3,766 research outputs found
Physics of Proximity Josephson Sensor
We study the proximity Josephson sensor (PJS) in both bolometric and
calorimetric operation and optimize it for different temperature ranges between
25 mK and a few Kelvin. We investigate how the radiation power is absorbed in
the sensor and find that the irradiated sensor is typically in a weak
nonequilibrium state. We show in detail how the proximity of the
superconductors affects the device response: for example via changes in
electron-phonon coupling and out-of-equilibrium noise. In addition, we estimate
the applicability of graphene as the absorber material.Comment: 13 pages, 11 figures, submitted to Journal of Applied Physics, v2:
Addition of a new section discussing the radiation coupling to the device,
several minor change
Electron-phonon heat transfer in monolayer and bilayer graphene
We calculate the heat transfer between electrons to acoustic and optical
phonons in monolayer and bilayer graphene (MLG and BLG) within the
quasiequilibrium approximation. For acoustic phonons, we show how the
temperature-power laws of the electron-phonon heat current for BLG differ from
those previously derived for MLG and note that the high-temperature
(neutral-regime) power laws for MLG and BLG are also different, with a weaker
dependence on the electronic temperature in the latter. In the general case we
evaluate the heat current numerically. We suggest that a measurement of the
heat current could be used for an experimental determination of the
electron-acoustic phonon coupling constants, which are not accurately known.
However, in a typical experiment heat dissipation by electrons at very low
temperatures is dominated by diffusion, and we estimate the crossover
temperature at which acoustic-phonon coupling takes over in a sample with Joule
heating. At even higher temperatures optical phonons begin to dominate. We
study some examples of potentially relevant types of optical modes, including
in particular the intrinsic in-plane modes, and additionally the remote surface
phonons of a possible dielectric substrate.Comment: 13 pages, 8 figures; moved details to appendixes, added discussion of
remote phonon
Theory of temperature fluctuation statistics in superconductor-normal metal tunnel structures
We describe the statistics of temperature fluctuations in a SINIS structure,
where a normal metal island (N) is coupled by tunnel junctions (I) to two
superconducting leads (S). We specify conditions under which this structure
exhibits manifestly non-Gaussian fluctuations of temperature. We consider both
the Gaussian and non-Gaussian regimes of these fluctuations, and the current
fluctuations that are caused by the fluctuating temperature. We also describe a
measurement setup that could be used to observe the temperature fluctuations.Comment: 10 pages, 9 figures, final versio
Cyclostationary shot noise in mesoscopic measurements
We discuss theoretically a setup where a time-dependent current consisting of
a DC bias and two sinusoidal harmonics is driven through a sample. If the
sample exhibits current-dependent shot noise, the down-converted noise power
spectrum varies depending on the local-oscillator phase of the mixer. The
theory of this phase-dependent noise is applied to discuss the measurement of
the radio-frequency single-electron transistor. We also show that this effect
can be used to measure the shot noise accurately even in nonlinear
high-impedance samples.Comment: 3 pages, 2 figure
Meridional transport and deposition of atmospheric 10Be
10Be concentrations measured in ice cores exhibit larger temporal variability than expected based on theoretical production calculations. To investigate whether this is due to atmospheric transport a general circulation model study is performed with the 10Be production divided into stratospheric, tropospheric tropical, tropospheric subtropical and tropospheric polar sources. A control run with present day 10Be production rate is compared with a run during a geomagnetic minimum. The present 10Be production rate is 4–5 times higher at high latitudes than in the tropics whereas during a period of no geomagnetic dipole field it is constant at all latitudes. The 10Be deposition fluxes, however, show a very similar latitudinal distribution in both the present day and the geomagnetic minimum run indicating that 10Be is well mixed in the atmosphere before its deposition. This is also confirmed by the fact that the contribution of 10Be produced in the stratosphere is dominant (55%–70%) and relatively constant at all latitudes. The contribution of stratospheric 10Be is approximately 70% in Greenland and 60% in Antarctica reflecting the weaker stratosphere-troposphere air exchange in the Southern Hemisphere
Modeling cosmogenic radionuclides 10Be and 7Be during the Maunder Minimum using the ECHAM5-HAM general circulation model
All existing 10Be records from Greenland and Antarctica show increasing concentrations during the Maunder Minimum period (MM), 1645–1715, when solar activity was very low and the climate was colder (little ice age). In detail, however, the 10Be records deviate from each other. We investigate to what extent climatic changes influence the 10Be measured in ice by modeling this period using the ECHAM5-HAM general circulation model. Production calculations show that during the MM the mean global 10Be production was higher by 32% than at present due to lower solar activity. Our modeling shows that the zonally averaged modeled 10Be deposition flux deviates by only ~8% from the average increase of 32%, indicating that climatic effects are much smaller than the production change. Due to increased stratospheric production, the 10Be content in the downward fluxes is larger during MM, leading to larger 10Be deposition fluxes in the subtropics, where stratosphere-troposphere exchange (STE) is strongest. In polar regions the effect is small. In Greenland the deposition change depends on latitude and altitude. In Antarctica the change is larger in the east than in the west. We use the 10Be/7Be ratio to study changes in STE. We find larger change between 20° N–40° N during spring, pointing to a stronger STE in the Northern Hemisphere during MM. In the Southern Hemisphere the change is small. These findings indicate that climate changes do influence the 10Be deposition fluxes, but not enough to significantly disturb the production signal. Climate-induced changes remain small, especially in polar regions
Thermal conductance of a proximity superconductor
We study heat transport in hybrid normal metal - superconductor - normal
metal (NSN) structures. We find the thermal conductance of a short
superconducting wire to be strongly enhanced beyond the BCS value due to
inverse proximity effect. The measurements agree with a model based on the
quasiclassical theory of superconductivity in the diffusive limit. We determine
a crossover temperature below which quasiparticle heat conduction dominates
over the electron-phonon relaxation.Comment: 4+ pages, 3 figure
Quantum transitions induced by the third cumulant of current fluctuations
We investigate the transitions induced by external current fluctuations on a
small probe quantum system. The rates for the transitions between the energy
states are calculated using the real-time Keldysh formalism for the density
matrix evolution. We especially detail the effects of the third cumulant of
current fluctuations inductively coupled to a quantum bit and propose a setup
for detecting the frequency-dependent third cumulant through the transitions it
induces.Comment: 4 pages, 3 figure
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