464 research outputs found
Quantum and Classical Orientational Ordering in Solid Hydrogen
We present a unified view of orientational ordering in phases I, II, and III
of solid hydrogen. Phases II and III are orientationally ordered, while the
ordering objects in phase II are angular momenta of rotating molecules, and in
phase III the molecules themselves. This concept provides quantitative
explanation of the vibron softening, libron and roton spectra, and increase of
the IR vibron oscillator strength in phase III. The temperature dependence of
the effective charge parallels the frequency shifts of the IR and Raman
vibrons. All three quantities are linear in the order parameter.Comment: Replaced with the final text, accepted for publication in PRL. 1 Fig.
added. Misc. text revision
A molecular perspective on the limits of life: Enzymes under pressure
From a purely operational standpoint, the existence of microbes that can grow
under extreme conditions, or "extremophiles", leads to the question of how the
molecules making up these microbes can maintain both their structure and
function. While microbes that live under extremes of temperature have been
heavily studied, those that live under extremes of pressure have been
neglected, in part due to the difficulty of collecting samples and performing
experiments under the ambient conditions of the microbe. However, thermodynamic
arguments imply that the effects of pressure might lead to different organismal
solutions than from the effects of temperature. Observationally, some of these
solutions might be in the condensed matter properties of the intracellular
milieu in addition to genetic modifications of the macromolecules or repair
mechanisms for the macromolecules. Here, the effects of pressure on enzymes,
which are proteins essential for the growth and reproduction of an organism,
and some adaptations against these effects are reviewed and amplified by the
results from molecular dynamics simulations. The aim is to provide biological
background for soft matter studies of these systems under pressure.Comment: 16 pages, 8 figure
High-Pressure Reentrant Ferroelectricity in PbTiO Revisited
We study ferroelectricity in the classic perovskite ferroelectric PbTiO
to very high pressures with density functional theory (DFT) and experimental
diamond anvil techniques. We use second harmonic generation (SHG) spectroscopy
to detect lack of inversion symmetry, if present. Consistent with early
understanding and experiments, we find that ferroelectricity disappears at
moderate pressures. However, the DFT calculations show that the disappearance
arises from the overtaking of zone boundaries instabilities, and not to the
squeezing out of the off-centering ferroelectric displacements with pressure,
as previously thought. Our computations also predict a dramatic double
reentrance of ferroelectricity at higher pressures, not yet seen in
experiments
Doming Modes and Dynamics of Model Heme Compounds
Synchrotron far-IR spectroscopy and density-functional calculations are used to characterize the low-frequency dynamics of model heme FeCO compounds. The “doming” vibrational mode in which the iron atom moves out of the porphyrin plane while the periphery of this ring moves in the opposite direction determines the reactivity of oxygen with this type of molecule in biological systems. Calculations of frequencies and absorption intensities and the measured pressure dependence of vibrational modes in the model compounds are used to identify the doming and related normal modes
The Application of Micro-Raman Spectroscopy to Analysis and Identification of Minerals in Thin Section
Micro-Raman spectroscopy is a useful analytical tool for studying minerals in thin section. The advantages of this technique as a structural probe for analysis of micron-size minerals are demonstrated with a study of polymorphism of SiO2 and MgSiO3. Three polymorphs of silica, -quartz, coesite, and glass, in a thin section of Coconino sandstone were identified in situ with a Raman microprobe. The Raman spectra of these phases were compared to that measured for stishovite obtained from the same rock. Spectra of protoenstatite, orthoenstatite, and clinoenstatite, three polymorphs of MgSiO3, are consistent with their similar pyroxene chain structures but different space groups. The characteristic Raman spectra in each instance could be used for finger-printing identification of the phases and their orientations
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