4,316 research outputs found
Enhancement of electron-hole superfluidity in double few-layer graphene
We propose two coupled electron-hole sheets of few-layer graphene as a new
nanostructure to observe superfluidity at enhanced densities and enhanced
transition temperatures. For ABC stacked few-layer graphene we show that the
strongly correlated electron-hole pairing regime is readily accessible
experimentally using current technologies. We find for double trilayer and
quadlayer graphene sheets spatially separated by a nano-thick hexagonal
boron-nitride insulating barrier, that the transition temperature for
electron-hole superfluidity can approach temperatures of 40 K.Comment: 17 pages, 5 figure
Wigner crystallization in transition metal dichalcogenides: A new approach to correlation energy
We introduce a new approach for the correlation energy of one- and two-valley
two-dimensional electron gas (2DEG) systems. Our approach is based on a random
phase approximation at high densities and a classical approach at low
densities, with interpolation between the two limits. This approach gives
excellent agreement with available Quantum Monte Carlo (QMC) calculations. We
employ the two-valley 2DEG model to describe the electron correlations in
monolayer transition metal dichalcogenides (TMDs). The zero-temperature
transition from a Fermi liquid to a quantum Wigner crystal phase in monolayer
TMDs is obtained using density-functional theory within the local-density
approximation. Consistent with QMC, we find that electrons crystallize at
in one-valley 2DEG. For two-valleys, we predict Wigner
crystallization at , indicating that valley degeneracy has little
effect on the critical , in contrast to an earlier claim.Comment: 5 pages, 3 figure
Development of improved amorphous materials for laser systems
Crystallization calculations were performed in order to determine the possibility of forming a particular type of laser glass with the avoidance of devitrification in an outer space laboratory. It was demonstrated that under the homogenuous nucleating conditions obtainable in a zero gravity laboratory this laser glass may be easily quenched to a virtually crystal-free product. Experimental evidence is provided that use of this material as a host in a neodymium glass laser would result in more than a 10 percent increase in efficiency when compared to laser glass rods of a similar composition currently commercially available. Differential thermal analysis, thermal gradient oven, X-ray diffraction, and liquidus determination experiments were carried out to determine the basics of the crystallization behavior of the glass, and small-angle X-ray scattering and splat-cooling experiments were performed in order to provide additional evidence for the feasibility of producing this laser glass material, crystal free, in an outer space environment
Multiband Mechanism for the Sign Reversal of Coulomb Drag Observed in Double Bilayer Graphene Heterostructures
Coupled 2D sheets of electrons and holes are predicted to support novel
quantum phases. Two experiments of Coulomb drag in electron-hole (e-h) double
bilayer graphene (DBLG) have reported an unexplained and puzzling sign reversal
of the drag signal. However, we show that this effect is due to the multiband
character of DBLG. Our multiband Fermi liquid theory produces excellent
agreement and captures the key features of the experimental drag resistance for
all temperatures. This demonstrates the importance of multiband effects in
DBLG: they have a strong effect not only on superfluidity, but also on the
drag.Comment: 5 pages, 3 figure
An investigation of methods of recording the electrical activity of the nervous system with particular reference to the occurrence and suppression of stimulus artefact
A theory of the mechanism of the production of
stimulus artefact in three dimensional preparations has been
advanced, in which the artefact is regarded as being composed
of four major components.That it has been possible to demonstrate these four
components separately, and to reduce a large artefact to
below the system noise level using methods based on the
theory, would support the view that these four components represent the only ones of practical significance.The theory is quantitative in that, if values are
assigned to the various transfer functions involved, the
amplitude and waveform of the artefact produced in as given
system is predictable. It has been found that where the
transfer functions involve the electrode impedances, in many
cases a sufficiently close approximation to the true transfer
function can be obtained by regarding the electrode impedance
as either a pure resistance, or a shunt combination of
resistance and capacitance. Values of resistance and capacitance
corresponding to the various electrodes used in this
laboratory have been indicated, and it has been shown that
these can be used to evaluate the overall transfer functions
of the recording system and stimulating circuit.A knowledge of the transfer impedances associated with
the preparation completes the information required to
estimate the amplitude and waveform of the artefact to be
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expected in a given situation. The lower limit of one of
transfer impedances, (Ce ), is set by the earth elec-
(P)
trode impedance, but the other three can assume any value,
including zero, over a very wide range. Since the other
transfer functions, associated with the electrode networks,
and the stimulating and recording apparatus, can also vary
within very wide limits, the resultant artefact, being a
function of all these variables, can assume an enormous
variety of amplitudes and waveforms.It is because the artefact is a function of so many
variables, most of which can effect a change of several order
of magnitude in one or more of the artefact components, that
the importance of viewing the stimulating/preparation/record
ing system as a whole, when considering stimulus artefact,
can hardly be overstressed.The usefulness of a quantitative theory of stimulus
artefact becomes apparent when an attempt is made to reduce
the artefact arising in a practical situation. Thus a
proper appreciation of the mechanism of artefact production
should enable the various components present to be recognised,
