88 research outputs found
Non-hexagonal-ring defects and structures induced by strain in graphene and in functionalized graphene
We perform {\textit ab initio} calculations for the strain-induced formation
of non-hexagonal-ring defects in graphene, graphane (planar CH), and graphenol
(planar COH). We find that the simplest of such topological defects, the
Stone-Wales defect, acts as a seed for strain-induced dissociation and
multiplication of topological defects. Through the application of inhomogeneous
deformations to graphene, graphane and graphenol with initially small
concentrations of pentagonal and heptagonal rings, we obtain several novel
stable structures that possess, at the same time, large concentrations of
non-hexagonal rings (from fourfold to elevenfold) and small formation energies
Wannier-function description of the electronic polarization and infrared absorption of high-pressure hydrogen
We have constructed maximally-localized Wannier functions for prototype
structures of solid molecular hydrogen under pressure, starting from LDA and
tight-binding Bloch wave functions. Each occupied Wannier function can be
associated with two paired protons, defining a ``Wannier molecule''. The sum of
the dipole moments of these ``molecules'' always gives the correct macroscopic
polarization, even under strong compression, when the overlap between nearby
Wannier functions becomes significant. We find that at megabar pressures the
contributions to the dipoles arising from the overlapping tails of the Wannier
functions is very large. The strong vibron infrared absorption experimentally
observed in phase III, above ~ 150 GPa, is analyzed in terms of the
vibron-induced fluctuations of the Wannier dipoles. We decompose these
fluctuations into ``static'' and ``dynamical'' contributions, and find that at
such high densities the latter term, which increases much more steeply with
pressure, is dominant.Comment: 17 pages, two-column style with 14 postscript figures embedded. Uses
REVTEX and epsf macro
A quantum fluid of metallic hydrogen suggested by first-principles calculations
It is generally assumed that solid hydrogen will transform into a metallic
alkali-like crystal at sufficiently high pressure. However, some theoretical
models have also suggested that compressed hydrogen may form an unusual
two-component (protons and electrons) metallic fluid at low temperature, or
possibly even a zero-temperature liquid ground state. The existence of these
new states of matter is conditional on the presence of a maximum in the melting
temperature versus pressure curve (the 'melt line'). Previous measurements of
the hydrogen melt line up to pressures of 44 GPa have led to controversial
conclusions regarding the existence of this maximum. Here we report ab initio
calculations that establish the melt line up to 200 GPa. We predict that subtle
changes in the intermolecular interactions lead to a decline of the melt line
above 90 GPa. The implication is that as solid molecular hydrogen is
compressed, it transforms into a low-temperature quantum fluid before becoming
a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and
its isotopes bears analogies with the familiar phases of 3He and 4He, the only
known zero-temperature liquids, but the long-range Coulombic interactions and
the large component mass ratio present in hydrogen would ensure dramatically
different propertiesComment: See related paper: cond-mat/041040
Suppression of decoherence via strong intra-environmental coupling
We examine the effects of intra-environmental coupling on decoherence by
constructing a low temperature spin--spin-bath model of an atomic impurity in a
Debye crystal. The impurity interacts with phonons of the crystal through
anti-ferromagnetic spin-spin interactions. The reduced density matrix of the
central spin representing the impurity is calculated by dynamically integrating
the full Schroedinger equation for the spin--spin-bath model for different
thermally weighted eigenstates of the spin-bath. Exact numerical results show
that increasing the intra-environmental coupling results in suppression of
decoherence. This effect could play an important role in the construction of
solid state quantum devices such as quantum computers.Comment: 4 pages, 3 figures, Revtex fil
Phyllosilicates as earth-abundant layered materials for electronics and optoelectronics: Prospects and challenges in their ultrathin limit
Phyllosilicate minerals are an emerging class of naturally occurring layered
insulators with large bandgap energy that have gained attention from the
scientific community. This class of lamellar materials has been recently
explored at the ultrathin two-dimensional level due to their specific
mechanical, electrical, magnetic, and optoelectronic properties, which are
crucial for engineering novel devices (including heterostructures). Due to
these properties, phyllosilicates minerals can be considered promising low-cost
nanomaterials for future applications. In this Perspective article, we will
present relevant features of these materials for their use in potential
2D-based electronic and optoelectronic applications, also discussing some of
the major challenges in working with them.Comment: 29 pages, 4 figure
Absence of Metallization in Solid Molecular Hydrogen
Being the simplest element with just one electron and proton the electronic
structure of the Hydrogen atom is known exactly. However, this does not hold
for the complex interplay between them in a solid and in particular not at high
pressure that is known to alter the crystal as well as the electronic
structure. Back in 1935 Wigner and Huntington predicted that at very high
pressure solid molecular hydrogen would dissociate and form an atomic solid
that is metallic. In spite of intense research efforts the experimental
realization, as well as the theoretical determination of the crystal structure
has remained elusive. Here we present a computational study showing that the
distorted hexagonal P6/m structure is the most likely candidate for Phase
III of solid hydrogen. We find that the pairing structure is very persistent
and insulating over the whole pressure range, which suggests that metallization
due to dissociation may precede eventual bandgap closure. Due to the fact that
this not only resolve one of major disagreement between theory and experiment,
but also excludes the conjectured existence of phonon-driven superconductivity
in solid molecular hydrogen, our results involve a complete revision of the
zero-temperature phase diagram of Phase III
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