9,227 research outputs found
Phase-reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138
We investigate a phase-reference VLBI observation that was conducted at 15.4
GHz by fast switching VLBA antennas between the compact steep-spectrum radio
source 3C 138 and the quasar PKS 0528+134 which are about 4 away on the
sky. By comparing the phase-reference mapping with the conventional hybrid
mapping, we demonstrate the feasibility of high precision astrometric
measurements for sources separated by 4. VLBI phase-reference mapping
preserves the relative phase information, and thus provides an accurate
relative position between 3C 138 and PKS 0528+134 of
and
(J2000.0) in right ascension and declination, respectively. This gives an
improved position of the nucleus (component A) of 3C 138 in J2000.0 to be
RA= and Dec= under the
assumption that the position of calibrator PKS 0528+134 is correct. We further
made a hybrid map by performing several iterations of CLEAN and
self-calibration on the phase-referenced data with the phase-reference map as
an input model for the first phase self-calibration. Compared with the hybrid
map from the limited visibility data directly obtained from fringe fitting 3C
138 data, this map has a similar dynamic range, but a higher angular
resolution. Therefore, phase-reference technique is not only a means of phase
connection, but also a means of increasing phase coherence time allowing
self-calibration technique to be applied to much weaker sources.Comment: 9 pages plus 2 figures, accepted by PASJ (Vol.58 No.6
Magnetocrystalline anisotropic effect in GdCoFeAsO ()
From a systematic study of the electrical resistivity , magnetic
susceptibility , isothermal magnetization and the specific
heat , a temperature-magnetic field (-) phase diagram has been
established for GdCoFeAsO ( and ) polycrystalline
compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic
(FM) transition of Co electrons () and
antiferromagnetic (AFM) transition of Gd electrons
(). For the Fe-doped sample (), an extra
magnetic reorientation transition takes place below ,
which is likely associated with Co moments. The two magnetic species of Gd and
Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM)
behavior in the magnetic susceptibility. Upon decreasing the temperature (), the magnetocrystalline anisotropy breaks up the FM
order of Co by aligning the moments with the local easy axes of the various
grains, leading to a spin reorientation transition at
. By applying a magnetic field,
monotonically decreases to lower temperatures, while
the is relatively robust against the external field.
On the other hand, the applied magnetic field pulls the magnetization of grains
from the local easy direction to the field direction via a first-order
reorientation transition, with the transition field () increasing
upon cooling the temperature.Comment: accepted by physical Review B 6 figures and 7 page
Bulk Rotational Symmetry Breaking in Kondo Insulator SmB6
Kondo insulator samarium hexaboride (SmB6) has been intensely studied in
recent years as a potential candidate of a strongly correlated topological
insulator. One of the most exciting phenomena observed in SmB6 is the clear
quantum oscillations appearing in magnetic torque at a low temperature despite
the insulating behavior in resistance. These quantum oscillations show multiple
frequencies and varied effective masses. The origin of quantum oscillation is,
however, still under debate with evidence of both two-dimensional Fermi
surfaces and three-dimensional Fermi surfaces. Here, we carry out
angle-resolved torque magnetometry measurements in a magnetic field up to 45 T
and a temperature range down to 40 mK. With the magnetic field rotated in the
(010) plane, the quantum oscillation frequency of the strongest oscillation
branch shows a four-fold rotational symmetry. However, in the angular
dependence of the amplitude of the same branch, this four-fold symmetry is
broken and, instead, a twofold symmetry shows up, which is consistent with the
prediction of a two-dimensional Lifshitz-Kosevich model. No deviation of
Lifshitz-Kosevich behavior is observed down to 40 mK. Our results suggest the
existence of multiple light-mass surface states in SmB6, with their mobility
significantly depending on the surface disorder level.Comment: 15 pages, 9 figure
Extraction of Plumes in Turbulent Thermal Convection
We present a scheme to extract information about plumes, a prominent coherent
structure in turbulent thermal convection, from simultaneous local velocity and
temperature measurements. Using this scheme, we study the temperature
dependence of the plume velocity and understand the results using the equations
of motion. We further obtain the average local heat flux in the vertical
direction at the cell center. Our result shows that heat is not mainly
transported through the central region but instead through the regions near the
sidewalls of the convection cell.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Electrical current-driven pinhole formation and insulator-metal transition in tunnel junctions
Current Induced Resistance Switching (CIS) was recently observed in thin
tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to
electromigration of metallic atoms in nanoconstrictions in the insulating
barrier. The CIS effect is here studied in TJs with two thin (20 \AA)
non-magnetic (NM) Ta electrodes inserted above and below the insulating
barrier. We observe resistance (R) switching for positive applied electrical
current (flowing from the bottom to the top lead), characterized by a
continuous resistance decrease and associated with current-driven displacement
of metallic ions from the bottom electrode into the barrier (thin barrier
state). For negative currents, displaced ions return into their initial
positions in the electrode and the electrical resistance gradually increases
(thick barrier state). We measured the temperature (T) dependence of the
electrical resistance of both thin- and thick-barrier states ( and R
respectively). Experiments showed a weaker R(T) variation when the tunnel
junction is in the state, associated with a smaller tunnel contribution.
