19,256 research outputs found
Improved position measurement of nano electromechanical systems using cross correlations
We consider position measurements using the cross-correlated output of two
tunnel junction position detectors. Using a fully quantum treatment, we
calculate the equation of motion for the density matrix of the coupled
detector-detector-mechanical oscillator system. After discussing the presence
of a bound on the peak-to-background ratio in a position measurement using a
single detector, we show how one can use detector cross correlations to
overcome this bound. We analyze two different possible experimental
realizations of the cross correlation measurement and show that in both cases
the maximum cross-correlated output is obtained when using twin detectors and
applying equal bias to each tunnel junction. Furthermore, we show how the
double-detector setup can be exploited to drastically reduce the added
displacement noise of the oscillator.Comment: 9 pages, 1 figure; v2: new Sec.
Charge and momentum transfer in supercooled melts: Why should their relaxation times differ?
The steady state values of the viscosity and the intrinsic ionic-conductivity
of quenched melts are computed, in terms of independently measurable
quantities. The frequency dependence of the ac dielectric response is
estimated. The discrepancy between the corresponding characteristic relaxation
times is only apparent; it does not imply distinct mechanisms, but stems from
the intrinsic barrier distribution for -relaxation in supercooled
fluids and glasses. This type of intrinsic ``decoupling'' is argued not to
exceed four orders in magnitude, for known glassformers. We explain the origin
of the discrepancy between the stretching exponent , as extracted from
and the dielectric modulus data. The actual width of the
barrier distribution always grows with lowering the temperature. The contrary
is an artifact of the large contribution of the dc-conductivity component to
the modulus data. The methodology allows one to single out other contributions
to the conductivity, as in ``superionic'' liquids or when charge carriers are
delocalized, implying that in those systems, charge transfer does not require
structural reconfiguration.Comment: submitted to J Chem Phy
Strong Correlation to Weak Correlation Phase Transition in Bilayer Quantum Hall Systems
At small layer separations, the ground state of a nu=1 bilayer quantum Hall
system exhibits spontaneous interlayer phase coherence and has a
charged-excitation gap E_g. The evolution of this state with increasing layer
separation d has been a matter of controversy. In this letter we report on
small system exact diagonalization calculations which suggest that a single
phase transition, likely of first order, separates coherent incompressible (E_g
>0) states with strong interlayer correlations from incoherent compressible
states with weak interlayer correlations. We find a dependence of the phase
boundary on d and interlayer tunneling amplitude that is in very good agreement
with recent experiments.Comment: 4 pages, 4 figures included, version to appear in Phys. Rev. Let
Temperature dependent carrier lifetime studies on Ti-doped multicrystalline silicon
Carrier lifetimemeasurements were performed on deliberately Ti-doped multicrystalline silicon wafers using a temperature controlled photoconductance device. The dominant recombination center was found to be the double-donor level associated with interstitial titanium. The interstitial Ti concentrations in multicrystalline silicon wafers were determined by measuring the ShockleyâReadâHall time constant for holes and using the known values of the thermal velocity and capture cross section for holes of the double-donor level at different temperatures. The measured values of the Ti concentration were then used to determine the electron capture cross section of the double-donor level over the temperature range of 140â270â°C via the measured values of the ShockleyâReadâHall time constant for electrons and the known thermal velocity. Multiphonon emission was found to be the most likely capture mechanism for this temperature range for electron capture into the double-donor level of Ti in silicon. The effective segregation coefficient for Ti was estimated by fitting Scheilâs equation to the measured values of the Ti concentrations and their respective vertical positions in the ingot. If all Ti were present as the interstitial double-donor, a lower limit of 1.8Ă10â»â¶ can be ascribed to the segregation coefficient, which is very close to the equilibrium value.This work was funded by an Australian Research
Council Linkage Grant between the Australian National
University, SierraTherm Production Furnaces, and
SunPower Corporation. D.H.M. is supported by an Australian
Research Council fellowship
Electrical transport through a single-electron transistor strongly coupled to an oscillator
We investigate electrical transport through a single-electron transistor
coupled to a nanomechanical oscillator. Using a combination of a
master-equation approach and a numerical Monte Carlo method, we calculate the
average current and the current noise in the strong-coupling regime, studying
deviations from previously derived analytic results valid in the limit of
weak-coupling. After generalizing the weak-coupling theory to enable the
calculation of higher cumulants of the current, we use our numerical approach
to study how the third cumulant is affected in the strong-coupling regime. In
this case, we find an interesting crossover between a weak-coupling transport
regime where the third cumulant heavily depends on the frequency of the
oscillator to one where it becomes practically independent of this parameter.
