324 research outputs found
Interaction Of Electrons With Spin Waves In The Bulk And In Multilayers
The exchange interaction between electrons and magnetic spins is considerably
enhanced near interfaces, in magnetic multilayers. As a result, a dc current
can be used to generate spin oscillations. We review theory and experimental
evidence. The s-d exchange interaction causes a rapid precession of itinerant
conduction-electron spins s around the localized spins S of magnetic electrons.
Because of the precession, the time-averaged interaction torque between s and S
vanishes. An interface between a magnetic layer and a spacer causes a local
coherence between the precession phases of differnt electrons, within 10 nm
from the interface, and restores the torque. Also, a second magnetic layer with
pinned S is used to prepare s in a specific direction. the current-induced
drive torque of s on S in the active layer may be calculated from the spin
current (Slonczewski) or from the spin imbalance Delta-mu (Berger). Spin
current and Delta-mu are proportional to each other, and can arise from
Fermi-surface translation, as well as from expansion/contraction.Comment: Invited paper at Seattle MMM01 Conference, Nov. 2001 (to appear in J.
Appl. Phys.
Extremely Inefficient Star Formation in the Outer Disks of Nearby Galaxies
(Abridged) We combine data from The HI Nearby Galaxy Survey and the GALEX
Nearby Galaxy Survey to study the relationship between atomic hydrogen (HI) and
far-ultraviolet (FUV) emission outside the optical radius (r25) in 17 spiral
and 5 dwarf galaxies. In this regime, HI is likely to represent most of the ISM
and FUV emission to trace recent star formation with little bias due to
extinction, so that the two quantities closely trace the underlying
relationship between gas and star formation rate (SFR). The azimuthally
averaged HI and FUV intensities both decline with increasing radius in this
regime, with the scale length of the FUV profile typically half that of the HI
profile. Despite the mismatch in profiles, there is a significant spatial
correlation (at 15" resolution) between local FUV and HI intensities; near r25
this correlation is quite strong, in fact stronger than anywhere inside r25,
and shows a decline towards larger radii. The star formation efficiency (SFE) -
defined as the ratio of FUV/HI and thus the inverse of the gas depletion time -
decreases with galactocentric radius across the outer disks, though much
shallower than across the optical disks. On average, we find the gas depletion
times to be well above a Hubble time (~10^11 yr). We observe a clear
relationship between FUV/HI and HI column in the outer disks, with the SFE
increasing with increasing HI column. Despite observing systematic variations
in FUV/HI, we find no clear evidence for step-function type star formation
thresholds. When compared with results from inside r25, we find outer disk star
formation to be distinct in several ways: it is extremely inefficient
(depletion times of many Hubble times) with column densities and SFRs lower
than found anywhere inside the optical disks. It appears that the HI column is
one of, perhaps even the key environmental factor in setting the SFR in outer
galaxy disks.Comment: Accepted for Publication in The Astronomical Journa
The mass-dependent star formation histories of disk galaxies: infall model versus observations
We introduce a simple model to explore the star formation histories of disk
galaxies. We assume that the disk origins and grows by continuous gas infall.
The gas infall rate is parametrized by the Gaussian formula with one free
parameter: infall-peak time . The Kennicutt star formation law is adopted
to describe how much cold gas turns into stars. The gas outflow process is also
considered in our model. We find that, at given galactic stellar mass ,
model adopting late infall-peak time results in blue colors, low
metallicity, high specific star formation rate and high gas fraction, while gas
outflow rate mainly influences the gas-phase metallicity and star formation
efficiency mainly influences the gas fraction. Motivated by the local observed
scaling relations, we construct a mass-dependent model by assuming low mass
galaxy has later infall-peak time and larger gas outflow rate than
massive systems. It is shown that this model can be in agreement with not only
the local observations, but also the observed correlations between specific
star formation rate and galactic stellar mass at
intermediate redshift . Comparison between the Gaussian-infall model and
exponential-infall model is also presented. It shows that the
exponential-infall model predicts higher star formation rate at early stage and
lower star formation rate later than that of Gaussian-infall. Our results
suggest that the Gaussian infall rate may be more reasonable to describe the
gas cooling process than the exponential infall rate, especially for low-mass
systems.Comment: 12 pages, 6 figures, ApJ, 2010, 722, 38
Combined effect of Zeeman splitting and spin-orbit interaction on the Josephson current in a S-2DEG-S structure
We analyze new spin effects in current-carrying state of superconductor-2D
electron gas-superconductor (S-2DEG-S) device with spin-polarized nuclei in
2DEG region. The hyperfine interaction of 2D electrons with nuclear spins,
described by the effective magnetic field B, produces Zeeman splitting of
Andreev levels without orbital effects, that leads to the interference pattern
of supercurrent oscillations over B. The spin-orbit effects in 2DEG cause
strongly anisotropic dependence of the Josephson current on the direction of B,
which may be used as a probe for the spin-orbit interaction intensity. Under
certain conditions, the system reveals the properties of pi-junction.Comment: 4 pages, 4 figure
The migration of nearby spirals from the blue to red sequence: AGN feedback or environmental effects?
We combine ultraviolet to near-infrared photometry with HI 21cm line
observations for a complete volume-limited sample of nearby galaxies in
different environments (from isolated galaxies to Virgo cluster members), to
study the migration of spirals from the blue to the red sequence. Although our
analysis confirms that, in the transition region between the two sequences, a
high fraction of spirals host active galactic nuclei (AGN), it clearly shows
that late-types with quenched star formation are mainly HI deficient galaxies
preferentially found in the Virgo cluster. This not only suggests that
environmental effects could play a significant role in driving the migration of
local galaxies from the blue sequence, but it also implies that a physical link
between AGN feedback and quenching may not be assumed from a correlation
between nuclear activity and colour.Comment: Accepted for publication in MNRAS. 6 pages, 1 figur
Is the magnetic field necessary for the Aharonov-Bohm effect in mesoscopics?
A new class of topological mesoscopic phenomena in absence of external
magnetic field (meso-nucleo-spinics)is predicted, which is based on combined
action of the nonequilibrium nuclear spin population and charge carriers
spin-orbit interaction . As an example, we show that Aharonov-Bohm like
oscillations of the persistent current in GaAs/AlGaAs based mesoscopic rings
may exist, in the absence of the external magnetic field, provided that a
topologically nontrivial strongly nonequilibrium nuclear spin population is
created. This phenomenon is due to the breaking, via the spin-orbit coupling,
of the clock wise - anti clock wise symmetry of the charge carriers momentum,
which results in the oscillatory in time persistent current.Comment: 14 pages, Late
The GALEX UV luminosity function of the cluster of galaxies Abell 1367
We present the GALEX NUV (2310 A) and FUV (1530 A) galaxy luminosity
functions of the nearby cluster of galaxies A1367 in the magnitude range -20.3<
M_AB < -13.3. The luminosity functions are consistent with previous (~ 2 mag
shallower) estimates based on the FOCA and FAUST experiments, but display a
steeper faint-end slope than the GALEX luminosity function for local field
galaxies. Using spectro-photometric optical data we select out star-forming
systems from quiescent galaxies and study their separate contributions to the
cluster luminosity function. We find that the UV luminosity function of cluster
star-forming galaxies is consistent with the field. The difference between the
cluster and field LF is entirely due to the contribution at low luminosities
(M_AB >-16 mag) of non star-forming, early-type galaxies that are significantly
over dense in clusters.Comment: 4 pages, 4 figures, 1 table. Accepted for publication in
Astrophysical Journal Letter
- …