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

Symmetry of the Gap in Bi2212 from Photoemission Spectroscopy

In a recent Letter, Shen et al have detected a large anisotropy of the superconducting gap in Bi2212, consistent with d-wave symmetry, from photoemission spectroscopy. Moreover, they claim that the change in their spectra as a function of aging is also consistent with such an intrepretation. In this Comment, I show that the latter statement is not entirely correct, in that the data as a function of aging are inconsistent with a d-wave gap but are consistent with an anisotropic s-wave gap.Comment: 3 pages (Plain TeX with macros), plus 1 postscript figur

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 $\sim$ 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

Complete relativistic equation of state for neutron stars

We construct the equation of state (EOS) in a wide density range for neutron stars using the relativistic mean field theory. The properties of neutron star matter with both uniform and non-uniform distributions are studied consistently. The inclusion of hyperons considerably softens the EOS at high densities. The Thomas-Fermi approximation is used to describe the non-uniform matter, which is composed of a lattice of heavy nuclei. The phase transition from uniform matter to non-uniform matter occurs around $0.06 \rm{fm^{-3}}$, and the free neutrons drip out of nuclei at about $2.4 \times 10^{-4}\ \rm{fm^{-3}}$. We apply the resulting EOS to investigate the neutron star properties such as maximum mass and composition of neutron stars.Comment: 23 pages, REVTeX, 9 ps figures, to appear in Phys. Rev.