6,776 research outputs found
Matched wideband low-noise amplifiers for radio astronomy
Two packaged low noise amplifiers for the 0.3–4 GHz frequency range are described. The amplifiers can be operated at temperatures of 300–4 K and achieve noise temperatures in the 5 K range (<0.1 dB noise figure) at 15 K physical temperature. One amplifier utilizes commercially available, plastic-packaged SiGe transistors for first and second stages; the second amplifier is identical except it utilizes an experimental chip transistor as the first stage. Both amplifiers use resistive feedback to provide input reflection coefficient S11<−10 dB over a decade bandwidth with gain over 30 dB. The amplifiers can be used as rf amplifiers in very low noise radio astronomy systems or as i.f. amplifiers following superconducting mixers operating in the millimeter and submillimeter frequency range
Self-consistent ac quantum transport using nonequilibrium Green functions
We develop an approach for self-consistent ac quantum transport in the
presence of time-dependent potentials at non-transport terminals. We apply the
approach to calculate the high-frequency characteristics of a nanotube
transistor with the ac signal applied at the gate terminal. We show that the
self-consistent feedback between the ac charge and potential is essential to
properly capture the transport properties of the system. In the on-state, this
feedback leads to the excitation of plasmons, which appear as pronounced
divergent peaks in the dynamic conductance at terahertz frequencies. In the
off-state, these collective features vanish, and the conductance exhibits
smooth oscillations, a signature of single-particle excitations. The proposed
approach is general and will allow the study of the high-frequency
characteristics of many other low-dimensional nanoscale materials such as
nanowires and graphene-based systems, which are attractive for terahertz
devices, including those that exploit plasmonic excitations.Comment: 11 pages, 5 figures, accepted in Physical Review
Flow reversals in turbulent convection via vortex reconnections
We employ detailed numerical simulations to probe the mechanism of flow
reversals in two-dimensional turbulent convection. We show that the reversals
occur via vortex reconnection of two attracting corner rolls having same sign
of vorticity, thus leading to major restructuring of the flow. Large
fluctuations in heat transport are observed during the reversal due to this
flow reconfiguration. The flow configurations during the reversals have been
analyzed quantitatively using large-scale modes. Using these tools, we also
show why flow reversals occur for a restricted range of Rayleigh and Prandt
numbers
A Predictive Model for Convective Flows Induced by Surface Reactivity Contrast
Concentration gradients in a fluid along a reactive surface due to contrast
in surface reactivity generate convective flows. These flows result from
contributions by electro and diffusio osmotic phenomena. In this study we have
analyzed reactive patterns that release and consume protons, analogous to
bimetallic catalytic conversion of peroxide. Here, we present a simple
analytical model that accurately predicts the induced potentials and consequent
velocities in such systems over a wide range of input parameters. Our model is
tested against direct numerical solutions to the coupled Poisson,
Nernst-Planck, and Navier-Stokes equations. Our analysis can be used to predict
enhancement of mass transport and the resulting impact on overall catalytic
conversion, and is also applicable to predicting the speed of catalytic
nanomotors
Understanding the Fano Resonance : through Toy Models
The Fano Resonance, involving the mixing between a quasi-bound `discrete'
state of an inelastic channel lying in the continuum of scattering states
belonging to the elastic channel, has several subtle features. The underlying
ideas have recently attracted attention in connection with interference effects
in quantum wires and mesoscopic transport phenomena. Simple toy models are
provided in the present study to illustrate the basics of the Fano resonance in
a simple and tractable setting.Comment: 17 pages, 1 figur
Quantum discord and non-Markovianity of quantum dynamics
The problem of recognizing (non-)Markovianity of a quantum dynamics is
revisited through analyzing quantum correlations. We argue that
instantaneously-vanishing quantum discord provides a necessary and sufficient
condition for Markovianity of a quantum map. This is used to introduce a
measure of non-Markovianity. This measure, however, requires demanding
knowledge about the system and the environment. By using a quantum correlation
monogamy property and an ancillary system, we propose a simplified measure with
less requirements. Non-Markovianity is thereby decided by quantum state
tomography of the system and the ancilla.Comment: 5 pages, 3 figure
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