and make it possible to diagnose which parts of the system
are responsible. Steps can then be taken to improve the performance
of the relevant parts of the system using the
techniques and apparatus described here and elsewhere.Consideration of possible methods of reducing stimulus
artefact in general has shown that three out of the
four major components could be reduced indefinitely by
sufficient improvement in the isolation of the stimulator,
and in the common mode rejection of the recording system.Thus it has been argued that the best way in which
the 'Escape' components of the artefact can be controlled
is to reduce to a minimum the capacitance to earth of the
stimulator circuit. The disadvantages of the conventional,
passive way of fulfilling this requirement, using a radio frequency isolating unit, can be overcome by the active system
using the Low Capacitance Stimulator described.It has been shown possible to construct such an
instrument having substantially less capacitance to earth
than the best R.F. units published, yet retaining all the
advantages of a conventional earthed stimulator, exemplified
in this case by the provision of constant current output
pulses of up to 20 mA.Measurements of the maximum value of the escape component
likely to be observed when using a conventional stimulator, were used to assess the required capacitance to earth
of an 'ideal' stimulator giving escape artefacts below the
recording system noise level in all circumstances. This
ideal capacitance was estimated to be approximately one pF. -
the value chosen as the design target in the development of
the Low Capacitance Stimulator. The conclusion that this
value represents the limit to which the capacitance to earth
of a stimulator may be usefully reduced, is supported by the
complete absence of escape artefact components always observed
when the stimulator was used under normal stimulating
conditions, i.e. not specially arranged to demonstrate
escape components.Nevertheless, the stimulator described here is not
presented as a fully engineered equipment, but rather as an
experimental apparatus, constructed to demonstrate the
feasibility of the active technique which it embodies.
There would therefore seem to be no reason why advantage
should not be taken of the possibility of further reduction
in stimulator capacitance, should this be considered desirable,
by using a miniature, transistorised construction, and
more sophisticated servo amplifiers in place of the auxiliary
cathode followers. In this way a standard of stimulus
isolation might be attained which would be quite unapproachable
by any passive technique.Similar remarks as to the experimental nature of the
High Rejection Ratio recording amplifier described in
chapter six can also be made. No doubt an improvement in
its performance could be obtained by increasing the complexity
of its auxiliary servo amplifier to increase its gain
and bandwidth while retaining adequate stability with high
'resistance recording electrodes, but it is questionable
whether a further increase in common mode rejection, already
over a hundred times that of a conventional system, could
often be employed. Certainly it can be said that when
normal stimulation was used, as distinct from the injection
of artificially large common mode potentials into the preparation,
the common components of the artefact obtained with the
smallest electrodes used in this laboratory were always reduced
to below the recording system noise level.An additional advantage of the much higher rejection
of common mode interference obtainable under practical conditions with this recording system, is the enhanced rejection
of 'in- phase' potentials induced in the preparation from the
supply mains, and of unwanted biological signals appearing
as common mode potentials at the recording electrodes.There would appear to be other applications for such
a purely 'differential' amplifier in instrumentation in
non-biological fields.A system using both the Low Capacitance Stimulator
and the High Rejection Ratio amplifier might be said to be
capable of reducing to below noise level three of the components
of any artefact likely to be met with in practice.
Were it possible to make a similar claim for the Differential
Attenuator Unit in dealing with the fourth component, a combination
cf the three units might have been held to constitute
an 'ideal' anti-artefact system.Unfortunately, there seems little chance that the
Differential Attenuator Unit could ever be relied on to
reduce every Differential Direct artefact component encountered
to below noise level, indeed experience has shown
that it is not always possible, with arbitrary electrode
positions, to achieve the standard of rejection obtained in
Fig. 7.3.1. On the other hand, it has been demonstrated
that, using non-polarizable electrodes in an artificial
resistive 'preparation', a much higher standard of rejection
of the whole artefact can be attained, so that the disappointing
results with real tissue can reasonably be ascribed to
the properties of the tissues themselves. This being so,
there is little to be gained by further development of the
Differential Attenuator UniteAlthough the combination of the three units developed
in the course of this study falls short of forming an
'ideal' anti -artefact system, in the sense of being able to
eliminate any conceivable artefact, it may be argued that
such a system comes near to being an optimum one in which
further development of the apparatus would yield no significant
improvement of the anti-artefact performance
Multi-mode horn
A horn has an input aperture and an output aperture, and comprises a conductive inner surface formed by rotating a curve about a central axis. The curve comprises a first arc having an input aperture end and a transition end, and a second arc having a transition end and an output aperture end. When rotated about the central axis, the first arc input aperture end forms an input aperture, and the second arc output aperture end forms an output aperture. The curve is then optimized to provide a mode conversion which maximizes the power transfer of input energy to the Gaussian mode at the output aperture
A Look at Kulstad v. Maniaci in Light of Changing Cultural Norms
A Look at Kulstad v. Maniac
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