By applying large enough electrical currents we induced large irreversible
R-decreases in the studied TJs, associated with barrier degradation. We then
monitored the evolution of the R(T) dependence for different stages of barrier
degradation. In particular, we observed a smooth transition from tunnel- to
metallic-dominated transport. The initial degradation-stages are related to
irreversible barrier thickness decreases (without the formation of pinholes).
Only for later barrier degradation stages do we have the appearance of metallic
paths between the two electrodes that, however, do not lead to metallic
dominated transport for small enough pinhole radius.Comment: 10 pages, 3 figure
Upper critical field and thermally activated flux flow in single crystalline TlRbFeSe
The upper critical field of
TlRbFeSe single crystals has been determined by
means of measuring the electrical resistivity in both a pulsed magnetic field
(60T) and a DC magnetic field (14T). It is found that
linearly increases with decreasing temperature for ,
reaching T. On the
other hand, a larger with a strong convex curvature
is observed for ((18K)60T). This compound shows a moderate anisotropy of the upper
critical field around , but decreases with decreasing temperature.
Analysis of the upper critical field based on the Werthamer-Helfand-Hohenberg
(WHH) method indicates that is orbitally limited for
, but the effect of spin paramagnetism may play an
important role on the pair breaking for . All these
experimental observations remarkably resemble those of the iron pnictide
superconductors, suggesting a unified scenario for the iron-based
superconductors. Moreover, the superconducting transition is significantly
broadened upon applying a magnetic field, indicating strong thermal fluctuation
effects in the superconducting state of
TlRbFeSe. The derived thermal activation energy
for vortex motion is compatible with those of the 1111-type iron pnictides.Comment: 7 pages, 6 figure
The Peculiar Atmospheric Chemistry of KELT-9b
The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the
transition between gas giants and stars, and therefore between two
traditionally distinct regimes of atmospheric chemistry. Previous theoretical
studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite
the high ultraviolet flux from KELT-9, we show using photochemical kinetics
calculations that the observable atmosphere of KELT-9b is predicted to be close
to chemical equilibrium, which greatly simplifies any theoretical
interpretation of its spectra. It also makes the atmosphere of KELT-9b, which
is expected to be cloudfree, a tightly constrained chemical system that lends
itself to a clean set of theoretical predictions. Due to the lower pressures
probed in transmission (compared to emission) spectroscopy, we predict the
abundance of water to vary by several orders of magnitude across the
atmospheric limb depending on temperature, which makes water a sensitive
thermometer. Carbon monoxide is predicted to be the dominant molecule under a
wide range of scenarios, rendering it a robust diagnostic of the metallicity
when analyzed in tandem with water. All of the other usual suspects (acetylene,
ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be
subdominant at solar metallicity, while atomic oxygen, iron and magnesium are
predicted to have relative abundances as high as 1 part in 10,000. Neutral
atomic iron is predicted to be seen through a forest of optical and
near-infrared lines, which makes KELT-9b suitable for high-resolution
ground-based spectroscopy with HARPS-N or CARMENES. We summarize future
observational prospects of characterizing the atmosphere of KELT-9b.Comment: Accepted by ApJ. 9 pages, 6 figures. Corrected minor errors in
Figures 1a and 1b (some line styles were switched by accident), text and
conclusions unchanged, these minor changes will be updated in final ApJ proo
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