National Energy Research Scientific Computing Center 2004 Annual Report
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materials sciences
Total Negative Refraction in Crystals
Y. Zhang, B. Fluegel, and A. Mascarenhas, “Total negative refraction in real crystals for ballistic electrons and light,” Phys. Rev. Lett. 91, 7404 (2003). BES
Zhang et al. have found theoretically that a ballistic electron beam passing a domain twin in an ordered semiconductor results in a negative refraction with 100% transmission (i.e., zero reflection). This result, which applies in principle to the full spectrum of electromagnetic waves, has led to the first experimental demonstration of total negative refraction of light in real crystalline materials. The ability to steer light without reflection could be extremely valuable for high-power optics.
Figure 11. Adhesive interaction between hexadecane molecules and gold atoms. The solid surfaces were modeled as a rough gold surface (a) or a flat gold (111) plane (b). The bottom panel shows ordered domains corresponding to the flat surface junction. The coefficient of friction is independent of the detailed nature of surface roughness.
Elucidating Amontons’ Law
J. Gao, W. D. Luedtke, D. Gourdon, M. Ruths, J. N. Israelachvili, and U. Landman, “Frictional forces and Amontons' Law: From the molecular to the macroscopic scale,” J. Phys. Chem. B 108, 3410 (2004). BES, AF
Amontons’ law states that for any two materials, friction force is directly proportional to applied load, with a constant of proportionality independent of the contact area, surface roughness, and sliding velocity (Figure 11). No one knows why this law works on macroscopic, microscopic, and nanoscopic scales. Using molecular dynamics simulations, Gao et al. found that local energy-dissipating mechanisms are thermodynamic as well as mechanical, and that a proper statistical description can be formulated through the use of the Weibull distribution of local friction forces. They also concluded that although the “real” area of contact is a nonfundamental quantity, it may be a convenient scaling parameter for describing the really fundamental parameters — the number density of atoms, molecules, or bonds involved in an adhesive or frictional interaction.
Excitonic Effects in Nanotubes
C. D. Spataru, S. Ismail-Beigi, L. X. Benedict, and S. G. Louie, “Excitonic effects and optical spectra of single-walled carbon nanotubes,” Phys. Rev. Lett. 92, 077402-1 (2004). BES, NSF
Optical response of individual single-walled carbon nanotubes (SWCNTs) can now be measured, but measurements have deviated greatly from theory. To understand why, Spataru et al. calculated electron–hole interaction effects (excitonic effects) on optical spectra of five SWCNTs. Results show bound excitons can exist in metallic SWCNTs — surprising news, since they do not exist in bulk metallic systems. The binding energy of excitons can be as large as 1 eV in semiconducting SWCNTs (at least one order of magnitude larger than exciton binding energy in bulk semiconductors of similar band gap) and 100 meV in metallic SWCNTs. These large many-electron effects explain discrepancies between previous theories and experiments.
Self-Healing of CdSe Nanocrystals
Figure 12. Unrelaxed (a) and relaxed (b),(c) wurtzite structures of CdxSex, x = 6, 15, 33, and 45. The Cd is green and the Se is white in the models. The side view is parallel to the c axis, while the top view is along the c axis.
A. Puzder, A. J. Williamson, F. Gygi, and G. Galli, “Self-healing of CdSe nanocrystals: First-principles calculations,” Phys. Rev. Lett. 92, 217401 (2004). BES
CdSe semiconductor nanocrystals are a promising building block for new nanoscale materials. Puzder et al. completed an ab initio study that shows how the surface reconstructions of CdSe nanoparticles affect their electronic and optical properties. Atomic structures of the clusters are relaxed both in vacuum and in the presence of surfactant ligands, indicating significant geometrical rearrangements of the nanoparticle surface in both environments while the wurtzite core is maintained (Figure 12). These reconstructions lead to the opening of an optical gap without the aid of passivating ligands, thus “self-healing” the surface electronic structure. Calculations also predict the existence of a midgap state responsible for recently observed subband emission.
Segregation of Platinum in Nanoparticles
G. Wang, M. A. Van Hove, P. N. Ross, and M. I. Baskes, “Monte Carlo simulations of segregation in Pt–Re catalyst nanoparticles,” J. Chem. Phys. 121, 5410 (2004). BES
Bimetallic mixtures of platinum (Pt) and rhenium (Re) on alumina are widely used as catalysts in petroleum refining and show promise for fuel cell applications. Wang et al. used the Monte Carlo simulation method to investigate the surface segregation and the core–shell structures of Pt–Re nanoparticles. They found that regardless of the shape of the nanoparticles, the Pt atoms segregate preferentially to the facet sites, less to edge sites, and least to vertex sites in the outermost atomic layer. Particularly interesting was the discovery of surface reconstruction on some of the facets, reflecting the preference of Pt atoms to form close-packed hexagonal structures at surfaces.
Excitonic Entanglement in Quantum Dot Pairs
G. Bester, J. Shumway and A. Zunger, “Theory of excitonic spectra and entanglement engineering in dot molecules,” Phys. Rev. Lett. 93, 047401 (2004). BES, SciDAC
Bester et al. calculated the correlated pseudopotential wave function of an exciton in a pair of vertically stacked InGaAs/GaAs dots, and found that competing effects of strain, geometry, and band mixing led to many unexpected features missing in contemporary models. The first four excitonic states were all optically active at small interdot separation, due to the broken symmetry of the single-particle states. The authors quantified the degree of entanglement of the exciton wavefunctions, showed its sensitivity to interdot separation, and suggested ways to spectroscopically identify and maximize the entanglement of exciton states.