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
Superconductivity in the Chalcogens up to Multimegabar Pressures
Highly sensitive magnetic susceptibility techniques were used to measure the
superconducting transition temperatures in S up to 231(5) GPa. S
transforms to a superconductor with T of 10 K and has a discontinuity in
T_c dependence at 160 GPa corresponding to bco to beta-Po phase transition.
Above this pressure T_c in S has a maximum reaching about 17.3(+/-0.5) K at 200
GPa and then slowly decreases with pressure to 15 K at 230 GPa.
This trend in the pressure dependence parallels the behavior of the heavier
members Se and Te. Superconductivity in Se was also observed from 15 to 25 GPa
with T_c changing from 4 to 6 K and above 150 GPa with T_c of 8 K.
Similiarities in the T_c dependences for S, Se, and Te, and the implications
for oxygen are discussed.Comment: 4 pages, 10 figure
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