Flux through a hole from a shaken granular medium

We have measured the flux of grains from a hole in the bottom of a shaken container of grains. We find that the peak velocity of the vibration, vmax, controls the flux, i.e., the flux is nearly independent of the frequency and acceleration amplitude for a given value of vmax. The flux decreases with increasing peak velocity and then becomes almost constant for the largest values of vmax. The data at low peak velocity can be quantitatively described by a simple model, but the crossover to nearly constant flux at larger peak velocity suggests a regime in which the granular density near the container bottom is independent of the energy input to the system.Comment: 14 pages, 4 figures. to appear in Physical Review

Thermoelectric power factor under strain-induced band-alignment in the half-Heuslers NbCoSn and TiCoSb

Band convergence is an effective strategy to improve the thermoelectric performance of complex bandstructure thermoelectric materials. Half-Heuslers are good candidates for band convergence studies because they have multiple bands near the valence bad edge that can be converged through various band engineering approaches providing power factor improvement opportunities. Theoretical calculations to identify the outcome of band convergence employ various approximations for the carrier scattering relaxation times (the most common being the constant relaxation time approximation) due to the high computational complexity involved in extracting them accurately. Here, we compare the outcome of strain-induced band convergence under two such scattering scenarios: i) the most commonly used constant relaxation time approximation and ii) energy dependent inter- and intra-valley scattering considerations for the half-Heuslers NbCoSn and TiCoSb. We show that the outcome of band convergence on the power factor depends on the carrier scattering assumptions, as well as the temperature. For both materials examined, band convergence improves the power factor. For NbCoSn, however, band convergence becomes more beneficial as temperature increases, under both scattering relaxation time assumptions. In the case of TiCoSb, on the other hand, constant relaxation time considerations also indicate that the relative power factor improvement increases with temperature, but under the energy dependent scattering time considerations, the relative improvement weakens with temperature. This indicates that the scattering details need to be accurately considered in band convergence studies to predict more accurate trends.Comment: 21 pages, 8 figures. arXiv admin note: text overlap with arXiv:1905.0795

High pressure CVD inside microstructured optical fibres

We report the fabrication of semiconductor structures within holey fibres via a pressure driven microfluidic chemical vapour deposition process, demonstrating templated growth of crystalline Group IV semiconductor structures and devices in extreme aspect ratio geometries

Recent advances on thermoelectric materials

By converting waste heat into electricity through the thermoelectric power of solids without producing greenhouse gas emissions, thermoelectric generators could be an important part of the solution to today's energy challenge. There has been a resurgence in the search for new materials for advanced thermoelectric energy conversion applications. In this paper, we will review recent efforts on improving thermoelectric efficiency. Particularly, several novel proof-of-principle approaches such as phonon disorder in phonon-glasselectron crystals, low dimensionality in nanostructured materials and charge-spin-orbital degeneracy in strongly correlated systems on thermoelectric performance will be discussed.Comment: 12 pages, 12 figure

Fabrication of extreme aspect ratio wires within photonic crystal fibers

We have recently fabricated continuous semiconducting micro and nanowires within the empty spaces of highly ordered microstructured (e.g., photonic crystal or holey) optical fibers (MOFs). These systems contain the highest aspect ratio semiconductor micro- and nanowires yet produced by any method: centimeters long and ~100 nm in diameter. These structures combine the flexible light guiding capabilities of an optical fiber with the electronic and optical functionalities of semiconductors and have many potential applications for in-fiber sensing, including in-fiber detection, modulation, and generation of light

Microstructured optical fibers embedded with semiconductors and metals: a potential route to fiberized metamaterials

Functional optoelectronic materials such as bulk crystalline semiconductors and plasmonic materials such as metals inside optical waveguides could lead to fiber devices with radically new electronic and photonic degrees of freedom. The growth of such materials inside microstructured optical fiber air-silica templates using our microfluidic ultrahigh pressure chemical fluid technique will be discussed, along with the performance of initial demonstrator devices and their putative metamaterials applications