5,688 research outputs found
Investigation of the shear-mechanical and dielectric relaxation processes in two mono-alcohols close to the glass transition
Shear-mechanical and dielectric measurements on the two monohydroxy
(mono-alcohol) molecular glass formers 2-ethyl-1-hexanol and 2-butanol close to
the glass transition temperature are presented. The shear-mechanical data are
obtained using the piezoelectric shear-modulus gauge method covering
frequencies from 1mHz to 10kHz. The shear-mechanical relaxation spectra show
two processes, which follow the typical scenario of a structural (alpha)
relaxation and an additional (Johari-Goldstein) beta relaxation. The dielectric
relaxation spectra are dominated by a Debye-type peak with an additional
non-Debye peak visible. This Debye-type relaxation is a common feature peculiar
to mono-alcohols. The time scale of the non-Debye dielectric relaxation process
is shown to correspond to the mechanical structural (alpha) relaxation.
Glass-transition temperatures and fragilities are reported based on the
mechanical alpha relaxation and the dielectric Debye-type process, showing that
the two glass-transition temperatures differ by approximately 10K and that the
fragility based on the Debye-type process is a factor of two smaller than the
structural fragility. If a mechanical signature of the Debye-type relaxation
exists in these liquids, its relaxation strength is at most 1% and 3% of the
full relaxation strength of 2-butanol and 2-ethyl-1-hexanol respectively. These
findings support the notion that it is the non-Debye dielectric relaxation
process that corresponds to the structural alpha relaxation in the liquid.Comment: 8 pages, 6 figures. Minor corrections, updated figures, more
dielectric data show
Photonic crystal fiber with a hybrid honeycomb cladding
We consider an air-silica honeycomb lattice and demonstrate a new approach to
the formation of a core defect. Typically, a high or low-index core is formed
by adding a high-index region or an additional air-hole (or other low-index
material) to the lattice, but here we discuss how a core defect can be formed
by manipulating the cladding region rather than the core region itself.
Germanium-doping of the honeycomb lattice has recently been suggested for the
formation of a photonic band-gap guiding silica-core and here we experimentally
demonstrate how an index-guiding silica-core can be formed by fluorine-doping
of the honeycomb lattice.Comment: 5 pages including 3 figures. Accepted for Optics Expres
Approximate square-root-time relaxation in glass-forming liquids
We present data for the dielectric relaxation of 43 glass-forming organic
liquids, showing that the primary (alpha) relaxation is often close to
square-root-time relaxation. The better an inverse power-law description of the
high-frequency loss applies, the more accurately is square-root-time relaxation
obeyed. These findings suggest that square-root-time relaxation is generic to
the alpha process, once a common view, but since long believed to be incorrect.
Only liquids with very large dielectric losses deviate from this picture by
having consistently narrower loss peaks. As a further challenge to the
prevailing opinion, we find that liquids with accurate square-root-time
relaxation cover a wide range of fragilities
X-ray diffraction microscopy based on refractive optics
A formalism is presented for dark-field X-ray microscopy using refractive optics. The new technique can produce three-dimensional maps of lattice orientation and axial strain within millimetre-sized sampling volumes and is particularly suited toin situstudies of materials at hard X-ray energies. An objective lens in the diffracted beam magnifies the image and acts as a very efficient filter in reciprocal space, enabling the imaging of individual domains of interest with a resolution of 100 nm. Analytical expressions for optical parameters such as numerical aperture, vignetting, and the resolution in both direct and reciprocal spaces are provided. It is shown that the resolution function in reciprocal space can be highly anisotropic and varies as a function of position in the field of view. Inserting a square aperture in front of the objective lens facilitates disjunct and space-filling sampling, which is key for three-dimensional reconstruction and analysis procedures based on the conservation of integrated intensity. A procedure for strain scanning is presented. Finally the formalism is validated experimentally at an X-ray energy of 17 keV.</jats:p
Ultra-large bandwidth hollow-core guiding in all-silica Bragg fibers with nano-supports
We demonstrate a new class of hollow-core Bragg fibers that are composed of
concentric cylindrical silica rings separated by nanoscale support bridges. We
theoretically predict and experimentally observe hollow-core confinement over
an octave frequency range. The bandwidth of bandgap guiding in this new class
of Bragg fibers exceeds that of other hollow-core fibers reported in the
literature. With only three rings of silica cladding layers, these Bragg fibers
achieve propagation loss of the order of 1 dB/m.Comment: 9 pages including 5 figure
Supercooled Liquid Dynamics Studied via Shear-Mechanical Spectroscopy
We report dynamical shear-modulus measurements for five glass-forming liquids
(pentaphenyl trimethyl trisiloxane, diethyl phthalate, dibutyl phthalate,
1,2-propanediol, and m-touluidine). The shear-mechanical spectra are obtained
by the piezoelectric shear-modulus gauge (PSG) method. This technique allows
one to measure the shear modulus ( Pa) of the liquid within a
frequency range from 1 mHz to 10 kHz. We analyze the frequency-dependent
response functions to investigate whether time-temperature superposition (TTS)
is obeyed. We also study the shear-modulus loss-peak position and its
high-frequency part. It has been suggested that when TTS applies, the
high-frequency side of the imaginary part of the dielectric response decreases
like a power law of the frequency with an exponent -1/2. This conjecture is
analyzed on the basis of the shear mechanical data. We find that TTS is obeyed
for pentaphenyl trimethyl trisiloxane and in 1,2-propanediol while in the
remaining liquids evidence of a mechanical process is found. Although
the the high-frequency power law behavior of the shear-loss
may approach a limiting value of when lowering the temperature, we
find that the exponent lies systematically above this value (around 0.4). For
the two liquids without beta relaxation (pentaphenyl trimethyl trisiloxane and
1,2-propanediol) we also test the shoving model prediction, according to which
the the relaxation-time activation energy is proportional to the instantaneous
shear modulus. We find that the data are well described by this model.Comment: 7 pages, 6 figure
A Concept for an STJ-based Spectrograph
We describe a multi-order spectrograph concept suitable for 8m-class
telescopes, using the intrinsic spectral resolution of Superconducting
Tunneling Junction detectors to sort the spectral orders. The spectrograph
works at low orders, 1-5 or 1-6, and provides spectral coverage with a
resolving power of R~8000 from the atmospheric cutoff at 320 nm to the long
wavelength end of the infrared H or K band at 1800 nm or 2400 nm. We calculate
that the spectrograph would provide substantial throughput and wavelength
coverage, together with high time resolution and sufficient dynamic range. The
concept uses currently available technology, or technologies with short
development horizons, restricting the spatial sampling to two linear arrays;
however an upgrade path to provide more spatial sampling is identified. All of
the other challenging aspects of the concept - the cryogenics, thermal baffling
and magnetic field biasing - are identified as being feasible.Comment: Accepted in Monthly Notices of the Royal Astronomical Society, 12
pages with 10 figure
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