48 research outputs found
The Nature of Interlayer Binding and Stacking of - Hybridized Carbon Layers: A Quantum Monte Carlo Study
-graphyne is a two-dimensional sheet of - hybridized carbon
atoms in a honeycomb lattice. While the geometrical structure is similar to
that of graphene, the hybridized triple bonds give rise to electronic structure
that is different from that of graphene. Similar to graphene, -graphyne
can be stacked in bilayers with two stable configurations, but the different
stackings have very different electronic structures: one is predicted to have
gapless parabolic bands and the other a tunable band gap which is attractive
for applications. In order to realize applications, it is crucial to understand
which stacking is more stable. This is difficult to model, as the stability is
a result of weak interlayer van der Waals interactions which are not well
captured by density functional theory (DFT). We have used quantum Monte Carlo
simulations that accurately include van der Waals interactions to calculate the
interlayer binding energy of bilayer graphyne and to determine its most stable
stacking mode. Our results show that interlayer bindings of - and
-bonded carbon networks are significantly underestimated in a Kohn-Sham
DFT approach, even with an exchange-correlation potential corrected to include,
in some approximation, van der Waals interactions. Finally, our quantum Monte
Carlo calculations reveal that the interlayer binding energy difference between
the two stacking modes is only 0.9(4) meV/atom. From this we conclude that the
two stable stacking modes of bilayer -graphyne are almost degenerate
with each other, and both will occur with about the same probability at room
temperature unless there is a synthesis path that prefers one stacking over the
other.Comment: 25 pages, 6 figure
Ab initio quantum Monte Carlo calculations of spin superexchange in cuprates: the benchmarking case of CaCuO
In view of the continuous theoretical efforts aimed at an accurate
microscopic description of the strongly correlated transition metal oxides and
related materials, we show that with continuum quantum Monte Carlo (QMC)
calculations it is possible to obtain the value of the spin superexchange
coupling constant of a copper oxide in a quantitatively excellent agreement
with experiment. The variational nature of the QMC total energy allows us to
identify the best trial wave function out of the available pool of wave
functions, which makes the approach essentially free from adjustable parameters
and thus truly ab initio. The present results on magnetic interactions suggest
that QMC is capable of accurately describing ground state properties of
strongly correlated materials.Comment: Published in Physical Review
Effects of morphology on phonons of nanoscopic silver grains
The morphology of nanoscopic Ag grains significantly affects the phonons.
Atomistic simulations show that realistic nanograin models display complex
vibrational properties. (1) Single-crystalline grains. Nearly-pure torsional
and radial phonons appear at low frequencies. For low-energy, faceted models,
the breathing mode and acoustic gap (lowest frequency) are about 10% lower than
predicted by elasticity theory (ET) for a continuum sphere of the same volume.
The sharp edges and the atomic lattice split the ET-acoustic-gap quintet into a
doublet and triplet. The surface protrusions associated with nearly spherical,
high-energy models produce a smaller acoustic gap and a higher vibrational
density of states (DOS) at frequencies \nu<2 THz. (2) Twined icosahedra. In
contrast to the single-crystal case, the inherent strain produce a larger
acoustic gap, while the core atoms yield a DOS tail extending beyond the
highest frequency of single-crystalline grains. (3) Mark's decahedra, in
contrast to (1) and (2), do not have a breathing mode; although twined and
strained, do not exhibit a high-frequency tail in the DOS. (4) Irregular
nanograins. Grain boundaries and surface disorder yield non-degenerate phonon
frequencies, and significantly smaller acoustic gap. Only these nanograins
exhibit a low-frequency \nu^2 DOS in the interval 1-2 THz.Comment: Version published in Phys. Rev.