7,580 research outputs found
Permafrost - physical aspects and carbon cycling, databases and uncertainties
Permafrost is defined as ground that remains below 0°C for at least 2 consecutive years. About 24% of the northern hemisphere land area is underlain by permafrost. The thawing of permafrost has the potential to influence the climate system through the release of carbon (C) from northern high latitude terrestrial ecosystems, but there is substantial uncertainty about the sensitivity of the C cycle to thawing permafrost. Soil C can be mobilized from permafrost in response to changes in air temperature, directional changes in water balance, fire, thermokarst, and flooding. Observation networks need to be implemented to understand responses of
permafrost and C at a range of temporal and spatial scales. The understanding gained from these observation networks needs to be integrated into modeling frameworks capable of representing how the responses of permafrost C will influence the trajectory of climate in the future
Static and Dynamic Magnetism in Underdoped Superconductor BaFeCoAs
We report neutron scattering measurements on single crystals of
BaFeCoAs. The magnetic Bragg peak intensity is reduced by
6 % upon cooling through T. The spin dynamics exhibit a gap of 8 meV with
anisotropic three-dimensional (3d) interactions. Below T additional
intensity appears at an energy of 4.5(0.5) meV similar to previous
observations of a spin resonance in other Fe-based superconductors. No further
gapping of the spin excitations is observed below T for energies down to 2
meV. These observations suggest the redistribution of spectral weight from the
magnetic Bragg position to a spin resonance demonstrating the direct
competition between static magnetic order and superconductivity.Comment: 4 pages, 4 figure
Anisotropic thermal expansion of Fe1.06Te and FeTe0.5Se0.5 single crystals
Heat capacity and anisotropic thermal expansion was measured for Fe1.06Te and
FeTe0.5Se0.5 single crystals. Previously reported phase transitions are clearly
seen in both measurements. In both cases the thermal expansion is anisotropic.
The uniaxial pressure derivatives of the superconducting transition temperature
in FeTe0.5Se0.5 inferred from the Ehrenfest relation have opposite signs for
in-plane and c-axis pressures. Whereas the Gruneisen parameters for both
materials are similar and only weakly temperature-dependent above ~ 80 K, at
low temperatures (in the magnetically ordered phase) the magnetic contribution
to the Gruneisen parameter in Fe1.06Te is significantly larger than electron
and phonon contributions combined
Effect of annealing on the specific heat of optimally doped Ba(FeCo)As
We report the temperature dependence of the low-temperature specific heat
down to 400 mK of the electron-doped Ba(FeCo)As
superconductors. We have measured two samples extracted from the same batch:
first sample has been measured just after preparation with no additional heat
treatment. The sample shows =20 K, residual specific heat
=3.6 mJ/mol K and a Schottky-like contribution at low
temperatures. A second sample has been annealed at 800 for two weeks
and shows = 25 K and =1.4 mJ/mol~K. By subtracting
the lattice specific heat, from pure BaFeAs, the temperature
dependence of the electronic specific heat has been obtained and studied. For
both samples the temperature dependence of clearly indicate the
presence of low-energy excitations in the system. Their specific heat data
cannot be described by single clean s- or d-wave models and the data requires
an anisotropic gap scenario which may or may not have nodesComment: SCES 2010, 5 pages, 2 figure
The Penguin: a Low Reynolds Number Powered Glider for Station Keeping Missions
The Penguin is a low Reynolds number (approx. 100,000) remotely piloted vehicle (RPV). It was designed to fly three laps indoors around two pylons in a figure-eight course while maximizing loiter time. The Penguin's low Reynolds number mission is an important one currently being studied for possible future flights in the atmospheres of other planets and for specialized military missions. Although the Penguin's mission seemed quite simple at first, the challenges of such low Reynolds number flight have proven to be quite unique. In addition to the constraint of low Reynolds number flight, the aircraft had to be robust in its control, highly durable, and it had to carry a small instrument package. The Penguin's flight plan, concept, performance, aerodynamic design, weight estimation, structural design, propulsion, stability and control, and cost estimate is detailed
Exact calculation of spectral properties of a particle interacting with a one dimensional fermionic system
Using the Bethe ansatz analysis as was reformulated by Edwards, we calculate
the spectral properties of a particle interacting with a bath of fermions in
one dimension for the case of equal particle-fermion masses. These are directly
related to singularities apparent in optical experiments in one dimensional
systems. The orthogonality catastrophe for the case of an infinite particle
mass survives in the limit of equal masses. We find that the exponent
of the quasiparticle weight, is different for the two
cases, and proportional to their respective phaseshifts at the Fermi surface;
we present a simple physical argument for this difference. We also show that
these exponents describe the low energy behavior of the spectral function, for
repulsive as well as attractive interaction.Comment: 22 pages + 1 postscript figure, REVTE
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