336 research outputs found
Two sub-band conductivity of Si quantum well
We report on two sub-band transport in double gate SiO-Si-SiO quantum
well with 14 nm thick Si layer at 270 mK. At symmetric well potential the
experimental sub-band spacing changes monotonically from 2.3 to 0.3 meV when
the total density is adjusted by gate voltages between
m. The conductivity is mapped in large gate bias
window and it shows strong non-monotonic features. At symmetric well potential
and high density these features are addressed to sub-band wave function
delocalization in the quantization direction and to different disorder of the
top and bottom interfaces of the Si well. Close to bi-layer/second sub-band
threshold the non-monotonic behavior is interpreted to arise from scattering
from localized band tail electrons.Comment: Presented at MSS12 conference July 10-15, 2005 Albuquerque, New
Mexico, USA. Post-deadline paper, Poster PA2-293. Version 2: typos corrected,
few clarifications added in the text, summary shortened, title removed from
Ref.
Acoustic Phonon Tunneling and Heat Transport due to Evanescent Electric Fields
The authors describe how acoustic phonons can directly tunnel through vacuum
and, therefore, transmit energy and conduct heat between bodies that are
separated by a vacuum gap. This effect is enabled by introducing a coupling
mechanism, such as piezoelectricity, that strongly couples electric field and
lattice deformation. The electric field leaks into the vacuum as an evanescent
field, which leads to finite solid-vacuum-solid transmission probability. Due
to strong resonances in the system some phonons can go through the vacuum gap
with (or close to) unity transmission, which leads to significant thermal
conductance and heat flux.Comment: main text, 3 figures, supplementary materia
Flux-driven Josephson parametric amplifier for sub-GHz frequencies fabricated with side-wall passivated spacer junction technology
We present experimental results on a Josephson parametric amplifier tailored
for readout of ultra-sensitive thermal microwave detectors. In particular, we
discuss the impact of fabrication details on the performance. We show that the
small volume of deposited dielectric materials enabled by the side-wall
passivated spacer niobium junction technology leads to robust operation across
a wide range of operating temperatures up to 1.5 K. The flux-pumped amplifier
has gain in excess of 20 dB in three-wave mixing and its center frequency is
tunable between 540 MHz and 640 MHz. At 600 MHz, the amplifier adds 105 mK
9 mK of noise, as determined with the hot/cold source method.
Phase-sensitive amplification is demonstrated with the device
Dielectric losses in multi-layer Josephson junction qubits
We have measured the excited state lifetimes in Josephson junction phase and
transmon qubits, all of which were fabricated with the same scalable
multi-layer process. We have compared the lifetimes of phase qubits before and
after removal of the isolating dielectric, SiNx, and find a four-fold
improvement of the relaxation time after the removal. Together with the results
from the transmon qubit and measurements on coplanar waveguide resonators,
these measurements indicate that the lifetimes are limited by losses from the
dielectric constituents of the qubits. We have extracted the individual loss
contributions from the dielectrics in the tunnel junction barrier, AlOx, the
isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighing the total
loss with the parts of electric field over the different dielectric materials.
Our results agree well and complement the findings from other studies,
demonstrating that superconducting qubits can be used as a reliable tool for
high-frequency characterization of dielectric materials. We conclude with a
discussion of how changes in design and material choice could improve qubit
lifetimes up to a factor of four.Comment: 10 pages, 4 figures,and 4 table
Diffusion-emission theory of photon enhanced thermionic emission solar energy harvesters
Numerical and semi-analytical models are presented for
photon-enhanced-thermionic-emission (PETE) devices. The models take diffusion
of electrons, inhomogeneous photogeneration, and bulk and surface recombination
into account. The efficiencies of PETE devices with silicon cathodes are
calculated. Our model predicts significantly different electron affinity and
temperature dependence for the device than the earlier model based on a
rate-equation description of the cathode. We show that surface recombination
can reduce the efficiency below 10% at the cathode temperature of 800 K and the
concentration of 1000 suns, but operating the device at high injection levels
can increase the efficiency to 15%.Comment: 5 pages, 4 figure
Thermoelectric bolometers based on ultra-thin heavily doped single-crystal silicon membranes
We present ultra-thin silicon membrane thermocouple bolometers suitable for
fast and sensitive detection of low levels of thermal power and infrared
radiation at room temperature. The devices are based on 40 nm-thick strain
tuned single crystalline silicon membranes shaped into heater/absorber area and
narrow n- and p-doped beams, which operate as the thermocouple. The
electro-thermal characterization of the devices reveal noise equivalent power
of 13 pW/rtHz and thermal time constant of 2.5 ms. The high sensitivity of the
devices is due to the high Seebeck coefficient of 0.39 mV/K and reduction of
thermal conductivity of the Si beams from the bulk value. The bolometers
operate in the Johnson-Nyquist noise limit of the thermocouple, and the
performance improvement towards the operation close to the temperature
fluctuation limit is discussed.Comment: 11 pages, 3 figure
Traceable Coulomb Blockade Thermometry
We present a measurement and analysis scheme for determining traceable
thermodynamic temperature at cryogenic temperatures using Coulomb blockade
thermometry. The uncertainty of the electrical measurement is improved by
utilizing two sampling digital voltmeters instead of the traditional lock-in
technique. The remaining uncertainty is dominated by that of the numerical
analysis of the measurement data. Two analysis methods are demonstrated:
numerical fitting of the full conductance curve and measuring the height of the
conductance dip. The complete uncertainty analysis shows that using either
analysis method the relative combined standard uncertainty (k = 1) in
determining the thermodynamic temperature in the temperature range from 20 mK
to 200 mK is below 0.5 %. In this temperature range, both analysis methods
produced temperature estimates that deviated from 0.39 % to 0.67 % from the
reference temperatures provided by a superconducting reference point device
calibrated against the Provisional Low Temperature Scale of 2000.Comment: 11 page
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