289 research outputs found
Optimization of sample-chip design for stub-matched radio-frequency reflectometry measurements
A radio-frequency (rf) matching circuit with an in situ tunable varactor
diode used for rf reflectometry measurements in semiconductor nanostructures is
investigated and used to optimize the sample-specific chip design. The samples
are integrated in a 2-4 GHz stub-matching circuit consisting of a waveguide
stub shunted to the terminated coplanar waveguide. Several quantum point
contacts fabricated on a GaAs/AlGaAs heterostructure with different chip
designs are compared. We show that the change of the reflection coefficient for
a fixed change in the quantum point contact conductance can be enhanced by a
factor of 3 compared to conventional designs by a suitable electrode geometry
Radio-frequency operation of a double-island single-electron transistor
We present results on a double-island single-electron transistor (DISET)
operated at radio-frequency (rf) for fast and highly sensitive detection of
charge motion in the solid state. Using an intuitive definition for the charge
sensitivity, we compare a DISET to a conventional single-electron transistor
(SET). We find that a DISET can be more sensitive than a SET for identical,
minimum device resistances in the Coulomb blockade regime. This is of
particular importance for rf operation where ideal impedance matching to 50 Ohm
transmission lines is only possible for a limited range of device resistances.
We report a charge sensitivity of 5.6E-6 e/sqrt(Hz) for a rf-DISET, together
with a demonstration of single-shot detection of small (<=0.1e) charge signals
on microsecond timescales.Comment: 6 pages, 6 figure
Sensitivity of spin-torque diodes for frequency-tunable resonant microwave detection
We calculate the efficiency with which magnetic tunnel junctions can be used
as resonant detectors of incident microwave radiation via the spin-torque diode
effect. The expression we derive is in good agreement with the sensitivities we
measure for MgO-based magnetic tunnel junctions with an extended (unpatterned)
magnetic pinned layer. However, the measured sensitivities are reduced below
our estimate for a second set of devices in which the pinned layer is a
patterned synthetic antiferromagnet (SAF). We suggest that this reduction may
be due to an undesirable coupling between the magnetic free layer and one of
the magnetic layers within the etched SAF. Our calculations suggest that
optimized tunnel junctions should achieve sensitivities for resonant detection
exceeding 10,000 mV/mW.Comment: 17 pages, 2 figure
Development and operation of the twin radio frequency single electron transistor for solid state qubit readout
Ultra-sensitive detectors and readout devices based on the radio frequency
single electron transistor (rf-SET) combine near quantum-limited sensitivity
with fast operation. Here we describe a twin rf-SET detector that uses two
superconducting rf-SETs to perform fast, real-time cross-correlated
measurements in order to distinguish sub-electron signals from charge noise on
microsecond time-scales. The twin rf-SET makes use of two tuned resonance
circuits to simultaneously and independently address both rf-SETs using
wavelength division multiplexing (WDM) and a single cryogenic amplifier. We
focus on the operation of the twin rf-SET as a charge detector and evaluate the
cross-talk between the two resonance circuits. Real time suppression of charge
noise is demonstrated by cross correlating the signals from the two rf-SETs.
