600 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
Novel Electronic Structure of Nitrogen-Doped Lutetium Hydrides
First-principles density functional theory (DFT) calculations of Lu-H-N
compounds reveal low-energy configurations of Fmm
LuHN structures that exhibit novel electronic properties such as
flat bands, sharply peaked densities of states (van Hove singularities, vHs),
and intersecting Dirac cones near the Fermi energy (E). These N-doped
LuH-based structures also exhibit an interconnected metallic hydrogen
network, which is a common feature of high-T hydride superconductors.
Electronic property systematics give estimates of T for optimally ordered
structures that are well above the critical temperatures predicted for
structures considered previously. The vHs and flat bands near E are
enhanced in DFT+U calculations, implying strong correlation physics should also
be considered for first-principles studies of these materials. These results
provide a basis for understanding the novel electronic properties observed for
nitrogen-doped lutetium hydride.Comment: To be submitted. 4 main figures, 15 SI Figures, 1 table. 5 pages (13
pgs SI
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
Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen
A new class of strongly infrared active structures is identified for phase
III of compressed molecular H2 by constant-pressure ab initio molecular
dynamics and density-functional perturbation calculations. These are planar
quadrupolar structures obtained as a distortion of low-pressure quadrupolar
phases, after they become unstable at about 150 GPa due to a zone-boundary soft
phonon. The nature of the II-III transition and the origin of the IR activity
are rationalized by means of simple electrostatics, as the onset of a
stabilizing dipole-quadrupole interaction.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let
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