26,332 research outputs found
Calibration of shielded microwave probes using bulk dielectrics
A stripline-type near-field microwave probe is microfabricated for microwave
impedance microscopy. Unlike the poorly shielded coplanar probe that senses the
sample tens of microns away, the stripline structure removes the stray fields
from the cantilever body and localizes the interaction only around the
focused-ion beam deposited Pt tip. The approaching curve of an oscillating tip
toward bulk dielectrics can be quantitatively simulated and fitted to the
finite-element analysis result. The peak signal of the approaching curve is a
measure of the sample dielectric constant and can be used to study unknown bulk
materials.Comment: 10 pages, 3 figure
Insights from ARPES for an undoped, four-layered, two-gap high-T_c superconductor
An undoped cuprate with apical fluorine and inner (i) and outer (o)
CuO2-layers is a 60 K superconductor whose Fermi surface (FS) has large n- and
p-doped sheets with the SC gap on the n-sheet twice that on the p -sheet (Y.
Chen et al.). The Fermi surface is not reproduced by the LDA, but the screening
must be substantially reduced due to electronic correlations, and oxygen in the
o-layers must be allowed to dimple outwards. This charges the i-layers by
0.01|e|, causes an 0.4 eV Madelung-potential difference between the i and o
-layers, quenches the i-o hopping, and localizes the n-sheets onto the
i-layers, thus protecting their d-wave pairs from being broken by scattering on
impurities in the BaF layers. The correlation-reduced screening strengthens the
coupling to z-axis phonons.Comment: 4 pages, 3 figure
The Dense Plasma Torus Around the Nucleus of an Active Galaxy NGC 1052
A subparsec-scale dense plasma torus around an active galactic nucleus (AGN)
is unveiled. We report on very-long-baseline interferometry (VLBI) observations
at 2.3, 8.4, and 15.4 GHz towards the active galaxy NGC 1052. The convex
spectra of the double-sided jets and the nucleus imply that synchrotron
emission is obscured through free--free absorption (FFA) by the foreground cold
dense plasma. A trichromatic image was produced to illustrate the distribution
of the FFA opacity. We found a central condensation of the plasma which covers
about 0.1 pc and 0.7 pc of the approaching and receding jets, respectively. A
simple explanation for the asymmetric distribution is the existence of a thick
plasma torus perpendicular to the jets. We also found an ambient FFA absorber,
whose density profile can be ascribed to a spherical distribution of the
isothermal King model. The coexistence of torus-like and spherical
distributions of the plasma suggests a transition from radial accretion to
rotational accretion around the nucleus.Comment: 10 pages, to appear in Publ. Astron. Soc. Japan, vol.53, No.2 (2001
Four dual AGN candidates observed with the VLBA
According to hierarchical structure formation models, merging galaxies are
expected to be seen in different stages of their coalescence. However,
currently there are no straightforward observational methods neither to select
nor to confirm a large number of dual active galactic nuclei (AGN) candidates.
Most attempts involve the better understanding of double-peaked narrow emission
line sources, to distinguish the objects where the emission lines originate
from narrow-line kinematics or jet-driven outflows from those which might
harbour dual AGN. We observed four such candidate sources with the Very Long
Baseline Array (VLBA) at 1.5 GHz with 10 milli-arcsecond angular
resolution where spectral profiles of AGN optical emission suggested the
existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3,
the radio structures are aligned with the optical emission features, thus the
double-peaked emission lines might be the results of jet-driven outflows. In
the third detected source SDSS J115523.74+150756.9, the radio structure is less
extended and oriented nearly perpendicular to the position angle derived from
optical spectroscopy. The fourth source remained undetected with the VLBA but
it has been imaged with the Very Large Array at arcsec resolution a few months
before our observations, suggesting the existence of extended radio structure.
In none of the four sources did we detect two radio-emitting cores, a
convincing signature of duality.Comment: 35 pages, 3 figures, 2 tables, accepted for publication in Ap
Apparent electron-phonon interaction in strongly correlated systems
We study the interaction of electrons with phonons in strongly correlated
solids, having high-T_c cuprates in mind. Using sum-rules, we show that the
apparent strength of this interaction strongly depends on the property studied.
If the solid has a small fraction (doping) delta of charge carriers, the
influence of the interaction on the phonon self-energy is reduced by a factor
delta, while there is no corresponding reduction of the coupling seen in the
electron self-energy. This supports the interpretation of recent photoemission
experiments, assuming a strong coupling to phonons.Comment: 4 pages, RevTeX, 2 eps figure
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