8,320 research outputs found
A time-dependent approach to electron pumping in open quantum systems
We propose a time-dependent approach to investigate the motion of electrons
in quantum pump device configurations. The occupied one-particle states are
propagated in real time and used to calculate the local electron density and
current. An advantage of the present computational scheme is that the same
computational effort is required to simulate monochromatic, polychromatic and
nonperiodic drivings. Furthermore, initial state dependence and history effects
are naturally accounted for. This approach can also be embedded in the
framework of time-dependent density functional theory to include
electron-electron interactions. In the special case of periodic drivings we
combine the Floquet theory with nonequilibrium Green's functions and obtain a
general expression for the pumped current in terms of inelastic transmission
probabilities. This latter result is used for benchmarking our propagation
scheme in the long-time limit. Finally, we discuss the limitations of
Floquet-based schemes and suggest our approach as a possible way to go beyond
them.Comment: 14 pages, 8 figure
Occurrence and persistence of magnetic elements in the quiet Sun
Turbulent convection efficiently transports energy up to the solar
photosphere, but its multi-scale nature and dynamic properties are still not
fully understood. Several works in the literature have investigated the
emergence of patterns of convective and magnetic nature in the quiet Sun at
spatial and temporal scales from granular to global. Aims. To shed light on the
scales of organisation at which turbulent convection operates, and its
relationship with the magnetic flux therein, we studied characteristic spatial
and temporal scales of magnetic features in the quiet Sun. Methods. Thanks to
an unprecedented data set entirely enclosing a supergranule, occurrence and
persistence analysis of magnetogram time series were used to detect spatial and
long-lived temporal correlations in the quiet Sun and to investigate their
nature. Results. A relation between occurrence and persistence representative
for the quiet Sun was found. In particular, highly recurrent and persistent
patterns were detected especially in the boundary of the supergranular cell.
These are due to moving magnetic elements undergoing motion that behaves like a
random walk together with longer decorrelations ( h) with respect to
regions inside the supergranule. In the vertices of the supegranular cell the
maximum observed occurrence is not associated with the maximum persistence,
suggesting that there are different dynamic regimes affecting the magnetic
elements
Evaluating shoreline identification using optical satellite images
A technique to extract the shoreline location from optical satellite images has been developed and evaluated for the case study site of Progreso, Yucatán, México. A novel method to extract a satellite-derived shoreline (SDS) was developed ensuring the maximum contrast between sea and land. The area under investigation is an 8. km length of shoreline that faces north into the Gulf of México.The SDS was validated using quasi-simultaneous in situ shoreline measurements, both adjusted to equal water levels. In situ shoreline measurements recorded the instantaneous shoreward extent of the wave run-up when walking along the beach.The validation of SDS revealed that the SDS is located consistently seawards of the in situ shoreline, explained by: a) the water depth that optical satellite image requires to identify a pixel either as sea or land, and b) the shoreward extent of the wave run-up. The overall distance between SDS and in situ shoreline is 5.6. m on average with a standard deviation of 1.37. m (in the horizontal) over 8. km of shoreline. Confidence bounds considering the shoreward extent of the wave run-up, inter-tidal beach slope variation and tidal uncertainty were computed to assess the accuracy of the SDS.The SDS has been shown to be capable of detecting shoreline changes of less than 10. m and abrupt changes due to storms. The success of our method suggests that it should be applicable to other locations, after adapting the confidence bounds to the beach conditions
Comment on "Density Functional Simulation of a Breaking Nanowire"
In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)]
described first principles calculations for a breaking Na nanocontact. Their
system consists of a periodic one-dimensional array of supercells, each of
which contains 39 Na atoms, originally forming a straight, crystalline wire
with a length of 6 atoms. The system is elongated by increasing the length of
the unit cell. At each step, the atomic configuration is relaxed to a new local
equilibrium, and the tensile force is evaluated from the change of the total
energy with elongation. Aside from a discontinuity of the force occuring at the
