914 research outputs found
Two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2: phase transitions and AFMorder as seen by 87Rb NMR
We report the Rb nuclear magnetic resonance (NMR) results in a recently
synthesized Na5RbCu4(AsO4)Cl2. This complex novel two-dimensional (2D) cuprate
is an unique magnetic material, which contains layers of coupled Cu4O4
tetramers. In zero applied magnetic field, it orders antiferromagnetically via
a second-order low-entropy phase transition at TN = 15(1) K. We characterise
the ordered state by 87Rb NMR, and suggest for it a noncollinear rather than
collinear arrangement of spins. We discuss the properties of Rb nuclear site
and point out the new structural phase transition(s) around 74 K and 110 K.Comment: 2 pages, 2 figures, Proceedings of SCES'05, Vienna 200
Space Shuttle Communications Coverage Analysis for Thermal Tile Inspection
The space shuttle ultra-high frequency Space-to-Space Communication System has to provide adequate communication coverage for astronauts who are performing thermal tile inspection and repair on the underside of the space shuttle orbiter (SSO). Careful planning and quantitative assessment are necessary to ensure successful system operations and mission safety in this work environment. This study assesses communication systems performance for astronauts who are working in the underside, non-line-of-sight shadow region on the space shuttle. All of the space shuttle and International Space Station (ISS) transmitting antennas are blocked by the SSO structure. To ensure communication coverage at planned inspection worksites, the signal strength and link margin between the SSO/ISS antennas and the extravehicular activity astronauts, whose line-of-sight is blocked by vehicle structure, was analyzed. Investigations were performed using rigorous computational electromagnetic modeling techniques. Signal strength was obtained by computing the reflected and diffracted fields along the signal propagation paths between transmitting and receiving antennas. Radio frequency (RF) coverage was determined for thermal tile inspection and repair missions using the results of this computation. Analysis results from this paper are important in formulating the limits on reliable communication range and RF coverage at planned underside inspection and repair worksites
Millimeter and submillimeter wave technology developments for the next generation of fusion devices
There is increasing demand for compact watt-level coherent sources in the millimeter and submillimeter wave region. The approach that we have taken to satisfy this need is to fabricate two-dimensional grids loaded with oscillators, electronic beam steerers, and frequency multipliers for quasioptical coherent spatial combining of the outputs of a large number of low-power devices
Highly efficient frequency triplers in the millimeter wave region incorporating a back-to-back configuration of two varactor diodes
This paper reports on the recent development of monolithic frequency tripler array employing a back-to-back configuration of varactor diodes. Even harmonic idler
circuits are unnecessary in this design. Furthermore, no external dc bias is required. The arrangement results in highly efficient, easily-fabricated and inexpensive frequency triplers
Nomenclature for the C60-Ih and C70-D5h(6) fullerenes (IUPAC Recommendations 2002)
Fullerenes are a new allotrope of carbon characterized by a closed-cage structure consisting of an even number of three-coordinate carbon atoms devoid of hydrogen atoms. This class was originally limited to closed-cage structures with 12 isolated five-membered rings, the rest being six-membered rings. Although it was recognized that existing organic ring nomenclature could be used for these structures, the resulting names would be extremely unwieldy and inconvenient for use. At the same time it was also recognized that established organic nomenclature principles could be used, or adapted, to provide a consistent nomenclature for this unique class of compounds based on the class name fullerene. However, it was necessary to develop an entirely new method for uniquely numbering closed-cage systems. This paper describes IUPAC recommendations for naming and uniquely numbering the two most common fullerenes with isolated pentagons, the icosahedral C60 fullerene and a D5h-C70 fullerene. It also describes recommendations for adapting organic nomenclature principles for naming fullerenes with nonclosed-cage structures, heterofullerenes, derivatives formed by substitution of hydrofullerenes, and the fusion of organic rings or ring systems to the fullerene cage. Finally, this paper suggests methods for describing structures consisting of two or more fullerene units and for denoting configurations of chiral fullerenes and their derivative
Surface characterization and surface electronic structure of organic quasi-one-dimensional charge transfer salts
We have thoroughly characterized the surfaces of the organic charge-transfer
salts TTF-TCNQ and (TMTSF)2PF6 which are generally acknowledged as prototypical
examples of one-dimensional conductors. In particular x-ray induced
photoemission spectroscopy turns out to be a valuable non-destructive
diagnostic tool. We show that the observation of generic one-dimensional
signatures in photoemission spectra of the valence band close to the Fermi
level can be strongly affected by surface effects. Especially, great care must
be exercised taking evidence for an unusual one-dimensional many-body state
exclusively from the observation of a pseudogap.Comment: 11 pages, 12 figures, v2: minor changes in text and figure labellin
GPU Concurrency: Weak Behaviours and Programming Assumptions
Concurrency is pervasive and perplexing, particularly on graphics processing units (GPUs). Current specifications of languages and hardware are inconclusive; thus programmers often rely on folklore assumptions when writing software.
