24,783 research outputs found
The Origin of the Boson Peak and the Thermal Conductivity Plateau in Low Temperature Glasses
We argue that the intrinsic glassy degrees of freedom in amorphous solids
giving rise to the thermal conductivity plateau and the ``boson peak'' in the
heat capacity at moderately low temperatures are directly connected to those
motions giving rise to the two-level like excitations seen at still lower
temperatures. These degrees of freedom can be thought of as strongly anharmonic
transitions between the local minima of the glassy energy landscape that are
accompanied by ripplon-like domain wall motions of the glassy mosaic structure
predicted to occur at by the random first order transition theory. The
energy spectrum of the vibrations of the mosaic depends on the glass transition
temperature, the Debye frequency and the molecular length scale. The resulting
spectrum reproduces the experimental low temperature Boson peak. The
``non-universality'' of the thermal conductivity plateau depends on and arises from calculable interactions with the phonons.Comment: 4 pages, submitted to PR
Large deviations for local times and intersection local times of fractional Brownian motions and Riemann-Liouville processes
In this paper we prove exact forms of large deviations for local times and
intersection local times of fractional Brownian motions and Riemann-Liouville
processes. We also show that a fractional Brownian motion and the related
Riemann-Liouville process behave like constant multiples of each other with
regard to large deviations for their local and intersection local times. As a
consequence of our large deviation estimates, we derive laws of iterated
logarithm for the corresponding local times. The key points of our methods: (1)
logarithmic superadditivity of a normalized sequence of moments of
exponentially randomized local time of a fractional Brownian motion; (2)
logarithmic subadditivity of a normalized sequence of moments of exponentially
randomized intersection local time of Riemann-Liouville processes; (3)
comparison of local and intersection local times based on embedding of a part
of a fractional Brownian motion into the reproducing kernel Hilbert space of
the Riemann-Liouville process.Comment: To appear in the Annals of Probabilit
High Thermoelectric Performance and Defect Energetics of Multipocketed Full Heusler Compounds
We report a first-principles density-functional study of electron-phonon interactions in and thermoelectric transport properties of the full Heusler compounds Sr2BiAu and Sr2SbAu. Our results show that ultrahigh intrinsic bulk thermoelectric performance across a wide range of temperatures is physically possible and point to the presence of multiply degenerate and highly dispersive carrier pockets as the key factor for achieving this. Sr2BiAu, which features ten energy-aligned low-effective-mass pockets (six along Γ-X and four at L), is predicted to deliver n-type zT=0.4-4.9 at T=100-700 K. Comparison with the previously investigated compound Ba2BiAu shows that the additional L pockets in Sr2BiAu significantly increase its low-temperature power factor to a maximum value of 12 mW m-1 K-2 near T=300 K. However, at high temperatures the power factor of Sr2BiAu drops below that of Ba2BiAu because the L states are heavier and subject to strong scattering by phonon deformation, as opposed to the lighter Γ-X states, which are limited by polar-optical scattering. Sr2SbAu is predicted to deliver a lower n-type zT=3.4 at T=750 K due to appreciable misalignment between the L and Γ-X carrier pockets, generally heavier scattering, and a slightly higher lattice thermal conductivity. Soft acoustic modes, which are responsible for the low lattice thermal conductivity, also increase the vibrational entropy and high-temperature stability of these Heusler compounds, suggesting that their experimental synthesis may be feasible. The dominant intrinsic defects are found to be Au vacancies, which drive the Fermi level towards the conduction band and work in favor of n-doping
High-quality positrons from a multi-proton bunch driven hollow plasma wakefield accelerator
By means of hollow plasma, multiple proton bunches work well in driving
nonlinear plasma wakefields and accelerate electrons to energy frontier with
preserved beam quality. However, the acceleration of positrons is different
because the accelerating structure is strongly charge dependent. There is a
discrepancy between keeping a small normalized emittance and a small energy
spread. This results from the conflict that the plasma electrons used to
provide focusing to the multiple proton bunches dilute the positron bunch. By
loading an extra electron bunch to repel the plasma electrons and meanwhile
reducing the plasma density slightly to shift the accelerating phase with a
conducive slope to the positron bunch, the positron bunch can be accelerate to
400 GeV (40% of the driver energy) with an energy spread as low as 1% and well
preserved normalized emittance. The successful generation of high quality and
high energy positrons paves the way to the future energy frontier lepton
colliders.Comment: 14 pages, 5 figure
Observation of the Presuperfluid Regime in a Two-Dimensional Bose Gas
In complementary images of coordinate-space and momentum-space density in a
trapped 2D Bose gas, we observe the emergence of pre-superfluid behavior. As
phase-space density increases toward degenerate values, we observe a
gradual divergence of the compressibility from the value predicted by
a bare-atom model, . grows to 1.7 before
reaches the value for which we observe the sudden emergence of a spike
at in momentum space. Momentum-space images are acquired by means of a 2D
focusing technique. Our data represent the first observation of non-meanfield
physics in the pre-superfluid but degenerate 2D Bose gas.Comment: Replace with the version appeared in PR
Characterization of the Soluble Nanoparticles Formed through Coulombic Interaction of Bovine Serum Albumin with Anionic Graft Copolymers at Low pH
A static light scattering (SLS) study of bovine serum albumin (BSA) mixtures
with two anionic graft copolymers of poly (sodium acrylate-co-sodium
2-acrylamido-2-methyl-1-propanesulphonate)-graft-poly (N,
N-dimethylacrylamide), with a high composition in poly (N,
N-dimethylacrylamide) (PDMAM) side chains, revealed the formation of oppositely
charged complexes, at pH lower than 4.9, the isoelectric point of BSA. The
core-corona nanoparticles formed at pH = 3.00, were characterized. Their
molecular weight and radius of gyration were determined by SLS, while their
hydrodynamic radius was determined by dynamic light scattering. Small angle
neutron scattering measurements were used to determine the radius of the
insoluble complexes, comprising the core of the particles. The values obtained
indicated that their size and aggregation number of the nanoparticles, were
smaller when the content of the graft copolymers in neutral PDMAM side chains
was higher. Such particles should be interesting drug delivery candidates, if
the gastrointestinal tract was to be used
- …