transition from a crytalline to an amorphous configuration during the early
stages of elongation, they were unable to identify any simple correlations
between the force and the number of electronic modes transmitted through the
contact. An important question is whether their model is realistic, i.e.,
whether it can be compared to experimental results obtained for a single
nanocontact between two macroscopic pieces of metal. In this Comment, we
demonstrate that with such a small unit cell, the interference effects between
neighboring contacts are of the same size as the force oscillations in a single
nanocontact.Comment: 1 pag
Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids
Calculations of the hyperpolarizability are typically much more difficult to
converge with basis set size than the linear polarizability. In order to
understand these convergence issues and hence obtain accurate ab initio values,
we compare calculations of the static hyperpolarizability of the gas-phase
chloroform molecule (CHCl_3) using three different kinds of basis sets:
Gaussian-type orbitals, numerical basis sets, and real-space grids. Although
all of these methods can yield similar results, surprisingly large, diffuse
basis sets are needed to achieve convergence to comparable values. These
results are interpreted in terms of local polarizability and
hyperpolarizability densities. We find that the hyperpolarizability is very
sensitive to the molecular structure, and we also assess the significance of
vibrational contributions and frequency dispersion
Understanding Charge Transfer in Donor-Acceptor/Metal Systems: A Combined Theoretical and Experimental Study
We develop an effective potential approach for assessing the flow of charge
within a two-dimensional donor-acceptor/metal network based on core-level
shifts. To do so, we perform both density functional theory (DFT) calculations
and x-ray photoemission spectroscopy (XPS) measurements of the core-level
shifts for three different monolayers adsorbed on a Ag substrate. Specifically,
we consider perfluorinated pentacene (PFP), copper phthalocyanine (CuPc) and
their 1:1 mixture (PFP+CuPc) adsorbed on Ag(111).Comment: 12 pages, 10 figure
Injerto autólogo cubital como complemento a una mandibulectomÃa parcial para el tratamiento de un tumor de células escamosas
Se presenta un caso clÃnico de un tumor de células escamosas en la cavidad oral del perro, resuelto mediante mandibulectomÃa parcial combinada con la implantación de un injerto óseo autólogo. En la evolución tras 30 meses no ha presentado recidivas ni metástasis
The Sargassum Invasion of the Eastern Caribbean and Dynamics of the Equatorial North Atlantic
Carbon fibre tips for scanning probe microscopy based on quartz tuning fork force sensors
We report the fabrication and the characterization of carbon fibre tips for
their use in combined scanning tunnelling and force microscopy based on
piezoelectric quartz tuning fork force sensors. We find that the use of carbon
fibre tips results in a minimum impact on the dynamics of quartz tuning fork
force sensors yielding a high quality factor and consequently a high force
gradient sensitivity. This high force sensitivity in combination with high
electrical conductivity and oxidation resistance of carbon fibre tips make them
very convenient for combined and simultaneous scanning tunnelling microscopy
and atomic force microscopy measurements. Interestingly, these tips are quite
robust against occasionally occurring tip crashes. An electrochemical
fabrication procedure to etch the tips is presented that produces a sub-100 nm
apex radius in a reproducible way which can yield high resolution images.Comment: 14 pages, 10 figure
Calibration of the length of a chain of single gold atoms
Using a scanning tunneling microscope or mechanically controllable break
junctions it has been shown that it is possible to control the formation of a
wire made of single gold atoms. In these experiments an interatomic distance
between atoms in the chain of ~3.6 Angstrom was reported which is not
consistent with recent theoretical calculations. Here, using precise
calibration procedures for both techniques, we measure length of the atomic
chains. Based on the distance between the peaks observed in the chain length
histogram we find the mean value of the inter-atomic distance before chain
rupture to be 2.6 +/- 0.2 A . This value agrees with the theoretical
calculations for the bond length. The discrepancy with the previous
experimental measurements was due to the presence of He gas, that was used to
promote the thermal contact, and which affects the value of the work function
that is commonly used to calibrate distances in scanning tunnelling microscopy
and mechanically controllable break junctions at low temperatures.Comment: 6 pages, 6 figure
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