561 research outputs found
Bewirtschaftungseffekte auf Wurzelbiomasse und Kohlenstoff-Rhizodeposition von Mais in zwei Schweizer Langzeitversuchen
Below ground carbon (BGC) inputs by agricultural plants into the soil are an important variable in soil carbon (C) modelling. The sources for BGC inputs are dead root biomass and C release by living roots (C rhizodeposition). Since management effects on BGC inputs are not profoundly understood, we address the following research questions in this research project: (i) What are the proportions of root biomass and C rhizodeposition of the total BGC input in the topsoil and subsoil under maize cultivation at different sites? (ii) Does long-term fertilization practice affect total root biomass, root distribution, shoot/root ratios, and C rhizodeposition of maize? Results obtained from field experiments in 2013 on two Swiss long-term experimental sites (“DOK” near Basel and “ZOFE” in Zurich) reveal no significant differences between total root biomasses and total C rhizodeposition of maize in different management treatments. While the proportion of topsoil (0-0.25 m) root biomass of the total (0-0.75 m) root biomass increases on the “DOK” site, the below ground/above ground C ratios decrease on both sites with increasing management intensity (trends only)
Dating late Cenozoic erosional surfaces in Victoria Land, Antarctica, with cosmogenic neon in pyroxenes
We present 21Ne exposure ages of erosional glaciogenic rock surfaces on nunataks in northern Victoria Land, Antarctica: i) in the Prince Albert Mountains and ii) near Mesa Range. These nunataks are located directly at the margin of the polar plateau and therefore provide an immediate record of ice volume changes of the East Antarctic Ice Sheet, not biased by ice shelf grounding or narrow valley sections downstream the outlet glaciers. The sampling locations overlook the present ice surface by less than 200 m, but were last covered by ice 3.5 Ma bp (minimum age, not corrected for erosion). This strongly indicates that the ice sheet has not been substantially thicker than today since at least the early Pliocene, which supports the hypothesis of a stable East Antarctic Ice Sheet. First absolute ages are reported for the alpine topography above the erosive trimline that typically marks the upper limit of glacial activity in northern Victoria Land. Unexpectedly low nuclide concentrations suggest that erosion rates on the alpine topography are considerably higher due to the steep slopes than those affecting flat erosional surfaces carrying Antarctic tor
Molecular states in carbon nanotube double quantum dots
We report electrical transport measurements through a semiconducting
single-walled carbon nanotube (SWNT) with three additional top-gates. At low
temperatures the system acts as a double quantum dot with large inter-dot
tunnel coupling allowing for the observation of tunnel-coupled molecular states
extending over the whole double-dot system. We precisely extract the tunnel
coupling and identify the molecular states by the sequential-tunneling line
shape of the resonances in differential conductance.Comment: 5 pages, 4 figure
Positive cross-correlations due to Dynamical Channel-Blockade in a three-terminal quantum dot
We investigate current fluctuations in a three-terminal quantum dot in the
sequential tunneling regime. In the voltage-bias configuration chosen here, the
circuit is operated like a beam splitter, i.e. one lead is used as an input and
the other two as outputs. In the limit where a double occupancy of the dot is
not possible, a super-Poissonian Fano factor of the current in the input lead
and positive cross-correlations between the current fluctuations in the two
output leads can be obtained, due to dynamical channel-blockade. When a single
orbital of the dot transports current, this effect can be obtained by lifting
the spin-degeneracy of the circuit with ferromagnetic leads or with a magnetic
field. When several orbitals participate in the electronic conduction, lifting
spin-degeneracy is not necessary. In all cases, we show that a super-Poissonian
Fano factor for the input current is not equivalent to positive
cross-correlations between the outputs. We identify the conditions for
obtaining these two effects and discuss possible experimental realizations.Comment: 18 pages, 20 Figures, submitted to Phys. rev.
Integrable theory of quantum transport in chaotic cavities
The problem of quantum transport in chaotic cavities with broken
time-reversal symmetry is shown to be completely integrable in the universal
limit. This observation is utilised to determine the cumulants and the
distribution function of conductance for a cavity with ideal leads supporting
an arbitrary number of propagating modes. Expressed in terms of solutions
to the fifth Painlev\'e transcendent and/or the Toda lattice equation, the
conductance distribution is further analysed in the large- limit that
reveals long exponential tails in the otherwise Gaussian curve.Comment: 4 pages; final version to appear in Physical Review Letter
Shot noise from action correlations
We consider universal shot noise in ballistic chaotic cavities from a
semiclassical point of view and show that it is due to action correlations
within certain groups of classical trajectories. Using quantum graphs as a
model system we sum these trajectories analytically and find agreement with
random-matrix theory. Unlike all action correlations which have been considered
before, the correlations relevant for shot noise involve four trajectories and
do not depend on the presence of any symmetry.Comment: 4 pages, 2 figures (a mistake in version 1 has been corrected
The 1/3-shot noise suppression in diffusive nanowires
We report low-temperature shot noise measurements of short diffusive Au wires
attached to electron reservoirs of varying sizes. The measured noise
suppression factor compared to the classical noise value
strongly depends on the electric heat conductance of the reservoirs. For small
reservoirs injection of hot electrons increases the measured noise and hence
the suppression factor. The universal 1/3-suppression factor can only
asymptotically be reached for macroscopically large and thick electron
reservoirs. A heating model based on the Wiedemann-Franz law is used to explain
this effect.Comment: 10 figure
Coulomb induced positive current-current correlations in normal conductors
In the white-noise limit current correlations measured at different contacts
of a mesoscopic conductor are negative due to the antisymmetry of the wave
function (Pauli principle). We show that current fluctuations at capacitive
contacts induced via the long range Coulomb interaction as consequence of
charge fluctuations in the mesoscopic sample can be {\it positively}
correlated. The positive correlations are a consequence of the extension of the
wave-functions into areas near both contacts. As an example we investigate in
detail a quantum point contact in a high magnetic field under conditions in
which transport is along an edge state.Comment: Revtex, 4 pages includes 2 figure
Shot noise in ferromagnet--normal metal systems
A semiclassical theory of the low frequency shot noise in ferromagnet -
normal metal systems is formulated. Non-collinear magnetization directions of
the ferromagnetic leads, arbitrary junctions and the elastic and inelastic
scattering regimes are considered. The shot noise is governed by a set of
mesoscopic parameters that are expressed in terms of the microscopic details of
the junctions in the circuit. Explicit results in the case of ballistic,
tunnel, and diffusive junctions are evaluated. The shot noise, the current and
the Fano factor are calculated for a double barrier ferromagnet - normal metal
- ferromagnet system. It is demonstrated that the shot noise can have a
non-monotonic behavior as a function of the relative angle between the
magnetizations of the ferromagnetic reservoirs.Comment: 17 pages, 7 figure
Shot Noise by Quantum Scattering in Chaotic Cavities
We have experimentally studied shot noise of chaotic cavities defined by two
quantum point contacts in series. The cavity noise is determined as 1/4*2e|I|
in agreement with theory and can be well distinguished from other contributions
to noise generated at the contacts. Subsequently, we have found that cavity
noise decreases if one of the contacts is further opened and reaches nearly
zero for a highly asymmetric cavity.Comment: 4 pages, 4 figures, REVTe
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