10,601 research outputs found
Nature-Inspired Interconnects for Self-Assembled Large-Scale Network-on-Chip Designs
Future nano-scale electronics built up from an Avogadro number of components
needs efficient, highly scalable, and robust means of communication in order to
be competitive with traditional silicon approaches. In recent years, the
Networks-on-Chip (NoC) paradigm emerged as a promising solution to interconnect
challenges in silicon-based electronics. Current NoC architectures are either
highly regular or fully customized, both of which represent implausible
assumptions for emerging bottom-up self-assembled molecular electronics that
are generally assumed to have a high degree of irregularity and imperfection.
Here, we pragmatically and experimentally investigate important design
trade-offs and properties of an irregular, abstract, yet physically plausible
3D small-world interconnect fabric that is inspired by modern network-on-chip
paradigms. We vary the framework's key parameters, such as the connectivity,
the number of switch nodes, the distribution of long- versus short-range
connections, and measure the network's relevant communication characteristics.
We further explore the robustness against link failures and the ability and
efficiency to solve a simple toy problem, the synchronization task. The results
confirm that (1) computation in irregular assemblies is a promising and
disruptive computing paradigm for self-assembled nano-scale electronics and (2)
that 3D small-world interconnect fabrics with a power-law decaying distribution
of shortcut lengths are physically plausible and have major advantages over
local 2D and 3D regular topologies
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On the prevalence of bridged macrocyclic pyrroloindolines formed in regiodivergent alkylations of tryptophan.
A Friedel-Crafts alkylation is described that efficiently transforms tryptophan-containing peptides into macrocycles of varying ring connectivity. Factors are surveyed that influence the distribution of regioisomers, with a focus on indole C3-alkylations leading to bridged endo-pyrroloindolines. We probe the stability and stereochemistry of these pyrroloindolines, study their rearrangement to C2-linked indolic macrocycles, and demonstrate a scalable, stereoselective synthesis of this compound class. Placing the macrocyclization in sequence with further template-initiated annulation leads to extraordinary polycyclic products and further demonstrates the potential for this chemistry to drive novel peptidomimetic lead discovery programs
Accelerated Test Development for Coil-coated Steel Building Panels
This paper discusses the experimental design and the preliminary findings of an ongoing project designed to establish an accelerated laboratory test that would rank coating system performance the same as their performance in atmospheric exposure. A total of ten materials are included in the program: four substrates each with two coating systems and one substrate with two additional coating systems. Samples were installed at four atmospheric exposure sites: Middletown, OH, Daytona Beach, FL, Monroeville, PA, and Halifax, NS, Canada. Three different orientations were utilized at each of the exposure sites and a variety of building panel features were included on the test panels (roll formed bends, laps, cut drip edges, standing seam closures, and scribes). The work discussed in this paper includes the program design and implementation and preliminary correlation\u27s of the three-year atmospheric exposure results to several standard accelerated test methods including: ASTM B117, ASTM G85, ASTM G87, and GM 9540
Stability of Fermi Surfaces and K-Theory
Nonrelativistic Fermi liquids in d+1 dimensions exhibit generalized Fermi
surfaces: (d-p)-dimensional submanifolds in the momentum-frequency space
supporting gapless excitations. We show that the universality classes of stable
Fermi surfaces are classified by K-theory, with the pattern of stability
determined by Bott periodicity. The Atiyah-Bott-Shapiro construction implies
that the low-energy modes near a Fermi surface exhibit relativistic invariance
in the transverse p+1 dimensions. This suggests an intriguing parallel between
norelativistic Fermi liquids and D-branes of string theory.Comment: 4 pages, revte
Unveiling a Population of X-ray Non-Detected AGN
We define a sample of 27 radio-excess AGN in the Chandra Deep Field North by
selecting galaxies that do not obey the radio/infrared correlation for
radio-quiet AGN and star-forming galaxies. Approximately 60% of these
radio-excess AGN are X-ray undetected in the 2 Ms Chandra catalog, even at
exposures of > 1 Ms; 25% lack even 2-sigma X-ray detections. The absorbing
columns to the faint X-ray-detected objects are 10^22 cm^-2 < N_H < 10^24
cm^-2, i.e., they are obscured but unlikely to be Compton thick. Using a local
sample of radio-selected AGN, we show that a low ratio of X-ray to radio
emission, as seen in the X-ray weakly- and non-detected samples, is correlated
with the viewing angle of the central engine, and therefore with obscuration.
