Hyeondeok Shin

Cohesion Energetics of Carbon Allotropes: Quantum Monte Carlo Study

We have performed quantum Monte Carlo calculations to study the cohesion energetics of carbon allotropes, including sp(3)-bonded diamond, sp(2)-bonded graphene, sp-sp(2) hybridized graphynes, and sp-bonded carbyne. The computed cohesive energies of diamond and graphene are found to be in excellent agreement with the corresponding values determined experimentally for diamond and graphite, respectively, when the zero-point energies, along with the interlayer binding in the case of graphite, are included. We have also found that the cohesive energy of graphyne decreases systematically as the ratio of sp-bonded carbon atoms increases. The cohesive energy of γ-graphyne, the most energetically stable graphyne, turns out to be 6.766(6) eV/atom, which is smaller than that of graphene by 0.698(12) eV/atom. Experimental difficulty in synthesizing graphynes could be explained by their significantly smaller cohesive energies. Finally, we conclude that the cohesive energy of a newly proposed graphyne can be accurately estimated with the carbon-carbon bond energies determined from the cohesive energies of graphene and three different graphynes considered here.

Commensurate-incommensurate transition of 4He adsorbed on a single C60 molecule.

Path-integral Monte Carlo calculations have been performed to study (4)He adsorption on a single C(60) molecule. Helium corrugations on the fullerene molecular surface are incorporated with the (4)He-C(60) interaction described by the sum of all (4)He-C interatomic pair potentials. Radial density distributions show a layer-by-layer growth of (4)He with the first adlayer being located at a distance of ~6.3 Å from the center of the C(60) molecule. The monolayer shows different quantum states as the number of (4)He adatoms N varies. For N = 32, we find a commensurate solid, with each of the 32 adsorption sites on the molecular surface being occupied by a single (4)He atom. Various domain-wall structures are observed as more (4)He atoms are added and the first layer crystallizes into an incommensurate solid when it is completely filled. This commensurate-incommensurate transition of the helium monolayer is found to be accompanied by re-entrant superfluid response at a low temperature of 0.31 K with the superfluidity being totally quenched at N = 32, 44, and 48. Finally, the different quantum states observed in the helium monolayer around C(60) are compared with phase diagrams proposed for the corresponding layer on a graphite surface.

QMCPACK: An open source ab initio quantum Monte Carlo package for the electronic structure of atoms, molecules and solids

QMCPACK is an open source quantum Monte Carlo package for ab initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow type trial wavefunctions in conjunction with a sophisticated optimizer capable of optimizing tens of thousands of parameters. The orbital space auxiliary-field quantum Monte Carlo method is also implemented, enabling cross validation between different highly accurate methods. The code is specifically optimized for calculations with large numbers of electrons on the latest high performance computing architectures, including multicore central processing unit and graphical processing unit systems. We detail the programs capabilities, outline its structure, and give examples of its use in current research calculations. The package is available at http://qmcpack.org.

Nature of Interlayer Binding and Stacking of sp–sp2 Hybridized Carbon Layers: A Quantum Monte Carlo Study

α-Graphyne is a two-dimensional sheet of sp-sp2 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 sp2-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.

Effects of 3He impurities on the superfluid response of the 4He monolayer on a C20 molecule

The path-integral Monte Carlo calculations have been performed to investigate the effects of (3)He impurities on structural and superfluid properties of the (4)He monolayer on a single C(20) molecule. According to our previous study, the helium monolayer exhibits different quantum states for different numbers of (4)He adatoms and is completed to form a commensurate solid where nanoscale supersolidity can be realized through the activation of mobile vacancy states. We first observe that different structures for different numbers of helium atoms are mostly preserved with the replacement of a few (4)He atoms with the same number of (3)He atoms, whether the helium layer is a fluid or a solid. However, the substitution of (3)He impurities is found to have different effects on the superfluid response of the helium layer, depending on its quantum state. For a partially-filled fluid layer the superfluid fraction decreases monotonically with the increasing (3)He concentration, which can be understood in terms of the suppression of exchange couplings among (4)He atoms due to the presence of (3)He impurities. On the other hand, the substitution of a few (3)He impurity atoms may increase the superfluid fraction of a near-complete monolayer that is in a crystalline solid state. The enhancement of superfluidity in a solid layer is interpreted to be due to interstitial and vacancy defects promoted by larger quantum fluctuations of lighter (3)He atoms. This provides strong evidence that the (4)He monolayer on C(20) shows the vacancy-based supersolidity near its completion.