1,284 research outputs found
Theoretical Study of Spin-dependent Electron Transport in Atomic Fe Nanocontacts
We present theoretical predictions of spintronic transport phenomena that
should be observable in ferromagnetic Fe nanocontacts bridged by chains of Fe
atoms. We develop appropriate model Hamiltonians based on semi-empirical
considerations and the known electronic structure of bulk Fe derived from ab
initio density functional calculations. Our model is shown to provide a
satisfactory description of the surface properties of Fe nano-clusters as well
as bulk properties. Lippmann-Schwinger and Green's function techniques are used
together with Landauer theory to predict the current, magneto-resistance, and
spin polarization of the current in Fe nanocontacts bridged by atomic chains
under applied bias. Unusual device characteristics are predicted including
negative magneto-resistance and spin polarization of the current, as well as
spin polarization of the current for anti-parallel magnetization of the Fe
nanocontacts under moderate applied bias. We explore the effects that
stretching the atomic chain has on the magneto-resistance and spin polarization
and predict a cross-over regime in which the spin polarization of the current
for parallel magnetization of the contacts switches from negative to positive.
We find resonant transmission due to dangling bond formation on tip atoms as
the chain is stretched through its breaking point to play an important role in
spin-dependent transport in this regime. The physical mechanisms underlying the
predicted phenomena are discussed.Comment: 13 pages, 6 figures, Accepted for publication in Physical Review
Inverse Magnetoresistance of Molecular Junctions
We present calculations of spin-dependent electron transport through single
organic molecules bridging pairs of iron nanocontacts. We predict the
magnetoresistance of these systems to switch from positive to negative with
increasing applied bias for both conducting and insulating molecules. This
novel inverse magnetoresistance phenomenon is robust, does not depend on the
presence of impurities, and is unique to molecular and atomic nanoscale
magnetic junctions. Its physical origin is identified and its relevance to
experiment and to potential technological applications is discussed.Comment: 5 pages, 3 figures; published version Phys. Rev.
Optical Transitions in Highly Excited States: RF LOG Spectrum of XeI
Pulsed laser optogalvanic spectroscopy of diluted gases in a
32MHz radio frequency discharge (RF LOG) was applied to xenon
in a pressure range from 0.01to several Torr. The optical transitions
caused by exciting the products of the discharge with laser energies from 14,000to 17,000cm? have been recorded and assigned. At low pressures, most transitions originate in the 5d states, whereas at higher pressures those from the 6p and 6s\u27 states become dominant. Results indicate that (il) coupling and selection rules /),J. = O, ± 1 and /),K. = O, ± 1 provide the most appropriate description of the observed transitions to the lower n-states. However, some transitions with /),K. = ± 2 also possess considerable intensity for some higher angular momentum transitions (p-d, d-f). The low pressure spectra are dominated by the d-f transitions for which all series have been observed. An important feature of these series is that at high n values (n> 20) the oscillatory potential of the RF field starts to populate high angular momentum states and causes a substantial broadening of the transitions to these states. In addition, close to the ionization limit, field induced ionization can take place and, under certain conditions,
this may cause certain high series members to disappear. The
corresponding results are presented and discussed
Comparison of fission and quasi-fission modes
Quantum shell effects are known to affect the formation of fragments in
nuclear fission. Shell effects also affect quasi-fission reactions occurring in
heavy-ion collisions. Systematic time-dependent Hartree-Fock simulations of
50Ca+176Yb collisions show that the mass equilibration between the fragments in
quasi-fission is stopped when they reach similar properties to those in the
asymmetric fission mode of the 226Th compound nucleus. Similar shell effects
are then expected to determine the final repartition of nucleons between the
nascent fragments in both mechanisms. Future experimental studies that could
test these observations are discussed.Comment: 8 pages, 4 figures, 1 tabl
Chandra observation of the central galaxies in A1060 cluster of galaxies
Chandra observation of the central region of the A1060 cluster of galaxies
resolved X-ray emission from two giant elliptical galaxies, NGC 3311 and NGC
3309. The emission from these galaxies consists of two components, namely the
hot interstellar medium (ISM) and the low-mass X-ray binaries (LMXBs). We found
the spatial extent of the ISM component was much smaller than that of stars for
both galaxies, while the ratios of X-ray to optical blue-band luminosities were
rather low but within the general scatter for elliptical galaxies. After
subtracting the LMXB component, the ISM is shown to be in pressure balance with
the intracluster medium of A1060 at the outer boundary of the ISM. These
results imply that the hot gas supplied from stellar mass loss is confined by
the external pressure of the intracluster medium, with the thermal conduction
likely to be suppressed. The cD galaxy NGC 3311 does not exhibit the extended
potential structure which is commonly seen in bright elliptical galaxies, and
we discuss the possible evolution history of the very isothermal cluster A1060.Comment: 12 pages, 7 figures, Latex2e(emulateapj5), accepted in Ap
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A physico-chemical study of some areas of fundamental significance to biophysics
This report discusses the following topics on Biophysics: Radiation signatures; electronic structure of steroids and vitamins; laser optogalvanic effect; vacuum ultraviolet spectroscopy; and ozone
Evidence for a Photospheric Component in the Prompt Emission of the Short GRB120323A and its Effects on the GRB Hardness-Luminosity Relation
The short GRB 120323A had the highest flux ever detected with the Fermi/GBM.
