712 research outputs found
Thermal properties of charge noise sources
Measurements of the temperature and bias dependence of Single Electron
Transistors (SETs) in a dilution refrigerator show that charge noise increases
linearly with refrigerator temperature above a voltage-dependent threshold
temperature, and that its low temperature saturation is due to SET
self-heating. We show further that the two-level fluctuators responsible for
charge noise are in strong thermal contact with the electrons in the SET, which
can be at a much higher temperature than the substrate. We suggest that the
noise is caused by electrons tunneling between the SET metal and nearby
potential wells
Propagating phonons coupled to an artificial atom
Quantum information can be stored in micromechanical resonators, encoded as
quanta of vibration known as phonons. The vibrational motion is then restricted
to the stationary eigenmodes of the resonator, which thus serves as local
storage for phonons. In contrast, we couple propagating phonons to an
artificial atom in the quantum regime, and reproduce findings from quantum
optics with sound taking over the role of light. Our results highlight the
similarities between phonons and photons, but also point to new opportunities
arising from the unique features of quantum mechanical sound. The low
propagation speed of phonons should enable new dynamic schemes for processing
quantum information, and the short wavelength allows regimes of atomic physics
to be explored which cannot be reached in photonic systems.Comment: 30 pages, 6 figures, 1 tabl
Nonequilibrium probing of two-level charge fluctuators using the step response of a single-electron transistor
We report a new method to study two-level fluctuators (TLFs) by measuring the offset charge induced after applying a sudden step voltage to the gate electrode of a single-electron transistor. The offset charge is measured for more than 20 h for samples made on three different substrates. We find that the offset charge drift follows a logarithmic increase over 4 orders of magnitude in time and that the logarithmic slope increases linearly with the step voltage. The charge drift is independent of temperature, ruling out thermally activated TLFs and demonstrating that the charge fluctuations involve tunneling. These observations are in agreement with expectations for an ensemble of TLFs driven out of equilibrium. From our model, we extract the density of TLFs assuming either a volume density or a surface density
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Remobilization of dormant carbon from Siberian-Arctic permafrost during three past warming events.
Carbon cycle models suggest that past warming events in the Arctic may have caused large-scale permafrost thaw and carbon remobilization, thus affecting atmospheric CO2 levels. However, observational records are sparse, preventing spatially extensive and time-continuous reconstructions of permafrost carbon release during the late Pleistocene and early Holocene. Using carbon isotopes and biomarkers, we demonstrate that the three most recent warming events recorded in Greenland ice cores-(i) Dansgaard-Oeschger event 3 (~28 ka B.P.), (ii) Bølling-Allerød (14.7 to 12.9 ka B.P.), and (iii) early Holocene (~11.7 ka B.P.)-caused massive remobilization and carbon degradation from permafrost across northeast Siberia. This amplified permafrost carbon release by one order of magnitude, particularly during the last deglaciation when global sea-level rise caused rapid flooding of the land area thereafter constituting the vast East Siberian Arctic Shelf. Demonstration of past warming-induced release of permafrost carbon provides a benchmark for the sensitivity of these large carbon pools to changing climate
Carbon Abundances of Three Carbon-Enhanced Metal-Poor Stars from High-Resolution Gemini-S/bHROS Spectra of the 8727A [C I] Line
We present the results from an analysis of the 8727ang forbidden [C I] line
in high-resolution Gemini-S/bHROS spectra of three CEMP stars. We find the
[C/Fe] ratios based on the [C I] abundances of the two most Fe-rich stars in
our sample (HIP 0507-1653: [Fe/H] = -1.42 and HIP 0054-2542: [Fe/H] = -2.66) to
be in good agreement with previously determined CH and C_2 line-based values.