Our technique can explore the proportion of obscured AGN in the distant
Universe; the results reported here for radio-excess objects are consistent
with but at the low end of the overall theoretical predictions for
Compton-thick objects.Comment: Accepted for publication in the Astrophysical Journal, 15 pages, 10
figures, 4 table
Topological Defects and Gapless Modes in Insulators and Superconductors
We develop a unified framework to classify topological defects in insulators
and superconductors described by spatially modulated Bloch and Bogoliubov de
Gennes Hamiltonians. We consider Hamiltonians H(k,r) that vary slowly with
adiabatic parameters r surrounding the defect and belong to any of the ten
symmetry classes defined by time reversal symmetry and particle-hole symmetry.
The topological classes for such defects are identified, and explicit formulas
for the topological invariants are presented. We introduce a generalization of
the bulk-boundary correspondence that relates the topological classes to defect
Hamiltonians to the presence of protected gapless modes at the defect. Many
examples of line and point defects in three dimensional systems will be
discussed. These can host one dimensional chiral Dirac fermions, helical Dirac
fermions, chiral Majorana fermions and helical Majorana fermions, as well as
zero dimensional chiral and Majorana zero modes. This approach can also be used
to classify temporal pumping cycles, such as the Thouless charge pump, as well
as a fermion parity pump, which is related to the Ising non-Abelian statistics
of defects that support Majorana zero modes.Comment: 27 pages, 15 figures, Published versio
Binarity as a key factor in protoplanetary disk evolution: Spitzer disk census of the eta Chamaeleontis cluster
The formation of planets is directly linked to the evolution of the
circumstellar (CS) disk from which they are born. The dissipation timescales of
CS disks are, therefore, of direct astrophysical importance in evaluating the
time available for planet formation. We employ Spitzer Space Telescope spectra
to complete the CS disk census for the late-type members of the ~8 Myr-old eta
Chamaeleontis star cluster. Of the 15 K- and M-type members, eight show excess
emission. We find that the presence of a CS disk is anti-correlated with
binarity, with all but one disk associated with single stars. With nine single
stars in total, about 80% retain a CS disk. Of the six known or suspected close
binaries the only CS disk is associated with the primary of RECX 9. No
circumbinary disks have been detected. We also find that stars with disks are
slow rotators with surface values of specific angular momentum j = 2-15 j_sun.
All high specific angular momentum systems with j = 20-30 j_sun are confined to
the primary stars of binaries. This provides novel empirical evidence for
rotational disk locking and again demonstrates the much shorter disk lifetimes
in close binary systems compared to single star systems. We estimate the
characteristic mean disk dissipation timescale to be ~5 Myr and ~9 Myr for the
binary and single star systems, respectively.Comment: Accepted by ApJ
Research notes: Characterization of several abnormal nodulation reactions in soybeans
Several abnormal nodulation reactions in soybeans are known. These range from a complete lack of nodules, caused by the non-nodulating gene (Williams and Lynch, 1954) to plants with normal-appearing nodules (Vest et al., 1973), but low nitrogen fixation as exemplified by the \u27Peking\u27-strain T23 combination. The purpose of the study reported here was threefold
Research notes: Inheritance of abnormal nodulation between Rhizobium japonicum strain 62 and the soybean variety Amsoy 71
To date, four genes are known that result in abnormal nodulation in soybeans. The gene rj1 (Williams and Lynch, 1954; Caldwell, 1966) prevents nodulation with almost all Rhizobium japonicum strains. The genes Rj2 (Caldwell, 1966 ) in combination with strains b7 and bl4 of the 3-24-44 serogroup and bl22 of the 122 serogroup, Rj3 (Vest, 1970) in combination with strain 33, and Rj4 (Vest and Caldwell, 1972) in combination with strain 61 all result in chlorotic plants with abnormal nodulation
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