The effect of optic flow cues on honeybee flight control in wind

To minimize the risk of colliding with the ground or other obstacles, flying animals need to control both their ground speed and ground height. This task is particularly challenging in wind, where head winds require an animal to increase its airspeed to maintain a constant ground speed and tail winds may generate negative airspeeds, rendering flight more difficult to control. In this study, we investigate how head and tail winds affect flight control in the honeybee Apis mellifera , which is known to rely on the pattern of visual motion generated across the eye—known as optic flow—to maintain constant ground speeds and heights. We find that, when provided with both longitudinal and transverse optic flow cues (in or perpendicular to the direction of flight, respectively), honeybees maintain a constant ground speed but fly lower in head winds and higher in tail winds, a response that is also observed when longitudinal optic flow cues are minimized. When the transverse component of optic flow is minimized, or when all optic flow cues are minimized, the effect of wind on ground height is abolished. We propose that the regular sidewards oscillations that the bees make as they fly may be used to extract information about the distance to the ground, independently of the longitudinal optic flow that they use for ground speed control. This computationally simple strategy could have potential uses in the development of lightweight and robust systems for guiding autonomous flying vehicles in natural environments.

Unconventional Co-Existence of Insulating Nano-Regions and Conducting Filaments in Reduced SrTiO3: Mode Softening, Local Piezoelectricity, and Metallicity

Doped SrTiO3 becomes a metal at extremely low doping concentrations n and is even superconducting at n < 1020 cm−3, with the superconducting transition temperature adopting a dome-like shape with increasing carrier concentration. In this paper it is shown within the polarizability model and from first principles calculations that up to a well-defined carrier concentration nc transverse optic mode softening takes place together with polar nano-domain formation, which provides evidence of inhomogeneity and a two-component type behavior with metallicity coexisting with polarity. Beyond this region, a conventional metal is formed where superconductivity as well as mode softening is absent. For n ≤ nc the effective electron-phonon coupling follows the superconducting transition temperature. Effusion measurements, as well as macroscopic and nanoscopic conductivity measurements, indicate that the distribution of oxygen vacancies is local and inhomogeneous, from which it is concluded that metallicity stems from filaments which are embedded in a polar matrix as long as n ≤ nc.

Strong Coupling of Folded Phonons with Plasmons in 6H-SiC Micro/Nanocrystals

Silicon carbide (SiC) has a large number of polytypes of which 3C-, 4H-, 6H-SiC are most common. Since different polytypes have different energy gaps and electrical properties, it is important to identify and characterize various SiC polytypes. Here, Raman scattering is performed on 6H-SiC micro/nanocrystal (MNC) films to investigate all four folded transverse optic (TO) and longitudinal optic (LO) modes. With increasing film thickness, the four folded TO modes exhibit the same frequency downshift, whereas the four folded LO modes show a gradually-reduced downshift. For the same film thickness, all the folded modes show larger frequency downshifts with decreasing MNC size. Based on plasmons on MNCs, these folded modes can be attributed to strong coupling of the folded phonons with plasmons which show different strengths for the different folded modes while changing the film thickness and MNC size. This work provides a useful technique to identify SiC polytypes from Raman scattering.

On the role of micro-inertia in enriched continuum mechanics

In this paper, the role of gradient micro-inertia terms η ¯ ∥ ∇ u , t ∥ 2 and free micro-inertia terms η ∥ P , t ∥ 2 is investigated to unveil their respective effects on the dynamic behaviour of band-gap metamaterials. We show that the term η ¯ ∥ ∇ u , t ∥ 2 alone is only able to disclose relatively simplified dispersive behaviour. On the other hand, the term η ∥ P , t ∥ 2 alone describes the full complex behaviour of band-gap metamaterials. A suitable mixing of the two micro-inertia terms allows us to describe a new feature of the relaxed-micromorphic model, i.e. the description of a second band-gap occurring for higher frequencies. We also show that a split of the gradient micro-inertia η ¯ ∥ ∇ u , t ∥ 2 , in the sense of Cartan–Lie decomposition of matrices, allows us to flatten separately the longitudinal and transverse optic branches, thus giving us the possibility of a second band-gap. Finally, we investigate the effect of the gradient inertia η ¯ ∥ ∇ u , t ∥ 2 on more classical enriched models such as the Mindlin–Eringen and the internal variable ones. We find that the addition of such a gradient micro-inertia allows for the onset of one band-gap in the Mindlin–Eringen model and three band-gaps in the internal variable model. In this last case, however, non-local effects cannot be accounted for, which is a too drastic simplification for most metamaterials. We conclude that, even when adding gradient micro-inertia terms, the relaxed micromorphic model remains the best performing one, among the considered enriched models, for the description of non-local band-gap metamaterials.