For the most Fe-deficient star in our sample (HIP 1005-1439: [Fe/H] = -3.08),
however, the [C/Fe] ratio is found to be 0.34 dex lower than the published
molecular-based value. We have carried out 3D local thermodynamic equilibrium
(LTE) calculations for [C I], and the resulting corrections are found to be
modest for all three stars, suggesting that the discrepancy between the [C I]
and molecular-based C abundances of HIP 1005-1439 is due to more severe 3D
effects on the molecular lines. Carbon abundances are also derived from C I
high-excitation lines and are found to be 0.45-0.64 dex higher than the [C
I]-based abundances. Previously published non-LTE C I abundance corrections
bring the [C I] and C I abundances into better agreement; however, targeted
NLTE calculations for CEMP stars are clearly needed. We have also derived the
abundances of N, K, and Fe for each star. The Fe abundances agree well with
previously derived values, and the K abundances are similar to those of
C-normal metal-poor stars. Nitrogen abundances have been derived from resolved
lines of the CN red system. The abundances are found to be approximately 0.44
dex larger than literature values, which have been derived from CN blue bands
near 3880 and 4215 ang. We discuss evidence that suggests that analyses of the
CN blue system bands underestimate the N abundances of metal-poor giants.Comment: Accepted for publication in AJ; 42 pages, 6 figures, 7 table
Two photon annihilation of Kaluza-Klein dark matter
We investigate the fermionic one-loop cross section for the two photon
annihilation of Kaluza-Klein (KK) dark matter particles in a model of universal
extra dimensions (UED). This process gives a nearly mono-energetic gamma-ray
line with energy equal to the KK dark matter particle mass. We find that the
cross section is large enough that if a continuum signature is detected, the
energy distribution of gamma-rays should end at the particle mass with a peak
that is visible for an energy resolution of the detector at the percent level.
This would give an unmistakable signature of a dark matter origin of the
gamma-rays, and a unique determination of the dark matter particle mass, which
in the case studied should be around 800 GeV. Unlike the situation for
supersymmetric models where the two-gamma peak may or may not be visible
depending on parameters, this feature seems to be quite robust in UED models,
and should be similar in other models where annihilation into fermions is not
helicity suppressed. The observability of the signal still depends on largely
unknown astrophysical parameters related to the structure of the dark matter
halo. If the dark matter near the galactic center is adiabatically contracted
by the central star cluster, or if the dark matter halo has substructure
surviving tidal effects, prospects for detection look promising.Comment: 17 pages, 3 figures; slightly revised versio
Organic matter composition and greenhouse gas production of thawing subsea permafrost in the Laptev Sea
Subsea permafrost represents a large carbon pool that might be or become a significant greenhouse gas source. Scarcity of observational data causes large uncertainties. We here use five 21-56 m long subsea permafrost cores from the Laptev Sea to constrain organic carbon (OC) storage and sources, degradation state and potential greenhouse gas production upon thaw. Grain sizes, optically-stimulated luminescence and biomarkers suggest deposition of aeolian silt and fluvial sand over 160 000 years, with dominant fluvial/alluvial deposition of forest- and tundra-derived organic matter. We estimate an annual thaw rate of 1.3 ± 0.6 kg OC m−2 in subsea permafrost in the area, nine-fold exceeding organic carbon thaw rates for terrestrial permafrost. During 20-month incubations, CH4 and CO2 production averaged 1.7 nmol and 2.4 µmol g−1 OC d−1, providing a baseline to assess the contribution of subsea permafrost to the high CH4 fluxes and strong ocean acidification observed in the region
Abundance Analysis of HE2148-1247, A Star With Extremely Enhanced Neutron Capture Elements
Abundances for 27 elements in the very metal poor dwarf star HE2148-1247 are
presented, including many of the neutron capture elements. We establish that
HE2148-1247 is a very highly s-process enhanced star with anomalously high Eu
as well, Eu/H about half Solar, demonstrating the large addition of heavy
nuclei at [Fe/H] = -2.3 dex. Ba and La are enhanced by a somewhat larger factor
and reach the solar abundance, while Pb significantly exceeds it. Ba/Eu is ten
times the solar r-process ratio but much less than that of the s-process,
indicating a substantial r-process addition as well. C and N are also very
highly enhanced. We have found that HE2148-1247 is a radial velocity variable.
The C, N and the s-process element enhancements thus presumably were produced
through mass transfer from a former AGB binary companion. The large enhancement
of heavy r-nuclides also requires an additional source as this is far above any
inventory in the ISM at such low [Fe/H]. We further hypothesize that accretion
onto the white dwarf from the envelope of the star caused accretion induced
collapse of the white dwarf, forming a neutron star, which then produced heavy
r-nuclides and again contaminated its companion. (abridged)Comment: Accepted by the Astrophysical Journal. Companion paper by Qian and
Wasserburg follow
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