Independent particle descriptions of tunneling from a many-body perspective

Currents across thin insulators are commonly taken as single electrons moving across classically forbidden regions; this independent particle picture is well-known to describe most tunneling phenomena. Examining quantum transport from a different perspective, i.e., by explicit treatment of electron-electron interactions, we evaluate different single particle approximations with specific application to tunneling in metal-molecule-metal junctions. We find maximizing the overlap of a Slater determinant composed of single particle states to the many-body current-carrying state is more important than energy minimization for defining single particle approximations in a system with open boundary conditions. Thus the most suitable single particle effective potential is not one commonly in use by electronic structure methods, such as the Hartree-Fock or Kohn-Sham approximations.Comment: 4+ pages, 4 figures; accepted to Phys. Rev. B Rapid Communication

Mechanical Response of Hollow Metallic Nanolattices: Combining Structural and Material Size Effects

Ordered cellular solids have higher compressive yield strength and stiffness compared to stochastic foams. The mechanical properties of cellular solids depend on their relative density and follow structural scaling laws. These scaling laws assume the mechanical properties of the constituent materials, like modulus and yield strength, to be constant and dictate that equivalent-density cellular solids made from the same material should have identical mechanical properties. We present the fabrication and mechanical properties of three-dimensional hollow gold nanolattices whose compressive responses demonstrate that strength and stiffness vary as a function of geometry and tube wall thickness. All nanolattices had octahedron geometry, a constant relative density, ρ ∼ 5%, a unit cell size of 5–20 μm, and a constant grain size in the Au film of 25–50 nm. Structural effects were explored by increasing the unit cell angle from 30 deg to 60 deg while keeping all other parameters constant; material size effects were probed by varying the tube wall thickness, t, from 200 nm to 635 nm, at a constant relative density and grain size. In situ uniaxial compression experiments revealed an order of magnitude increase in yield stress and modulus in nanolattices with greater lattice angles, and a 150% increase in the yield strength without a concomitant change in modulus in thicker-walled nanolattices for fixed lattice angles. These results imply that independent control of structural and material size effects enables tunability of mechanical properties of three-dimensional architected metamaterials and highlight the importance of material, geometric, and microstructural effects in small-scale mechanics

Assessing N competition between outplanted conifer seedlings and early successional plants using ion-exchange membranes

Non-Peer ReviewedDuring the early establishment phase, outplanted white spruce (Picea glauca (Moench) Voss.) and jack pine (Pinus banksiana Lamb.) seedlings are vulnerable to lethargic growth or mortality because of interspecific competition for soil nutrients, particularly nitrogen (N). Accurately quantifying the degree of N competition is essential for supporting effective vegetation management decisions. This study evaluated N competition at four boreal forest sites, three years following outplanting, using two-week in situ burials of ion-exchange membrane (IEM) in plots with and without vegetation management (VM). The effect of noncrop N uptake on soil N availability also was assessed using conventional 2N KCl extractions. Vegetation management continued to support increased conifer seedling growth, with no effect on survival compared to control plots. Although the N supply rate measured using IEM (Plant Root Simulator™-probes) were not correlated (P >0.05) with 2N KCl-extracted N concentration, there was a correlation (R2 = 0.68 to 0.76, P <0.01) between N supply rate and seedling growth. Ammonium-N supply rate was better correlated than NO3--N with conifer seedling growth, which is in agreement with preferential NH4+-N uptake by conifer species. The results of this study support the use of in situ IEM burials for monitoring soil N bioavailability during the early establishment phase

Higher compressive strengths and the Bauschinger effect in conformally passivated copper nanopillars

Our current understanding of size-dependent strength in nano- and microscale crystals is centered around the idea that the overall strength is determined by the stress required to propagate dislocation sources. The nature and type of these dislocation sources is the subject of extensive debate, however, one commonality amongst these theories is that the ability of the free surface to absorb dislocations is a necessary condition for transition to a source controlled regime. In this work we demonstrate that atomic layer deposition (ALD) of conformal 5–25 nm thick TiO_2/Al_(2)O_3 coatings onto electroplated single crystalline copper pillars with diameters ranging from 75 nm to 1 μm generally inhibits the ability of a dislocation to vanish at the free surface. Uniaxial compression tests reveal increased strength and hardening relative to uncoated pillars at equivalent diameters, as well as a notable recovery of plastic strain during unloading, i.e. the Bauschinger effect. Unlike previous reports, these coated pillars retained the stochastic signature in their stress–strain curves. We explain these observations within the framework of a size-dependent strength theory based on a single arm source model, dislocation theory, and microstructural analysis by transmission electron microscopy

Strain rate effects in the mechanical response of polymer anchored carbon nanotube foams

Super-compressible foam-like carbon nanotube films have been reported to exhibit highly nonlinear viscoelastic behaviour in compression similar to soft tissue. Their unique combination of light weight and exceptional electrical, thermal and mechanical properties have helped identify them as viable building blocks for more complex nanosystems and as stand-alone structures for a variety of different applications. In the as-grown state, their mechanical performance is limited by the weak adhesion between the tubes, controlled by the van der Waals forces, and the substrate allowing the forests to split easily and to have low resistance in shear. Under axial compression loading carbon nanotubes have demonstrated bending, buckling8 and fracture9 (or a combination of the above) depending on the loading conditions and on the number of loading cycles. In this work, we partially anchor dense vertically aligned foam-like forests of carbon nanotubes on a thin, flexible polymer layer to provide structural stability, and report the mechanical response of such systems as a function of the strain rate. We test the sample under quasi-static indentation loading and under impact loading and report a variable nonlinear response and different elastic recovery with varying strain rates. A Bauschinger-like effect is observed at very low strain rates while buckling and the formation of permanent defects in the tube structure is reported at very high strain rates. Using high-resolution transmission microscopyComment: 19 Pages, 4 Figure

Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses

The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of ≈2 μm, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these “designed” composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems

Designing core-shell 3D photonic crystal lattices for negative refraction

We use a plane wave expansion method to define parameters for the fabrication of 3-dimensional (3D) core-shell photonic crystals (PhCs) with lattice geometries that are capable of all-angle negative refraction (AANR) in the midinfrared centered around 8.0 μm. We discuss the dependence of the AANR frequency range on the volume fraction of solid within the lattice and on the ratio of the low index core material to the high index shell material. Following the constraints set by simulations, we fabricate two types of nanolattice PhCs: (1) polymer core-germanium shell and (2) amorphous carbon core-germanium shell to enable experimental observation of 3D negative refraction and related dispersion phenomena at infrared and eventually optical frequencies