25,247 research outputs found
Aging vs crystallisation dynamics in hyperquenched glasses and a resolution of the water Tg controversy
The possibility of observing a glass transition in water before
crystallisation occurs has been debated vigorously but inconclusively over five
decades [1,2]. For two decades a glass transition at 136K [2,3] was accepted
but this transition has perplexing qualities [4]. Recently it has been
argued[2,5],that this assignment must be wrong. The re-assignment of Tg to
temperatures above the 150K crystallisation was vigorously contested [6]. Here
we use detailed anneal-and-scan studies of a hyperquenched inorganic glass,
which does not crystallize on heating, to interpret the perplexing aspects of
the 136K water phenomenon. We show that it is indeed linked to a glass
transition, though only via a cross-over phenomenon. The thermal history that
gives the same behaviour ("shadow" glass transition) in the inorganic glass is
linked by crossover to a "normal" glass transition 23% higher in temperature.
Thus a Tg is indeed unobservable for water, while the vitreous nature of
hyperquenched glassy water is strongly supported. The shadow Tg is reproducible
in the inorganic glass as it is in H2O. The observed aging dynamics are very
relevant to current glass theory, particularly to dynamical heterogeneity which
is seen to have an energy manifestation.Comment: 23 pages, 4 figure
A slave mode expansion for obtaining ab-initio interatomic potentials
Here we propose a new approach for performing a Taylor series expansion of
the first-principles computed energy of a crystal as a function of the nuclear
displacements. We enlarge the dimensionality of the existing displacement space
and form new variables (ie. slave modes) which transform like irreducible
representations of the space group and satisfy homogeneity of free space.
Standard group theoretical techniques can then be applied to deduce the
non-zero expansion coefficients a priori. At a given order, the translation
group can be used to contract the products and eliminate terms which are not
linearly independent, resulting in a final set of slave mode products. While
the expansion coefficients can be computed in a variety of ways, we demonstrate
that finite difference is effective up to fourth order. We demonstrate the
power of the method in the strongly anharmonic system PbTe. All anharmonic
terms within an octahedron are computed up to fourth order. A proper unitary
transformation demonstrates that the vast majority of the anharmonicity can be
attributed to just two terms, indicating that a minimal model of phonon
interactions is achievable. The ability to straightforwardly generate
polynomial potentials will allow precise simulations at length and time scales
which were previously unrealizable
On the [CII]-SFR relation in high redshift galaxies
After two ALMA observing cycles, only a handful of [CII]
emission line searches in z>6 galaxies have reported a positive detection,
questioning the applicability of the local [CII]-SFR relation to high-z
systems. To investigate this issue we use the Vallini et al. 2013 (V13) model,
based on high-resolution, radiative transfer cosmological simulations to
predict the [CII] emission from the interstellar medium of a z~7 (halo mass
) galaxy. We improve the V13 model by including
(a) a physically-motivated metallicity (Z) distribution of the gas, (b) the
contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic
Microwave Background on the [CII] line luminosity. We study the relative
contribution of diffuse neutral gas to the total [CII] emission () for different SFR and Z values. We find that the [CII]
emission arises predominantly from PDRs: regardless of the galaxy properties,
% since, at these early epochs, the CMB temperature
approaches the spin temperature of the [CII] transition in the cold neutral
medium ( K). Our model predicts a high-z
[CII]-SFR relation consistent with observations of local dwarf galaxies
(). The [CII] deficit suggested by actual data
( in BDF3299 at z~7.1) if confirmed by deeper
ALMA observations, can be ascribed to negative stellar feedback disrupting
molecular clouds around star formation sites. The deviation from the local
[CII]-SFR would then imply a modified Kennicutt-Schmidt relation in z>6
galaxies. Alternatively/in addition, the deficit might be explained by low gas
metallicities ().Comment: 9 pages, 6 figures, replaced with the version accepted for
pubblication in Ap
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