Here we study its remarkable spectral properties and their evolution using two
spectral models: (i) a single emission component scenario, where the spectrum
is modeled by the empirical Band function, and (ii) a two component scenario,
where thermal (Planck-like) emission is observed simultaneously with a
non-thermal component (a Band function). We find that the latter model fits the
integrated burst spectrum significantly better than the former, and that their
respective spectral parameters are dramatically different: when fit with a Band
function only, the Epeak of the event is unusually soft for a short GRB, while
adding a thermal component leads to more typical short GRB values. Our
time-resolved spectral analysis produces similar results. We argue here that
the two-component model is the preferred interpretation for GRB 120323A, based
on: (i) the values and evolution of the Band function parameters of the two
component scenario, which are more typical for a short GRB, and (ii) the
appearance in the data of a significant hardness-intensity correlation,
commonly found in GRBs, when we employee two-component model fits; the
correlation is non-existent in the Band-only fits. GRB 110721A, a long burst
with an intense photospheric emission, exhibits the exact same behavior. We
conclude that GRB 120323A has a strong photospheric emission contribution,
first time observed in a short GRB. Magnetic dissipation models are difficult
to reconcile with these results, which instead favor photospheric thermal
emission and fast cooling synchrotron radiation from internal shocks. Finally,
we derive a possibly universal hardness-luminosity relation in the source frame
using a larger set of GRBs L,i=(1.59+/-0.84).10^50 (Epeak,i)^(1.33+/-0.07)
erg/s), which could be used as a possible redshift estimator for cosmology.Comment: 27 pages, 13 figures, Accepted by ApJ (April, 7th 2013
The frontier of darkness: the cases of GRB 040223, GRB 040422, GRB 040624
Understanding the reasons for the faintness of the optical/near-infrared
afterglows of the so-called dark bursts is essential to assess whether they
form a subclass of GRBs, and hence for the use of GRBs in cosmology. With VLT
and other ground-based telescopes, we searched for the afterglows of the
INTEGRAL bursts GRB 040223, GRB 040422 and GRB 040624 in the first hours after
the triggers. A detection of a faint afterglow and of the host galaxy in the K
band was achieved for GRB 040422, while only upper limits were obtained for GRB
040223 and GRB 040624, although in the former case the X-ray afterglow was
observed. A comparison with the magnitudes of a sample of afterglows clearly
shows the faintness of these bursts, which are good examples of a population
that an increasing usage of large diameter telescopes is beginning to unveil.Comment: 4 pages, 2 figures. To appear in the proceedings of the 16th Annual
October Astrophysics Conference in Maryland "Gamma Ray Bursts in the Swift
Era", eds. S. Holt, N. Gehrels & J. Nouse
Current-Driven Conformational Changes, Charging and Negative Differential Resistance in Molecular Wires
We introduce a theoretical approach based on scattering theory and total
energy methods that treats transport non-linearities, conformational changes
and charging effects in molecular wires in a unified way. We apply this
approach to molecular wires consisting of chain molecules with different
electronic and structural properties bonded to metal contacts. We show that
non-linear transport in all of these systems can be understood in terms of a
single physical mechanism and predict that negative differential resistance at
high bias should be a generic property of such molecular wires.Comment: 9 pages, 4 figure
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