4,949 research outputs found
Conductance of a quantum point contact based on spin-density-functional theory
We present full quantum mechanical conductance calculations of a quantum
point contact (QPC) performed in the framework of the density functional theory
(DFT) in the local spin-density approximation (LDA). We show that a
spin-degeneracy of the conductance channels is lifted and the total conductance
exhibits a broad plateau-like feature at 0.5*2e^{2}/h. The lifting of the
spin-degeneracy is a generic feature of all studied QPC structures (both very
short and very long ones; with the lengths in the range 40<l<500 nm). The
calculated conductance also shows a hysteresis for forward- and backward sweeps
of the gate voltage. These features in the conductance can be traced to the
formation of weakly coupled quasi-bound states (magnetic impurities) inside the
QPC (also predicted in previous DFT-based studies). A comparison of obtained
results with the experimental data shows however, that while the spin-DFT based
"first-principle" calculations exhibits the spin polarization in the QPC, the
calculated conductance clearly does not reproduce the 0.7 anomaly observed in
almost all QPCs of various geometries. We critically examine major features of
the standard DFT-based approach to the conductance calculations and argue that
its inability to reproduce the 0.7 anomaly might be related to the infamous
derivative discontinuity problem of the DFT leading to spurious
self-interaction errors not corrected in the standard LDA. Our results indicate
that the formation of the magnetic impurities in the QPC might be an artefact
of the LDA when localization of charge is expected to occur. We thus argue that
an accurate description of the QPC structure would require approaches that go
beyond the standard DFT+LDA schemes.Comment: 9 pages, 5 figure
Inelastic Scattering in Metal-H2-Metal Junctions
We present first-principles calculations of the dI/dV characteristics of an
H2 molecule sandwiched between Au and Pt electrodes in the presence of
electron-phonon interactions. The conductance is found to decrease by a few
percentage at threshold voltages corresponding to the excitation energy of
longitudinal vibrations of the H2 molecule. In the case of Pt electrodes, the
transverse vibrations can mediate transport through otherwise non-transmitting
Pt -channels leading to an increase in the differential conductance even
though the hydrogen junction is characterized predominately by a single almost
fully open transport channel. In the case of Au, the transverse modes do not
affect the dI/dV because the Au d-states are too far below the Fermi level. A
simple explanation of the first-principles results is given using scattering
theory. Finally, we compare and discuss our results in relation to experimental
data.Comment: Accepted in Phys. Rev.
[OI]63micron jets in class 0 sources detected by Herschel
We present Herschel PACS mapping observations of the [OI]63 micron line
towards protostellar outflows in the L1448, NGC1333-IRAS4, HH46, BHR71 and
VLA1623 star forming regions. We detect emission spatially resolved along the
outflow direction, which can be associated with a low excitation atomic jet. In
the L1448-C, HH46 IRS and BHR71 IRS1 outflows this emission is kinematically
resolved into blue- and red-shifted jet lobes, having radial velocities up to
200 km/s. In the L1448-C atomic jet the velocity increases with the distance
from the protostar, similarly to what observed in the SiO jet associated with
this source. This suggests that [OI] and molecular gas are kinematically
connected and that this latter could represent the colder cocoon of a jet at
higher excitation. Mass flux rates (\.M(OI)) have been measured from
the [OI]63micron luminosity adopting two independent methods. We find values in
the range 1-4 10 Mo/yr for all sources but HH46, for which an order of
magnitude higher value is estimated. \.M(OI) are compared with mass
accretion rates (\.M) onto the protostar and with \.M derived
from ground-based CO observations. \.M(OI)/\.M ratios are in
the range 0.05-0.5, similar to the values for more evolved sources.
\.M(OI) in HH46 IRS and IRAS4A are comparable to \.M(CO), while
those of the remaining sources are significantly lower than the corresponding
\.M(CO). We speculate that for these three sources most of the mass
flux is carried out by a molecular jet, while the warm atomic gas does not
significantly contribute to the dynamics of the system.Comment: 37 pages and 12 figures, accepted for publication on Astrophysical
Journa
On the unitarity problem in space/time noncommutative theories
It is shown that the violation of unitarity observed in space/time
noncommutative field theories is due to an improper definition of quantum field
theory on noncommutative spacetime.Comment: 7 pages; typos corrected, sign convention changed on p.
Rotational properties of dipolar Bose-Einstein condensates confined in anisotropic harmonic potentials
We study the rotational properties of a dipolar Bose-Einstein condensate
confined in a quasi-two- dimensional anisotropic trap, for an arbitrary
orientation of the dipoles with respect to their plane of motion. Within the
mean-field approximation we find that the lowest-energy state of the system
depends strongly on the relative strength between the dipolar and the contact
interactions, as well as on the size and the orientation of the dipoles, and
the size and the orientation of the deformation of the trapping potential.Comment: 6 pages, 5 figure
Resolving the shocked gas in HH54 with Herschel: CO line mapping at high spatial and spectral resolution
The HH54 shock is a Herbig-Haro object, located in the nearby Chamaeleon II
cloud. Observed CO line profiles are due to a complex distribution in density,
temperature, velocity, and geometry. Resolving the HH54 shock wave in the
far-infrared cooling lines of CO constrain the kinematics, morphology, and
physical conditions of the shocked region. We used the PACS and SPIRE
instruments on board the Herschel space observatory to map the full FIR
spectrum in a region covering the HH54 shock wave. Complementary Herschel-HIFI,
APEX, and Spitzer data are used in the analysis as well. The observed features
in the line profiles are reproduced using a 3D radiative transfer model of a
bow-shock, constructed with the Line Modeling Engine code (LIME). The FIR
emission is confined to the HH54 region and a coherent displacement of the
location of the emission maximum of CO with increasing J is observed. The peak
positions of the high-J CO lines are shifted upstream from the lower J CO lines
and coincide with the position of the spectral feature identified previously in
CO(10-9) profiles with HIFI. This indicates a hotter molecular component in the
upstream gas with distinct dynamics. The coherent displacement with increasing
J for CO is consistent with a scenario where IRAS12500-7658 is the exciting
source of the flow, and the 180 K bow-shock is accompanied by a hot (800 K)
molecular component located upstream from the apex of the shock and blueshifted
by -7 km s. The spatial proximity of this knot to the peaks of the
atomic fine-structure emission lines observed with Spitzer and PACS ([OI]63,
145 m) suggests that it may be associated with the dissociative shock as
the jet impacts slower moving gas in the HH54 bow-shock.Comment: 6 pages, 5 figure
Winter wheat roots grow twice as deep as spring wheat roots, is this important for N uptake and N leaching losses?
Cropping systems comprising winter catch crops followed by spring wheat could reduce N leaching risks compared to traditional winter wheat systems in humid climates. We studied the soil mineral N (Ninorg) and root growth of winter- and spring wheat to 2.5 m depth during three years. Root depth of winter wheat (2.2 m) was twice that of spring wheat, and this was related to much lower amounts of Ninorg in the 1 to 2.5 m layer after winter wheat (81 kg Ninorg ha-1 less). When growing winter catch crops before spring wheat, N content in the 1 to 2.5 m layer after spring wheat was not different from that after winter wheat. The results suggest that by virtue of its deep rooting, winter wheat may not lead to high levels of leaching as it is often assumed in humid climates. Deep soil and root measurements (below 1 m) in this experiment were essential to answer the questions we posed
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