5,496 research outputs found
Near-field coupling of gold plasmonic antennas for sub-100 nm magneto-thermal microscopy
The development of spintronic technology with increasingly dense, high-speed,
and complex devices will be accelerated by accessible microscopy techniques
capable of probing magnetic phenomena on picosecond time scales and at deeply
sub-micron length scales. A recently developed time-resolved magneto-thermal
microscope provides a path towards this goal if it is augmented with a
picosecond, nanoscale heat source. We theoretically study adiabatic
nanofocusing and near-field heat induction using conical gold plasmonic
antennas to generate sub-100 nm thermal gradients for time-resolved
magneto-thermal imaging. Finite element calculations of antenna-sample
interactions reveal focused electromagnetic loss profiles that are either
peaked directly under the antenna or are annular, depending on the sample's
conductivity, the antenna's apex radius, and the tip-sample separation. We find
that the thermal gradient is confined to 40 nm to 60 nm full width at half
maximum for realistic ranges of sample conductivity and apex radius. To
mitigate this variation, which is undesirable for microscopy, we investigate
the use of a platinum capping layer on top of the sample as a thermal
transduction layer to produce heat uniformly across different sample materials.
After determining the optimal capping layer thickness, we simulate the
evolution of the thermal gradient in the underlying sample layer, and find that
the temporal width is below 10 ps. These results lay a theoretical foundation
for nanoscale, time-resolved magneto-thermal imaging.Comment: 24 pages including Supporting Information, 6 figures in the main
text, 4 supporting figure
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A refined model for the Earth's global energy balance
A commonly-used model of the global radiative budget assumes that the radiative response to forcing, R, is proportional to global surface air temperature T, R=λT . Previous studies have highlighted two unresolved issues with this model: first, the feedback parameter λ depends on the forcing agent; second, λ varies with time. Here, we investigate the factors controlling R in two atmosphere–slab ocean climate models subjected to a wide range of abrupt climate forcings. It is found that R scales not only with T, but also with the large-scale tropospheric stability S (defined here as the estimated inversion strength area-averaged over ocean regions equatorward of 50∘). Positive S promotes negative R, mainly through shortwave cloud and lapse-rate changes. A refined model of the global energy balance is proposed that accounts for both temperature and stability effects. This refined model quantitatively explains (1) the dependence of climate feedbacks on forcing agent (or equivalently, differences in forcing efficacy), and (2) the time evolution of feedbacks in coupled climate model experiments. Furthermore, a similar relationship between R and S is found in observations compared with models, lending confidence that the refined energy balance model is applicable to the real world
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The inconstancy of the transient climate response parameter under increasing COâ‚‚
In the Coupled Model Intercomparison Project Phase 5 (CMIP5), the model-mean increase in global mean surface air temperature T under the 1pctCO2 scenario (atmospheric CO2 increasing at 1% yr−1) during the second doubling of CO2 is 40% larger than the transient climate response (TCR), i.e. the increase in T during the first doubling. We identify four possible contributory effects. First, the surface climate system loses heat less readily into the ocean beneath as the latter warms. The model spread in the thermal coupling between the upper and deep ocean largely explains the model spread in ocean heat uptake efficiency. Second, CO2 radiative forcing may rise more rapidly than logarithmically with CO2 concentration. Third, the climate feedback parameter may decline as the CO2 concentration rises. With CMIP5 data, we cannot distinguish the second and third possibilities. Fourth, the climate feedback parameter declines as time passes or T rises; in 1pctCO2, this effect is less important than the others. We find that T projected for the end of the twenty-first century correlates more highly with T at the time of quadrupled CO2 in 1pctCO2 than with the TCR, and we suggest that the TCR may be underestimated from observed climate change
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Global reconstruction of historical ocean heat storage and transport
Most of the excess energy stored in the climate system due to anthropogenic
greenhouse gas emissions has been taken up by the oceans,
leading to thermal expansion and sea level rise. The oceans thus
have an important role in the Earth’s energy imbalance. Observational
constraints on future anthropogenic warming critically depend
on accurate estimates of past ocean heat content (OHC) change. We
present a novel reconstruction of OHC since 1871, with global coverage
of the full ocean depth. Our estimates combine timeseries of
observed sea surface temperatures, with much longer historical coverage
than those in the ocean interior, together with a representation
(a Green’s function) of time-independent ocean transport processes.
For 1955-2017, our estimates are comparable to direct estimates
made by infilling the available 3D time-dependent ocean temperature
observations. We find that the global ocean absorbed heat during
this period at a rate of 0.30 ± 0.06 W/m2
in the upper 2000 m and
0.028 ± 0.026 W/m2 below 2000 m, with large decadal fluctuations.
The total OHC change since 1871 is estimated at 436 ±91 × 1021 J,
with an increase during 1921-1946 (145 ± 62× 1021 J) that is as large
as during 1990-2015. By comparing with direct estimates, we also
infer that, during 1955-2017, up to half of the Atlantic Ocean warming
and thermosteric sea level rise at low-to-mid latitudes emerged due
to heat convergence from changes in ocean transport
Teaching Accounting Courses Online: One Instructors Experience
Accounting programs at colleges and universities across the country are offering more online courses, and in some cases entire degree programs. Given increasing enrollments in online accounting education, it is important that accounting educators become aware of the things that work and the things that dont work when delivering courses over the internet. This paper addresses the good and bad aspects of teaching accounting online. It looks at optimal ways to integrate state-of-the-art technology into an online teaching model and explores ways to increase effectiveness of online delivery as compared to traditional courses. This paper examines teaching accounting courses online at a small liberal arts school. Our research identifies features in the course that led to its effectiveness
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The dependence of radiative forcing and feedback on evolving patterns of surface temperature change in climate models
Experiments with CO2 instantaneously quadrupled and then held constant are used to show that the relationship between the global-mean net heat input to the climate system and the global-mean surface-air-temperature change is nonlinear in Coupled Model Intercomparison Project phase 5 (CMIP5) Atmosphere-Ocean General Circulation Models (AOGCMs). The nonlinearity is shown to arise from a change in strength of climate feedbacks driven by an evolving pattern of surface warming. In 23 out of the 27 AOGCMs examined the climate feedback parameter becomes significantly (95% confidence) less negative – i.e. the effective climate sensitivity increases – as time passes. Cloud feedback parameters show the largest changes. In the AOGCM-mean approximately 60% of the change in feedback parameter comes from the topics (30N-30S). An important region involved is the tropical Pacific where the surface warming intensifies in the east after a few decades. The dependence of climate feedbacks on an evolving pattern of surface warming is confirmed using the HadGEM2 and HadCM3 atmosphere GCMs (AGCMs). With monthly evolving sea-surface-temperatures and sea-ice prescribed from its AOGCM counterpart each AGCM reproduces the time-varying feedbacks, but when a fixed pattern of warming is prescribed the radiative response is linear with global temperature change or nearly so. We also demonstrate that the regression and fixed-SST methods for evaluating effective radiative forcing are in principle different, because rapid SST adjustment when CO2 is changed can produce a pattern of surface temperature change with zero global mean but non-zero change in net radiation at the top of the atmosphere (~ -0.5 Wm-2 in HadCM3)
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