Electronic and quantum transport properties of a graphene-BN dot-ring hetero-nanostructure

Quantum dots, quantum rings, and, most recently, quantum dot-ring nanostructures have been studied for their interesting potential applications in nanoelectronic applications. Here, the electronic properties of a dot-ring hetero-nanostructure consisting of a graphene ring and graphene dot with a hexagonal boron nitride (h-BN) ring serving as barrier between ring and dot are investigated using density functional theory. Analysis of the character of the wave functions near the Fermi level and of the charge distribution of this dot-ring structure and calculations of the quantum transport properties find asymmetry in the conductance resonances leading to asymmetric I–V characteristics which can be modified by applying a negative voltage potential to the central graphene dot

Self-consistent calculations of strain-induced band gap changes in semiconducting (n, 0) carbon nanotubes

First-principles density-functional calculations of the electronic structure, energy band gaps (Eg), and strain-induced band gap changes in moderate-gap single-walled (n,0) carbon nanotubes (SWNTs) are presented. It is confirmed that (n,0) SWNTs fall into two classes depending upon n mod 3=1 or 2. Eg is always lower for “mod 1” than for “mod 2” SWNTs of similar diameter. For n\u3c10, strong curvature effects dominate Eg; from n=10 to 17, the Eg oscillations, amplified due to σ−π mixing, decrease and can be explained very well with a tight-binding model which includes trigonal warping. Under strain, the two families of semiconducting SWNTs are distinguished by equal and opposite energy shifts for these gaps. For (10,0) and (20,0) tubes, the potential surface and band gap changes are explored up to approximately ±6% strain or compression. For each strain value, full internal geometry relaxation is allowed. The calculated band gap changes are ±(115±10) meV per 1% strain, positive for the mod 1 and negative for the mod 2 family, about 10% larger than the tight-binding result of ±97 meV and twice as large as the shift predicted from a tight-binding model that includes internal sublattice relaxation

Atomic clusters and cluster models in solid state physics

Since it became possible to study experimentally the evolution of a metal and the transition from atomic and molecular to surface and bulk properties, the study of clusters, especially metal clusters due to their role in heterogeneous catalysis, represents a highly active research area. On the other hand, clusters serve also as very useful models in solid state physics to investigate impurity, surface, and interface problems. Both cluster aspects will be discussed in addition to the role of cluster calculations in aperiodic polymer and band structure computations. We also report results of Li‐cluster studies. Correlation effects, calculated with the coupled cluster method, are discussed in connection with the onset of Pierls\u27 distortion in Li‐rings. Finally, it is demonstrated how the cluster model is used to investigate polymer‐metal surface interactions and how these interactions influence and change the polymer conformation

Valence in chemisorption studies

In the spirit of Coulson\u27s Valence, large-scale ab initio computations on chemisorption problems are revisited. Cluster calculations which model the interactions of halogen atoms with the silicon surface are reviewed and compared to the interaction of atomic hydrogen on the same surface. A rich structure in bonding mechanisms is revealed. They range from ionic to covalent wavefunctions and exhibit the primitive pattern of understanding Coulson so strongly believed in. © 1992

The relativistic gravity train

© 2018 European Physical Society. The gravity train that takes 42.2 min from any point A to any other point B that is connected by a straight-line tunnel through Earth has captured the imagination more than most other applications in calculus or introductory physics courses. Brachystochron and, most recently, nonlinear density solutions have been discussed. Here relativistic corrections are presented. It is discussed how the corrections affect the time to fall through Earth, the Sun, a white dwarf, a neutron star, and - the ultimate limit - the difference in time measured by a moving, a stationary and the fiducial observer at infinity if the density of the sphere approaches the density of a black hole. The relativistic gravity train can serve as a problem with approximate and exact analytic solutions and as numerical exercise in any introductory course on relativity

Ab initio electronic structure of superionic conductor Li \u3c inf\u3e 3 P

Lithium phosphide (Li3P) has recently been introduced as a good lithium ion conductor. Results of ab initio Hartree-Fock calculations for the electronic structure and the optimized lattice parameters for the hexagonal P6/mmm space group are reported. The total energy, band structure, density of states and charge densities are obtained. The results demonstrate how the band structure of the insulator Li3P can be derived from the band structure of its metallic constituent Li2P and Li monolayers. The metal-insulator transition occurs if the inter-plane distance falls below 4.24 Å. © 1992