For the case of simultaneous operation, the rf-SETs had charge sensitivities of
and .Comment: Updated version, including new content. Comments most welcome:
[email protected] or [email protected]
Formation of robust and completely tunable resonant photonic band gaps
We identify different types of the photonic band gaps (PBGs) of two
dimensional magnetic photonic crystals (MPCs) consisting of arrays of magnetic
cylinders and study the different tunability (by an external static magnetic
field) of these PBGs. One type of the band gaps comes from infinitely
degenerate flat bands and is closely related to those in the study of
plasmonics. In addition, such PBGs are magnetically tunable and robust against
position disorder. We calcualte the transmission of the PBG's and found
excellent agreement with the results of the photonic band structure
calculation. Positional disorder of the lattice structure affects the different
types of PBGs differently.Comment: 4 pages, 5 figure
Nanoscale broadband transmission lines for spin qubit control
The intense interest in spin-based quantum information processing has caused
an increasing overlap between two traditionally distinct disciplines, such as
magnetic resonance and nanotechnology. In this work we discuss rigourous design
guidelines to integrate microwave circuits with charge-sensitive
nanostructures, and describe how to simulate such structures accurately and
efficiently. We present a new design for an on-chip, broadband, nanoscale
microwave line that optimizes the magnetic field driving a spin qubit, while
minimizing the disturbance on a nearby charge sensor. This new structure was
successfully employed in a single-spin qubit experiment, and shows that the
simulations accurately predict the magnetic field values even at frequencies as
high as 30 GHz.Comment: 18 pages, 8 figures, 1 table, pdflate
The microscopic structure of cold aqueous methanol mixtures
The evolution of the micro-segregated structure of aqueous methanol mixtures, in the temperature range 300 K-120 K, is studied with computer simulations, from the static structural point of view. The structural heterogeneity of water is reinforced at lower temperatures, as witnessed by a pre-peak in the oxygen-oxygen structure factor. Water tends to form predominantly chain-like clusters at lower temperatures and smaller concentrations. Methanol domains have essentially the same chain-like cluster structure as the pure liquid at high concentrations and becomes mono- meric at smaller ones. Concentration uctuations decrease with temperature, leading to quasi-ideal Kirkwood-Bu integrals, despite the enhanced molecular interactions, which we interpret as the signature of non-interacting segregated water and methanol clusters. This study throws a new light on the nature of the micro-heterogeneous structure of this mixture: the domain segregation is essentially based on the appearance of linear water clusters, unlike other alcohol aqueous mixtures, such as with propanol or butanol, where the water domains are more bulky.
Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces
The concept of optical phase discontinuities is applied to the design and
demonstration of aberration-free planar lenses and axicons, comprising a phased
array of ultrathin subwavelength spaced optical antennas. The lenses and
axicons consist of radial distributions of V-shaped nanoantennas that generate
respectively spherical wavefronts and non-diffracting Bessel beams at telecom
wavelengths. Simulations are also presented to show that our aberration-free
designs are applicable to high numerical aperture lenses such as flat
microscope objectives
A dual tuned Complementary Structure Frequency Selective surface for WLAN applications
Nosema ceranae (Microsporidia: Nosematidae) Does Not Cause Collapse of Colonies of Africanized Apis mellifera (Hymenoptera: Apidae) in Tropical Climate
Nosemosis is an important disease that affects honey bees (Apis mellifera Lineu), caused by obligate intracellular parasites, Nosema apis and/or Nosema ceranae. Since the initial detection of N. ceranae in A. mellifera coincided with recent large-scale losses of bee colonies worldwide, the impacts of this parasite under field conditions are of great interest. Here we test two hypotheses, the first one, whether the climatic variables (temperature, air humidity and precipitation) influence the intensity of infection of the microsporidium Nosema spp. in Africanized honey bees (Apis mellifera), and the second, whether the local of hive installation (outdoor or roofed) influences the intensity of infection of these spores in Africanized honey bees. Between August 2013 and August 2016, samples of Africanized bees were collected weekly from 20 colonies, of which ten were located in an open area (outdoor apiary) and ten under a roof on a concrete floor (roofed apiary). N. ceranae was the only species present. The type of apiary did not influence (p > 0.05) the number of spores of N. ceranae in Africanized bees. However, the infection intensities of the roofed apiary colonies were lower in the autumn. Regarding the meteorological parameters, there was a negative correlation between the winter infection intensities and the minimum temperature in the roofed apiary and the humidity in the outdoor apiary. The highest infection intensities occurred in both apiaries in the spring and summer, which may be related to higher pollen production. On average, the infection intensity was 16.19 ± 15.81 x 105, ranging from zero to 100.5 x105, and there were no records of collapse during the three years
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