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 $r_s=30.5$ in one-valley 2DEG. For two-valleys, we predict Wigner crystallization at $r_s= 29.5$, indicating that valley degeneracy has little effect on the critical $r_s$, 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

The Cepheid mass discrepancy and pulsation-driven mass loss

Context. A longstanding challenge for understanding classical Cepheids is the Cepheid mass discrepancy, where theoretical mass estimates using stellar evolution and stellar pulsation calculations have been found to differ by approximately 10 - 20%. Aims. We study the role of pulsation-driven mass loss during the Cepheid stage of evolution as a possible solution to this mass discrepancy. Methods. We computed stellar evolution models with a Cepheid mass-loss prescription and various amounts of convective core overshooting. The contribution of mass loss towards the mass discrepancy is determined using these models, Results. Pulsation-driven mass loss is found to trap Cepheid evolution on the instability strip, allowing them to lose about 5 - 10% of their total mass when moderate convective core overshooting, an amount consistent with observations of other stars, is included in the stellar models. Conclusions. We find that the combination of moderate convective core overshooting and pulsation-driven mass loss can solve the Cepheid mass discrepancy.Comment: 4 pages, 2 figures and 2 tables. Accepted for publication A&A Letter

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

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 130 - 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