Proton and hydrogen transport through two-dimensional monolayers

© 2016 IOP Publishing Ltd. Diffusion of protons and hydrogen atoms in representative two-dimensional materials is investigated. Specifically, density functional calculations were performed on graphene, hexagonal boron nitride (h-BN),phosphorene, silicene, and molybdenum disulfide (MoS2) monolayersto study the surface interaction and penetration barriers for protons and hydrogen atoms employing finite cluster models. The calculated barrier heights correlate approximately with the size of the opening formed by the three-fold open sitesinthe monolayers considered. They range from 1.56 eV(proton) and 4.61 eV(H) for graphene to 0.12 eV (proton) and 0.20 eV (H) for silicene. The results indicate that only graphene and h-BN monolayers have the potential for membranes with high selective permeability. The MoS2 monolayer behaves differently: protons and Hatoms become trapped between the outer Slayersinthe Mo planeinawell withadepth of1.56 eV (proton) and 1.5 eV(Hatom), possibly explaining why no proton transport was detected, suggesting MoS2 asa hydrogen storage material instead. For graphene and h-BN, off-center proton penetration reduces the barrier to1.38 eV for graphene and 0.11 eVfor h-BN. Furthermore, Pt acting as a substrate was found to have a negligible effect on the barrier height. In defective graphene, the smallest barrier for proton diffusion (1.05 eV) is found for an oxygenterminated defect. Therefore, it seems more likely that thermal protons can penetrate a monolayer of h-BN but not graphene and defects are necessary to facilitate the proton transport in graphene

A coupled cluster study of the stability of lithium clusters

Coupled cluster studies on Li2, on the Li6 ring and on other Li6 clusters are reported. In its linear approximation the coupled cluster method gives a larger fraction of the correlation energy for Li2 than the nonlinear version, although other physical properties like force constant and bond length are described unsatisfactory. The planar Li6 ring is predicted to be stable in the equidistant form. Larger rings tend to have a Peierl\u27s distorted alternant geometry on the Hartree-Fock level. Thus Li behaves somewhat similar to (CH)n, while for H n also the n = 6 ring is distorted. The stability of equidistant six-membered rings is therefore attributed to the existence of rather delocalized 2s electrons. The comparison of the results for Li6 clusters of different symmetry (D6h,Oh,C5v) with similar calculations reported in the literature indicates that the inclusion of p-functions is essential, whereas the size of the s function subspace is not very important. © 1987 American Institute of Physics

Comparative Ab Initio study of electronic and ionic properties of lithium nitride (Li3N), lithium phosphide (Li3P), and lithium arsenide (Li3As)

Ab initio Hartree–Fock band structure calculations are presented for the first time for lithium phosphide (Li3P) and lithium arsenide (Li3As) in the hexagonal P6/mmm crystal structure. Results are compared to those for lithium nitride (Li3N). The new calculations for Li3N agree with previous Hartree–Fock calculations, except for the valence band structure where results of previous pseudopotential calculations are confirmed. Geometry optimization for Li3P yields a lattice parameter a of 4.45 Å and a c value of 4.80 Å. These values differ markedly from experimental results reported to be 4.271 and 7.590 Å, respectively. A similar discrepancy is found for lithium arsenide: a = 4.60 Å and c = 4.96 Å have to be compared to the reported experimental values of 4.397 Å for a and 7.824 Å for c. Force constants are derived for in‐plane and interplane vibrations. The band structures for Li3P and Li3As are found to be very similar to the one calculated for Li3N. Using Li3P as an example, it is shown how the band structure of the insulator can be derived from the band structures of the two metallic constitutent Li2P and Li monolayers. The metal–insulator transition occurs if the inter‐plane distance falls below 4.25 Å. Contrary to expectations raised earlier, it is found that the 3d electrons in arsenic are strongly localized, evidenced by a very narrow d band width of 0.1 eV. They cannot be used to explain the difference in conductivity between the phosphide and the arsenide. A Mulliken population analysis gives charge distributions close to the ideal ionic structure (Li+)3X3−, X = N, P, and As. Overall it is found that hexagonal lithium phosphide and lithium arsenide arsenide are more similar to lithium nitride and less anisotropic than suggested previously. This discrepancy could be due to the use of polycrystalline samples in earlier experiments