4,298 research outputs found
Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface
Although the creation of spin polarization in various non-magnetic media via
electrical spin injection from a ferromagnetic tunnel contact has been
demonstrated, much of the basic behavior is heavily debated. It is reported
here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs
that local magnetostatic fields arising from interface roughness dramatically
alter and even dominate the accumulation and dynamics of spins in the
semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to
reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous
and non-collinear with the injector magnetization. The inverted Hanle effect
serves as experimental signature. This interaction needs to be taken into
account in the analysis of experimental data, particularly in extracting the
spin lifetime and its variation with different parameters (temperature, doping
concentration). It produces a broadening of the standard Hanle curve and
thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si
at room temperature it is shown that the spin lifetime is larger than
previously determined, and a new lower bound of 0.29 ns is obtained. The
results are expected to be general and occur for spins near a magnetic
interface not only in semiconductors but also in metals, organic and
carbon-based materials including graphene, and in various spintronic device
structures.Comment: Final version, with text restructured and appendices added (25 pages,
9 figures). To appear in Phys. Rev.
Numerical Investigation of the Interface Tension in the three-dimensional Ising Model
The interface tension in the three-dimensional Ising model in the low
temperature phase is investigated by means of the Monte Carlo method. Together
with other physically relevant quantities it is obtained from a calculation of
time-slice correlation functions in a cylindrical geometry. The results at
three different values of the temperature are compared with the predictions
from a semiclassical approximation in the framework of renormalized
theory in three dimensions, and are in good agreement with them.Comment: 17 pages, MS-TPI-92-13 (replaced due to correction of minor numerical
errors
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
Effects of Dust on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of an optical QSO by a spiral galaxy is often
counteracted by dust obscuration, since the line-of-sight to the QSO passes
close to the center of the galactic disk. The dust in the lens is likely to be
correlated with neutral hydrogen, which in turn should leave a Lyman-alpha
absorption signature on the QSO spectrum. We use the estimated dust-to-gas
ratio of the Milky-Way galaxy as a mean and allow a spread in its values to
calculate the effects of dust on lensing by low redshift spiral galaxies. Using
a no-evolution model for spirals at z<1 we find (in Lambda=0 cosmologies) that
the magnification bias due to lensing is stronger than dust obscuration for QSO
samples with a magnitude limit B<16. The density parameter of neutral hydrogen,
Omega_HI, is overestimated in such samples and is underestimated for fainter
QSOs.Comment: 18 pages, 4 figures, ApJ, in pres
The Ha luminosity function and star formation rate up to z~1
We describe ISAAC/ESO-VLT observations of the Ha(6563) Balmer line of 33
field galaxies from the Canada-France Redshift Survey (CFRS) with redshifts
selected between 0.5 and 1.1. We detect Ha in emission in 30 galaxies and
compare the properties of this sample with the low-redshift sample of CFRS
galaxies at z~0.2 (Tresse & Maddox 1998). We find that the Ha luminosity,
L(Ha), is tightly correlated to M(B(AB)) in the same way for both the low- and
high-redshift samples. L(Ha) is also correlated to L([OII]3727), and again the
relation appears to be similar at low and high redshifts. The ratio
L([OII])/L(Ha) decreases for brighter galaxies by as much as a factor 2 on
average. Derived from the Ha luminosity function, the comoving Ha luminosity
density increases by a factor 12 from =0.2 to =1.3. Our results confirm a
strong rise of the star formation rate (SFR) at z<1.3, proportional to
(1+z)^{4.1+/-0.3} (with H_0=50 km/s/Mpc, q_0=0.5). We find an average SFR(2800
Ang)/SFR(Ha) ratio of 3.2 using the Kennicutt (1998) SFR transformations. This
corresponds to the dust correction that is required to make the near UV data
consistent with the reddening-corrected Ha data within the self-contained,
I-selected CFRS sample.Comment: 16 pages, 16 figures and 3 tables included, figures and text updated,
same results as in the 1st version, accepted in MNRA
Spitzer Mapping of PAHs and H2 in Photodissociation Regions
The mid-infrared (MIR) spectra of dense photodissociation regions (PDRs) are
typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs)
and the lowest pure rotational states of molecular hydrogen (H2); two species
which are probes of the physical properties of gas and dust in intense UV
radiation fields. We utilize the high angular resolution of the Infrared
