1,292 research outputs found
Midinfrared Conductivity in Orientationally Disordered Doped Fullerides
The coupling between the intramolecular vibrational modes and the doped
conduction electrons in is studied by a calculation of the
electronic contributions to the phonon self energies. The calculations are
carried out for an orientationally ordered reference solid with symmetry and for a model with quenched orientational disorder on the
fullerene sites. In both cases, the dispersion and symmetry of the renormalized
modes is governed by the electronic contributions. The current current
correlation functions and frequency dependent conductivity through the
midinfrared are calculated for both models. In the disordered structures, the
renormalized modes derived from even parity intramolecular phonons are resonant
with the dipole excited single particle spectrum, and modulate the predicted
midinfrared conductivity. The spectra for this coupled system are calculated
for several recently proposed microscopic models for the electron phonon
coupling, and a comparison is made with recent experimental data which
demonstrate this effect.Comment: 32 pages + 9 postscript figures (on request), REVTeX 3.
Nonperturbative approach to the Hubbard model in C60 cluster
We propose a computational scheme for the Hubbard model in the C60 cluster in
which the interaction with the Fermi sea of charges added to the neutral
molecule is switched on sequentially. This is applied to the calculation of the
balance of charging energies, within a low-energy truncation of the space of
states which produces moderate errors for an intermediate range of the
interaction strength.Comment: 5 pages, Revtex, 2 figure
In-Situ Infrared Transmission Study of Rb- and K-Doped Fullerenes
We have measured the four IR active molecular vibrations in
as a function of doping . We observe
discontinuous changes in the vibrational spectra showing four distinct phases
(presumably , and 6). The and modes
show the largest changes shifting downward in frequency in four steps as the
doping increases. Several new very weak modes are visible in the phase
and are possibly Raman modes becoming weakly optically active. We present
quantitative fits of the data and calculate the electron-phonon coupling of the
IR mode.Comment: 3 pages, Figure 1 included, 3 more figures available by request.
REVTEX v3.0 IRC60DO
Energy gap in superconducting fullerides: optical and tunneling studies
Tunneling and optical transmission studies have been performed on
superconducting samples of Rb3C60. At temperatures much below the
superconducting transition temperature Tc the energy gap is 2 Delta=5.2 +-
0.2meV, corresponding to 2 Delta/kB Tc = 4.2. The low temperature density of
states, and the temperature dependence of the optical conductivity resembles
the BCS behavior, although there is an enhanced ``normal state" contribution.
The results indicate that this fulleride material is an s-wave superconductor,
but the superconductivity cannot be described in the weak coupling limit.Comment: RevTex file with four .EPS figures. Prints to four pages. Also
available at http://buckminster.physics.sunysb.edu/papers/pubrece.htm
Community inclusion of wheelchair users during the long-term recovery phase following the 2010/2011 Canterbury earthquakes
Research exploring how people living with disability experience community inclusion during the medium to long-term recovery following natural disasters is scant. Yet such information is vital to ensure that recovering communities are inclusive of all members within the population. This study explored the perspectives of people with specific functional needs, wheelchair users, regarding their experience of community inclusion in the four years following the 2010/2011 earthquakes in Christchurch, New Zealand. Thirteen adult wheelchair users were interviewed one-to-one and then invited to attend a group interview. The group interview presented a summary of the interview data for discussion to help clarify and prioritise elements of community inclusion. All data were subjected to thematic analysis. Four interrelated themes described the key elements of the participants’ experience of community post-earthquakes: 1) earthquakes magnified barriers, 2) community inclusion requires energy, 3) social connections are important, and 4) an unprecedented opportunity for change. Findings emphasized the need for recovery energies at a local and national level to move from conceptualizing disability in terms of individual vulnerability, to instead, focusing on reducing environmental barriers that inhibit community inclusion. Of critical importance is creating pathways for people who experience disability to be co-creators of this change
Theory of Superconducting of doped fullerenes
We develop the nonadiabatic polaron theory of superconductivity of
taking into account the polaron band narrowing and realistic
electron-phonon and Coulomb interactions. We argue that the crossover from the
BCS weak-coupling superconductivity to the strong-coupling polaronic and
bipolaronic superconductivity occurs at the BCS coupling constant independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's
theorem except small polarons for any realistic electron-phonon interaction. By
the use of the polaronic-type function and the ``exact'' diagonalization in the
truncated Hilbert space of vibrons (``phonons'') we calculate the ground state
energy and the electron spectral density of the molecule. This
allows us to describe the photoemission spectrum of in a wide
energy region and determine the electron-phonon interaction. The strongest
coupling is found with the high-frequency pinch mode and with the
Frenkel exciton. We clarify the crucial role of high-frequency bosonic
excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb
repulsion allowing the intermediate and low-frequency phonons to couple two
small polarons in a Cooper pair. The Eliashberg-type equations are solved for
low-frequency phonons. The value of the superconducting , its pressure
dependence and the isotope effect are found to be in a remarkable agreement
with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Multilingual representations for low resource speech recognition and keyword search
© 2015 IEEE. This paper examines the impact of multilingual (ML) acoustic representations on Automatic Speech Recognition (ASR) and keyword search (KWS) for low resource languages in the context of the OpenKWS15 evaluation of the IARPA Babel program. The task is to develop Swahili ASR and KWS systems within two weeks using as little as 3 hours of transcribed data. Multilingual acoustic representations proved to be crucial for building these systems under strict time constraints. The paper discusses several key insights on how these representations are derived and used. First, we present a data sampling strategy that can speed up the training of multilingual representations without appreciable loss in ASR performance. Second, we show that fusion of diverse multilingual representations developed at different LORELEI sites yields substantial ASR and KWS gains. Speaker adaptation and data augmentation of these representations improves both ASR and KWS performance (up to 8.7% relative). Third, incorporating un-transcribed data through semi-supervised learning, improves WER and KWS performance. Finally, we show that these multilingual representations significantly improve ASR and KWS performance (relative 9% for WER and 5% for MTWV) even when forty hours of transcribed audio in the target language is available. Multilingual representations significantly contributed to the LORELEI KWS systems winning the OpenKWS15 evaluation
(Micro)evolutionary changes and the evolutionary potential of bird migration
Seasonal migration is the yearly long-distance movement of individuals between their breeding and wintering grounds. Individuals from nearly every animal group exhibit this behavior, but probably the most iconic migration is carried out by birds, from the classic V-shape formation of geese on migration to the amazing nonstop long-distance flights undertaken by Arctic Terns Sterna paradisaea. In this chapter, we discuss how seasonal migration has shaped the field of evolution. First, this behavior is known to turn on and off quite rapidly, but controversy remains concerning where this behavior first evolved geographically and whether the ancestral state was sedentary or migratory (Fig. 7.1d, e). We review recent work using new analytical techniques to provide insight into this topic. Second, it is widely accepted that there is a large genetic basis to this trait, especially in groups like songbirds that migrate alone and at night precluding any opportunity for learning. Key hypotheses on this topic include shared genetic variation used by different populations to migrate and only few genes being involved in its control. We summarize recent work using new techniques for both phenotype and genotype characterization to evaluate and challenge these hypotheses. Finally, one topic that has received less attention is the role these differences in migratory phenotype could play in the process of speciation. Specifically, many populations breed next to one another but take drastically different routes on migration (Fig. 7.2). This difference could play an important role in reducing gene flow between populations, but our inability to track most birds on migration has so far precluded evaluations of this hypothesis. The advent of new tracking techniques means we can track many more birds with increasing accuracy on migration, and this work has provided important insight into migration's role in speciation that we will review here
Post-stroke inhibition of induced NADPH oxidase type 4 prevents oxidative stress and neurodegeneration
Ischemic stroke is the second leading cause of death worldwide. Only one moderately effective therapy exists, albeit with contraindications that exclude 90% of the patients. This medical need contrasts with a high failure rate of more than 1,000 pre-clinical drug candidates for stroke therapies. Thus, there is a need for translatable mechanisms of neuroprotection and more rigid thresholds of relevance in pre-clinical stroke models. One such candidate mechanism is oxidative stress. However, antioxidant approaches have failed in clinical trials, and the significant sources of oxidative stress in stroke are unknown. We here identify NADPH oxidase type 4 (NOX4) as a major source of oxidative stress and an effective therapeutic target in acute stroke. Upon ischemia, NOX4 was induced in human and mouse brain. Mice deficient in NOX4 (Nox4(-/-)) of either sex, but not those deficient for NOX1 or NOX2, were largely protected from oxidative stress, blood-brain-barrier leakage, and neuronal apoptosis, after both transient and permanent cerebral ischemia. This effect was independent of age, as elderly mice were equally protected. Restoration of oxidative stress reversed the stroke-protective phenotype in Nox4(-/-) mice. Application of the only validated low-molecular-weight pharmacological NADPH oxidase inhibitor, VAS2870, several hours after ischemia was as protective as deleting NOX4. The extent of neuroprotection was exceptional, resulting in significantly improved long-term neurological functions and reduced mortality. NOX4 therefore represents a major source of oxidative stress and novel class of drug target for stroke therapy
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