Finally, we study the spectrum of the transport noise and show that the two
peaks found in the weak-coupling limit merge on increasing the coupling
strength. Our calculation of the frequency-dependence of the noise also allows
to describe how transport-induced damping of the mechanical oscillations is
affected in the strong-coupling regime.Comment: 11 pages, 9 figure
Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves
We have experimentally determined the spin-dependent Seebeck coefficient of
permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices.
The devices were specifically designed to completely separate heat related
effects from charge related effects. A pure heat current through the nanopillar
spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic
layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck
coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where
S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and
spin-down electrons. By using a three-dimensional finite-element model (3D-FEM)
based on spin-dependent thermoelectric theory, whose input material parameters
were measured in separate devices, we were able to accurately determine a
spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5
microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a
Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where
S_{F} is the Seebeck coefficient of the ferromagnet. The results are in
agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent
Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve
structures
Edge spin accumulation in semiconductor two-dimensional hole gases
The controlled generation of localized spin densities is a key enabler of
semiconductor spintronics In this work, we study spin Hall effect induced edge
spin accumulation in a two-dimensional hole gas with strong spin orbit
interactions. We argue that it is an intrinsic property, in the sense that it
is independent of the strength of disorder scattering. We show numerically that
the spin polarization near the edge induced by this mechanism can be large, and
that it becomes larger and more strongly localized as the spin-orbit coupling
strength increases, and is independent of the width of the conducting strip
once this exceeds the elastic scattering mean-free-path. Our experiments in
two-dimensional hole gas microdevices confirm this remarkable spin Hall effect
phenomenology. Achieving comparable levels of spin polarization by external
magnetic fields would require laboratory equipment whose physical dimensions
and operating electrical currents are million times larger than those of our
spin Hall effect devices.Comment: 6 pages, 5 figure
Variations in signal-to-noise characteristics of tissue-equivalent attenuators for mammographic automatic exposure control system performance evaluation
PURPOSE: This work investigates the impact of tissue-equivalent attenuator choice on measured signal-to-noise ratio (SNR) for automatic exposure control (AEC) performance evaluation in digital mammography. It also investigates how the SNR changes for each material when used to evaluate AEC performance across different mammography systems.
METHODS: AEC performance was evaluated for four mammography systems using seven attenuator sets at two thicknesses (4 and 8 cm). All systems were evaluated in 2D imaging mode, and one system was evaluated in digital breast tomosynthesis (DBT) mode. The methodology followed the 2018 ACR digital mammography quality control (DMQC) manual. Each system-attenuator-thickness combination was evaluated using For Processing images in ImageJ with standard ROI size and location. The closest annual physicist testing results were used to explore the impact of varying measured AEC performance on image quality.
RESULTS: The measured SNR varied by 44%-54% within each system across all attenuators at 4 cm thickness in 2D mode. The variation appeared to be largely due to changes in measured noise, with variations of 46%-67% within each system across all attenuators at 4 cm thickness in 2D mode. Two systems had failing SNR levels for two of the materials using the minimum SNR criterion specified in the ACR DMQC manual. Similar trends were seen in DBT mode and at 8 cm thickness. Within each material, there was 115%-131% variation at 4 cm and 82%-114% variation at 8 cm in the measured SNR across the four imaging systems. Variation in SNR did not correlate with system operating level based on visual image quality and average glandular dose (AGD).
CONCLUSION: Choice of tissue-equivalent attenuator for AEC performance evaluation affects measured SNR values. Depending on the material, the difference may be enough to result in failure following the longitudinal and absolute thresholds specified in the ACR DMQC manual
Chalcogenide phase change materials for nanoscale switching
Since the demonstration of threshold switching in chalcogenide alloys over forty five years ago, phase change materials have been extensively investigated for switching and data storage applications. Phase change switching is based on the reversible change between crystalline and amorphous states of a material and in many chalcogenides this change of state takes place in nanoseconds
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