88 research outputs found
Determination of complex absorbing potentials from the electron self-energy
The electronic conductance of a molecule making contact to electrodes is
determined by the coupling of discrete molecular states to the continuum
electrode density of states. Interactions between bound states and continua can
be modeled exactly by using the (energy-dependent) self-energy, or
approximately by using a complex potential. We discuss the relation between the
two approaches and give a prescription for using the self-energy to construct
an energy-independent, non-local, complex potential. We apply our scheme to
studying single-electron transmission in an atomic chain, obtaining excellent
agreement with the exact result. Our approach allows us to treat
electron-reservoir couplings independent of single electron energies, allowing
for the definition of a one-body operator suitable for inclusion into
correlated electron transport calculations.Comment: 11 pages, 8 figures; to be published in the J. Chem. Phy
Magnetically controlled current flow in coupled-dot arrays
Quantum transport through an open periodic array of up to five dots is
investigated in the presence of a magnetic field. The device spectrum exhibits
clear features of the band structure of the corresponding one-dimensional
artificial crystal which evolves with varying field. A significant magnetically
controlled current flow is induced with changes up to many orders of magnitude
depending on temperature and material parameters. Our results put forward a
simple design for measuring with current technology the magnetic subband
formation of quasi one-dimensional Bloch electrons.Comment: 9 pages, 5 figure
Absolute properties of the main-sequence eclipsing binary FM Leo
First spectroscopic and new photometric observations of the eclipsing binary
FM Leo are presented. The main aims were to determine orbital and stellar
parameters of two components and their evolutionary stage. First spectroscopic
observations of the system were obtained with DDO and PST spectrographs. The
results of the orbital solution from radial velocity curves are combined with
those derived from the light-curve analysis (ASAS-3 photometry and
supplementary observations of eclipses with 1 m and 0.35 m telescopes) to
derive orbital and stellar parameters. JKTEBOP, Wilson-Devinney binary
modelling codes and a two-dimensional cross-correlation (TODCOR) method were
applied for the analysis. We find the masses to be M_1 = 1.318 0.007 and
M_2 = 1.287 0.007 M_sun, the radii to be R_1 = 1.648 0.043 and R_2
= 1.511 0.049 R_sun for primary and secondary stars, respectively. The
evolutionary stage of the system is briefly discussed by comparing physical
parameters with current stellar evolution models. We find the components are
located at the main sequence, with an age of about 3 Gyr.Comment: 5 pages, 4 figures, to appear in MNRA
Geometrical enhancement of the proximity effect in quantum wires with extended superconducting tunnel contacts
We study Andreev reflection in a ballistic one-dimensional channel coupled in
parallel to a superconductor via a tunnel barrier of finite length . The
dependence of the low-energy Andreev reflection probability on
reveals the existence of a characteristic length scale beyond which
is enhanced up to unity despite the low interfacial transparency. The
Andreev reflection enhancement is due to the strong mixing of particle and hole
states that builds up in contacts exceeding the coherence length ,
leading to a small energy gap (minigap) in the density of states of the normal
system. The role of the geometry of such hybrid contacts is discussed in the
context of the experimental observation of zero-bias Andreev anomalies in the
resistance of extended carbon nanotube/superconductor junctions in field effect
transistor setups.Comment: 11 pages, 8 figures; minor revisions including added Ref. 7 and inset
to Fig. 3b; version as accepted for publication to Phys. Rev.
Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478
Blue supergiant stars are known to display photometric and spectroscopic
variability that is suggested to be linked to stellar pulsations. Pulsational
activity in massive stars strongly depends on the star's evolutionary stage and
is assumed to be connected with mass-loss episodes, the appearance of
macroturbulent line broadening, and the formation of clumps in the wind. To
investigate a possible interplay between pulsations and mass-loss, we carried
out an observational campaign of the supergiant 55 Cyg over a period of five
years to search for photospheric activity and cyclic mass-loss variability in
the stellar wind. We modeled the H, He I, Si II and Si III lines using the
nonlocal thermal equilibrium atmosphere code FASTWIND and derived the
photospheric and wind parameters. In addition, we searched for variability in
the intensity and radial velocity of photospheric lines and performed a moment
analysis of the line profiles to derive frequencies and amplitudes of the
variations. The Halpha line varies with time in both intensity and shape,
displaying various types of profiles: P Cygni, pure emission, almost complete
absence, and double or multiple peaked. The star undergoes episodes of variable
mass-loss rates that change by a factor of 1.7-2 on different timescales. We
also observe changes in the ionization rate of Si II and determine a
multiperiodic oscillation in the He I absorption lines, with periods ranging
from a few hours to 22.5 days. We interpret the photospheric line variations in
terms of oscillations in p-, g-, and strange modes. We suggest that these
pulsations can lead to phases of enhanced mass loss. Furthermore, they can
mislead the determination of the stellar rotation. We classify the star as a
post-red supergiant, belonging to the group of alpha Cyg variables.Comment: 20 pages, 18 figures, 3 tables, accepted to Astronomy & Astrophysic
Transport properties and electrical device characteristics with the TiMeS computational platform: Application in silicon nanowires
Electron transport in nanotube--molecular wire hybrids
We study contact effects on electron transport across a molecular wire
sandwiched between two semi-infinite (carbon) nanotube leads as a model for
nanoelectrodes. Employing the Landauer scattering matrix approach we find that
the conductance is very sensitive to parameters such as the coupling strength
and geometry of the contact. The conductance exhibits markedly different
behavior in the two limiting scenarios of single contact and multiple contacts
between the molecular wire and the nanotube interfacial atoms. In contrast to a
single contact the multiple-contact configuration acts as a filter selecting
single transport channels. It exhibits a scaling law for the conductance as a
function of coupling strength and tube diameter. We also observe an unusual
narrow-to-broad-to-narrow behavior of conductance resonances upon decreasing
the coupling.Comment: 4 pages, figures include
Phonon-mediated thermal conductance of mesoscopic wires with rough edges
We present an analysis of acoustic phonon propagation through long,
free-standing, insulating wires with rough surfaces. Due to a crossover from
ballistic propagation of the lowest-frequency phonon mode at to a diffusive (or even localized) behavior upon the increase of
phonon frequency, followed by re-entrance into the quasi-ballistic regime, the
heat conductance of a wire acquires an intermediate tendency to saturate within
the temperature range .Comment: 4 pages, 3 figures included; minor changes and corrections, figures 1
and 2 replaced by better versions; to appear in PRB Brief Report
Lattice-dynamics of a Disordered solid-solid Interface
Generic properties of elastic phonon transport at a disordered interface are
studied. The results show that phonon transmittance is a strong function of
frequency and the disorder correlation length. At frequencies lower than the
van Hove singularity the transmittance at a given frequency increases as the
correlation length decreases. At low frequencies, this is reflected by
different power-laws for phonon conductance across correlated and uncorrelated
disordered interfaces which are in approximate agreement with perturbation
theory of an elastic continuum. These results can be understood in terms of
simple mosaic and two-colour models of the interface.Comment: 17 pages, 5 figures, submitted to PR
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