584 research outputs found
Direct thermal conductance measurements on suspended monocrystalline nanostructures
We describe and demonstrate a new class of devices that enable direct thermal conductance measurements on monocrystalline nanostructures. These are possible through our newly developed techniques for three-dimensional, successive surface nanomachining of GaAs-based heterostructures. Our methods allow the patterning of complex devices comprising electrically insulating, mesoscopic thermal conductors with separate, thermal transducers in situ. Intimate thermal contact between these elements is provided by their epitaxial registry. Low-temperature thermal conductance measurements indicate that phonon boundary scattering in these initial nanometer is scale structures is partially specular. These devices offer promise for ultrasensitive bolometry and calorimetry
Force balance in canonical ensembles of static granular packings
We investigate the role of local force balance in the transition from a
microcanonical ensemble of static granular packings, characterized by an
invariant stress, to a canonical ensemble. Packings in two dimensions admit a
reciprocal tiling, and a collective effect of force balance is that the area of
this tiling is also invariant in a microcanonical ensemble. We present
analytical relations between stress, tiling area and tiling area fluctuations,
and show that a canonical ensemble can be characterized by an intensive
thermodynamic parameter conjugate to one or the other. We test the equivalence
of different ensembles through the first canonical simulations of the force
network ensemble, a model system.Comment: 9 pages, 9 figures, submitted to JSTA
Identification of an alternative G{alpha}q-dependent chemokine receptor signal transduction pathway in dendritic cells and granulocytes
CD38 controls the chemotaxis of leukocytes to some, but not all, chemokines, suggesting that chemokine receptor signaling in leukocytes is more diverse than previously appreciated. To determine the basis for this signaling heterogeneity, we examined the chemokine receptors that signal in a CD38-dependent manner and identified a novel "alternative" chemokine receptor signaling pathway. Similar to the "classical" signaling pathway, the alternative chemokine receptor pathway is activated by G{alpha}i2-containing Gi proteins. However, unlike the classical pathway, the alternative pathway is also dependent on the Gq class of G proteins. We show that G{alpha}q-deficient neutrophils and dendritic cells (DCs) make defective calcium and chemotactic responses upon stimulation with N-formyl methionyl leucyl phenylalanine and CC chemokine ligand (CCL) 3 (neutrophils), or upon stimulation with CCL2, CCL19, CCL21, and CXC chemokine ligand (CXCL) 12 (DCs). In contrast, G{alpha}q-deficient T cell responses to CXCL12 and CCL19 remain intact. Thus, the alternative chemokine receptor pathway controls the migration of only a subset of cells. Regardless, the novel alternative chemokine receptor signaling pathway appears to be critically important for the initiation of inflammatory responses, as G{alpha}q is required for the migration of DCs from the skin to draining lymph nodes after fluorescein isothiocyanate sensitization and the emigration of monocytes from the bone marrow into inflamed skin after contact sensitization
Angoricity and compactivity describe the jamming transition in soft particulate matter
The application of concepts from equilibrium statistical mechanics to
out-of-equilibrium systems has a long history of describing diverse systems
ranging from glasses to granular materials. For dissipative jammed systems--
particulate grains or droplets-- a key concept is to replace the energy
ensemble describing conservative systems by the volume-stress ensemble. Here,
we test the applicability of the volume-stress ensemble to describe the jamming
transition by comparing the jammed configurations obtained by dynamics with
those averaged over the ensemble as a probe of ergodicity. Agreement between
both methods suggests the idea of "thermalization" at a given angoricity and
compactivity. We elucidate the thermodynamic order of the jamming transition by
showing the absence of critical fluctuations in static observables like
pressure and volume. The approach allows to calculate observables such as the
entropy, volume, pressure, coordination number and distribution of forces to
characterize the scaling laws near the jamming transition from a statistical
mechanics viewpoint.Comment: 27 pages, 13 figure
Heat conduction in the disordered harmonic chain revisited
A general formulation is developed to study heat conduction in disordered
harmonic chains with arbitrary heat baths that satisfy the
fluctuation-dissipation theorem. A simple formal expression for the heat
current J is obtained, from which its asymptotic system-size (N) dependence is
extracted. It is shown that the ``thermal conductivity'' depends not just on
the system itself but also on the spectral properties of the fluctuation and
noise used to model the heat baths. As special cases of our heat baths we
recover earlier results which reported that for fixed boundaries , while for free boundaries . For other choices we