Spectrograph on the Spitzer Space Telescope to construct spectral maps of the
PAH and H2 features for three of the best studied PDRs in the galaxy, NGC 7023,
NGC 2023 and IC 63. We present spatially resolved maps of the physical
properties, including the H2 ortho-to-para ratio, temperature, and G_o/n_H. We
also present evidence for PAH dehydrogenation, which may support theories of H2
formation on PAH surfaces, and a detection of preferential self-shielding of
ortho-H2. All PDRs studied exhibit average temperatures of ~500 - 800K, warm H2
column densities of ~10^20 cm^-2, G_o/n_H ~ 0.1 - 0.8, and ortho-to-para ratios
of ~ 1.8. We find that while the average of each of these properties is
consistent with previous single value measurements of these PDRs, when
available, the addition of spatial resolution yields a diversity of values with
gas temperatures as high as 1500 K, column densities spanning ~ 2 orders of
magnitude, and extreme ortho-to-para ratios of 3.Comment: 14 figure
CN and HCN in Dense Interstellar Clouds
We present a theoretical investigation of CN and HCN molecule formation in
dense interstellar clouds. We study the gas-phase CN and HCN production
efficiencies from the outer photon-dominated regions (PDRs) into the opaque
cosmic-ray dominated cores. We calculate the equilibrium densities of CN and
HCN, and of the associated species C+, C, and CO, as functions of the
far-ultraviolet (FUV) optical depth. We consider isothermal gas at 50 K, with
hydrogen particle densities from 10^2 to 10^6 cm^-3. We study clouds that are
exposed to FUV fields with intensities 20 to 2*10^5 times the mean interstellar
FUV intensity. We assume cosmic-ray H2 ionization rates ranging from 5*10^-17
s^-1, to an enhanced value of 5*10^-16 s^-1. We also examine the sensitivity of
the density profiles to the gas-phase sulfur abundance.Comment: Accepted for publication in ApJ, 33 pages, 8 figure
H2CO and CH3OH maps of the Orion Bar photodissociation region
A previous analysis of methanol and formaldehyde towards the Orion Bar
concluded that the two molecular species may trace different physical
components, methanol the clumpy material, and formaldehyde the interclump
medium. To verify this hypothesis, we performed multi-line mapping observations
of the two molecules to study their spatial distributions. The observations
were performed with the IRAM-30m telescope at 218 and 241 GHz, with an angular
resolution of ~11''. Additional data for H2CO from the Plateau de Bure array
are also discussed. The data were analysed using an LVG approach.
Both molecules are detected in our single-dish data. Our data show that CH3OH
peaks towards the clumps of the Bar, but its intensity decreases below the
detection threshold in the interclump material. When averaging over a large
region of the interclump medium, the strongest CH3OH line is detected with a
peak intensity of ~0.06K. Formaldehyde also peaks on the clumps, but it is also
detected in the interclump gas. We verified that the weak intensity of CH3OH in
the interclump medium is not caused by the different excitation conditions of
the interclump material, but reflects a decrease in the column density of
methanol. The abundance of CH3OH relative to H2CO decreases by at least one
order of magnitude from the dense clumps to the interclump medium.Comment: 11 pages, accepted for publication in A&
The Ionization Fraction in Dense Molecular Gas II: Massive Cores
We present an observational and theoretical study of the ionization fraction
in several massive cores located in regions that are currently forming stellar
clusters. Maps of the emission from the J = 1-> O transitions of C18O, DCO+,
N2H+, and H13CO+, as well as the J = 2 -> 1 and J = 3 -> 2 transitions of CS,
were obtained for each core. Core densities are determined via a large velocity
gradient analysis with values typically 10^5 cm^-3. With the use of
observations to constrain variables in the chemical calculations we derive
electron fractions for our overall sample of 5 cores directly associated with
star formation and 2 apparently starless cores. The electron abundances are
found to lie within a small range, -6.9 < log10(x_e) < -7.3, and are consistent
with previous work. We find no difference in the amount of ionization fraction
between cores with and without associated star formation activity, nor is any
difference found in electron abundances between the edge and center of the
emission region. Thus our models are in agreement with the standard picture of
cosmic rays as the primary source of ionization for molecular ions. With the
addition of previously determined electron abundances for low mass cores, and
even more massive cores associated with O and B clusters, we systematically
examine the ionization fraction as a function of star formation activity. This
analysis demonstrates that the most massive sources stand out as having the
lowest electron abundances (x_e < 10^-8).Comment: 35 pages (8 figures), using aaspp4.sty, to be published in
Astrophysical Journa
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
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