4,966 research outputs found
Driven localized excitations in the acoustic spectrum of small nonlinear macroscopic and microscopic lattices
Both bright and dark traveling, locked, intrinsic localized modes (ILMs) have
been generated with a spatially uniform driver at a frequency in the acoustic
spectrum of a nonlinear micromechanical cantilever array. Complementary
numerical simulations show that a minimum density of modes, hence array size,
is required for the formation of such locked smoothly running excitations.
Additional simulations on a small 1-D antiferromagnetic spin system are used to
illustrate that such uniformly driven running ILMs should be a generic feature
of a nanoscale atomic lattice.Comment: Physical Review Letters, accepte
Microanalytical identification of barium sulphate crystals in statoliths of Chara Rhizoids
In contrast to higher plants, Chara rhizoids contain statolith vacuoles filled with biocrystallites of BaS04 in the form of rods composed of globular subunits ca. 7 nm in diameter. The revelation of the crystallites under electron microscopy is dependent on the fixative; best structural preservation was observed after fixation in a buffered glutaraldehyde + acrolein solution; 0s04 and KMnO4 partially dissolved both the biocrystallites and synthetic BaS04
Quantum dynamics of localized excitations in a symmetric trimer molecule
We study the time evolution of localized (local bond) excitations in a
symmetric quantum trimer molecule. We relate the dynamical properties of
localized excitations such as their spectral intensity and their temporal
evolution (survival probability and tunneling of bosons) to their degree of
overlap with quantum tunneling pair states. We report on the existence of
degeneracy points in the trimer eigenvalue spectrum for specific values of
parameters due to avoided crossings between tunneling pair states and
additional states. The tunneling of localized excitations which overlap with
these degenerate states is suppressed on all times. As a result local bond
excitations may be strongly localized forever, similar to their classical
counterparts.Comment: 9 pages, 12 figures. Improved version with more discussions. Some
figures were replaced for better understanding. Accepted in Phys. Rev.
Experimental Generation and Observation of Intrinsic Localized Spin Wave Modes in an Antiferromagnet
By driving with a microwave pulse the lowest frequency antiferromagnetic
resonance of the quasi 1-D biaxial antiferromagnet (C_2 H_5 NH_3)_2 CuCl_4 into
an unstable region intrinsic localized spin waves have been generated and
detected in the spin wave gap. These findings are consistent with the
prediction that nonlinearity plus lattice discreteness can lead to localized
excitations with dimensions comparable to the lattice constant.Comment: 10 pages, 4 figures, accepted for publication in Physical Review
Letter
The dust SED in the dwarf galaxy NGC 1569: Indications for an altered dust composition?
We discuss the interpretation of the dust SED from the mid-infrared to the
millimeter range of NGC 1569. The model developed by D\'esert et al. (1990)
including three dust components (Polyaromatic Hydrocarbons, Very Small Grains
and big grains) can explain the data using a realistic interstellar radiation
field and adopting an enhanced abundance of VSGs. A simple three-temperature
model is also able to reproduce the data but requires a very low dust
temperature which is considered to be unlikely in this low-metallicity
starburst galaxy. The high abundance of Very Small Grains might be due to large
grain destruction in supernova shocks. This possibility is supported by ISO
data showing that the emission at 14.3 m, tracing VSGs, is enhanced with
respect to the emission at 6.7 m and 850 m in regions of high star
formation.Comment: 4 pages, conference proceedings paper, "The Spectral Energy
Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg,
4-8 Oct. 2004, eds. C.C. Popescu & R.J. Tuffs, AIP Conf. Ser., in pres
A survey of string teachers' opinions regarding the teaching of violin/viola shifting.
String teachers agreed that shifting is best taught in a private studio setting. Most respondents introduce shifting after 1st position is secure. Other commonalities included 1st position taught before 3 rd position, the importance of ear training, and the use of scale studies.This study examined string teachers' opinions regarding the technical aspects of shifting on the violin and viola. Areas of interest included how to teach shifting, when to introduce shifting, what teaching methods were most effective, and what instructional materials were used by string teachers. Two hundred twenty-nine participants completed a questionnaire consisting of three sections: demographic information, pedagogical issues regarding shifting, and open-ended questions. The data were interpreted using statistical analysis, including a MANOVA to compare primary teaching areas to find any commonalities and/or significant main effects as well as significant interactions between teaching experience and education among the respondents. The open-ended data revealed various response trends and themes.However, many items had mixed responses. Findings revealed that K-12 teachers and college/private teachers' attitudes were polarized regarding teaching shifting. Also, interactions between teaching experience and education revealed differences between respondents with a bachelor's, master's, or doctoral degree combined with a range of a few to many years of teaching experience. Opinions differed regarding posture, needing more literature, and shifting taught during the third year. Further research is needed to isolate these differences to see what teaching methods regarding introducing shifting are more effective within various settings
Draft crystal structure of the vault shell at 9-A resolution.
Vaults are the largest known cytoplasmic ribonucleoprotein structures and may function in innate immunity. The vault shell self-assembles from 96 copies of major vault protein and encapsulates two other proteins and a small RNA. We crystallized rat liver vaults and several recombinant vaults, all among the largest non-icosahedral particles to have been crystallized. The best crystals thus far were formed from empty vaults built from a cysteine-tag construct of major vault protein (termed cpMVP vaults), diffracting to about 9-A resolution. The asymmetric unit contains a half vault of molecular mass 4.65 MDa. X-ray phasing was initiated by molecular replacement, using density from cryo-electron microscopy (cryo-EM). Phases were improved by density modification, including concentric 24- and 48-fold rotational symmetry averaging. From this, the continuous cryo-EM electron density separated into domain-like blocks. A draft atomic model of cpMVP was fit to this improved density from 15 domain models. Three domains were adapted from a nuclear magnetic resonance substructure. Nine domain models originated in ab initio tertiary structure prediction. Three C-terminal domains were built by fitting poly-alanine to the electron density. Locations of loops in this model provide sites to test vault functions and to exploit vaults as nanocapsules
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