the subtle differences between the different species
of oscillation. No significant differences are observed in the electronic
parameters derived from quantum oscillations and AMRO for the two isotopic
substitutions. The interlayer transfer integrals are determined for both
isotopic substitutions and a slight difference is observed which may account
for the negative isotope effect previously reported [1]. The success of the
semi-classical simulations suggests that non-Fermi liquid effects are not
required to explain the interlayer-transport in this system.Comment: 15 pages, 16 figure
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