1,570,471 research outputs found
Elastic collapse in disordered isostatic networks
Isostatic networks are minimally rigid and therefore have, generically,
nonzero elastic moduli. Regular isostatic networks have finite moduli in the
limit of large sizes. However, numerical simulations show that all elastic
moduli of geometrically disordered isostatic networks go to zero with system
size. This holds true for positional as well as for topological disorder. In
most cases, elastic moduli decrease as inverse power-laws of system size. On
directed isostatic networks, however, of which the square and cubic lattices
are particular cases, the decrease of the moduli is exponential with size. For
these, the observed elastic weakening can be quantitatively described in terms
of the multiplicative growth of stresses with system size, giving rise to bulk
and shear moduli of order exp{-bL}. The case of sphere packings, which only
accept compressive contact forces, is considered separately. It is argued that
these have a finite bulk modulus because of specific correlations in contact
disorder, introduced by the constraint of compressivity. We discuss why their
shear modulus, nevertheless, is again zero for large sizes. A quantitative
model is proposed that describes the numerically measured shear modulus, both
as a function of the loading angle and system size. In all cases, if a density
p>0 of overconstraints is present, as when a packing is deformed by
compression, or when a glass is outside its isostatic composition window, all
asymptotic moduli become finite. For square networks with periodic boundary
conditions, these are of order sqrt{p}. For directed networks, elastic moduli
are of order exp{-c/p}, indicating the existence of an "isostatic length scale"
of order 1/p.Comment: 6 pages, 6 figues, to appear in Europhysics Letter
High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)
We present multi-frequency, high resolution VLBA circular polarization images
of the radio source 3C 84 in the center of NGC 1275. Our images reveal a
complex distribution of circular polarization in the inner parsec of the radio
jet, with local levels exceeding 3% polarization, the highest yet detected with
VLBI techniques. The circular polarization changes sign along the jet, making
3C 84 also the first radio jet to show both signs of circular polarization
simultaneously. The spectrum and changing sign of the circular polarization
indicate that it is unlikely to be purely intrinsic to the emitted synchrotron
radiation. The Faraday conversion process makes a significant and perhaps
dominant contribution to the circular polarization, and the observed spectrum
suggests the conversion process is near saturation. The sign change in the
circular polarization along the jet may result from this saturation or may be
due to a change in magnetic field order after an apparent bend in the jet. From
the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels
of circular polarization inferred for the intra-day variable source PKS
1519-273, we suggest a connection between small spatial scales and efficient
production of circular polarization.Comment: 4 pages, accepted in ApJ Letter
Influence of moving breathers on vacancies migration
A vacancy defect is described by a Frenkel--Kontorova model with a
discommensuration. This vacancy can migrate when interacts with a moving
breather. We establish that the width of the interaction potential must be
larger than a threshold value in order that the vacancy can move forward. This
value is related to the existence of a breather centred at the particles
adjacent to the vacancy.Comment: 11 pages, 10 figure
Tuning the thermal conductance of molecular junctions with interference effects
We present an \emph{ab initio} study of the role of interference effects in
the thermal conductance of single-molecule junctions. To be precise, using a
first-principles transport method based on density functional theory, we
analyze the coherent phonon transport in single-molecule junctions based on
several benzene and oligo-phenylene-ethynylene derivatives. We show that the
thermal conductance of these junctions can be tuned via the inclusion of
substituents, which induces destructive interference effects and results in a
decrease of the thermal conductance with respect to the unmodified molecules.
In particular, we demonstrate that these interference effects manifest as
antiresonances in the phonon transmission, whose energy positions can be
controlled by varying the mass of the substituents. Our work provides clear
strategies for the heat management in molecular junctions and more generally in
nanostructured metal-organic hybrid systems, which are important to determine,
how these systems can function as efficient energy-conversion devices such as
thermoelectric generators and refrigerators
Genetic Variation of MtDNA Cytochrome Oxidase Subunit I (COI) in Local Swamp Buffaloes in Indonesia
The objective of this research was to identify genetic variation of mitochondria DNA especially in cytochrome oxidase subunit I (COI) among population of Indonesian buffaloes. Samples of swamp buffaloes were collected from Aceh (n= 3), North Sumatra (n= 3), Riau (n= 3), Banten (n= 3), Central Java (n= 3), West Nusa Tenggara (n= 3) and South Sulawesi (n= 3), and riverine buffalo from North Sumatra (n= 1) out of group for comparison. Sequence of COI was analyzed using MEGA 5.10 software with neighbor-joining method kimura 2-parameter model to reconstruct phylogeny tree. The result showed that three haplotypes for swamp buffalo and one haplotype for riverine buffalo in Indonesia resulted from 41 polymorphic sites. This finding showed that the COI gene could be considered as a marker to distinguish among swamp buffaloes in Indonesia
Workpiece positioning vise
A pair of jaw assemblies simultaneously driven in opposed reciprocation by a single shaft has oppositely threaded sections to automatically center delicate or brittle workpieces such as lithium fluoride crystal beneath the blade of a crystal cleaving machine. Both jaw assemblies are suspended above the vise bed by a pair of parallel guide shafts attached to the vise bed. Linear rolling bearings, fitted around the guide shafts and firmly held by opposite ends of the jaw assemblies, provide rolling friction between the guide shafts and the jaw assemblies. A belleville washer at one end of the drive shaft and thrust bearings at both drive shaft ends hold the shaft in compression between the vise bed, thereby preventing wobble of the jaw assemblies due to wear between the shaft and vise bed
Dynamic cyclic performance of phenol-formaldehyde resin derived carbons for pre-combustion CO2 capture : An experimental study
Acknowledgments This work was carried out with financial support from the Spanish MINECO (Project ENE2011-23467), co-financed by the European Regional Development Fund (ERDF).Peer reviewedPublisher PD
Electrooptical scanning of film
Scan-in scan-out flying spot scanning system recognizes three different levels of transmissivity within a frame. It selectively acts on these levels either to intensify the illumination or to extend the duration of the illuminating spot to any picture element. Thus it improves the ratio of signal to tube noise in the cameras output
Structural and electronic properties of grain boundaries in graphite: Planes of periodically distributed point defects
We report on scanning tunneling microscopy and spectroscopy of grain
boundaries in highly oriented pyrolytic graphite. Grain boundaries showed a
periodic structure and an enhanced charge density compared to the bare graphite
surface. Two possible periodic structures have been observed along grain
boundaries. A geometrical model producing periodically distributed point
defects on the basal plane of graphite has been proposed to explain the
structure of grain boundaries. Scanning tunneling spectroscopy on grain
boundaries revealed two strong localized states at -0.3 V and 0.4 V.Comment: 5 pages, 5 figure
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