807 research outputs found
Josephson parametric reflection amplifier with integrated directionality
A directional superconducting parametric amplifier in the GHz frequency range
is designed and analyzed, suitable for low-power read-out of microwave kinetic
inductance detectors employed in astrophysics and when combined with a
nonreciprocal device at its input also for circuit quantum electrodynamics
(cQED). It consists of an one wavelength long nondegenerate Josephson
parametric reflection amplifier circuit. The device has two Josephson junction
oscillators, connected via a tailored impedance to an on-chip passive circuit
which directs the in- to the output port. The amplifier provides a gain of 20
dB over a bandwidth of 220 MHz on the signal as well as on the idler portion of
the amplified input and the total photon shot noise referred to the input
corresponds to maximally 1.3 photons per second per Hertz of bandwidth. We
predict a factor of four increase in dynamic range compared to conventional
Josephson parametric amplifiers.Comment: Main article (5 pages plus 2 pages references) plus supplemental
material (14 pages
Engineering physics of superconducting hot-electron bolometer mixers
Superconducting hot-electron bolometers are presently the best performing
mixing devices for the frequency range beyond 1.2 THz, where good quality
superconductor-insulator-superconductor (SIS) devices do not exist. Their
physical appearance is very simple: an antenna consisting of a normal metal,
sometimes a normal metal-superconductor bilayer, connected to a thin film of a
narrow, short superconductor with a high resistivity in the normal state. The
device is brought into an optimal operating regime by applying a dc current and
a certain amount of local- oscillator power. Despite this technological
simplicity its operation has been found to be controlled by many different
aspects of superconductivity, all occurring simultaneously. A core ingredient
is the understanding that there are two sources of resistance in a
superconductor: a charge conversion resistance occurring at an
normal-metal-superconductor interface and a resistance due to time- dependent
changes of the superconducting phase. The latter is responsible for the actual
mixing process in a non-uniform superconducting environment set up by the
bias-conditions and the geometry. The present understanding indicates that
further improvement needs to be found in the use of other materials with a
faster energy-relaxation rate. Meanwhile several empirical parameters have
become physically meaningful indicators of the devices, which will facilitate
the technological developments.Comment: This is an author-processed copy of an Invited contribution to the
Special Issue of the IEEE Transactions on Terahertz Science and Technology
dedicated to the 28th IEEE International Symposium on Space Terahertz
Technology (ISSTT2017
Glass Transition in a Two-Dimensional Electron System in Silicon in a Parallel Magnetic Field
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional electron system (2DES) in Si in the vicinity of the
metal-insulator transition (MIT) persists in parallel magnetic fields B of up
to 9 T. At low B, both the glass transition density and , the
critical density for the MIT, increase with B such that the width of the
metallic glass phase () increases with B. At higher B, where the
2DES is spin polarized, and no longer depend on B. Our results
demonstrate that charge, as opposed to spin, degrees of freedom are responsible
for glassy ordering of the 2DES near the MIT.Comment: 4 pages, 5 figure
Transport properties of an electron-hole bilayer/superconductor hybrid junction
We investigate the transport properties of a junction consisting of an
electron-hole bilayer in contact with normal and superconducting leads. The
electron-hole bilayer is considered as a semi-metal with two electronic bands.
We assume that in the region between the contacts the system hosts an exciton
condensate described by a BCS-like model with a gap in the
quasiparticle density of states. We first discuss how the subgap electronic
transport through the junction is mainly governed by the interplay between two
kinds of reflection processes at the interfaces: The standard Andreev
reflection at the interface between the superconductor and the exciton
condensate, and a coherent crossed reflection at the
semi-metal/exciton-condensate interface that converts electrons from one layer
into the other. We show that the differential conductance of the junction shows
a minimum at voltages of the order of . Such a minimum can be seen as
a direct hallmark of the existence of the gapped excitonic state
Anaerobic membrane bioreactors: Are membranes really necessary?
Membranes themselves represent a significant cost for the full scale application of anaerobic membrane bioreactors (AnMBR). The possibility of operating an AnMBR with a self-forming dynamic membrane generated by the substances present in the reactor liquor would translate into an important saving. A self-forming dynamic membrane only requires a support material over which a cake layer is formed, which determines the rejection properties of the system. The present research studies the application of self-forming dynamic membranes in AnMBRs. An AnMBR was operated under thermophilic and mesophilic conditions, using woven and non woven materials as support for the dynamic membranes. Results showed that the formation of a cake layer over the support materials enables the retention of more than 99% of the solids present in the reactor. However, only low levels of flux were achieved, up to 3 L/m2 x h, and reactor operation was unstable, with sudden increases in filtration resistance, due to excessive cake layer formation. Further fine-tuning of the proposed technology involves looking for conditions that can control effectively cake layer formatio
Metal-insulator transition and glassy behavior in two-dimensional electron systems
Studies of low-frequency resistance noise demonstrate that glassy freezing
occurs in a two-dimensional electron system in silicon in the vicinity of the
metal-insulator transition (MIT). The width of the metallic glass phase, which
separates the 2D metal and the (glassy) insulator, depends strongly on
disorder, becoming extremely small in high-mobility (low-disorder) samples. The
glass transition is manifested by a sudden and dramatic slowing down of the
electron dynamics, and by a very abrupt change to the sort of statistics
characteristic of complicated multistate systems. In particular, the behavior
of the second spectrum, an important fourth-order noise statistic, indicates
the presence of long-range correlations between fluctuators in the glassy
phase, consistent with the hierarchical picture of glassy dynamics.Comment: Contribution to conference on "Noise as a tool for studying
materials" (SPIE), Santa Fe, New Mexico, June 2003; 15 pages, 12 figs.
(includes some low-quality figs; send e-mail to get high-quality figs.
Dynamics of spin transport in voltage-biased Josephson junctions
We investigate spin transport in voltage-biased spin-active Josephson
junctions. The interplay of spin filtering, spin mixing, and multiple Andreev
reflection leads to nonlinear voltage dependence of the dc and ac spin current.
We compute the voltage characteristics of the spin current (I_S) for
superconductor-ferromagnet-superconductor (SFS) Josephson junctions. The
sub-harmonic gap structure of I_S(V) is shown to be sensitive to the degree of
spin mixing generated by the ferromagnetic interface, and exhibits a pronounced
even-odd effect associated with spin transport during multiple Andreev
reflection processes. For strong spin mixing both the magnitude and the
direction of the dc spin current can be sensitively controlled by the bias
voltage.Comment: 4 pages, 3 figure
Scaling of nano-Schottky-diodes
A generally applicable model is presented to describe the potential barrier
shape in ultra small Schottky diodes. It is shown that for diodes smaller than
a characteristic length (associated with the semiconductor doping level)
the conventional description no longer holds. For such small diodes the
Schottky barrier thickness decreases with decreasing diode size. As a
consequence, the resistance of the diode is strongly reduced, due to enhanced
tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky
barrier height, this effect provides an explanation for several experimental
observations of enhanced conduction in small Schottky diodes.Comment: 4 pages, 4 figures, accepted for publication in Appl. Phys. Lett.,
some minor additions and correction
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