4,221 research outputs found
Design and Performance of a Practical Variable-Temperature Scanning Tunneling Potentiometry System
We have constructed a scanning tunneling potentiometry system capable of
simultaneously mapping the transport-related electrochemical potential of a
biased sample along with its surface topography. Combining a novel sample
biasing technique with a continuous current-nulling feedback scheme pushes the
noise performance of the measurement to its fundamental limit - the Johnson
noise of the STM tunnel junction. The resulting 130 nV voltage sensitivity
allows us to spatially resolve local potentials at scales down to 2 nm, while
maintaining angstrom scale STM imaging, all at scan sizes of up to 15 um. A
mm-range two-dimensional coarse positioning stage and the ability to operate
from liquid helium to room temperature with a fast turn-around time greatly
expand the versatility of the instrument. By performing studies of several
model systems, we discuss the implications of various types of surface
morphology for potentiometric measurements.Comment: 16 pages, 17 figures, accepted to Review of Scientific Instruments v2
- minor changes: cleaned up figures/figure caption
Spectroscopy of Globular Clusters in the Sculptor Group Galaxies NGC 253 and NGC 55
We have obtained spectra for 103 published globular cluster candidates in the
Sculptor Group galaxies NGC 253 and NGC 55. On the basis of radial velocities
and digitized plate images, 14 globular clusters are identified in NGC 253 and
one probable globular cluster is identified in NGC 55. The majority of the
objects in the sample appear to be background galaxies. We have obtained and
analysed COSMOS plate scans of NGC 253 and NGC 55 and use these along with the
spectroscopically identified clusters to define new samples of globular cluster
candidates in the two galaxies which should have reduced contamination.Comment: 11 pages, 9 postscript figures, accepted for publication in MNRA
Toric Duality Is Seiberg Duality
We study four N=1 SU(N)^6 gauge theories, with bi-fundamental chiral matter
and a superpotential. In the infrared, these gauge theories all realize the
low-energy world-volume description of N coincident D3-branes transverse to the
complex cone over a del Pezzo surface dP_3 which is the blowup of P^2 at three
generic points. Therefore, the four gauge theories are expected to fall into
the same universality class--an example of a phenomenon that has been termed
"toric duality." However, little independent evidence has been given that such
theories are infrared-equivalent.
In fact, we show that the four gauge theories are related by the N=1 duality
of Seiberg, vindicating this expectation. We also study holographic aspects of
these gauge theories. In particular we relate the spectrum of chiral operators
in the gauge theories to wrapped D3-brane states in the AdS dual description.
We finally demonstrate that the other known examples of toric duality are
related by N=1 duality, a fact which we conjecture holds generally.Comment: 46 pages, 2 figures, harvma
The Atacama Large Millimeter/submillimeter Array: Overview & status
The Atacama Large Millimeter/submillimeter Array (ALMA) is an international
radio telescope under construction in the Atacama Desert of northern Chile.
ALMA will be situated on a high-altitude site at 5000 m elevation which
provides excellent atmospheric transmission over the instrument wavelength
range of 0.3 to 3 mm. ALMA will be comprised of two key observing components:
an array of up to sixty-four 12-m diameter antennas arranged in a multiple
configurations ranging in size from 0.15 to ~14 km, and a set of four 12-m and
twelve 7-m antennas operating in closely-packed configurations ~50m in diameter
(known as the Atacama Compact Array, or ACA), providing both interferometric
and total-power astronomical information. High-sensitivity dual-polarization 8
GHz-bandwidth spectral-line and continuum measurements between all antennas
will be available from two flexible digital correlators
Some Experimental Signatures to look for Time-reversal Violating superconductors
We discuss some experimental signatures associated with the topological
structures of unconventional superconductor order parameters of form
, where , or . Specifically, we study
the topological surface states on the and equivalent surfaces of such
superconductors which are observable in Andreev tunneling experiments, as well
as evaluate the magnetic flux trapped in superconducting rings of such
superconductors with multiple grain-boundary Josephson junctions. Previous
experiments are examined and several new experiments suggested.Comment: 11 pages, 3 figure
Quenching of Impurity Spins at Cu/CuO Interfaces: An Antiferromagnetic Proximity Effect
It is observed that the magnetoconductance of bilayer films of copper (Cu)
and copper monoxide (CuO) has distinct features compared of that of Cu films on
conventional band insulator substrates. We analyze the data above 2 K by the
theory of weak antilocalization in two-dimensional metals and suggest that
spin-flip scatterings by magnetic impurities inside Cu are suppressed in Cu/CuO
samples. Plausibly the results imply a proximity effect of antiferromagnetism
inside the Cu layer, which can be understood in the framework of
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. The data below 1 K, which
exhibit slow relaxation reminiscent of spin glass, are consistent with this
interpretation.Comment: 6 pages, 4 figures, 2 tables. Added a supplementary materia
Low-speed aerodynamic characteristics of a 13-percent-thick airfoil section designed for general aviation applications
Wind-tunnel tests were conducted to determine the low-speed section characteristics of a 13 percent-thick airfoil designed for general aviation applications. The results were compared with NACA 12 percent-thick sections and with the 17 percent-thick NASA airfoil. The tests were conducted ovar a Mach number range from 0.10 to 0.35. Chord Reynolds numbers varied from about 2,000,000 to 9,000,000
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