5,016 research outputs found
Evaluation of the optical conductivity tensor in terms of contour integrations
For the case of finite life-time broadening the standard Kubo-formula for the
optical conductivity tensor is rederived in terms of Green's functions by using
contour integrations, whereby finite temperatures are accounted for by using
the Fermi-Dirac distribution function. For zero life-time broadening, the
present formalism is related to expressions well-known in the literature.
Numerical aspects of how to calculate the corresponding contour integrals are
also outlined.Comment: 8 pages, Latex + 2 figure (Encapsulated Postscript
Color Gradients Detected in the HD 15115 Circumstellar Disk
We report HST/NICMOS coronagraphic images of the HD 15115 circumstellar disk
at 1.1\micron. We find a similar morphology to that seen in the visible and at
H band--an edge-on disk that is asymmetric in surface brightness. Several
aspects of the 1.1\micron data are different, highlighting the need for
multi-wavelength images of each circumstellar disk. We find a flattening to the
western surface brightness profile at 1.1\micron interior to 2\arcsec (90 AU)
and a warp in the western half of the disk. We measure the surface brightness
profiles of the two disk lobes and create a measure of the dust scattering
efficiency between 0.55-1.65\micron at 1\arcsec, 2\arcsec, and 3\arcsec. At
2\arcsec the western lobe has a neutral spectrum up to 1.1\micron and a strong
absorption or blue spectrum 1.1\micron, while a blue trend is seen in the
eastern lobe. At 1\arcsec the disk has a red F110W-H color in both lobes.Comment: 4 pages, 4 figures, emulateapj; accepted to ApJ
Electrical transport properties of bulk NiFe alloys and related spin-valve systems
Within the Kubo-Greenwood formalism we use the fully relativistic,
spin-polarized, screened Korringa-Kohn-Rostoker method together with the
coherent-potential approximation for layered systems to calculate the
resistivity for the permalloy series NiFe. We are able to
reproduce the variation of the resistivity across the entire series; notably
the discontinuous behavior in the vicinity of the structural phase transition
from bcc to fcc. The absolute values for the resistivity are within a factor of
two of the experimental data. Also the giant magnetoresistance of a series of
permalloy-based spin-valve structures is estimated; we are able to reproduce
the trends and values observed on prototypical spin-valve structures.Comment: 6 pages, ReVTeX + 4 figures (Encapsulated Postscript), submitted to
PR
Tuning independently Fermi energy and spin splitting in Rashba systems: Ternary surface alloys on Ag(111)
By detailed first-principles calculations we show that the Fermi energy and
the Rashba splitting in disordered ternary surface alloys (BiPbSb)/Ag(111) can
be independently tuned by choosing the concentrations of Bi and Pb. The
findings are explained by three fundamental mechanisms, namely the relaxation
of the adatoms, the strength of the atomic spin-orbit coupling, and band
filling. By mapping the Rashba characteristics,i.e.the splitting and the Rashba
energy, and the Fermi energy of the surface states in the complete range of
concentrations. Our results suggest to investigate experimentally effects which
rely on the Rashba spin-orbit coupling in dependence on spin-orbit splitting
and band filling.Comment: 11 pages, 3 figure
Spin-polarized surface states close to adatoms on Cu(111)
We present a theoretical study of surface states close to 3d transition metal
adatoms (Cr, Mn, Fe, Co, Ni and Cu) on a Cu(111) surface in terms of an
embedding technique using the fully relativistic Korringa-Kohn-Rostoker method.
For each of the adatoms we found resonances in the s-like states to be
attributed to a localization of the surface states in the presence of an
impurity. We studied the change of the s-like densities of states in the
vicinity of the surface state band-edge due to scattering effects mediated via
the adatom's d-orbitals. The obtained results show that a magnetic impurity
causes spin-polarization of the surface states. In particular, the long-range
oscillations of the spin-polarized s-like density of states around an Fe adatom
are demonstrated.Comment: 5 pages, 5 figures, submitted to PR
First Evidence of a Precessing Jet Excavating a Protostellar Envelope
We present new, sensitive, near-infrared images of the Class I protostar,
Elias 29, in the Ophiuchus cloud core. To explore the relationship between the
infall envelope and the outflow, narrowband H2 1-0 S(1), Br-gamma, and
narrowband K-continuum filters were used to image the source with the
Wide-Field Infrared Camera on the Hale 5m telescope and with Persson's
Auxiliary Nasmyth Infrared Camera on the Baade 6.5 m telescope. The source
appears as a bipolar, scattered light nebula, with a wide opening angle in all
filters, as is typical for late-stage protostars. However, the pure H2
emission-line images point to the presence of a heretofore undetected
precessing jet. It is argued that high-velocity, narrow, precessing jets
provide the mechanism for creating the observed wide-angled outflow cavity in
this source.Comment: 11 pages, 1 figure, 1 tabl
Investigating the flyby scenario for the HD 141569 system
HD 141569, a triple star system, has been intensively observed and studied
for its massive debris disk. It was rather regarded as a gravitationally bound
triple system but recent measurements of the HD 141569A radial velocity seem to
invalidate this hypothesis. The flyby scenario has therefore to be investigated
to test its compatibility with the observations. We present a study of the
flyby scenario for the HD141569 system, by considering 3 variants: a sole
flyby, a flyby associated with one planet and a flyby with two planets. We use
analytical calculations and perform N-body numerical simulations of the flyby
encounter. The binary orbit is found to be almost fixed by the observational
constraint on a edge-on plane with respect to the observers. If the binary has
had an influence on the disk structure, it should have a passing time at the
periapsis between 5000 and 8000 years ago and a distance at periapsis between
600 and 900 AU. The best scenario for reproducing the disk morphology is a
flyby with only 1 planet. For a 2 Mj (resp. 8 Mj) planet, its eccentricity must
be around 0.2 (resp. below 0.1). In the two cases, its apoapsis is about 130
AU. Although the global disk shape is reasonably well reproduced, some features
cannot be explain by the present model and the likehood of the flyby event
remains an issue. Dynamically speaking, HD 141569 is still a puzzling system
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