To remedy this state of affairs, we conducted a large empirical study of the concurrent behaviour of deployed GPUs. Armed with litmus tests (i.e. short concurrent programs), we questioned the assumptions in programming guides and vendor documentation about the guarantees provided by hardware. We developed a tool to generate thousands of litmus tests and run them under stressful workloads. We observed a litany of previously elusive weak behaviours, and exposed folklore beliefs about GPU programming---often supported by official tutorials---as false.
As a way forward, we propose a model of Nvidia GPU hardware, which correctly models every behaviour witnessed in our experiments. The model is a variant of SPARC Relaxed Memory Order (RMO), structured following the GPU concurrency hierarchy
Critical Properties of Spectral Functions for the 1D Anisotropic t-J Models with an Energy Gap
We exactly calculate the momentum-dependent critical exponents for spectral
functions in the one-dimensional anisotropic t-J models with a gap either in
the spin or charge excitation spectrum. Our approach is based on the Bethe
ansatz technique combined with finite-size scaling techniques in conformal
field theory. It is found that the spectral functions show a power-law
singularity, which occurs at frequencies determined by the dispersion of a
massive spin (or charge) excitation.We discuss how the nontrivial contribution
of a massive excitation controls the singular behavior in optical response
functions.Comment: 4 pages, REVTeX, 2 figures(available upon request), accepted for
publication in JPSJ 66 (1997) No.
Microscopic theory of the pseudogap and Peierls transition in quasi-one-dimensional materials
The problem of deriving from microscopic theory a Ginzburg-Landau free energy
functional to describe the Peierls or charge-density-wave transition in
quasi-one-dimensional materials is considered. Particular attention is given to
how the thermal lattice motion affects the electronic states. Near the
transition temperature the thermal lattice motion produces a pseudogap in the
density of states at the Fermi level. Perturbation theory diverges and the
traditional quasi-particle or Fermi liquid picture breaks down. The pseudogap
causes a significant modification of the coefficients in the Ginzburg-Landau
functional from their values in the rigid lattice approximation, which neglects
the effect of the thermal lattice motion. To appear in Physical Review B.Comment: 21 pages, RevTeX, 5 figures in uuencoded compressed tar fil
DC and millimeter-wave performance of watt-level barrier-intrinsic-n+ diode-grid frequency multiplier fabricated on III-V compound semiconductors
This paper reports the fabrication and millimeter-wave performance of a new class of monolithic metal-semiconductor heterostructure devices, the Barrier- Intrinsic-N+ (BIN) diode-grid frequency multipliers, which are fabricated on III-V compound semiconductors. This work also involves the measurement of the DC and low frequency
electrical properties of the BIN diode-grid frequency multiplier. In addition, a new analytical model which accurately describes the structure has been developed and is presented for the first time
- …