1,426 research outputs found
Effective Hamiltonians for holes in antiferromagnets: a new approach to implement forbidden double occupancy
A coherent state representation for the electrons of ordered antiferromagnets
is used to derive effective Hamiltonians for the dynamics of holes in such
systems. By an appropriate choice of these states, the constraint of forbidden
double occupancy can be implemented rigorously. Using these coherent states,
one arrives at a path integral representation of the partition function of the
systems, from which the effective Hamiltonians can be read off. We apply this
method to the t-J model on the square lattice and on the triangular lattice. In
the former case, we reproduce the well-known fermion-boson Hamiltonian for a
hole in a collinear antiferromagnet. We demonstrate that our method also works
for non-collinear antiferromagnets by calculating the spectrum of a hole in the
triangular antiferromagnet in the self-consistent Born approximation and by
comparing it with numerically exact results.Comment: 9 pages, Latex, 6 figure
Requirements for a transformerless power conditioning system
Requirements for development of a Transformerless Power Conditioning Subsystem (TPCS) that will meet utility, manufacturer, and customer needs are detailed. Issues analyzed include current utility guidelines, safety and grounding issues that appear as local codes, various kinds of TPCS connections that can be developed, dc injection, and a brief survey of TPCS circuit topologies that will meet requirements. The major result is that a finite time exists for control operation before dc injection into the distribution transformer causes customer outage (on the order of seconds). This time permits the control system to sense a dc injection condition and remove the TPCS from the utility system. Requirements for such a control system are specified. A three wire connection will ensure balanced operation for customer loads and two wire connections caused average value dc to be injected into single phase loads. This type of connection also allows for the lowest array voltage. The conclusion is that requirements for a TPCS can be determined and that there are not showstopping issues preventing implementation. The actual design and topology of the TPCS was left for further study
Devon Island Programs, 1965
The Arctic Institute of North America has made available for future research the facilities left by the Devon Island Expedition of 1960-63. These facilities consist of four Jamesway huts equipped to support research workers at the base, two Massey Ferguson tractors, one weasel and a large range of equipment designed to outfit scientists working in the field away from the base camp. In 1965, taking advantage of these facilities, two scientific programs were followed. The first, in geomorphology, was essentially a base-camp-oriented project run by two men. The second, in glaciology, was a field-oriented program run by two men and one woman working on the ice cap and three outlet valley glaciers. The geomorphology program was supported entirely by the Arctic Institute of North America. The glaciology program received financial support from the Arctic Institute of North America, U.S. Army Natick Laboratories, and the Royal Geographical Society. ..
A New Brown Dwarf Desert? A Scarcity of Wide Ultracool Binaries
We present the results of a deep-imaging search for wide companions to
low-mass stars and brown dwarfs using NSFCam on IRTF. We searched a sample of
132 M7-L8 dwarfs to magnitude limits of and ,
corresponding to secondary-primary mass ratios of . No companions
were found with separations between 2{\arcsec} to 31{\arcsec} (40 AU
to 1000 AU). This null result implies a wide companion frequency below
2.3% at the 95% confidence level within the sensitivity limits of the survey.
Preliminary modeling efforts indicate that we could have detected 85% of
companions more massive than and 50% above .Comment: 27 pages, 8 figures, 3 tables: accepted to the Astronomical Journa
Keck Imaging of Binary L Dwarfs
We present Keck near-infrared imaging of three binary L dwarf systems, all of
which are likely to be sub-stellar. Two are lithium dwarfs, and a third
exhibits an L7 spectral type, making it the coolest binary known to date. All
have component flux ratios near 1 and projected physical separations between 5
and 10 AU, assuming distances of 18 to 26 pc from recent measurements of
trigonometric parallax. These surprisingly similar binaries represent the sole
detections of companions in ten L dwarf systems which were analyzed in the
preliminary phase of a much larger dual-epoch imaging survey. The detection
rate prompts us to speculate that binary companions to L dwarfs are common,
that similar-mass systems predominate, and that their distribution peaks at
radial distances in accord both with M dwarf binaries and with the radial
location of Jovian planets in our own solar system. To fully establish these
conjectures against doubts raised by biases inherent in this small preliminary
survey, however, will require quantitative analysis of a larger volume-limited
sample which has been observed with high resolution and dynamic range.Comment: LaTex manuscript in 13 pages, 3 postscript figures, Accepted for
publication in the Letters of the Astrophysical Journal; Postscript pre-print
version available at: http://www.hep.upenn.edu/PORG/papers/koerner99a.p
Imaging the Small-Scale Circumstellar Gas Around T~Tauri Stars
We have detected circumstellar molecular gas around a small sample of T Tauri
stars through aperture synthesis imaging of CO(2-1) emission at ~2-3''
resolution. RY Tauri, DL Tauri, DO Tauri, and AS 209 show resolved and
elongated gaseous emission. For RY Tau, the deconvolved, half-maximum radius
along the direction of elongation, PA~48deg, is 110 AU. Corresponding radii and
orientations for the other sources are: DL Tau -- 250 AU at PA~84deg; DO Tau --
350 AU at PA~160deg; AS 209 -- 290 AU at PA~138deg. RY Tau, DL Tau, and AS 209
show velocity gradients parallel to the elongation, suggesting that the
circumstellar material is rotating. RY Tau and AS 209 also exhibit
double-peaked spectra characteristic of a rotating disk. Line emission from DO
Tau is dominated by high-velocity blue-shifted gas which complicates the
interpretation. Nevertheless, there is in each case sufficient evidence to
speculate that the circumstellar emission may arise from a protoplanetary disk
similar to that from which our solar system formed.Comment: TeX file with associated macros, no figures; AJ in press. Also
available as a compressed postscript file (including figures) by anonymous
ftp at ftp://cougar.jpl.nasa.gov/pub/davidk/preprints/COinTTS.ps.
Imaging the Haro 6-10 Infrared Companion
We present an infrared imaging study of the low-mass pre-main-sequence binary system Haro 6-10. This system is one of a handful in which the optically visible primary has the characteristics of a normal T Tauri star, while the secondary is a so-called "infrared companion" (IRC), a strongly extincted object that emits most of its luminosity in the infrared. A speckle holographic technique was used to produce nearly diffraction-limited images on three nights over a 1 yr period starting in late 1997. The images show that the IRC is obscured and surrounded by a compact, irregular, and variable nebula. This structure is in striking contrast to the well-ordered edge-on disk associated with HK Tauri B, the extincted companion to another T Tauri star of similar age. A new, resolved intensity peak was found 0".4 southwest of the IRC. We suggest that it may represent light scattered by a clump of dusty material illuminated by starlight escaping along an outflow-carved cavity in the IRC envelope. The primary star became fainter and the companion became more extended during the observing period
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