409 research outputs found
Monolayer graphene bolometer as a sensitive far-IR detector
In this paper we give a detailed analysis of the expected sensitivity and
operating conditions in the power detection mode of a hot-electron bolometer
(HEB) made from a few {\mu}m of monolayer graphene (MLG) flake which can be
embedded into either a planar antenna or waveguide circuit via NbN (or NbTiN)
superconducting contacts with critical temperature ~ 14 K. Recent data on the
strength of the electron-phonon coupling are used in the present analysis and
the contribution of the readout noise to the Noise Equivalent Power (NEP) is
explicitly computed. The readout scheme utilizes Johnson Noise Thermometry
(JNT) allowing for Frequency-Domain Multiplexing (FDM) using narrowband filter
coupling of the HEBs. In general, the filter bandwidth and the summing
amplifier noise have a significant effect on the overall system sensitivity.
The analysis shows that the readout contribution can be reduced to that of the
bolometer phonon noise if the detector device is operated at 0.05 K and the JNT
signal is read at about 10 GHz where the Johnson noise emitted in equilibrium
is substantially reduced. Beside the high sensitivity (NEP < 10
W/Hz, this bolometer does not have any hard saturation limit and thus
can be used for far-IR sky imaging with arbitrary contrast. By changing the
operating temperature of the bolometer the sensitivity can be fine tuned to
accommodate the background photon flux in a particular application. By using a
broadband low-noise kinetic inductance parametric amplifier, ~100s of graphene
HEBs can be read simultaneously without saturation of the system output.Comment: 9 pages. 6 figure, SPIE Astronomical Telescopes + Instrumentation,
Montr\'eal, Quebec, Canada, 22-27 June, 201
Energy resolution of terahertz single-photon-sensitive bolometric detectors
We report measurements of the energy resolution of ultra-sensitive
superconducting bolometric detectors. The device is a superconducting titanium
nanobridge with niobium contacts. A fast microwave pulse is used to simulate a
single higher-frequency photon, where the absorbed energy of the pulse is equal
to the photon energy. This technique allows precise calibration of the input
coupling and avoids problems with unwanted background photons. Present devices
have an intrinsic full-width at half-maximum energy resolution of approximately
23 terahertz, near the predicted value due to intrinsic thermal fluctuation
noise.Comment: 11 pages (double-spaced), 5 figures; minor revision
Decisions at the Brink: Locomotor Experience Affects Infantsâ Use of Social Information on an Adjustable Drop-off
How do infants decide what to do at the brink of a precipice? Infants could use two sources of information to guide their actions: perceptual information generated by their own exploratory activity and social information offered by their caregivers. The current study investigated the role of locomotor experience in using social informationâboth encouragement and discouragementâfor descending drop-offs. Mothers of 30 infants (experienced 12-month-old crawlers, novice 12-month-old walkers, and experienced 18-month-old walkers) encouraged and discouraged descent on a gradation of dropoffs (safe âstepsâ and risky âcliffsâ). Novice walkers descended more frequently than experienced crawlers and walkers and fell while attempting to walk over impossibly high cliffs. All infants showed evidence of integrating perceptual and social information, but locomotor experience affected infantsâ use of social messages, especially on risky dropoffs. Experienced crawlers and walkers selectively deferred to social information when perceptual information is ambiguous. In contrast, novice walkers took mothersâ advice inconsistently and only at extreme drop-offs
Small Black Holes on Branes: Is the horizon regular or singular ?
We investigate the following question: Consider a small mass, with
(the ratio of the Schwarzschild radius and the bulk curvature length) much
smaller than 1, that is confined to the TeV brane in the Randall-Sundrum I
scenario. Does it form a black hole with a regular horizon, or a naked
singularity? The metric is expanded in and the asymptotic form of
the metric is given by the weak field approximation (linear in the mass). In
first order of we show that the iteration of the weak field
solution, which includes only integer powers of the mass, leads to a solution
that has a singular horizon. We find a solution with a regular horizon but its
asymptotic expansion in the mass also contains half integer powers.Comment: Accepted for publication in PR
The ties that bind: Cradling in Tajikistan
A traditional childrearing practiceââgahvoraâ cradlingâin Tajikistan and other parts of Central Asia purportedly restricts movement of infantsâ body and limbs. However, the practice has been documented only informally in anecdotal reports. Thus, this study had two research questions: (1) To what extent are infantsâ movements restricted in the gahvora? (2) How is time in the gahvora distributed over a 24-hour day in infants from 1â24 months of age? To answer these questions, we video-recorded 146 mothers cradling their infants and interviewed them using 24-hour time diaries to determine the distribution of time infants spent in the gahvora within a day and across age. Infantsâ movements were indeed severely restricted. Although mothers showed striking uniformity in how they restricted infantsâ movements, they showed large individual differences in amount and distribution of daily use. Machine learning algorithms yielded three patterns of use: day and nighttime cradling, mostly nighttime cradling, and mostly daytime cradling, suggesting multiple functions of the cradling practice. Across age, time in the gahvora decreased, yet 20% of 12- to 24-montholds spent more than 15 hours bound in the gahvora. We discuss the challenges and benefits of cultural research, and how the discovery of new phenomena may defy Western assumptions about childrearing and development. Future work will determine whether the extent and timing of restriction impacts infantsâ physical and psychological development
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