35,497 research outputs found
A broadband microwave Corbino spectrometer at He temperatures and high magnetic fields
We present the technical details of a broadband microwave spectrometer for
measuring the complex conductance of thin films covering the range from 50 MHz
up to 16 GHz in the temperature range 300 mK to 6 K and at applied magnetic
fields up to 8 Tesla. We measure the complex reflection from a sample
terminating a coaxial transmission line and calibrate the signals with three
standards with known reflection coefficients. Thermal isolation of the heat
load from the inner conductor is accomplished by including a section of NbTi
superconducting cable (transition temperature around 8 9 K) and hermetic
seal glass bead adapters. This enables us to stabilize the base temperature of
the sample stage at 300 mK. However, the inclusion of this superconducting
cable complicates the calibration procedure. We document the effects of the
superconducting cable on our calibration procedure and the effects of applied
magnetic fields and how we control the temperature with great repeatability for
each measurement. We have successfully extracted reliable data in this
frequency, temperature and field range for thin superconducting films and
highly resistive graphene samples
Reduction of Effective Terahertz Focal Spot Size By Means Of Nested Concentric Parabolic Reflectors
An ongoing limitation of terahertz spectroscopy is that the technique is
generally limited to the study of relatively large samples of order 4 mm across
due to the generally large size of the focal beam spot. We present a nested
concentric parabolic reflector design which can reduce the terahertz focal spot
size. This parabolic reflector design takes advantage of the feature that
reflected rays experience a relative time delay which is the same for all
paths. The increase in effective optical path for reflected light is equivalent
to the aperture diameter itself. We have shown that the light throughput of an
aperture of 2 mm can be increased by a factor 15 as compared to a regular
aperture of the same size at low frequencies. This technique can potentially be
used to reduce the focal spot size in terahertz spectroscopy and enable the
study of smaller samples
The effect of electromechanical coupling on the strain in AlGaN/GaN heterojunction field effect transistors
The strain in AlGaN/GaN heterojunction field-effect transistors (HFETs) is
examined theoretically in the context of the fully-coupled equation of state
for piezoelectric materials. Using a simple analytical model, it is shown that,
in the absence of a two-dimensional electron gas (2DEG), the out-of-plane
strain obtained without electromechanical coupling is in error by about 30% for
an Al fraction of 0.3. This result has consequences for the calculation of
quantities that depend directly on the strain tensor. These quantities include
the eigenstates and electrostatic potential in AlGaN/GaN heterostructures. It
is shown that for an HFET, the electromechanical coupling is screened by the
2DEG. Results for the electromechanical model, including the 2DEG, indicate
that the standard (decoupled) strain model is a reasonable approximation for
HFET calculataions. The analytical results are supported by a self-consistent
Schr\"odinger-Poisson calculation that includes the fully-coupled equation of
state together with the charge-balance equation.Comment: 6 figures, revte
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