Bright photoluminescence from ordered arrays of SiGe nanowires grown on Si(111)

We report on the optical properties of SiGe nanowires (NWs) grown by molecular beam epitaxy (MBE) in ordered arrays on SiO2/Si(111) substrates. The production method employs Au catalysts with self-limited sizes deposited in SiO2-free sites opened-up in the substrate by focused ion beam patterning for the preferential nucleation and growth of these well-organized NWs. The NWs thus produced have a diameter of 200 nm, a length of 200 nm, and a Ge concentration x = 0.15. Their photoluminescence (PL) spectra were measured at low temperatures (from 6 to 25 K) with excitation at 405 and 458 nm. There are four major features in the energy range of interest (980–1120 meV) at energies of 1040.7, 1082.8, 1092.5, and 1098.5 meV, which are assigned to the NW-transverse optic (TO) Si–Si mode, NW-transverse acoustic (TA), Si–substrate–TO and NW-no-phonon (NP) lines, respectively. From these results the NW TA and TO phonon energies are found to be 15.7 and 57.8 meV, respectively, which agree very well with the values expected for bulk Si1− x Ge x with x = 0.15, while the measured NW NP energy of 1099 meV would indicate a bulk-like Ge concentration of x = 0.14. Both of these concentrations values, as determined from PL, are in agreement with the target value. The NWs are too large in diameter for a quantum confinement induced energy shift in the band gap. Nevertheless, NW PL is readily observed, indicating that efficient carrier recombination is occurring within the NWs.

Magnetism in a Spintronic Compound Zr0.8Cr0.2O2 of Small Crystallites

A transition metal ion Cr4+(3d2 spins) doped ZrO2 of small crystallites presents tailored magnetic, electrical and optical properties useful for magnetodielectric and spintronic applications. A liquid polymer precursor gel (Cr4+ and Zr4+ complex with glycerol) when heated in an autoclave at a small pressure 0.6-0.7 atm at 420–470 K results in a compound Zr0.8Cr0.2O2 with an average 5 nm crystallite size of a stabilized cubic (c)-ZrO2-type phase (after 2 h annealing at 773 K in air). A broad Raman band is observed at ~300 cm-1 in acoustic and transverse optic phonons characteristic of the c-ZrO2-type structure, with a prominent O–Cr–O stretching band at 878 cm-1 in a ferromagnetic order. A CrO2/Cr2O3 surface layer exhibits two weak bands at ~1011 cm-1 and 1032 cm-1. The sample exhibits an ‘S’-shaped ferromagnetic hysteresis loop (does not saturate below 60 kOe fields) at 5 K, with a magnetization M = 85.79 emu/g (in the CrO2 part) at 60 kOe and coercivity Hc = 100 Oe. With warming above 5 K, the loop converges progressively with only a weak ferromagnetism at room temperature, M = 9.08 emu/g and Hc = 54.2 Oe. As a pinning barrier, the uncompensated spins in the surface layer supports coercivity at low temperature. A model magnetic structure describes the magnetic properties in correlation to the microstructure.

Nitrogen induced optical phonon shift in GaNyAs1-y studied by Raman Scattering

ABSTRACTWe present a Raman study of pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y grown on GaAs(100) with y ranging from 0 to 0.06. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes for y = 0.06 are represented by the strong longitudinal optic (LO1) mode at 287.4 cm-1 and the weaker transverse optic (TO1) mode at 269.0 cm-1, while the GaN-like LO2 mode is observed at 475.6 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration in the alloy increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the linearly increasing GaN-like LO2 phonon band deviates from linearity at higher nitrogen concentration (y ≥ 0.03). The reason for the deviation of the LO2 phonon from linearity is discussed.

Infrared Dielectric Properties of In1-xGaxAs Epilayers on InP (100)

ABSTRACTThe concentration dependence of optical phonons in strained In1-xGaxAs epilayers grown on InP (100) by chemical beam epitaxy has been characterized with oblique angle polarized far-infrared reflectivity measurements. In this powerful method, the reflectance spectra contain sharp Berreman peaks exactly at the optical phonon frequencies. For radiation polarized in the plane of incidence (p-polarized), peaks for both the TO and LO phonons were observed. For s-polarization only the TO modes were observed. For heavily doped substrates the TO film phonons were observed as reflectance minima, whereas for lightly doped substrates they were seen as maxima. The measured spectra were curve resolved to separate the effects of the various modes which included GaAs-like longitudinal and transverse optic (LO and TO), a disorder induced, and InAs-like LO and TO phonons. The dielectric response function and phonon frequency dependences for all modes were obtained versus Ga fraction for x from 0.25 to 0.75 and the latter showed a quadratic dependence on x over this range. The effects of strain on the phonon frequencies could then be evaluated.

Raman Characterization of Strained GaNyAs1-y and InxGa1-xNyAs1-y Epilayers

ABSTRACTPseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y have been grown on GaAs(100) with y ranging from 0 to 0.05. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes are represented at y = 0.05 by the strong longitudinal optic (LO1) mode at 288.5 cm-1 and the weaker transverse optic (TO1) mode at 268.3 cm-1, while the GaN-like LO2 mode is observed at 474.8 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the GaN-like LO2 phonon band displays a sub-linear increase in wavenumber. Raman results for the unstrained quaternary alloy In0.06Ga0.94N0.02As0.98 are compared with those of GaN0.02As0.98.