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Shin, Hyeondeok,Kim, Jeongnim,Lee, Hoonkyung,Heinonen, Olle,Benali, Anouar,Kwon, Yongkyung American Chemical Society 2017 Journal of chemical theory and computation Vol.13 No.11
<P>α-Graphyne is a two-dimensional sheet of sp–sp<SUP>2</SUP> 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, α-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 sp- and sp<SUP>2</SUP>-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 α-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.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jctcce/2017/jctcce.2017.13.issue-11/acs.jctc.7b00747/production/images/medium/ct-2017-00747w_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ct7b00747'>ACS Electronic Supporting Info</A></P>
Interlayer Exchange Coupling and Local Superfluidity in (4He)N around C20
Hyeondeok Shin,권용경 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.60 No.1
The path-integral Monte Carlo method has been employed to study local superfluidity of 4He layers adsorbed on a C20 molecular surface. Strong inhomogeneity in the superfluid response between the first two helium layers around the fullerene molecule is found. Furthermore we find that even the strongly-bound first layer exhibiting a clear structural order has a finite superfluid fraction when the second layer is filled with itinerant helium atoms. This first-layer superfluidity is understood to be due to exchange coupling mediated by the second-layer helium atoms.