find that one can get other power laws including the ``Fourier behaviour'' .Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Parity Effect and Charge Binding Transition in Submicron Josephson Junction Arrays
We reconsider the issue of Berezinskii-Kosterlitz-Thouless (BKT) transition
into an insulating state in the Coulomb-dominated Josephson junction arrays. We
show that previously predicted picture of the Cooper-pair BKT transtion at T =
T_2 is valid only under the condition that T_2 is considerably below the
parity-effect temperature (which is usually almost 10 times below the value of
superconductive transition temperature), and even in this case it is not a
rigorous phase transition but only a crossover, whereas the real phase
transition takes place at T_1 = T_2/4. Our theory is in agreement with
available experimental data on Coulomb-dominated Josephson arrays and also
sheds some light on the origin of unusual reentrant temperature dependence of
resistivity in the array with nearly-criticial ratio of Coulomb to Josephson
energies.Comment: 4 pages, Revtex, to be published in JETP Letters, April 9
Playing for Data: Ground Truth from Computer Games
Recent progress in computer vision has been driven by high-capacity models
trained on large datasets. Unfortunately, creating large datasets with
pixel-level labels has been extremely costly due to the amount of human effort
required. In this paper, we present an approach to rapidly creating
pixel-accurate semantic label maps for images extracted from modern computer
games. Although the source code and the internal operation of commercial games
are inaccessible, we show that associations between image patches can be
reconstructed from the communication between the game and the graphics
hardware. This enables rapid propagation of semantic labels within and across
images synthesized by the game, with no access to the source code or the
content. We validate the presented approach by producing dense pixel-level
semantic annotations for 25 thousand images synthesized by a photorealistic
open-world computer game. Experiments on semantic segmentation datasets show
that using the acquired data to supplement real-world images significantly
increases accuracy and that the acquired data enables reducing the amount of
hand-labeled real-world data: models trained with game data and just 1/3 of the
CamVid training set outperform models trained on the complete CamVid training
set.Comment: Accepted to the 14th European Conference on Computer Vision (ECCV
2016
Elastic Wave Transmission at an Abrupt Junction in a Thin Plate, with Application to Heat Transport and Vibrations in Mesoscopic Systems
The transmission coefficient for vibrational waves crossing an abrupt
junction between two thin elastic plates of different widths is calculated.
These calculations are relevant to ballistic phonon thermal transport at low
temperatures in mesoscopic systems and the Q for vibrations in mesoscopic
oscillators. Complete results are calculated in a simple scalar model of the
elastic waves, and results for long wavelength modes are calculated using the
full elasticity theory calculation. We suggest that thin plate elasticty theory
provide a useful and tractable approximation to the full three dimensional
geometry.Comment: 35 pages, including 12 figure
Full capacitance-matrix effects in driven Josephson-junction arrays
We study the dynamic response to external currents of periodic arrays of
Josephson junctions, in a resistively capacitively shunted junction (RCSJ)
model, including full capacitance-matrix effects}. We define and study three
different models of the capacitance matrix : Model A
includes only mutual capacitances; Model B includes mutual and self
capacitances, leading to exponential screening of the electrostatic fields;
Model C includes a dense matrix that is constructed
approximately from superposition of an exact analytic solution for the
capacitance between two disks of finite radius and thickness. In the latter
case the electrostatic fields decay algebraically. For comparison, we have also
evaluated the full capacitance matrix using the MIT fastcap algorithm, good for
small lattices, as well as a corresponding continuum effective-medium analytic
evaluation of a finite voltage disk inside a zero-potential plane. In all cases
the effective decays algebraically with distance, with
different powers. We have then calculated current voltage characteristics for
DC+AC currents for all models. We find that there are novel giant capacitive
fractional steps in the I-V's for Models B and C, strongly dependent on the
amount of screening involved. We find that these fractional steps are quantized
in units inversely proportional to the lattice sizes and depend on the
properties of . We also show that the capacitive steps
are not related to vortex oscillations but to localized screened phase-locking
of a few rows in the lattice. The possible experimental relevance of these
results is also discussed.Comment: 12 pages 18 Postscript figures, REVTEX style. Paper to appear in July
1, Vol. 58, Phys. Rev. B 1998 All PS figures include
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